WorldWideScience

Sample records for band electronic structures

  1. Banded electron structures in the plasmasphere

    Energy Technology Data Exchange (ETDEWEB)

    Burke, W.J.; Rubin, A.G.; Hardy, D.A.; Holeman, E.G.

    1995-05-01

    The low-energy plasma analyzer on CRRES has detected significant fluxes of 10-eV to 30-keV electrons trapped on plasmaspheric field lines. On energy versus time spectrograms these electrons appear as banded structures that can span the 2 < L < 6 range of magnetic shells. The authors present an example of banded electron structures, encountered in the nightside plasmasphere during the magnetically quiet January 30, 1991. Empirical analysis suggests that two clouds of low energy electrons were injected from the plasma sheet to L < 4 on January 26 and 27 while the convective electric field was elevated. The energies of electrons in the first cloud were greater than those in the second. DMSP F8 measurements show that after the second injection, the polar cap potential rapidly decreased from >50 to <20 kY. Subsequent encounters with the lower energy cloud on alternating CRRES orbits over the next 2 days showed a progressive, earthward movement of the electrons, inner boundary. Whistler and electron cyclotron harmonic emissions accompanied the most intense manifestations of cloud electrons. The simplest explanation of these measurements is that after initial injection, the AIfven boundary moved Outward, leaving the cloud electrons on closed drift paths. Subsequent fluctuations of the convective electric field penetrated the plasmasphere, transporting cloud elements inward. The magnetic shell distribution of electron temperatures in one of the banded structures suggests that radiative energy losses may be comparable in magnitude to gains due to adiabatic compression.

  2. Complex band structure and superlattice electronic states

    Science.gov (United States)

    Schulman, J. N.; McGill, T. C.

    1981-04-01

    The complex band structures of the bulk materials which constitute the alternating layer (001) semiconductor-semiconductor superlattice are investigated. The complex bands near the center of the Brillouin zone in the [001] direction are studied in detail. The decay lengths of superlattice states whose energies lie in the bulk band gaps of one of the semiconductors are determined from the dispersion curves of these bands for imaginary k-->. This method is applied using a tight-binding band-structure calculation to two superlattices: the AlAs-GaAs superlattice and the CdTe-HgTe superlattice. The decay lengths of AlAs-GaAs superlattice conduction-band minimum states are found to be substantially shorter than those for the CdTe-HgTe superlattice. These differences in the decay of the states in the two superlattices result in differences in the variation of the conduction-band effective masses with the thickness of the AlAs and CdTe layers. The conduction-band effective masses increase more rapidly with AlAs thickness in the AlAs-GaAs superlattice than with CdTe thickness in the CdTe-HgTe superlattice.

  3. Electronic structure of heavy fermions: narrow temperature-independent bands

    Energy Technology Data Exchange (ETDEWEB)

    Arko, A.J.; Joyce, J.J.; Andrews, A.B.; Thompson, J.D.; Smith, J.L. [Los Alamos National Lab., NM (United States); Moshopoulou, E.; Fisk, Z. [NHMFL, Florida State Univ., Tallahassee, FL (United States); Menovsky, A.A. [Amsterdam Univ. (Netherlands). Natuurkundig Lab.; Canfield, P.C.; Olson, C.G. [Iowa State Univ., Ames, IA (United States). Ames Lab.

    1997-02-01

    The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from ARPES data reported here for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable. (orig.).

  4. The electronic structure of heavy fermions: Narrow temperature independent bands

    Energy Technology Data Exchange (ETDEWEB)

    Arko, A.J.; Joyce, J.J.; Smith, J.L.; Andrews, A.B. [and others

    1996-08-01

    The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from photoemission for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable.

  5. Electronic structure of NiO: Correlation and band effects

    Energy Technology Data Exchange (ETDEWEB)

    Shen, Z. (Stanford Electronics Laboratory, Stanford University, Stanford, California (USA)); List, R.S. (Los Alamos National Laboratory, Los Alamos, New Mexico (USA)); Dessau, D.S.; Wells, B.O. (Stanford Electronics Laboratory, Stanford University, Stanford, California (USA)); Jepsen, O. (Max-Planck-Institute for Solid State Research, D-7000 Stuttgart 80 (Federal Republic of Germany)); Arko, A.J.; Barttlet, R. (Los Alamos National Laboratory, Los Alamos, New Mexico (USA)); Shih, C.K. (Department of Physics, University of Texas, Austin, Texas (USA)); Parmigiani, F. (IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California (USA)); Huang, J.C.; Lindberg, P.A.P. (Stanford Electronics Laboratory, Stanford University, Stanford, California (USA))

    1991-08-15

    We have performed angle-resolved-photoemission experiments and local-density-functional (LDA) band calculations on NiO to study correlation and band effects of this conceptually important compound. Our experimental result suggests a dual nature of the electronic structure of NiO. On the one hand, the LDA band calculation has some relevance to the electronic structure of NiO, and the inclusion of the antiferromagnetic order is essential. For the lower O 2{ital p} bands, the LDA calculation agrees almost perfectly with experimental energy positions and dispersion relations. On the other hand, discrepancies between the experiment and the LDA calculation do exist, especially for the Ni 3{ital d} bands and the O 2{ital p} bands that are heavily mixed with the Ni 3{ital d} bands. It appears that the main discrepancies between the experimental results and the LDA calculation are concentrated in the regions of the insulating gap and the valence-band satellite. In addition to these results, we also report the interesting angle and photon-energy dependence of the satellite emission. The above results show that the angle-resolved-photoemission studies can provide much additional information about the electronic structure of correlated materials like NiO.

  6. Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.

    Science.gov (United States)

    Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun

    2015-05-27

    We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.

  7. Electronic structure of the valence band of II--VI wide band gap semiconductor interfaces

    OpenAIRE

    1996-01-01

    In this work we present the electronic band structure for (001)--CdTe interfaces with some other II--VI zinc blende semiconductors. We assume ideal interfaces. We use tight binding Hamiltonians with an orthogonal basis ($s p^3 s^*$). We make use of the well--known Surface Green's Function Matching method to calculate the interface band structure. In our calculation the dominion of the interface is constituted by four atomic layers. We consider here anion--anion interfaces only. We have includ...

  8. Crossing points in the electronic band structure of vanadium oxide

    Directory of Open Access Journals (Sweden)

    Keshav N. Shrivastava

    2010-03-01

    Full Text Available The electronic band structures of several models of vanadium oxide are calculated. In the models 1-3, every vanadium atom is connected to 4 oxygen atoms and every oxygen atom is connected to 4 vanadium atoms. In model 1, a=b=c 2.3574 Å; in model 2, a= 4.7148 Å, b= 2.3574 Å and c= 2.3574 Å; and in model 3, a= 4.7148 Å, b= 2.3574 Å and c= 4.7148 Å. In the models 4-6, every vanadium atom is connected to 4 oxygen atoms and every oxygen atom is connected to 2 vanadium atoms. In model 4, a=b= 4.551 Å and c= 2.851 Å; in model 5, a=b=c= 3.468 Å; and in model 6, a=b=c= 3.171 Å. We have searched for a crossing point in the band structure of all the models. In model 1 there is a point at which five bands appear to meet but the gap is 7.3 meV. In model 2 there is a crossing point between G and F points and there is a point between F and Q with the gap ≈ 3.6608 meV. In model 3, the gap is very small, ~ 10-5 eV. In model 4, the gap is 5.25 meV. In model 5, the gap between Z and G points is 2.035 meV, and in model 6 the gap at Z point is 4.3175 meV. The crossing point in model 2 looks like one line is bent so that the supersymmetry is broken. When pseudopotentials are replaced by a full band calculation, the crossing point changes into a gap of 2.72 x 10-4 eV.

  9. Electronic band structure of calcium selenide under pressure

    Energy Technology Data Exchange (ETDEWEB)

    Louail, L. [Faculty of Sciences, Department of Physics, University of Setif, Setif (Algeria)], E-mail: llouail@yahoo.fr; Haddadi, K.; Maouche, D.; Ali Sahraoui, F.; Hachemi, A. [Faculty of Sciences, Department of Physics, University of Setif, Setif (Algeria)

    2008-09-01

    Energy band structures under pressure of calcium selenide (CaSe) were calculated using the plane-wave pseudopotential code CASTEP. The results show a progressive transition from a direct to an indirect gap semiconductor at a pressure of about 2 GPa, in the B1 phase. An insulator-conductor change was also observed at 70 GPa, in the B2 phase. Concerning CaSe, these two results could not be evidenced in previous literature. Hence, our work is a first attempt in this direction.

  10. Determination of conduction and valence band electronic structure of anatase and rutile TiO2

    Indian Academy of Sciences (India)

    Jakub Szlachetko; Katarzyna Michalow-Mauke; Maarten Nachtegaal; Jacinto Sá

    2014-03-01

    Electronic structures of rutile and anatase polymorph of TiO2 were determined by resonant inelastic X-ray scattering measurements and FEFF9.0 calculations. Difference between crystalline structures led to shifts in the rutile Ti -band to lower energy with respect to anatase, i.e., decrease in band gap. Anatase possesses localized states located in the band gap where electrons can be trapped, which are almost absent in the rutile structure. This could well explain the reported longer lifetimes in anatase. It was revealed that HR-XAS is insufficient to study in-depth unoccupied states of investigated materials because it overlooks the shallow traps.

  11. Band structure of semiconductors

    CERN Document Server

    Tsidilkovski, I M

    2013-01-01

    Band Structure of Semiconductors provides a review of the theoretical and experimental methods of investigating band structure and an analysis of the results of the developments in this field. The book presents the problems, methods, and applications in the study of band structure. Topics on the computational methods of band structure; band structures of important semiconducting materials; behavior of an electron in a perturbed periodic field; effective masses and g-factors for the most commonly encountered band structures; and the treatment of cyclotron resonance, Shubnikov-de Haas oscillatio

  12. Electronic band structure effects in monolayer, bilayer, and hybrid graphene structures

    Science.gov (United States)

    Puls, Conor

    Since its discovery in 2005, graphene has been the focus of intense theoretical and experimental study owing to its unique two-dimensional band structure and related electronic properties. In this thesis, we explore the electronic properties of graphene structures from several perspectives including the magnetoelectrical transport properties of monolayer graphene, gap engineering and measurements in bilayer graphene, and anomalous quantum oscillation in the monolayer-bilayer graphene hybrids. We also explored the device implications of our findings, and the application of some experimental techniques developed for the graphene work to the study of a complex oxide, Ca3Ru2O7, exhibiting properties of strongly correlated electrons. Graphene's high mobility and ballistic transport over device length scales, make it suitable for numerous applications. However, two big challenges remain in the way: maintaining high mobility in fabricated devices, and engineering a band gap to make graphene compatible with logical electronics and various optical devices. We address the first challenge by experimentally evaluating mobilities in scalable monolayer graphene-based field effect transistors (FETs) and dielectric-covered Hall bars. We find that the mobility is limited in these devices, and is roughly inversely proportional to doping. By considering interaction of graphene's Dirac fermions with local charged impurities at the interface between graphene and the top-gate dielectric, we find that Coulomb scattering is responsible for degraded mobility. Even in the cleanest devices, a band gap is still desirable for electronic applications of graphene. We address this challenge by probing the band structure of bilayer graphene, in which a field-tunable energy band gap has been theoretically proposed. We use planar tunneling spectroscopy of exfoliated bilayer graphene flakes demonstrate both measurement and control of the energy band gap. We find that both the Fermi level and

  13. Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

    Science.gov (United States)

    Wang, Zefang; Zhao, Liang; Mak, Kin Fai; Shan, Jie

    2017-02-01

    We study the electronic band structure in the K/K' valleys of the Brillouin zone of monolayer WSe2 and MoSe2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe2, and parallel spins in monolayer MoSe2. The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first principles calculations. These values also suggest that both n- and p-type WSe2 and MoSe2 can be relevant for spin- and valley-based applications

  14. Effects of extrinsic and intrinsic perturbations on the electronic structure of graphene: Retaining an effective primitive cell band structure by band unfolding

    Science.gov (United States)

    Medeiros, Paulo V. C.; Stafström, Sven; Björk, Jonas

    2014-01-01

    We use a band unfolding technique to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations. This involves intrinsic perturbations, such as structural defects, and external ones, comprising nitrogen substitutions and the inclusion of graphene in adsorbed systems. In such cases, the band unfolding provides a reliable and efficient tool for quantitatively analyzing the effect of doping and defects on the electronic structure of graphene. We envision that this approach will become a standard method in the computational analysis of graphene's electronic structure in related systems.

  15. Electronic- and band-structure evolution in low-doped (Ga,Mn)As

    OpenAIRE

    Yastrubchak, O.; J. Sadowski; Krzyzanowska, H.; Gluba, L.; Zuk, J.; Domagala, J. Z.; Andrearczyk, T.; Wosinski, T.

    2013-01-01

    Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap transition energy with increasing Mn content in very low-doped ...

  16. Coexisting Honeycomb and Kagome Characteristics in the Electronic Band Structure of Molecular Graphene.

    Science.gov (United States)

    Paavilainen, Sami; Ropo, Matti; Nieminen, Jouko; Akola, Jaakko; Räsänen, Esa

    2016-06-08

    We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures.

  17. Engineering the electronic structure and band gap of boron nitride nanoribbon via external electric field

    Science.gov (United States)

    Chegel, Raad

    2016-06-01

    By using the third nearest neighbor modified tight binding (3NN-TB) method, the electronic structure and band gap of BNNRs under transverse electric fields are explored. The band gap of the BNNRs has a decreasing with increasing the intensity of the applied electric field, independent on the ribbon edge types. Furthermore, an analytic model for the dependence of the band gap in armchair and zigzag BNNRs on the electric field is proposed. The reduction of E g is similar for some N a armchair and N z zigzag BNNRs independent of their edges.

  18. The LDA+U calculation of electronic band structure of GaAs

    Science.gov (United States)

    Bahuguna, B. P.; Sharma, R. O.; Saini, L. K.

    2016-05-01

    We present the electronic band structure of bulk gallium arsenide (GaAs) using first principle approach. A series of calculations has been performed by applying norm-conserving pseudopotentials and ultrasoft non-norm-conserving pseudopotentials within the density functional theory. These calculations yield too small band gap as compare to experiment. Thus, we use semiemperical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U), which is quite effective in order to describe the band gap of GaAs.

  19. Electronic band structure effects in the stopping of protons in copper

    Science.gov (United States)

    Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.

    2016-10-01

    We present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v =0.02 -10 a .u . where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ˜1.5 ) in the velocity range v =0.07 -0.3 a .u . , which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v →0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.

  20. Electronic Band Structures of TiO2 with Heavy Nitrogen Doping

    Institute of Scientific and Technical Information of China (English)

    XUE Jinbo; LI Qi; LIANG Wei; SHANG Jianku

    2008-01-01

    The first-principles density-functional calculation was conducted to investigate the electronic band structures of titanium dioxide with heavy nitrogen doping (TiO2-xNx).The calculation results indicate that when x≤0.25,isolated N 2p states appear above the valence-band maximum of TiO2 without a band-gap narrowing between O 2p and Ti 3d states.When x≥0.50,an obvious band gap narrowing between O 2p and Ti 3d states was observed along with the existence of isolated N 2p states above the valence-band of TiO2,indicating that the mechanism proposed by Asahi et al operates under heavy nitrogen doping condition.

  1. Direct Measurement of the Band Structure of a Buried Two-Dimensional Electron Gas

    DEFF Research Database (Denmark)

    Miwa, Jill; Hofmann, Philip; Simmons, Michelle Y.;

    2013-01-01

    We directly measure the band structure of a buried two dimensional electron gas (2DEG) using angle resolved photoemission spectroscopy. The buried 2DEG forms 2 nm beneath the surface of p-type silicon, because of a dense delta-type layer of phosphorus n-type dopants which have been placed there. ...

  2. Doping and strain dependence of the electronic band structure in Ge and GeSn alloys

    Science.gov (United States)

    Xu, Chi; Gallagher, James; Senaratne, Charutha; Brown, Christopher; Fernando, Nalin; Zollner, Stefan; Kouvetakis, John; Menendez, Jose

    2015-03-01

    A systematic study of the effect of dopants and strain on the electronic structure of Ge and GeSn alloys is presented. Samples were grown by UHV-CVD on Ge-buffered Si using Ge3H8 and SnD4 as the sources of Ge and Sn, and B2H6/P(GeH3)3 as dopants. High-energy critical points in the joint-density of electronic states were studied using spectroscopic ellipsometry, which yields detailed information on the strain and doping dependence of the so-called E1, E1 +Δ1 , E0' and E2 transitions. The corresponding dependencies of the lowest direct band gap E0 and the fundamental indirect band gap Eindwere studied via room-T photoluminescence spectroscopy. Of particular interest for this work were the determination of deformation potentials, band gap renormalization effects, Burstein-Moss shifts due to the presence of carriers at band minima, and the dependence of other critical point parameters, such as amplitudes and phase angles, on the doping concentration. The selective blocking of transitions due to high doping makes it possible to investigate the precise k-space location of critical points. These studies are complemented with detailed band-structure calculations within a full-zone k-dot- p approach. Supported by AFOSR under DOD AFOSR FA9550-12-1-0208 and DOD AFOSR FA9550-13-1-0022.

  3. Electronic band structure and optical properties of the cubic, Sc, Y and La hydride systems

    Energy Technology Data Exchange (ETDEWEB)

    Peterman, D.J.

    1980-01-01

    Electronic band structure calculations are used to interpret the optical spectra of the cubic Sc, Y and La hydride systems. Self-consistent band calculations of ScH/sub 2/ and YH/sub 2/ were carried out. The respective joint densities of states are computed and compared to the dielectric functions determined from the optical measurements. Additional calculations were performed in which the Fermi level or band gap energies are rigidly shifted by a small energy increment. These calculations are then used to simulate the derivative structure in thermomodulation spectra and relate the origin of experimental interband features to the calculated energy bands. While good systematic agreement is obtained for several spectral features, the origin of low-energy interband transitions in YH/sub 2/ cannot be explained by these calculated bands. A lattice-size-dependent premature occupation of octahedral sites by hydrogen atoms in the fcc metal lattice is suggested to account for this discrepancy. Various non-self-consistent calculations are used to examine the effect of such a premature occupation. Measurements of the optical absorptivity of LaH/sub x/ with 1.6 < x < 2.9 are presented which, as expected, indicate a more premature occupation of the octahedral sites in the larger LaH/sub 2/ lattice. These experimental results also suggest that, in contrast to recent calculations, LaH/sub 3/ is a small-band-gap semiconductor.

  4. Impact of the electronic band structure in high-harmonic generation spectra of solids

    CERN Document Server

    Tancogne-Dejean, Nicolas; Kärtner, Franz X; Rubio, Angel

    2016-01-01

    An accurate analytic model describing high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that: (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals, (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential, the yield is increased for heavier atoms, and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.

  5. Banded structures in electron pitch angle diffusion coefficients from resonant wave-particle interactions

    Science.gov (United States)

    Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.

    2016-04-01

    Electron pitch angle (Dαα) and momentum (Dpp) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies ≤10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = ±1, ±2, … ±5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (α) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in Dαα and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than Dαα coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than Dαα coefficients for the case n ≠ 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of Dαα coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89° and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle 10° and Landau

  6. Banding and electronic structures of metal azides——Sensitivity and conductivity

    Institute of Scientific and Technical Information of China (English)

    肖鹤鸣; 李永富

    1995-01-01

    By using both DV-Xα and EH-CO methods, the calculation studies of the structure-property relationships of a series of metal azides, of their clusters’ electronic structures in ground and excited states, of their systems with cation vacancy and the doped Pb(N3)2, as well as their crystal band structures have been conducted. The results show that the sensitivity of ionic-type metal azides varies with the degree of difficulty of electronic transition of the losing charge on N3. A metal azide with cation vacancies has a greater sensitivity than the perfect one. When doped with monovalent metal ions, lead azide’s sensitivity increased; when with trivalent ones, its sensitivity decreased; when with divalent ones, little of it changed. Compared with heavy metal azides. an alkali metal azide has a larger band gap, a smaller band width and a greater transition energy of frontier electron with a smaller amount of losing charge on N3, and thus has lower sensitivity and conductivity than heavy metal azides.

  7. Electronic band structure of a Tl/Sn atomic sandwich on Si(111)

    Science.gov (United States)

    Gruznev, D. V.; Bondarenko, L. V.; Matetskiy, A. V.; Tupchaya, A. Y.; Alekseev, A. A.; Hsing, C. R.; Wei, C. M.; Eremeev, S. V.; Zotov, A. V.; Saranin, A. A.

    2015-01-01

    A two-dimensional compound made of one monolayer of Tl and one monolayer of Sn on Si(111) has been found to have a sandwichlike structure in which the Sn layer (having the milk-stool arrangement) resides on the bulklike terminated Si(111) surface and the Tl layer (having the honeycomb-chained-trimer arrangement) is located above the Sn layer. The electronic band structure of the compound contains two spin-split surface-state bands, of which one is nonmetallic and the other is metallic. Near the Fermi level the metallic band is split with the momentum splitting Δ k∥=0.037 Å-1 and energy splitting Δ EF=167 meV. The steep dispersion of the band when crossing the Fermi level corresponds to an electron velocity of ≈8.5 ×105 m/s, which is comparable to the value reported for graphene. The 2D Fermi contours have almost circular shape with spin texture typical for hexagonal surfaces.

  8. Ab initio electronic band structure study of III-VI layered semiconductors

    Science.gov (United States)

    Olguín, Daniel; Rubio-Ponce, Alberto; Cantarero, Andrés

    2013-08-01

    We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal ɛ-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.

  9. Correlation between morphology, electron band structure, and resistivity of Pb atomic chains on the Si(5 5 3)-Au surface.

    Science.gov (United States)

    Jałochowski, M; Kwapiński, T; Łukasik, P; Nita, P; Kopciuszyński, M

    2016-07-20

    Structural and electron transport properties of multiple Pb atomic chains fabricated on the Si(5 5 3)-Au surface are investigated using scanning tunneling spectroscopy, reflection high electron energy diffraction, angular resolved photoemission electron spectroscopy and in situ electrical resistance. The study shows that Pb atomic chains growth modulates the electron band structure of pristine Si(5 5 3)-Au surface and hence changes its sheet resistivity. Strong correlation between chains morphology, electron band structure and electron transport properties is found. To explain experimental findings a theoretical tight-binding model of multiple atomic chains interacting on effective substrate is proposed.

  10. Giant Amplification in Degenerate Band Edge Slow-Wave Structures Interacting with an Electron Beam

    CERN Document Server

    Othman, Mohamed A K; Figotin, Alexander; Capolino, Filippo

    2015-01-01

    We advance here a new amplification regime based on synchronous operation of four degenerate electromagnetic (EM) modes and the electron beam referred to as super synchronization. These four EM modes arise in a Fabry-Perot cavity (FPC) when degenerate band edge (DBE) condition is satisfied. The modes interact constructively with the electron beam resulting in superior amplification. In particular, much larger gains are achieved for smaller beam currents compared to conventional structures allowing for synchronization with only a single EM mode. We construct a mutli transmission line (MTL) model for a loaded waveguide slow-wave structure exhibiting a DBE, and investigate the phenomenon of giant gain via super synchronization using generalized Pierce model.

  11. First principles electronic band structure and phonon dispersion curves for zinc blend beryllium chalcogenide

    Energy Technology Data Exchange (ETDEWEB)

    Dabhi, Shweta, E-mail: venu.mankad@gmail.com; Mankad, Venu, E-mail: venu.mankad@gmail.com; Jha, Prafulla K., E-mail: venu.mankad@gmail.com [Department of Physics, Maharaja Krishnakumasinhji Bhavnagar University, Bhavnagar-364001 (India)

    2014-04-24

    A detailed theoretical study of structural, electronic and Vibrational properties of BeX compound is presented by performing ab-initio calculations based on density-functional theory using the Espresso package. The calculated value of lattice constant and bulk modulus are compared with the available experimental and other theoretical data and agree reasonably well. BeX (X = S,Se,Te) compounds in the ZB phase are indirect wide band gap semiconductors with an ionic contribution. The phonon dispersion curves are represented which shows that these compounds are dynamically stable in ZB phase.

  12. Physical properties and electronic band structure of noncentrosymmetric Th7Co3 superconductor.

    Science.gov (United States)

    Sahakyan, M; Tran, V H

    2016-05-25

    The physical properties of the noncentrosymmetric superconductor Th7Co3 have been investigated by means of ac-magnetic susceptibility, magnetization, specific heat, electrical resistivity, magnetoresistance and Hall effect measurements. From these data it is established that Th7Co3 is a dirty type-II superconductor with [Formula: see text] K, [Formula: see text] and moderate electron-phonon coupling [Formula: see text]. Some evidences for anisotropic superconducting gap are found, including e.g. reduced specific heat jump ([Formula: see text]) at T c, diminished superconducting energy gap ([Formula: see text]) as compared to the BCS values, power law field dependence of the Sommerfeld coefficient at 0.4 K ([Formula: see text]), and a concave curvature of the [Formula: see text] line. The magnitudes of the thermodynamic critical field and the energy gap are consistent with mean-squared anisotropy parameter [Formula: see text]. The electronic specific heat in the superconducting state is reasonably fitted to an oblate spheroidal gap model. Calculations of scalar relativistic and fully relativistic electronic band structures reveal considerable differences in the degenerate structure, resulting from asymmetric spin-orbit coupling (ASOC). A large splitting energy of spin-up spin-down bands at the Fermi level E F, [Formula: see text] meV is observed and a sizeable ratio [Formula: see text] could classify the studied compound into the class of noncentrosymmetric superconductors with strong ASOC. The noncentrosymmetry of the crystal structure and the atomic relativistic effects are both responsible for an importance of ASOC in Th7Co3. The calculated results for the density of states show a Van Hove singularity just below E F and dominant role of the 6d electrons of Th to the superconductivity.

  13. From Metal Cluster to Metal Nanowire: A Topological Analysis of Electron Density and Band Structure Calculation

    Directory of Open Access Journals (Sweden)

    Yu Wang

    2002-01-01

    Full Text Available Abstract:We investigate a theoretical model of molecular metalwire constructed from linear polynuclear metal complexes. In particular we study the linear Crn metal complex and Cr molecular metalwire. The electron density distributions of the model nanowire and the linear Crn metal complexes, with n = 3, 5, and 7, are calculated by employing CRYSTAL98 package with topological analysis. The preliminary results indicate that the bonding types between any two neighboring Cr are all the same, namely the polarized open-shell interaction. The pattern of electron density distribution in metal complexes resembles that of the model Cr nanowire as the number of metal ions increases. The conductivity of the model Cr nanowire is also tested by performing the band structure calculation.

  14. The electronic and transport properties of monolayer transition metal dichalcogenides: a complex band structure analysis

    Science.gov (United States)

    Szczesniak, Dominik

    Recently, monolayer transition metal dichalcogenides have attracted much attention due to their potential use in both nano- and opto-electronics. In such applications, the electronic and transport properties of group-VIB transition metal dichalcogenides (MX2 , where M=Mo, W; X=S, Se, Te) are particularly important. Herein, new insight into these properties is presented by studying the complex band structures (CBS's) of MX2 monolayers while accounting for spin-orbit coupling effects. By using the symmetry-based tight-binding model a nonlinear generalized eigenvalue problem for CBS's is obtained. An efficient method for solving such class of problems is presented and gives a complete set of physically relevant solutions. Next, these solutions are characterized and classified into propagating and evanescent states, where the latter states present not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gaps, which describe the tunneling currents in the MX2 materials. The importance of CBS's and tunneling currents is demonstrated by the analysis of the quantum transport across MX2 monolayers within phase field matching theory. Present work has been prepared within the Qatar Energy and Environment Research Institute (QEERI) grand challenge ATHLOC project (Project No. QEERI- GC-3008).

  15. Influence of variations in the electron-electron interaction on the ground state metric space "band structure" of a two-electron magnetic system

    Science.gov (United States)

    Sharp, P. M.; D'Amico, I.

    2016-02-01

    We consider a model system of two electrons confined in a two-dimensional harmonic oscillator potential, with the electrons interacting via an α / r2 potential, and subject to a magnetic field applied perpendicular to the plane of confinement. Our results show that variations in the strength of the electron-electron interaction generate a "band structure" in ground state metric spaces, which shares many characteristics with those generated as a result of varying the confinement potential. In particular, the metric spaces for wavefunctions, particle densities, and paramagnetic current densities all exhibit distinct "bands" and "gaps". The behavior of the polar angle of the bands also shares traits with that obtained by varying the confinement potential, but the behavior of the arc lengths of the bands on the metric space spheres can be seen to be different for the two cases and opposite for a large range of angular momentum values. The findings here and in Refs. [1,2] demonstrate that the "band structure" that arises in ground state metric spaces when a magnetic field is applied is a robust feature.

  16. Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

    Science.gov (United States)

    Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

    2017-04-01

    We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

  17. Physical properties and electronic band structure of noncentrosymmetric Th7Co3 superconductor

    Science.gov (United States)

    Sahakyan, M.; Tran, V. H.

    2016-05-01

    The physical properties of the noncentrosymmetric superconductor Th7Co3 have been investigated by means of ac-magnetic susceptibility, magnetization, specific heat, electrical resistivity, magnetoresistance and Hall effect measurements. From these data it is established that Th7Co3 is a dirty type-II superconductor with {{T}\\text{c}}=1.8+/- 0.02 K, Hc2\\text{orb}text{kOe}c2p and moderate electron-phonon coupling {λ\\text{el-\\text{ph}}}=0.56 . Some evidences for anisotropic superconducting gap are found, including e.g. reduced specific heat jump (Δ {{C}p}/γ {{T}\\text{c}}=1.01 ) at T c, diminished superconducting energy gap ({{Δ }0}/{{k}\\text{B}}{{T}\\text{c}}=2.17 ) as compared to the BCS values, power law field dependence of the Sommerfeld coefficient at 0.4 K ({{C}p}/T\\propto {{H}0.6} ), and a concave curvature of the {{H}c2}≤ft({{T}\\text{c}}\\right) line. The magnitudes of the thermodynamic critical field and the energy gap are consistent with mean-squared anisotropy parameter ˜ 0.23 . The electronic specific heat in the superconducting state is reasonably fitted to an oblate spheroidal gap model. Calculations of scalar relativistic and fully relativistic electronic band structures reveal considerable differences in the degenerate structure, resulting from asymmetric spin-orbit coupling (ASOC). A large splitting energy of spin-up spin-down bands at the Fermi level E F, Δ {{E}\\text{ASOC}}˜ 100 meV is observed and a sizeable ratio Δ {{E}\\text{ASOC}}/{{k}\\text{B}}{{T}\\text{c}}˜ 640 could classify the studied compound into the class of noncentrosymmetric superconductors with strong ASOC. The noncentrosymmetry of the crystal structure and the atomic relativistic effects are both responsible for an importance of ASOC in Th7Co3. The calculated results for the density of states show a Van Hove singularity just below E F and dominant role of the 6d electrons of Th to the superconductivity.

  18. Electronic band structure of a type-Ⅱ 'W' quantum well calculated by an eight-band k·p model

    Institute of Scientific and Technical Information of China (English)

    Yu Xiu; Gu Yong-Xian; Wang Qing; Wei Xin; Chen Liang-Hui

    2011-01-01

    In this paper, we present an investigation of type-Ⅱ 'W' quantum wells for the InAs/Ga1-xInxSb/AlSb family, where 'W' denotes the conduction profile of the material. We focus our attention on using the eight-band k·p model to calculate the band structures within the framework of finite element method. For the sake of clarity, the simulation in this paper is simplified and based on only one period-AlSb/InAs/Ga1-xInxSb/InAs/AlSb. The obtained numerical results include the energy levels and wavefunctions of carriers. We discuss the variations of the electronic properties by changing several important parameters, such as the thickness of either InAs or Ga1-xInxSb layer and the alloy composition in Ga1-xInxSb separately. In the last part, in order to compare the eight-band k·p model, we recalculate the conduction bands of the 'W' structure using the one-band k·p model and then discuss the difference between the two results, showing that conduction bands are strongly coupled with valence bands in the narrow band gap structure. The in-plane energy dispersions, which illustrate the suppression of the Auger recombination process, are also obtained.

  19. Electronic band structure of LaCoO3/Y/Mn compounds

    Science.gov (United States)

    Rahnamaye Aliabad, H. A.; Hesam, V.; Ahmad, Iftikhar; Khan, Imad

    2013-02-01

    Spin polarization effects on electronic properties of pure LaCoO3 and doped compounds (La0.5Y0.5CoO3, LaCo0.5Mn0.5O3) in the rhombohedral phase have been studied. We have employed the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA+U) under density functional theory (DFT). The calculated band structures along with total as well as partial densities of states reveal that Y and Mn impurities have a significant effect on the structural and electronic properties of LaCoO3. It is found that Mn alters insulating behavior of this compound to the half metallic for spin up state. Obtained results show that the magnetic moment for the Co-3d state is near 3.12μB in LaCoO3 compound which increases and decreases with addition of Y and Mn dopants respectively.

  20. First-Principles Band Calculations on Electronic Structures of Ag-Doped Rutile and Anatase TiO2

    Institute of Scientific and Technical Information of China (English)

    HOU Xing-Gang; LIU An-Dong; HUANG Mei-Dong; LIAO Bin; WU Xiao-Ling

    2009-01-01

    The electronic structures of Ag-doped rutile and anatase TiO2 are studied by first-principles band calculations based on density funetionai theory with the full-potentiai linearized-augraented-plane-wave method.New occupied bands ore found between the band gaps of both Ag-doped rutile and anatase TiO2.The formation of these new bands Capri be explained mainly by their orbitals of Ag 4d states mixed with Ti 3d states and are supposed to contribute to their visible light absorption.

  1. Cr-doping effects on unoccupied d-band electronic structure of TiO2

    Science.gov (United States)

    Wojtaszek, K.; Tyrala, K.; Czapla-Masztafiak, J.; Sa, J.; Szlachetko, J.

    2016-11-01

    Resonant X-ray emission spectroscopy (RXES) was applied to probe the electronic states above Fermi level for Cr-doped anatase TiO2. The results reveal doping-induced electronic band on low energy side of unoccupied electronic states. The experimental RXES data in combination with theoretical simulations allowed distinguishing electronic state changes on both, d-localized and d-delocalized orbitals of Cr-doped anatase TiO2. The detailed information on unoccupied electronic states is crucial in order to correlate enhanced light absorption properties with catalytic activity of the material.

  2. Electronic Band Structure of Transition Metal Dichalcogenides from Ab Initio and Slater–Koster Tight-Binding Model

    Directory of Open Access Journals (Sweden)

    Jose Ángel Silva-Guillén

    2016-10-01

    Full Text Available Semiconducting transition metal dichalcogenides present a complex electronic band structure with a rich orbital contribution to their valence and conduction bands. The possibility to consider the electronic states from a tight-binding model is highly useful for the calculation of many physical properties, for which first principle calculations are more demanding in computational terms when having a large number of atoms. Here, we present a set of Slater–Koster parameters for a tight-binding model that accurately reproduce the structure and the orbital character of the valence and conduction bands of single layer MX 2 , where M = Mo, W and X = S, Se. The fit of the analytical tight-binding Hamiltonian is done based on band structure from ab initio calculations. The model is used to calculate the optical conductivity of the different compounds from the Kubo formula.

  3. The electronic band structures of gadolinium chalcogenides: a first-principles prediction for neutron detecting

    Science.gov (United States)

    Li, Kexue; Liu, Lei; Yu, Peter Y.; Chen, Xiaobo; Shen, D. Z.

    2016-05-01

    By converting the energy of nuclear radiation to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess enough capability for capturing these neutral particles. While the element Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that γ-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 eV for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on the condition that their crystals can be grown with good quality.

  4. Experimental and theoretical investigations of the electronic band structure of metal-organic frameworks of HKUST-1 type

    Energy Technology Data Exchange (ETDEWEB)

    Gu, Zhi-Gang [Institut für Funktionelle Grenzflächen (IFG), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 350002 Fuzhou (China); Heinke, Lars, E-mail: Lars.Heinke@KIT.edu; Wöll, Christof [Institut für Funktionelle Grenzflächen (IFG), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); Neumann, Tobias; Wenzel, Wolfgang; Li, Qiang; Fink, Karin [Institute of Nanotechnology (INT), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen (Germany); Gordan, Ovidiu D.; Zahn, Dietrich R. T. [Semiconductor Physics, Technische Universität Chemnitz, 09107 Chemnitz (Germany)

    2015-11-02

    The electronic properties of metal-organic frameworks (MOFs) are increasingly attracting the attention due to potential applications in sensor techniques and (micro-) electronic engineering, for instance, as low-k-dielectric in semiconductor technology. Here, the band gap and the band structure of MOFs of type HKUST-1 are studied in detail by means of spectroscopic ellipsometry applied to thin surface-mounted MOF films and by means of quantum chemical calculations. The analysis of the density of states, the band structure, and the excitation spectrum reveal the importance of the empty Cu-3d orbitals for the electronic properties of HKUST-1. This study shows that, in contrast to common belief, even in the case of this fairly “simple” MOF, the excitation spectra cannot be explained by a superposition of “intra-unit” excitations within the individual building blocks. Instead, “inter-unit” excitations also have to be considered.

  5. Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

    Institute of Scientific and Technical Information of China (English)

    Kang Ling-Ling; Liu Ting-Yu; Zhang Qi-Ren; Xu Ling-Zhi; Zhang Fei-Wu

    2011-01-01

    The electronic structures of BaMgF4 crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slate-theory, using a numerically discrete variational (DV-Xα)method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band.The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.

  6. Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure.

    Science.gov (United States)

    Tolhurst, Thomas M; Leedahl, Brett; Andrews, Justin L; Marley, Peter M; Banerjee, Sarbajit; Moewes, Alexander

    2016-06-21

    New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.

  7. Dynamical and anharmonic effects on the electron-phonon coupling and the zero-point renormalization of the band structure

    Science.gov (United States)

    Antonius, Gabriel; Poncé, Samuel; Lantagne-Hurtubise, Étienne; Auclair, Gabriel; Côté, Michel; Gonze, Xavier

    2015-03-01

    The electron-phonon coupling in solids renormalizes the band structure, reducing the band gap by several tenths of an eV in light-atoms semiconductors. Using the Allen-Heine-Cardona theory (AHC), we compute the zero-point renormalization (ZPR) as well as the quasiparticle lifetimes of the full band structure in diamond, BN, LiF and MgO. We show how dynamical effects can be included in the AHC theory, and still allow for the use of a Sternheimer equation to avoid the summation over unoccupied bands. The convergence properties of the electron-phonon coupling self-energy with respect to the Brillouin zone sampling prove to be strongly affected by dynamical effects. We complement our study with a frozen-phonon approach, which reproduces the static AHC theory, but also allows to probe the phonon wavefunctions at finite displacements and include anharmonic effects in the self-energy. We show that these high-order components tend to reduce the strongest electron-phonon coupling elements, which affects significantly the band gap ZPR.

  8. Effect of low-temperature annealing on the electronic- and band-structures of (Ga,Mn)As epitaxial layers

    Science.gov (United States)

    Yastrubchak, O.; Wosinski, T.; Gluba, L.; Andrearczyk, T.; Domagala, J. Z.; Żuk, J.; Sadowski, J.

    2014-01-01

    The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and superconducting quantum interference device magnetometry, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results demonstrating a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the GaAs valence band. Whereas an increase in the band-gap-transition energy caused by the annealing treatment of the (Ga,Mn)As layers is interpreted as a result of annealing-induced enhancement of the free-hole concentration and the Fermi level location within the valence band. The experimental results are consistent with the valence-band origin of itinerant holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.

  9. Unified band-theoretic description of structural, electronic, and magnetic properties of vanadium dioxide phases

    Science.gov (United States)

    Xu, Sheng; Shen, Xiao; Hallman, Kent A.; Haglund, Richard F.; Pantelides, Sokrates T.

    2017-03-01

    The debate about whether the insulating phases of vanadium dioxide (V O2 ) can be described by band theory or whether it requires a theory of strong electron correlations remains unresolved even after decades of research. Energy-band calculations using hybrid exchange functionals or including self-energy corrections account for the insulating or metallic nature of different phases but have not yet successfully accounted for the observed magnetic orderings. Strongly correlated theories have had limited quantitative success. Here we report that by using hard pseudopotentials and an optimized hybrid exchange functional, the energy gaps and magnetic orderings of both monoclinic V O2 phases and the metallic nature of the high-temperature rutile phase are consistent with available experimental data, obviating an explicit role for strong correlations. We also identify a potential candidate for the newly found metallic monoclinic phase.

  10. Application of mid-infrared free-electron laser tuned to amide bands for dissociation of aggregate structure of protein.

    Science.gov (United States)

    Kawasaki, Takayasu; Yaji, Toyonari; Ohta, Toshiaki; Tsukiyama, Koichi

    2016-01-01

    A mid-infrared free-electron laser (FEL) is a linearly polarized, high-peak powered pulse laser with tunable wavelength within the mid-infrared absorption region. It was recently found that pathogenic amyloid fibrils could be partially dissociated to the monomer form by the irradiation of the FEL targeting the amide I band (C=O stretching vibration), amide II band (N-H bending vibration) and amide III band (C-N stretching vibration). In this study, the irradiation effect of the FEL on keratin aggregate was tested as another model to demonstrate an applicability of the FEL for dissociation of protein aggregates. Synchrotron radiation infrared microscopy analysis showed that the α-helix content in the aggregate structure decreased to almost the same level as that in the monomer state after FEL irradiation tuned to 6.06 µm (amide I band). Both irradiations at 6.51 µm (amide II band) and 8.06 µm (amide III band) also decreased the content of the aggregate but to a lesser extent than for the irradiation at the amide I band. On the contrary, the irradiation tuned to 5.6 µm (non-absorbance region) changed little the secondary structure of the aggregate. Scanning-electron microscopy observation at the submicrometer order showed that the angular solid of the aggregate was converted to non-ordered fragments by the irradiation at each amide band, while the aggregate was hardly deformed by the irradiation at 5.6 µm. These results demonstrate that the amide-specific irradiation by the FEL was effective for dissociation of the protein aggregate to the monomer form.

  11. Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures

    Science.gov (United States)

    Zhang, Chenxi; Gong, Cheng; Nie, Yifan; Min, Kyung-Ah; Liang, Chaoping; Oh, Young Jun; Zhang, Hengji; Wang, Weihua; Hong, Suklyun; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae

    2017-03-01

    Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.

  12. 2D Tl-Pb compounds on Ge(1 1 1) surface: atomic arrangement and electronic band structure.

    Science.gov (United States)

    Gruznev, D V; Bondarenko, L V; Tupchaya, A Y; Eremeev, S V; Mihalyuk, A N; Chou, J P; Wei, C M; Zotov, A V; Saranin, A A

    2017-01-25

    Structural transformations and evolution of the electron band structure in the (Tl, Pb)/Ge(1 1 1) system have been studied using low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density functional theory calculations. The two 2D Tl-Pb compounds on Ge(1 1 1), [Formula: see text]-(Tl, Pb) and [Formula: see text]-(Tl, Pb), have been found and their composition, atomic arrangement and electron properties has been characterized. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] compound is almost identical to the alike (Tl, Pb)/Si(1 1 1)[Formula: see text] system from the viewpoint of its atomic structure and electronic properties. They contain 1.0 ML of Tl atoms arranged into a honeycomb network of chained trimers and 1/3 ML of Pb atoms occupying the centers of the honeycomb units. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] compound contains six Tl atoms and seven Pb atoms per [Formula: see text] unit cell (i.e.  ∼0.67 ML Tl and  ∼0.78 ML Pb). Its atomic structure can be visualized as consisting of Pb hexagons surrounded by Tl trimers. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] and (Tl, Pb)/Ge(1 1 1)[Formula: see text] compounds are metallic and their band structures contain spin-split surface-state bands. By analogy with the (Tl, Pb)/Si(1 1 1)[Formula: see text], these (Tl, Pb)/Ge(1 1 1) compounds are believed to be promising objects for prospective studies of superconductivity in one-atom-layer systems.

  13. 2D Tl-Pb compounds on Ge(1 1 1) surface: atomic arrangement and electronic band structure

    Science.gov (United States)

    Gruznev, D. V.; Bondarenko, L. V.; Tupchaya, A. Y.; Eremeev, S. V.; Mihalyuk, A. N.; Chou, J. P.; Wei, C. M.; Zotov, A. V.; Saranin, A. A.

    2017-01-01

    Structural transformations and evolution of the electron band structure in the (Tl, Pb)/Ge(1 1 1) system have been studied using low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density functional theory calculations. The two 2D Tl-Pb compounds on Ge(1 1 1), \\sqrt{3}× \\sqrt{3} -(Tl, Pb) and 3× 3 -(Tl, Pb), have been found and their composition, atomic arrangement and electron properties has been characterized. The (Tl, Pb)/Ge(1 1 1)\\sqrt{3}× \\sqrt{3} compound is almost identical to the alike (Tl, Pb)/Si(1 1 1)\\sqrt{3}× \\sqrt{3} system from the viewpoint of its atomic structure and electronic properties. They contain 1.0 ML of Tl atoms arranged into a honeycomb network of chained trimers and 1/3 ML of Pb atoms occupying the centers of the honeycomb units. The (Tl, Pb)/Ge(1 1 1)3× 3 compound contains six Tl atoms and seven Pb atoms per 3× 3 unit cell (i.e.  ˜0.67 ML Tl and  ˜0.78 ML Pb). Its atomic structure can be visualized as consisting of Pb hexagons surrounded by Tl trimers. The (Tl, Pb)/Ge(1 1 1)\\sqrt{3}× \\sqrt{3} and (Tl, Pb)/Ge(1 1 1)3× 3 compounds are metallic and their band structures contain spin-split surface-state bands. By analogy with the (Tl, Pb)/Si(1 1 1)\\sqrt{3}× \\sqrt{3} , these (Tl, Pb)/Ge(1 1 1) compounds are believed to be promising objects for prospective studies of superconductivity in one-atom-layer systems.

  14. Transverse C-band deflecting structure for longitudinal electron-bunch-diagnosis in XFEL “SACLA”

    Energy Technology Data Exchange (ETDEWEB)

    Ego, H., E-mail: ego@spring8.or.jp [Japan Synchrotron Radiation Research Institute (JASRI), Kouto, Sayo, Hyogo (Japan); Maesaka, H.; Sakurai, T.; Otake, Y. [RIKEN SPring-8 Center, Kouto, Sayo, Hyogo (Japan); Hashirano, T.; Miura, S. [Mitsubishi Heavy Industries, Ltd. (MHI), Itozaki, Mihara, Hiroshima (Japan)

    2015-09-21

    In the 8 GeV compact X-ray FEL “SACLA,” a single bunch of electrons is compressed to a duration of approximately 30 fs to yield a peak current of 3 kA, which creates brilliant self-amplified spontaneous emission. To measure the longitudinal profile of an ultrashort electron bunch and verify the compression, we developed a high-gradient C-band RF deflecting structure 1.8 m long and periodically loaded with racetrack-shaped irises. The irises generated a high deflection gradient for the vertically deflecting HEM11-5π/6 dipole mode and suppressed rotation of the deflection plane. The two structures were fabricated and generated a stable total deflecting voltage exceeding 60 MV and revealed the longitudinal electron-bunch profile with an effective time resolution of approximately 10 fs.

  15. 8-band and 14-band kp modeling of electronic band structure and material gain in Ga(In)AsBi quantum wells grown on GaAs and InP substrates

    Energy Technology Data Exchange (ETDEWEB)

    Gladysiewicz, M.; Wartak, M. S. [Faculty of Fundamental Problems of Technology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw (Poland); Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5 (Canada); Kudrawiec, R. [Faculty of Fundamental Problems of Technology, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw (Poland)

    2015-08-07

    The electronic band structure and material gain have been calculated for GaAsBi/GaAs quantum wells (QWs) with various bismuth concentrations (Bi ≤ 15%) within the 8-band and 14-band kp models. The 14-band kp model was obtained by extending the standard 8-band kp Hamiltonian by the valence band anticrossing (VBAC) Hamiltonian, which is widely used to describe Bi-related changes in the electronic band structure of dilute bismides. It has been shown that in the range of low carrier concentrations n < 5 × 10{sup 18 }cm{sup −3}, material gain spectra calculated within 8- and 14-band kp Hamiltonians are similar. It means that the 8-band kp model can be used to calculate material gain in dilute bismides QWs. Therefore, it can be applied to analyze QWs containing new dilute bismides for which the VBAC parameters are unknown. Thus, the energy gap and electron effective mass for Bi-containing materials are used instead of VBAC parameters. The electronic band structure and material gain have been calculated for 8 nm wide GaInAsBi QWs on GaAs and InP substrates with various compositions. In these QWs, Bi concentration was varied from 0% to 5% and indium concentration was tuned in order to keep the same compressive strain (ε = 2%) in QW region. For GaInAsBi/GaAs QW with 5% Bi, gain peak was determined to be at about 1.5 μm. It means that it can be possible to achieve emission at telecommunication windows (i.e., 1.3 μm and 1.55 μm) for GaAs-based lasers containing GaInAsBi/GaAs QWs. For GaInAsBi/Ga{sub 0.47}In{sub 0.53}As/InP QWs with 5% Bi, gain peak is predicted to be at about 4.0 μm, i.e., at the wavelengths that are not available in current InP-based lasers.

  16. Valence band electronic structure and band alignment of LaAlO{sub 3}/SrTiO{sub 3}(111) heterointerfaces

    Energy Technology Data Exchange (ETDEWEB)

    Gabel, J.; Scheiderer, P.; Zapf, M.; Schuetz, P.; Sing, M.; Claessen, R. [Physikalisches Institut and Roentgen Center for Complex Material Systems (RCCM), Universitaet Wuerzburg (Germany); Schlueter, C.; Lee, T.L. [Diamond Light Source, Didcot (United Kingdom)

    2015-07-01

    As in the famous LaAlO{sub 3}(LAO)/SrTiO{sub 3}(STO) (001) a two-dimensional electron system (2DES) also forms at the interface between LAO and STO in (111) orientation. A distinct feature of the (111) interface is its peculiar real space topology. Each bilayer represents a buckled honeycomb lattice similar to graphene which is known theoretically to host various topologically non-trivial states. Bilayer STO in proximity to the interface can be regarded as a three-orbital generalization of graphene with enhanced electron correlations making it a promising candidate for the realization of strongly correlated topological phases. We have investigated the electronic structure of the LAO/STO (111) heterostructure in relation to the oxygen vacancy concentration which we can control by synchrotron light irradiation and oxygen dosing. With hard X-ray photoemission we study the core levels, whereas resonant soft X-ray photoemission is used to probe the interfacial valence band (VB) states. Two VB features are found: a peak at the Fermi level associated with the 2DES and in-gap states at higher binding energies attributed to oxygen vacancies. By varying the oxygen vacancy contribution we can tune the emergence of the VB states and engineer the interfacial band alignment.

  17. Electron transport and Goos-Haenchen shift in graphene with electric and magnetic barriers: optical analogy and band structure

    Energy Technology Data Exchange (ETDEWEB)

    Sharma, Manish [Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, New Delhi-110016 (India); Ghosh, Sankalpa [Department of Physics, Indian Institute of Technology Delhi, New Delhi-110016 (India)

    2011-02-09

    Transport of massless Dirac fermions in graphene monolayers is analysed in the presence of a combination of singular magnetic barriers and applied electrostatic potential. Extending a recently proposed (Ghosh and Sharma 2009 J. Phys.: Condens. Matter 21 292204) analogy between the transmission of light through a medium with modulated refractive index and electron transmission in graphene through singular magnetic barriers to the present case, we find the addition of a scalar potential profoundly changes the transmission. We calculate the quantum version of the Goos-Haenchen shift that the electron wave suffers upon being totally reflected by such barriers. The combined electric and magnetic barriers substantially modify the band structure near the Dirac point. This affects transport near the Dirac point significantly and has important consequences for graphene-based electronics.

  18. Electron transport and Goos-Hänchen shift in graphene with electric and magnetic barriers: optical analogy and band structure.

    Science.gov (United States)

    Sharma, Manish; Ghosh, Sankalpa

    2011-02-09

    Transport of massless Dirac fermions in graphene monolayers is analysed in the presence of a combination of singular magnetic barriers and applied electrostatic potential. Extending a recently proposed (Ghosh and Sharma 2009 J. Phys.: Condens. Matter 21 292204) analogy between the transmission of light through a medium with modulated refractive index and electron transmission in graphene through singular magnetic barriers to the present case, we find the addition of a scalar potential profoundly changes the transmission. We calculate the quantum version of the Goos-Hänchen shift that the electron wave suffers upon being totally reflected by such barriers. The combined electric and magnetic barriers substantially modify the band structure near the Dirac point. This affects transport near the Dirac point significantly and has important consequences for graphene-based electronics.

  19. Electronic structure of reconstructed InAs(001) surfaces - identification of bulk and surface bands based on their symmetries

    Science.gov (United States)

    Olszowska, Natalia; Kolodziej, Jacek J.

    2016-02-01

    Using angle-resolved photoelectron spectroscopy (ARPES) band structures of indium- and arsenic-terminated InAs(001) surfaces are investigated. These surfaces are highly reconstructed, elementary cells of their lattices contain many atoms in different chemical configurations, and moreover, they are composed of domains having related but different reconstructions. These domain-type surface reconstructions result in the reciprocal spaces containing regions with well-defined k→∥-vector and regions with not-well-defined one. In the ARPES spectra most of the surface related features appear as straight lines in the indeterminate k→∥-vector space. It is shown that, thanks to differences in crystal and surface symmetries, the single photon energy ARPES may be successfully used for classification of surface and bulk bands of electronic states on complex, highly reconstructed surfaces instead of the most often used variable photon energy studies.

  20. Atomic and electronic structures evolution of the narrow band gap semiconductor Ag2Se under high pressure

    Science.gov (United States)

    Naumov, P.; Barkalov, O.; Mirhosseini, H.; Felser, C.; Medvedev, S. A.

    2016-09-01

    Non-trivial electronic properties of silver telluride and other chalcogenides, such as the presence of a topological insulator state, electronic topological transitions, metallization, and the possible emergence of superconductivity under pressure have attracted attention in recent years. In this work, we studied the electronic properties of silver selenide (Ag2Se). We performed direct current electrical resistivity measurements, in situ Raman spectroscopy, and synchrotron x-ray diffraction accompanied by ab initio calculations to explore pressure-induced changes to the atomic and electronic structure of Ag2Se. The temperature dependence of the electrical resistivity was measured up to 30 GPa in the 4-300 K temperature interval. Resistivity data showed an unusual increase in the thermal energy gap of phase I, which is a semiconductor under ambient conditions. Recently, a similar effect was reported for the 3D topological insulator Bi2Se3. Raman spectroscopy studies revealed lattice instability in phase I indicated by the softening of observed vibrational modes with pressure. Our hybrid functional band structure calculations predicted that phase I of Ag2Se would be a narrow band gap semiconductor, in accordance with experimental results. At a pressure of ~7.5 GPa, Ag2Se underwent a structural transition to phase II with an orthorhombic Pnma structure. The temperature dependence of the resistivity of Ag2Se phase II demonstrated its metallic character. Ag2Se phase III, which is stable above 16.5 GPa, is also metallic according to the resistivity data. No indication of the superconducting transition is found above 4 K in the studied pressure range.

  1. Low-energy electron microscopy on two-dimensional systems : : growth, potentiometry and band structure mapping

    NARCIS (Netherlands)

    Kautz, Jaap

    2015-01-01

    Low Energy Electron Microscopy (LEEM) is a microscopy technique typically used to study surface processes. The sample is illuminated with a parallel beam of electrons under normal incidence and the reflected electrons are projected onto a pixelated detector, where an image is formed. In the first

  2. Electronic band structure and specific features of Sm{sub 2}NiMnO{sub 6} compound: DFT calculation

    Energy Technology Data Exchange (ETDEWEB)

    Reshak, A.H. [Institute of complex systems, FFPW, CENAKVA, University of South Bohemia in CB, Nove Hrady 37333 (Czech Republic); Center of Excellence Geopolymer and Green Technology, School of Material Engineering, University Malaysia Perlis, 01007 Kangar, Perlis (Malaysia); Azam, Sikander, E-mail: sikander.physicst@gmail.com [Institute of complex systems, FFPW, CENAKVA, University of South Bohemia in CB, Nove Hrady 37333 (Czech Republic)

    2013-09-15

    The band structure, density of states, electronic charge density, Fermi surface and optical properties of Sm{sub 2}NiMnO{sub 6} compound have been investigated with the support of density functional theory (DFT). The atomic positions of Sm{sub 2}NiMnO{sub 6} compound were optimized by minimizing the forces acting on the atoms, using the full potential linear augmented plane wave method. We employed the local density approximation (LDA), generalized gradient approximation (GGA) and Engel–Vosko GGA (EVGGA) to treat the exchange correlation potential by solving Kohn–Sham equations. The calculation shows that the compound is metallic with strong hybridization near the Fermi energy level (E{sub F}). The calculated density of states at the E{sub F} is about 21.60, 24.52 and 26.21 states/eV, and the bare linear low-temperature electronic specific heat coefficient (γ) is found to be 3.74, 4.25 and 4.54 mJ/mol K{sup 2} for EVGGA, GGA and LDA, respectively. The Fermi surface is composed of two sheets. The bonding features of the compounds are analyzed using the electronic charge density in the (011) crystallographic plane. The dispersion of the optical constants was calculated and discussed. - Highlights: • The compound is metallic with strong hybridization near the Fermi energy. • The density of states at the Fermi energy is calculated. • The bare linear low-temperature electronic specific heat coefficient is obtained. • Fermi surface is composed of two sheets. • The bonding features are analyzed using the electronic charge density.

  3. Multiband model of the valence-band electronic structure in cylindrical GaAs nanowires

    Directory of Open Access Journals (Sweden)

    Čukarić Nemanja A.

    2010-01-01

    Full Text Available We compute the hole states in the GaAs free-standing nanowires, and in the GaAs/(Al,GaAs core-shell nanowires of type I-s, which are grown along the [100] direction. The hole states are extracted from the 4-band Luttinger-Kohn Hamiltonian, which explicitly takes into account mixing between the light and heavy holes. The axial aproximation is adopted, which allowed classification of states according to the total angular monentum (fz when expressed in units of the Planck constant. The envelope functions are expanded in Bessel functions of the first kind. The dispersion relations of the subbands E(kz obtained by the devised method do not resemble parabolas, which is otherwise a feature of the dispersion relations of the conduction subbands. Furthermore, the energy levels of holes whose total orbital momentum is fz=1/2 are shown to cross for a free-standing wire. The low energy fz=1/2 states are found to anticross, but these anticrossings turn into crossings when the ratio of the inner and outer radius of the core-shell wire takes a certain value. The influence of the geometric parameters on the dispersion relations is considered for both free standing and core-shell nanowires.

  4. Strain Dependent Electronic Structure and Band Offset Tuning at Heterointerfaces of ASnO3 (A=Ca, Sr, and Ba) and SrTiO3

    Science.gov (United States)

    Baniecki, John D.; Yamazaki, Takashi; Ricinschi, Dan; Van Overmeere, Quentin; Aso, Hiroyuki; Miyata, Yusuke; Yamada, Hiroaki; Fujimura, Norifumi; Maran, Ronald; Anazawa, Toshihisa; Valanoor, Nagarajan; Imanaka, Yoshihiko

    2017-01-01

    The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices. PMID:28195149

  5. Importance of doping, dopant distribution, and defects on electronic band structure alteration of metal oxide nanoparticles: Implications for reactive oxygen species.

    Science.gov (United States)

    Saleh, Navid B; Milliron, Delia J; Aich, Nirupam; Katz, Lynn E; Liljestrand, Howard M; Kirisits, Mary Jo

    2016-10-15

    Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics.

  6. Coherent Optical Control of Electronic Excitations in Wide-Band-Gap Semiconductor Structures

    Science.gov (United States)

    2015-05-01

    structures. 4 The Hamiltonian describing the system contains 2 parts: H0, which includes the parts that can be treated exactly using a single...phonons, and the carrier–carrier interactions via Coulomb potential. We describe the dynamics of the system using the density matrix method.2,3 The...algebra, we obtain the following form of the Hamiltonian for the effective 2-level system : 0 * 1 ( ) ( ) , ( ) ( ) 22 ac eff eff ac t t t t

  7. Electronic band structures of AV{sub 2} (A = Ta, Ti, Hf and Nb) Laves phase compounds

    Energy Technology Data Exchange (ETDEWEB)

    Charifi, Z; Baaziz, H [Physics Department, Faculty of Science and Engineering, University of M' sila, 28000 M' sila (Algeria); Reshak, Ali Hussain [Institute of Physical Biology, South Bohemia University, Nove Hrady 37333 (Czech Republic)], E-mail: maalidph@yahoo.co.uk

    2009-01-14

    First-principles density functional calculations, using the all-electron full potential linearized augmented plane wave method, have been performed in order to investigate the structural and electronic properties for Laves phase AV{sub 2} (A = Ta, Ti, Hf and Nb) compounds. The generalized gradient approximation and the Engel-Vosko-generalized gradient approximation were used. Our calculations show that these compounds are metallic with more bands cutting the Fermi energy (E{sub F}) as we move from Nb to Ta, Hf and Ti, consistent with the increase in the values of the density of states at the Fermi level N(E{sub F}). N(E{sub F}) is controlled by the overlapping of V-p/d, A-d and A-p states around the Fermi energy. The ground state properties of these compounds, such as equilibrium lattice constant, are calculated and compared with the available literature. There is a strong/weak hybridization between the states, V-s states are strongly hybridized with A-s states below and above E{sub F}. Around the Fermi energy we notice that V-p shows strong hybridization with A-p states.

  8. Crystal symmetry and pressure effects on the valence band structure of γ -InSe and ɛ -GaSe: Transport measurements and electronic structure calculations

    Science.gov (United States)

    Errandonea, D.; Segura, A.; Manjón, F. J.; Chevy, A.; Machado, E.; Tobias, G.; Ordejón, P.; Canadell, E.

    2005-03-01

    This paper reports on Hall effect and resistivity measurements under high pressure up to 3-4 GPa in p -type γ -indium selenide (InSe) (doped with As, Cd, or Zn) and ɛ -gallium selenide (GaSe) (doped with N or Sn). The pressure behavior of the hole concentration and mobility exhibits dramatic differences between the two layered compounds. While the hole concentration and mobility increase moderately and monotonously in ɛ -GaSe, a large increase of the hole concentration near 0.8 GPa and a large continuous increase of the hole mobility, which doubled its ambient pressure value by 3.2 GPa, is observed in γ -InSe. Electronic structure calculations show that the different pressure behavior of hole transport parameters can be accounted for by the evolution of the valence-band maximum in each material under compression. While the shape of the valence band maximum is virtually pressure-insensitive in ɛ -GaSe, it changes dramatically in γ -InSe, with the emergence of a ring-shaped subsidiary maximum that becomes the absolute valence-band maximum as pressure increases. These differences are shown to be a consequence of the presence or absence of a symmetry element (mirror plane perpendicular to the anisotropy axis) in the point group of each polytype ( D3h for the ɛ -polytype and C3v for the γ -polytype), resulting in different selection rules that affect the k⃗•p⃗ interaction between valence bands.

  9. Calculation of the Energy-Band Structure of the Kronig-Penney Model Using the Nearly-Free and Tightly-Bound-Electron Approximations

    Science.gov (United States)

    Wetsel, Grover C., Jr.

    1978-01-01

    Calculates the energy-band structure of noninteracting electrons in a one-dimensional crystal using exact and approximate methods for a rectangular-well atomic potential. A comparison of the two solutions as a function of potential-well depth and ratio of lattice spacing to well width is presented. (Author/GA)

  10. Bulk Modulus and Electronic Band Structure of ZnGa2X4 (X=S,Se): a First-Principles Study

    Institute of Scientific and Technical Information of China (English)

    JIANG Xiao-Shu; MI Shu; SUN eeng-Jun; LU Yuan; LIANG Jiu-Qing

    2009-01-01

    First-principles local density functional calculations are presented for the compounds ZnGa2X4 (X = S, Se). We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters axe optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa2X4 by fitting the Birch-Murnaghan's equation of state. The extended Cohen's empirical formula agrees well with our ab initio results.

  11. Valence-band electronic structure of Zn3P2 as a function of annealing as studied by synchrotron radiation photoemission

    Science.gov (United States)

    Nelson, Art J.; Kazmerski, L. L.; Engelhardt, Mike; Hochst, Hartmut

    1990-02-01

    Ultraviolet photoemission (UPS) utilizing synchrotron radiation has been used to characterize changes in the valence-band electronic structure of crystalline Zn3P2 as a function of annealing temperature. The Zn3P2 crystal was etched in bromine-methanol prior to analysis and annealing was performed in vacuum at 300 and 350 °C after sputter cleaning. The UPS spectra for the virgin material are qualitatively similar to the photoemission results for various II-VI Zn compound semiconductors and a comparison of the Zn 3d binding energies with respect to the valence band maximum is presented. The results for the virgin material and the 300 °C anneal are further compared with the theoretically predicted band structure of Zn3P2 as determined by a pseudopotential energy band calculation. Loss of phosphorus from the surface and the presence of elemental zinc on the surface after the 350 °C anneal is evident.

  12. Effects of the Born-Oppenheimer approximation in the electronic band structure of MgB{sub 2} and ZrB{sub 2}.

    Energy Technology Data Exchange (ETDEWEB)

    Petzold, Vivien; Rosner, Helge [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Dresden (Germany)

    2011-07-01

    Electronic band structure calculations are routinely applied to many problems in chemistry and physics. The methods rely on a number of approximations, where the treatment of exchange and correlation is a very prominent issue, probably the most prominent in the development of new density functionals in the framework of density functional theory (DFT). The present work highlights effects that arise from the more fundamental Born-Oppenheimer approximation. Based on this approximation, the original problem - the quantum-mechanical description of matter consisting of nuclei and electrons - is decomposed into a nuclear and an electronic problem, the latter of which is treated by electronic band structure methods. Utilizing the most common density functionals, the local density approximation (LDA) and the generalized gradient approximation (GGA), we observe deviations between experimental and theoretical de Haas van Alphen (dHvA) frequencies for MgB{sub 2} and ZrB{sub 2} that can be consistently understood by electron-phonon coupling effects, which the theory is lacking. The explanation is based on a highly accurate computation of dHvA frequencies indicating an electron-phonon coupling-induced shift of the electronic bands.

  13. Physics of electrons in solids: Volume 1 - Solid state physics, band structure, superconductivity and magnetism; Volume 2 - Solid state physics, exercises and problems with keys; Physique des electrons dans les solides: Tome 1, structure de bandes, supraconductivite et magnetisme. Physique des electrons dans les solides: Tome 2, recueil d'exercices et de problemes

    Energy Technology Data Exchange (ETDEWEB)

    Alloul, H. [Paris-11 Univ., 91 - Orsay (France). Lab. de Physique des Solides

    2007-07-01

    The diversity of the macroscopic properties of solids like magnetism or superconductivity stems from the quantum states of electrons. Today only the experimental approach reveals the spectacular effects of these properties but basic concepts of quantum mechanics and of statistical physics are necessary to give an account of the link between the microscopic scale and the macroscopic world. The simple approach involving independent electrons gives a description of the electronic structure as energy bands that explains the existence of metals, isolators and semi-conductors. Magnetism and superconductivity can be understood only by taking into account the existence of correlations between the electrons in the solids. The first tome presents the formalism of quantum mechanics applied to the system formed by nuclei and the electrons in solids. Different issues like electronic structures in solids, electron transport, the microscopic origin of superconductivity, the magnetism of isolators, the dynamics of spin and magnetic resonance are explained. The second volume can be divided into 2 parts, the first part giving the keys of the questions arisen in the first volume while the second part propose a series of problems (with keys). These problems illustrate the topics presented in the first volume and deal with issues like optical properties of solids, electron bands, Peierls transition, phonons in solids, isolator-metal transition, cyclotron resonance, superconductivity of NbSe{sub 2}, electronic properties of La{sub 2}CuO{sub 4}, or the magnetism of thin films. (A.C.)

  14. Effect of cation substitution on electronic band structure of ZnGeAs{sub 2} pnictides: A mBJLDA approach

    Energy Technology Data Exchange (ETDEWEB)

    Saini, Hardev S.; Singh, Mukhtiyar [Department of Physics, Kurukshetra University, Kurukshetra 136119 (Haryana) (India); Reshak, Ali H. [School of Complex Systems, FFWP - South Bohemia University, Nove Hrady 37333 (Czech Republic); School of Material Engineering, Malaysia University of Perlis, P.O. Box 77, d/a Pejabat Pos Besar, 01007 Kangar, Perlis (Malaysia); Kashyap, Manish K., E-mail: manishdft@gmail.com [Department of Physics, Kurukshetra University, Kurukshetra 136119 (Haryana) (India)

    2012-03-25

    Highlights: Black-Right-Pointing-Pointer These compounds are characterized as narrow band gap semiconductors with a maximum gap (1.27 eV) for ZnGeAs{sub 2}. Black-Right-Pointing-Pointer A good agreement of band gaps with experiments is obtained within mBJLDA formalism. Black-Right-Pointing-Pointer The band gap decreases with the substitution of either one or both cations in reference compound, ZnGeAs{sub 2}. Black-Right-Pointing-Pointer The ionic/covalent character for A-As/B-As bond has been described on the basis of electro-negativity difference of the atoms. Black-Right-Pointing-Pointer The d-states of transition metal, Zn are localized deeper in valence band (E < 5 eV), showing no effective role to decide the magnitude of semiconducting band gap. - Abstract: The electronic properties of ABAs{sub 2} (A = Zn, Cd; B = Ge, Sn) compounds have been investigated using WIEN2k implementation of full potential linearized augmented plane wave (FPLAPW) method with an aim to study the effect of changing local environment by substituting cation(s) with corresponding next group element in reference compound (ZnGeAs{sub 2}) on these properties. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke-Johnson (mBJ) potential as coupled with Local Density Approximation (LDA) for these calculations. We predict a direct band gap in all these compounds and observe that the band gap decreases with the change of either one or both cations. The calculated band gaps are in better agreement with corresponding experimental ones as compared to other calculations. The electronic band structure is analyzed in terms of contributions from various electrons and the covalency of two bonds, Zn-As and Ge-As has been discussed with respect to substitutions.

  15. Intermetallic solid solution Fe{sub 1-x}Co{sub x}Ga{sub 3}: Synthesis, structure, NQR study and electronic band structure calculations

    Energy Technology Data Exchange (ETDEWEB)

    Verchenko, V.Yu.; Likhanov, M.S.; Kirsanova, M.A. [Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 (Russian Federation); Gippius, A.A; Tkachev, A.V.; Gervits, N.E. [Faculty of Physics, Lomonosov Moscow State University, Moscow 119991 (Russian Federation); A.V. Shubnikov Institute of Crystallography, Moscow 119333 (Russian Federation); Galeeva, A.V. [Faculty of Physics, Lomonosov Moscow State University, Moscow 119991 (Russian Federation); Buettgen, N.; Kraetschmer, W. [Institut fuer Physik, University of Augsburg, Augsburg D-86135 (Germany); Lue, C.S. [Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan (China); Okhotnikov, K.S. [Materials and Environmental Chemistry, Stockholm University, Stockholm (Sweden); Shevelkov, A.V., E-mail: shev@inorg.chem.msu.ru [Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 (Russian Federation)

    2012-10-15

    Unlimited solid solution Fe{sub 1-x}Co{sub x}Ga{sub 3} was prepared from Ga flux. Its crystal structure was refined for Fe{sub 0.5}Co{sub 0.5}Ga{sub 3} (P4{sub 2}/mnm, a=6.2436(9), c=6.4654(13), Z=4) and showed no ordering of the metal atoms. A combination of the electronic band structure calculations within the density functional theory (DFT) approach and {sup 69,71}Ga nuclear quadrupole resonance (NQR) spectroscopy clearly shows that the Fe-Fe and Co-Co dumbbells are preferred to the Fe-Co dumbbells in the crystals structure. The band structure features a band gap of about 0.4 eV, with the Fermi level crossing peaks of a substantial density of electronic states above the gap for x>0. The solid solution is metallic for x>0.025. The study of the nuclear spin-lattice relaxation shows that the rate of the relaxation, 1/T{sub 1}, is very sensitive to the Co concentration and correlates well with the square of the density of states at the Fermi level, N{sup 2}(E{sub F}). - Graphical abstract: Rate of the nuclear spin-lattice relaxation, 1/T{sub 1}, observed in the {sup 69}Ga NQR experiments for the intermetallic solid solution Fe{sub 1-x}Co{sub x}Ga{sub 3} is the highest for x=0.25 with the highest calculated density of electronic states at the Fermi level, N(E{sub F}); in general, 1/T{sub 1} correlates with N{sup 2}(E{sub F}). Highlights: Black-Right-Pointing-Pointer Fe{sub 1-x}Co{sub x}Ga{sub 3} solid solution is prepared in single crystalline form from Ga flux. Black-Right-Pointing-Pointer In the crystal structure Fe-Fe and Co-Co dumbbells are preferred to Fe-Co dumbbells. Black-Right-Pointing-Pointer Metal-to-semiconductor transition occurs at 0

  16. Electronic band structure and optical gain of GaNxBiyAs1-x-y/GaAs pyramidal quantum dots

    Science.gov (United States)

    Song, Zhi-Gang; Bose, Sumanta; Fan, Wei-Jun; Li, Shu-Shen

    2016-04-01

    The electronic band structure and optical gain of GaNxBiyAs1-x-y/GaAs pyramidal quantum dots (QDs) are investigated using the 16-band k ṡ p model with constant strain. The optical gain is calculated taking both homogeneous and inhomogeneous broadenings into consideration. The effective band gap falls as we increase the composition of nitrogen (N) and bismuth (Bi) and with an appropriate choice of composition we can tune the emission wavelength to span within 1.3 μm-1.55 μm, for device application in fiber technology. The extent of this red shift is more profound in QDs compared with bulk material due to quantum confinement. Other factors affecting the emission characteristics include virtual crystal, strain profile, band anticrossing (BAC), and valence band anticrossing (VBAC). The strain profile has a profound impact on the electronic structure, specially the valence band of QDs, which can be determined using the composition distribution of wave functions. All these factors eventually affect the optical gain spectrum. With an increase in QD size, we observe a red shift in the emission energy and emergence of secondary peaks owing to transitions or greater energy compared with the fundamental transition.

  17. Effects of Side-Chain and Electron Exchange Correlation on the Band Structure of Perylene Diimide Liquid Crystals: A Density Functional Study

    Energy Technology Data Exchange (ETDEWEB)

    Arantes, J. T.; Lima, M. P.; Fazzio, A.; Xiang, H.; Wei, S. H.; Dalpian, G. M.

    2009-04-01

    The structural and electronic properties of perylene diimide liquid crystal PPEEB are studied using ab initio methods based on the density functional theory (DFT). Using available experimental crystallographic data as a guide, we propose a detailed structural model for the packing of solid PPEEB. We find that due to the localized nature of the band edge wave function, theoretical approaches beyond the standard method, such as hybrid functional (PBE0), are required to correctly characterize the band structure of this material. Moreover, unlike previous assumptions, we observe the formation of hydrogen bonds between the side chains of different molecules, which leads to a dispersion of the energy levels. This result indicates that the side chains of the molecular crystal not only are responsible for its structural conformation but also can be used for tuning the electronic and optical properties of these materials.

  18. Simultaneous Out-of-band Interference Rejection and Radiation Enhancement in an Electronic Product via an EBG Structure

    DEFF Research Database (Denmark)

    Ruaro, Andrea; Thaysen, Jesper; Jakobsen, Kaj Bjarne

    2014-01-01

    to achieve simultaneously both the enhancement of the antenna radiation efficiency and the shrinking of its dimensions, while making the device more resilient to out-of-band electromagnetic interference (EMI). The patterning of the ground plane allows, in fact, to effectively suppress higher-order resonances......This work presents an application of a planar electromagnetic band gap (EBG) structure with a perspective product implementation in the back of the mind. The focus is on the integration of such structure under the constraint of space and system coexistence. It is discovered that it is possible...... (alternatively, parallel plate noise) and decrease the radiation efficiency of the structure forbidding higher-order modes to propagate and subsequently be diffracted by the ground plane....

  19. Band-edge electronic structure of β-In2S3: the role of s or p orbitals of atoms at different lattice positions.

    Science.gov (United States)

    Zhao, Zongyan; Cao, Yuechan; Yi, Juan; He, Xijia; Ma, Chenshuo; Qiu, Jianbei

    2012-04-23

    As a promising solar-energy material, the electronic structure and optical properties of Beta phase indium sulfide (β-In(2)S(3)) are still not thoroughly understood. This paper devotes to solve these issues using density functional theory calculations. β-In(2)S(3) is found to be an indirect band gap semiconductor. The roles of its atoms at different lattice positions are not exactly identical because of the unique crystal structure. Additonally, a significant phenomenon of optical anisotropy was observed near the absorption edge. Owing to the low coordination numbers of the In3 and S2 atoms, the corresponding In3-5s states and S2-3p states are crucial for the composition of the band-edge electronic structure, leading to special optical properties and excellent optoelectronic performances.

  20. Electronic transport and band structures of GaAs/AlAs nanostructures superlattices for near-infrared detection

    Science.gov (United States)

    Barkissy, Driss; Nafidi, Abdelhakim; Boutramine, Abderrazak; Benchtaber, Nassima; Khalal, Ali; El Gouti, Thami

    2017-01-01

    We report here the theoretical calculations of band structures E( d 1), E( k z , k p ) and effective mass along the growth axis and in the plane of GaAs/Al x Ga1- x As superlattices, in the envelope function formalism. The effect of valence band offset, well thickness and temperature on the band structures, has been also studied. Our results show that a transition from indirect to direct band gap in (GaAs) m /(AlAs)4 takes place between m = 5 and 6 monolayers at room temperature. Samples (GaAs)9/(AlAs)4 and GaAs( d 1 = 10 nm)/Al0.15Ga0.85As( d 2 = 15 nm) have a direct band gap of 1.747 eV at room temperature and 1.546 eV at T = 30 mK, respectively. Their corresponding cutoff wavelengths are located in the near infrared region. We have interpreted the photoluminescence measurements of Ledentsov et al. in GaAs( d 1 = 2.52 nm)/AlAs ( d 1 = 1.16 nm) and the oscillations in the magnetoresistance observed by Kawamura et al. in GaAs/Al0.15Ga0.85As superlattice. In the later, the existence of discrete quantized levels along the growth direction z indicates extremely low interactions between adjacent wells leading to the use in parallel transport. The position of Fermi level predicts that this sample exhibits n-type conductivity. These results were compared and discussed with the available data in the literature and can be used as a guide for the design of infrared nanostructured detectors.

  1. Reconstructing the energy band electronic structure of pulsed laser deposited CZTS thin films intended for solar cell absorber applications

    Science.gov (United States)

    Pandiyan, Rajesh; Oulad Elhmaidi, Zakaria; Sekkat, Zouheir; Abd-lefdil, Mohammed; El Khakani, My Ali

    2017-02-01

    We report here on the use of pulsed KrF-laser deposition (PLD) technique for the growth of high-quality Cu2ZnSnS4 (CZTS) thin films onto Si, and glass substrates without resorting to any post sulfurization process. The PLD-CZTS films were deposited at room temperature (RT) and then subjected to post annealing at different temperatures ranging from 200 to 500 °C in Argon atmosphere. The X-ray diffraction and Raman spectroscopy confirmed that the PLD films crystallize in the characteristic kesterite CZTS structure regardless of their annealing temperature (Ta), but their crystallinity is much improved for Ta ≥ 400 °C. The PLD-CZTS films were found to exhibit a relatively dense morphology with a surface roughness (RMS) that increases with Ta (from ∼14 nm at RT to 70 nm at Ta = 500 °C with a value around 40 nm for Ta = 300-400 °C). The optical bandgap of the PLD-CZTS films, was derived from UV-vis transmission spectra analysis, and found to decrease from 1.73 eV for non-annealed films to ∼1.58 eV for those annealed at Ta = 300 °C. These band gap values are very close to the optimum value needed for an ideal solar cell absorber. In order to achieve a complete reconstruction of the one-dimensional energy band structure of these PLD-CZTS absorbers, we have combined both XPS and UPS spectroscopies to determine their chemical bondings, the position of their valence band maximum (relative to Fermi level), and their work function values. This enabled us to sketch out, as accurately as possible, the band alignment of the heterojunction interface formed between CZTS and both CdS and ZnS buffer layer materials.

  2. Microstrip microwave band gap structures

    Indian Academy of Sciences (India)

    V Subramanian

    2008-04-01

    Microwave band gap structures exhibit certain stop band characteristics based on the periodicity, impedance contrast and effective refractive index contrast. These structures though formed in one-, two- and three-dimensional periodicity, are huge in size. In this paper, microstrip-based microwave band gap structures are formed by removing the substrate material in a periodic manner. This paper also demonstrates that these structures can serve as a non-destructive characterization tool for materials, a duplexor and frequency selective coupler. The paper presents both experimental results and theoretical simulation based on a commercially available finite element methodology for comparison.

  3. Electronic properties of the III-VI layer compounds GaS, GaSe and InSe. Part I: Band structure

    Energy Technology Data Exchange (ETDEWEB)

    Doni, E. (Pisa Univ. (Italy). Ist. di Fisica); Girlanda, R.; Grasso, V. (Messina Univ. (Italy). Ist. di Struttura della Materia); Balzarotti, A. (L' Aquila Univ. (Italy). Ist. di Fisica); Piacentini, M. (Comitato Nazionale per l' Energia Nucleare, Frascati (Italy). Lab. Nazionali di Frascati)

    1979-05-11

    The electronic properties of the semiconducting layer compounds GaS, GaSe and InSe are considered comparatively in a series of papers. In this first paper the band structures of the three compounds are computed by using the same method and the same approximations for all of them. The overlap-reduced semi-empirical tight-binding method (ORSTB) is outlined at first. The method consists in scaling only the overlap integrals in the secular equation. In this way it takes into account the localization of the atomiclike functions used in the Bloch sums in a more effective and simpler way than previous tight-binding semi-empirical schemes. For GaS, GaSe and InSe bands only two scaling factors are used as matching parameters to reproduce optical and photo-emission experimental data. The agreement between computed and experimental bands is very good. In particular, the shape of the bonding metal-metal bands which all previous tight-binding calculations failed a dipolar moment. The SEP is the macroscopic results of the alignment and the ''freezing-in'' of those structures. The study of the SEP promises to be useful in understanding the kinetic of the vapour-solid phase transition and in acquiring information about the structural transformations and phase transitions of the solids.

  4. Band structure engineering in organic semiconductors

    Science.gov (United States)

    Schwarze, Martin; Tress, Wolfgang; Beyer, Beatrice; Gao, Feng; Scholz, Reinhard; Poelking, Carl; Ortstein, Katrin; Günther, Alrun A.; Kasemann, Daniel; Andrienko, Denis; Leo, Karl

    2016-06-01

    A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

  5. The electronic structure of Nb3Al/Nb3Sn, a new test case for flat/steep band model of superconductivity

    Institute of Scientific and Technical Information of China (English)

    Yanlong Ding; Shuiquan Deng; Yong Zhao

    2014-01-01

    In this work, we choose Nb3Al/Nb3Sn as a new test case for flat/steep band model of superconductivity. Based on the density functional theory in the generalized gradient approximation, the electronic structure of Nb3Al/Nb3Sn has been studied. The obtained results agree well with those of the earlier studies and show clearly flat bands around the Fermi level. The steep bands as characterized in this work locate around the M point in the first Brillouin zone. The obtained results reveal that Nb3Al/Nb3Sn fits more to the ‘‘Flat/steep’’ band model than to the van-Hove singularity scenario. The flat/steep band condition for superconductivity implies a different thermodynamic behavior of superconductors other than that predicted from the conventional BCS theory. This observation sets up an indicator for selecting a suitable superconductor when its large-scale industrial use is needed, for example, in superconducting maglev system or ITER project.

  6. Band offsets and electronic structures of interface between In{sub 0.5}Ga{sub 0.5}As and InP

    Energy Technology Data Exchange (ETDEWEB)

    Cai, Genwang [School of Physical Science and Engineering and Key Laboratory of Materials Physics of Ministry of Education of China, Zhengzhou University, Zhengzhou 450052 (China); College of Science, Henan University of Technology, Zhengzhou 450001 (China); Wang, Changhong; Wang, Weichao [College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300071 (China); Liang, Erjun, E-mail: ejliang@zzu.edu.cn [School of Physical Science and Engineering and Key Laboratory of Materials Physics of Ministry of Education of China, Zhengzhou University, Zhengzhou 450052 (China)

    2016-02-07

    III–V semiconductor interfacing with high-κ gate oxide is crucial for the high mobility metal-oxide-semiconductor field transistor device. With density functional theory calculations, we explored the band offsets and electronic structures of the In{sub 0.5}Ga{sub 0.5}As/InP interfaces with various interfacial bondings. Among six different bonding interfaces, we found that P-In(Ga) bonding interface showed the highest stability. Local density of states calculations was adopted to calculate the band offsets. Except for the metallic interface, we noticed that neither valence band offset nor conduction band offset depended on the interfacial bondings. For the most stable P-In(Ga) interface, we did not observe any gap states. Furthermore, we explored the P-In(Ga) interfaces with interfacial P-As exchange defects, which slightly modified the interface stability and the band offsets but did not produce any gap states. These findings provide solid evidence that InP could serve as a promising interfacial passivation layer between III–V material and high-κ oxide in the application of high mobility devices.

  7. Tl{sub 4}CdI{sub 6} – Wide band gap semiconductor: First principles modelling of the structural, electronic, optical and elastic properties

    Energy Technology Data Exchange (ETDEWEB)

    Piasecki, M., E-mail: m.piasecki@ajd.czest.pl [Institute of Physics, Jan Dlugosz University, Armii Krajowej 13/15, 42-200 Czestochowa (Poland); Brik, M.G. [College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065 (China); Institute of Physics, University of Tartu, Ravila 14C, Tartu 50411 (Estonia); Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw (Poland); Kityk, I.V. [Faculty of Electrical Engineering, Czestochowa University of Technology, Armii Krajowej 17, 42-200 Czestochowa (Poland)

    2015-08-01

    A novel infrared optoelectronic material Tl{sub 4}CdI{sub 6} was studied using the density functional theory (DFT)-based techniques. Its structural, electronic, optical and elastic properties were all calculated in the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) and the local density approximation (LDA) with the Ceperley-Alder–Perdew-Zunger (CA–PZ) functionals. The studied material is a direct band gap semiconductor with the calculated band gaps of 2.043 eV (GGA) and 1.627 eV (LDA). The wavelength dependence of the refractive index was fitted to the Sellmeier equation in the spectral range from 400 to 2000 nm. Good agreement between the GGA-calculated values of refractive index and experimental data was achieved. To the best of our knowledge, this is the first consistent theoretical description of the title compound, which includes calculations and analysis of the structural, electronic, optical and elastic properties. - Graphical abstract: Display Omitted - Highlights: • Infrared optoelectronic material Tl{sub 4}CdI{sub 6} was studied using ab initio methods. • Structural, electronic, optical and elastic properties were calculated. • Independent components of the elastic constants tensor were calculated. • Good agreement with available experimental results was achieved.

  8. Valence-band electronic structure of Zn sub 3 P sub 2 as a function of annealing as studied by synchrotron radiation photoemission

    Energy Technology Data Exchange (ETDEWEB)

    Nelson, A.J.; Kazmerski, L.L. (Solar Energy Research Institute, 1617 Cole Boulevard, Golden, Colorado 80401 (US)); Engelhardt, M.; Hochst, H. (Synchrotron Radiation Center, University of Wisconsin-Madison, 3731 Schneider Drive, Stoughton, Wisconsin 53589 (US))

    1990-02-01

    Ultraviolet photoemission (UPS) utilizing synchrotron radiation has been used to characterize changes in the valence-band electronic structure of crystalline Zn{sub 3}P{sub 2} as a function of annealing temperature. The Zn{sub 3}P{sub 2} crystal was etched in bromine-methanol prior to analysis and annealing was performed in vacuum at 300 and 350 {degree}C after sputter cleaning. The UPS spectra for the virgin material are qualitatively similar to the photoemission results for various II-VI Zn compound semiconductors and a comparison of the Zn 3{ital d} binding energies with respect to the valence band maximum is presented. The results for the virgin material and the 300 {degree}C anneal are further compared with the theoretically predicted band structure of Zn{sub 3}P{sub 2} as determined by a pseudopotential energy band calculation. Loss of phosphorus from the surface and the presence of elemental zinc on the surface after the 350 {degree}C anneal is evident.

  9. Fowler-Nordheim tunneling and conduction-band discontinuity in GaAs/GaAsAl high electron mobility transistor structures

    Science.gov (United States)

    Smoliner, J.; Christanell, R.; Hauser, M.; Gornik, E.; Weimann, G.

    1987-06-01

    Oscillatory structure is observed in the dI/dV characteristics of conventional GaAs/GaAlAs high electron mobility transistor samples at liquid-helium temperature, which can be explained using a Fowler-Nordheim tunneling theory. The position of the oscillations allows a determination of the conduction-band discontinuity, and the depth of the deep donor levels in the GaAlAs for high aluminum concentrations. The fit of the data gives a value of Delta Ec/Delta Eg = 0.61 + or - 0.04 for aluminum concentration 30, 36, and 40 percent. The deep donor level in the GaAlAs was determined to be 130 meV below the conduction band.

  10. Quasiparticle band structure of antiferromagnetic Eu Te

    Energy Technology Data Exchange (ETDEWEB)

    Mathi Jaya, S.; Nolting, W. [Humboldt-Universitaet zu Berlin, Institut fuer Physik, Lehrstuhl Festkoerpertheorie, Invalidenstrasse 110, D-10115 Berlin (Germany)

    1997-11-24

    The temperature-dependent electronic quasiparticle spectrum of the antiferromagnetic semiconductor Eu Te is derived by use of a combination of a many-body model procedure with a tight-binding-'linear muffin tin orbital' (TB - LMTO) band structure calculation. The central part is the d-f model for a single band electron ('test electron') being exchange coupled to the anti ferromagnetically ordered localized moments of the Eu ions. The single-electron Bloch energies of the d-f model are taken from a TB-LMTO calculation for paramagnetic Eu Te. The d-f model is evaluated by a recently proposed moment conserving Green function technique to get the temperature-dependent sublattice-quasiparticle band structure (S-QBS) and sublattice-quasiparticle density of states (S-QDOS) of the unoccupied 5 d-6 s energy bands. Unconventional correlation effects and the appearance of characteristic quasiparticles ('magnetic polarons') are worked out in detail. The temperature dependence of the S-QDOS and S-QBS is mainly provoked by the spectral weights of the energy dispersions. Minority- and majority-spin spectra coincide for all temperatures but with different densities of states. Upon cooling from T{sub N} to T = 0 K the lower conduction band edge exhibits a small blue shift of -0.025 eV in accordance with the experiment. Quasiparticle damping manifesting itself in a temperature-dependent broadening of the spectral density peaks arises from spin exchange processes between (5 d-6 s) conduction band electrons and localized 4 f moments. (author)

  11. Preparation, crystal structures, experimental and theoretical electronic band structures of cobalt tellurides in the composition range CoTe{sub 1.3}-CoTe{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Muhler, M. [Institut fuer Technische Chemie, Ruhr-Universitaet Bochum, D-44780 Bochum (Germany); Bensch, W.; Schur, M. [Institut fuer Anorganische Chemie, Christian-Albrechts-Universitaet Kiel, Olshausenstrasse 40, D-24098 Kiel (Germany)

    1998-04-06

    Cobalt tellurides in the composition range CoTe{sub 1.3}-CoTe{sub 2} crystallize in a CdI{sub 2}-type structure with short intra- and interslab Te-Te contacts indicating a polymeric network with multiple Te-Te bonds explaining the very low c/a values of 1.38 to 1.41 of the hexagonal cells. Single-crystal x-ray investigations performed on CoTe{sub 2} confirm the marcasite-type structure in the centrosymmetric space group Pnnm. Experimental valence band spectra (UPS) confirm that the Co tellurides in the composition range CoTe{sub x} (1.3 < x < 2) are metals. The emission at the Fermi level E{sub F} decreases with the Te content and is due to Co 3d and Te 5p states. This assignment is supported by the results of the calculated density of states curve (DOS) which demonstrates that Te p states contribute about 50% in the CdI{sub 2}-type and about 35% in the marcasite-type structure. The Te d states contribute about 15% to the total Te contributions. This behaviour cannot be understood on the basis of a simple tight-binding description, ignoring d-valence states of Te. Core level spectra (XPS) suggest that all CoTe{sub x} samples are best described as intermetallic compounds. Small chemical shifts between the different samples are mainly due to the different Madelung contributions rather than to changes of the electron density located on the Co atoms. An oxidation number for Te <-1 in all CoTe{sub x} samples is deduced, in good agreement with the value of about -1.3 for the Te in CoTe{sub 2} that would be deduced from the relation between the Te-Te distances versus oxidation states of the anion in (Te{sub 2}){sup -11}, Te{sup -11}. The three-dimensional character of the Co tellurides deduced from the crystal structure is further confirmed by the calculated energy dispersion E(k). (author)

  12. Electronic band structure and Fermi surfaces of the quasi-two-dimensional monophosphate tungsten bronze, P4W12O44

    Science.gov (United States)

    Paul, S.; Ghosh, A.; Sato, T.; Sarma, D. D.; Takahashi, T.; Wang, E.; Greenblatt, M.; Raj, S.

    2014-02-01

    The electronic structure of quasi-two-dimensional monophosphate tungsten bronze, P4W12O44, has been investigated by high-resolution angle-resolved photoemission spectroscopy and density functional theoretical calculations. Experimental electron-like bands around \\Gamma point and Fermi surfaces have similar shapes as predicted by calculations. Fermi surface mapping at different temperatures shows a depletion of density of states at low temperature in certain flat portions of the Fermi surfaces. These flat portions of the Fermi surfaces satisfy the partial nesting condition with incommensurate nesting vectors q_1 and q_2 , which leads to the formation of charge density waves in this phosphate tungsten bronzes. The setting up of charge density wave in these bronzes can well explain the anomaly observed in its transport properties.

  13. Multi-band model of quantum electron devices

    Science.gov (United States)

    Unlu, Mehmet Burcin

    Wigner function equations for multi-band quantum devices are presented in this presentation. These quantum transport equations are derived from the equations of motion of non-equilibrium Green's function with the generalized Kadanoff Baym ansatz, and the multi-band k.p Hamiltonian including the spin-orbit interaction. The results are applied to a two-band resonant inter-band tunneling structure. A Wigner function representation is developed for the quantum transport theory of the conduction band electrons in Rashba effect resonant tunneling structures with a phonon bath. In narrow band gap heterostructures, spin splitting occurs mainly as a result of inversion asymmetry in the spatial dependence of the potential or as a result of external electric field. This "zero magnetic field spin splitting" is due to the Rashba term in the effective mass Hamiltonian. A theoretical study of the spin-dependent resonant tunneling structure based on multi-band non-equilibrium Green's functions is also presented in this work. Again, the quantum transport equations are derived using multiband non-equilibrium Green's function formulation in generalized Kadanoff-Baym ansatz. Finally, numerical results are presented based on the multi-band Wigner-Poisson code. This code is able to simulate multi-band resonant tunneling structures.

  14. Systematic study of the effect of HSE functional internal parameters on the electronic structure and band gap of a representative set of metal oxides.

    Science.gov (United States)

    Viñes, Francesc; Lamiel-García, Oriol; Chul Ko, Kyoung; Yong Lee, Jin; Illas, Francesc

    2017-04-30

    The effect of the amount of Hartree-Fock mixing parameter (α) and of the screening parameter (w) defining the range separated HSE type hybrid functional is systematically studied for a series of seven metal oxides: TiO2 , ZrO2 , CuO2 , ZnO, MgO, SnO2 , and SrTiO3 . First, reliable band gap values were determined by comparing the optimal α reproducing the experiment with the inverse of the experimental dielectric constant. Then, the effect of the w in the HSE functional on the calculated band gap was explored in detail. Results evidence the existence of a virtually infinite number of combinations of the two parameters which are able to reproduce the experimental band gap, without a unique pair able to describe the full studied set of materials. Nevertheless, the results point out the possibility of describing the electronic structure of these materials through a functional including a screened HF exchange and an appropriate correlation contribution. © 2017 Wiley Periodicals, Inc.

  15. The Electronic Structure of Calcium

    DEFF Research Database (Denmark)

    Jan, J.-P.; Skriver, Hans Lomholt

    1981-01-01

    The electronic structure of calcium under pressure is re-examined by means of self-consistent energy band calculations based on the local density approximation and using the linear muffin-tin orbitals (LMTO) method with corrections to the atomic sphere approximation included. At zero pressure.......149 Ryd, respectively, relative to the s band, give the best possible agreement. Under increasing pressure the s and p electrons are found to transfer into the d band, and Ca undergoes metal-semimetal-metal electronic transitions. Calculations of the bandstructure and the electronic pressure, including...

  16. Mn Doping Effects on the Electronic Band Structure of PbS Quantum Dot Thin Films: A Scanning Tunneling Microscopy Analysis

    Science.gov (United States)

    Yost, Andrew J.; Rimal, Gaurab; Tang, Jinke; Chien, Teyu

    A thorough understanding of the phenomena associated with doping of transition metals in semiconductors is important for the development of semiconducting electronic technologies such as semiconducting quantum dot sensitized solar cells (QDSSC). Manganese doping is of particular interest in a PbS QD as it is potentially capable of increasing overall QDSSC performance. Here we present scanning tunneling microscopy and spectroscopy studies about the effects of Manganese doping on the energy band structures of PbS semiconducting QD thin films, grown using pulsed laser deposition. As a result of Manganese doping in the PbS QD thin films, a widening of the electronic band gap was observed, which is responsible for the observed increase in resistivity. Furthermore, a loss of long range periodicity observed by XRD, upon incorporation of Manganese, indicates that the Manganese dopants also induce a large amount of grain boundaries. This work was supported by the following: U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, DEFG02-10ER46728 and the National Science Foundation Grant #0948027.

  17. Characterization of Structural Defects in Wide Band-Gap Compound Materials for Semiconductor and Opto-Electronic Applications

    Science.gov (United States)

    Goue, Ouloide Yannick

    Single crystals of binary and ternary compounds are touted to replace silicon for specialized applications in the semiconductor industry. However, the relative high density of structural defects in those crystals has hampered the performance of devices built on them. In order to enhance the performance of those devices, structurally perfect single crystals must be grown. The aim of this thesis is to investigate the interplay between crystal growth process and crystal quality as well as structural defect types and transport property. To this end, the thesis is divided into two parts. The first part provides a general review of the theory of crystal growth (chapter I), an introduction to the materials being investigated (chapter II and III) and the characterization techniques being used (chapter IV). • In chapter I, a brief description of the theory of crystal growth is provided with an eye towards the driving force behind crystal nucleation and growth along with the kinetic factors affecting crystal growth. The case of crystal growth of silicon carbide (SiC) by physical vapor transport (PVT) and chemical vapor deposition (CVD) is discussed. The Bridgman, travelling heater method (THM) and physical transport growth of cadmium zinc telluride (CZT) is also treated. In chapters II and III, we introduce the compound materials being investigated in this study. While a description of their crystal structure and properties is provided, the issues associated with their growth are discussed. In chapter IV, a description of the characterization techniques used in these studies is presented. These techniques are synchrotron X-ray topography (SXRT), transmission electron microscopy, transmission infrared microscopy (TIM), micro-Raman spectroscopy (muRS) and light microscopy. Extensive treatment of SXRT technique is also provided. In the second part, the experimental results obtained in the course of these studies are presented and discussed. These results are divided into

  18. The utility of band theory in strongly correlated electron systems

    Science.gov (United States)

    Zwicknagl, Gertrud

    2016-12-01

    This article attempts to review how band structure calculations can help to better understand the intriguing behavior of materials with strongly correlated electrons. Prominent examples are heavy-fermion systems whose highly anomalous low-temperature properties result from quantum correlations not captured by standard methods of electronic structure calculations. It is shown how the band approach can be modified to incorporate the typical many-body effects which characterize the low-energy excitations. Examples underlining the predictive power of this ansatz are discussed.

  19. First-principles study of spin-polarized electronic band structures in ferromagnetic Zn1-xTMxS (TM = Fe, Co and Ni)

    KAUST Repository

    Saeed, Yasir

    2010-10-01

    We report a first-principles study of structural, electronic and magnetic properties of crystalline alloys Zn1-xTMxS (TM = Fe, Co and Ni) at x = 0.25. Structural properties are computed from the total ground state energy convergence and it is found that the cohesive energies of Zn 1-xTMxS are greater than that of zincblende ZnS. We also study the spin-polarized electronic band structures, total and partial density of states and the effect of TM 3d states. Our results exhibit that Zn 0.75Fe0.25S, Zn0.75Co0.25S and Zn0.75Ni0.25S are half-metallic ferromagnetic with a magnetic moment of 4μB, 3μB and 2μB, respectively. Furthermore, we calculate the TM 3d spin-exchange-splitting energies Δx (d), Δx (x-d), exchange constants N0α and N0β, crystal field splitting (ΔEcrystEt2g-Eeg), and find that p-d hybridization reduces the local magnetic moment of TM from its free space charge value. Moreover, robustness of Zn1-xTMxS with respect to the variation of lattice constants is also discussed. © 2010 Elsevier B.V. All rights reserved.

  20. Correlation effects of π electrons on the band structures of conjugated polymers using the self-consistent GW approximation with vertex corrections.

    Science.gov (United States)

    Chang, Yao-Wen; Jin, Bih-Yaw

    2012-01-14

    Many-body perturbation theory is used to investigate the effect of π-electron correlations on the quasi-particle band structures of conjugated polymers at the level of the Pariser-Parr-Pople model. The self-consistent GW approximation with vertex corrections to both the self-energy and the polarization in Hedin's equations is employed in order to eliminate self-interaction errors and include the effects of electron-hole attraction in screening processes. The dynamic inverse dielectric function is constructed from the generalized plasmon-pole approximation with the static dressed polarization given by the coupled-perturbed Hartree-Fock equation. The bandgaps of trans-polyacetylene, trans-polyphenylenevinylene and poly(para)phenylene are calculated by both the Hartree-Fock and GW approximation, and a lowering of bandgaps due to electron correlations is found. We conclude that both dielectric screening and vertex corrections are important for calculating the quasi-particle bandgaps of conjugated polymers.

  1. Electronic structures of bare and terephthalic acid adsorbed TiO2(110)-(1 × 2) reconstructed surfaces: origin and reactivity of the band gap states.

    Science.gov (United States)

    Zhang, Wenhua; Liu, Liming; Wan, Li; Liu, Lingyun; Cao, Liang; Xu, Faqiang; Zhao, Jin; Wu, Ziyu

    2015-08-21

    Combined core level spectroscopy, valence spectroscopy and density functional theory studies have probed the terephthalic acid (TPA) adsorption behavior and the electronic structure of the rutile TiO2(110)-(1 × 2) reconstructed surface at room temperature. The TiO2(110)-(1 × 2) reconstructed surface exhibits an electron rich nature owing to the unsaturated coordination of the surface terminated Ti2O3 rows. Deprotonation of TPA molecules upon adsorption produces both surface bridging hydroxyl (ObH) and bidentate terephthalate species with a saturation coverage of nearly 0.5 monolayers (ML). In contrast to the TiO2(110)-(1 × 1) surface, the band gap states (BGSs) on the bare (1 × 2) surface exhibit an asymmetric spectral feature, which is originated from integrated contributions of the Ti2O3 termination and the defects in the near-surface region. The Ti2O3 originated BGSs are found to be highly sensitive to the TPA adsorption, a phenomenon well reproduced by the density functional theory (DFT) calculations. Theoretical simulations of the adsorption process also suggest that the redistribution of the electronic density on the (1 × 2) reconstructed surface accompanying the hydroxyl formation promotes the disappearance of the Ti2O3-row derived BGS.

  2. Electronic structure of tin monosulfide

    Science.gov (United States)

    Bletskan, D. I.; Bletskan, M. M.; Glukhov, K. E.

    2017-01-01

    The band structure of three-dimensional and two-dimensional tin monosulfide was calculated by the density functional method in LDA and LDA+U approximations. Group-theoretical analysis of the electronic band structure of SnS crystallized in the orthorhombic structure with space group D2h16- Pcmn is carried out, the symmetry of wave functions of the valence band and the bottom of the conduction band is found. The selection rules for direct and indirect optical transitions at different incident light polarization are determined. The group-theoretical analysis of energy states of the three-dimensional and two-dimensional SnS structures explains the formation of the band structure including the Davydov splitting. The calculated total density of states is compared with the known experimental XPS and UPS spectra, providing the assignment of their main features.

  3. An Improved Study of Electronic Band Structure and Optical Parameters of X-Phosphides (X--B, AL, Ga, In) by Modified Becke-Johnson Potential%An Improved Study of Electronic Band Structure and Optical Parameters of X-Phosphides (X--B, AL, Ga, In) by Modified Becke-Johnson Potential

    Institute of Scientific and Technical Information of China (English)

    Masood Yousaf; M.A. Saeed; R. Ahmed; M.M. Alsardia; Ahmad Radzi Mat Isa; A. Shaari

    2012-01-01

    We report the electronic band structure and optical parameters of X-Phosphides (X=B, AI, Ga, In) by first-principles technique based on a new approximation known as modified Becke-Johnson (roB J). This potential is considered more accurate in elaborating excited states properties of insulators and semiconductors as compared to LDA and GGA. The present calculated band gaps values of BP, AlP, GaP, and InP are 1.867 eV, 2.268 eV, 2.090 eV, and 1.377 eV respectively, which are in close agreement to the experimental results. The band gap values trend in this study is as: E9 (mBJ-GGA/LDA) 〉 E9 (GGA) 〉 Eg (LDA). Optical parametric quantities (dielectric constant, refractive index, reflectivity and optical conductivity) which based on the band structure are aiso presented and discussed. BP, AlP, GaP, and InP have strong absorption in between the energy range 4-9 eV, 4-7 ev, 3-7 eV, and 2-7 eV respectively. Static dielectric constant, static refractive index and coefficient of reflectivity at zero frequency, within mBJ-GGA, are also calculated. BP, AIP, GaP, and InP show significant optical conductivity in the range 5.2-10 eV, 4.3-8 eV, 3.5- 7.2 eV, and 3.2-8 eV respectively. The present study endorses that the said compounds can be used in opto-electronic applications, for different energy ranges.

  4. First-principle calculation of the elastic, band structure, electronic states, and optical properties of Cu-doped ZnS nanolayers

    Science.gov (United States)

    Lahiji, Mohammadreza Askaripour; Ziabari, Ali Abdolahzadeh

    2016-11-01

    The structural, elastic, electronic, and optical properties of undoped and Cu-doped ZnS nanostructured layers have been studied in the zincblende (ZB) phase, by first-principle approach. Density functional theory (DFT) has been employed to calculate the fundamental properties of the layers using full-potential linearized augmented plane-wave (FPLAPW) method. Mechanical analysis revealed that the bulk modulus increases with the increase of Cu content. Cu doping was found to reduce the band gap value of the material. In addition, DOS effective mass of the electrons and heavy holes was evaluated. Adding Cu caused the decrement/increment of transmission/reflectance of nanolayers in the UV-vis region. The substitution by Cu increased the intensity of the peaks, and a slight red shift was observed in the absorption peak. Moreover, the static dielectric constant, and static refractive index increased with Cu content. The optical conductivity also followed a similar trend to that of the dielectric constants. Energy loss function of the modeled compounds was also evaluated. All calculated parameters were compared with the available experimental and other theoretical results.

  5. Band Structure of the Growth Rate of the Two-Stream Instability of an Electron Beam Propagating in a Bounded Plasma

    CERN Document Server

    Kaganovich, I D

    2015-01-01

    This paper presents a study of the two-stream instability of an electron beam propagating in a finite-size plasma placed between two electrodes. It is shown that the growth rate in such a system is much smaller than that of an infinite plasma or a finite size plasma with periodic boundary conditions. Even if the width of the plasma matches the resonance condition for a standing wave, a spatially growing wave is excited instead with the growth rate small compared to that of the standing wave in a periodic system. The approximate expression for this growth rate is $\\gamma \\approx (1/13)\\omega_{pe}(n_{b}/n_{p})(L\\omega_{pe}/v_{b})\\ln (L\\omega_{pe}/v_{b})[ 1-0.18\\cos ( L\\omega_{pe}/v_{b}+{\\pi }/{2}) ]$, where $\\omega_{pe}$ is the electron plasma frequency, $n_{b}$ and $n_{p}$ are the beam and the plasma densities, respectively, $v_{b}$ is the beam velocity, and $L$ is the plasma width. The frequency, wave number and the spatial and temporal growth rates as functions of the plasma size exhibit band structure.

  6. Band structure of the growth rate of the two-stream instability of an electron beam propagating in a bounded plasma

    Science.gov (United States)

    Kaganovich, I. D.; Sydorenko, D.

    2016-11-01

    This paper presents a study of the two-stream instability of an electron beam propagating in a finite-size plasma placed between two electrodes. It is shown that the growth rate in such a system is much smaller than that of an infinite plasma or a finite size plasma with periodic boundary conditions. Even if the width of the plasma matches the resonance condition for a standing wave, a spatially growing wave is excited instead with the growth rate small compared to that of the standing wave in a periodic system. The approximate expression for this growth rate is γ≈(1 /13 )ωpe(nb/np)(L ωpe/vb)ln (L ωpe/vb)[1 -0.18 cos (L ωpe/vb+π/2 ) ] , where ωpe is the electron plasma frequency, nb and np are the beam and the plasma densities, respectively, vb is the beam velocity, and L is the plasma width. The frequency, wave number, and the spatial and temporal growth rates, as functions of the plasma size, exhibit band structure. The amplitude of saturation of the instability depends on the system length, not on the beam current. For short systems, the amplitude may exceed values predicted for infinite plasmas by more than an order of magnitude.

  7. Emergence of Electron Distributions Related to Banded Chorus Excitation

    Science.gov (United States)

    Liu, K.; Min, K.; Fu, X.; Gary, S. P.; Winske, D.; Cowee, M.

    2014-12-01

    Two-dimensional electromagnetic particle-in-cell simulations are performed to investigate the emergence of electron velocity distributions with an "anisotropy gap" in the radiation belts. Such distributions have been shown to be able to excite banded chorus as anisotropic electrons with T⊥/T||>1 at different T|| , where || and ⊥ denote directions relative to the background geomagnetic field, can drive the whistler anisotropy instability in the upper- and lower-bands, independently. Energy-dependent convection and loss of the electrons in the radiation belts can generally lead to anisotropic electrons which, subsequently, excite whistler waves. As a simplification of reality, the simulations in the present study start with an electron distribution which is constantly anisotropic at all energies. The anisotropic electrons lead to growth of whistlers in the system. The enhanced waves, in turn, scatter the electrons and reduce their anisotropy. Interestingly, the reduction of anisotropy is more dramatic for electrons with intermediate energies. An anisotropy gap arises as a natural consequence of the self-consistent wave-particle interactions and, in the quasi-steady phase of the simulations, produces the banded structure in the spectrum of the enhanced whistler waves.

  8. Dominance of many-body effects over the one-electron mechanism for band structure doping dependence in Nd{sub 2-x}Ce{sub x}CuO{sub 4}: the LDA+GTB approach

    Energy Technology Data Exchange (ETDEWEB)

    Korshunov, M M [L V Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation); Gavrichkov, V A [L V Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation); Ovchinnikov, S G [L V Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation); Nekrasov, I A [Institute of Electrophysics, Russian Academy of Sciences-Ural Division, 620016 Yekaterinburg, Amundsena 106 (Russian Federation); Kokorina, E E [Institute of Electrophysics, Russian Academy of Sciences-Ural Division, 620016 Yekaterinburg, Amundsena 106 (Russian Federation); Pchelkina, Z V [Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620041 Yekaterinburg, GSP-170 (Russian Federation)

    2007-12-05

    In the present work we report band structure calculations for the high-temperature superconductor Nd{sub 2-x}Ce{sub x}CuO{sub 4} in the regime of strong electronic correlations within an LDA+GTB method, which combines the local density approximation (LDA) and the generalized tight-binding method (GTB). The two mechanisms of band structure doping dependence were taken into account. Namely, the one-electron mechanism provided by the doping dependence of the crystal structure, and the many-body mechanism provided by the strong renormalization of the fermionic quasiparticles due to the large on-site Coulomb repulsion. We have shown that, in the antiferromagnetic and in the strongly correlated paramagnetic phases of the underdoped cuprates, the main contribution to the doping evolution of the band structure and Fermi surface comes from the many-body mechanism.

  9. Elucidating the stop bands of structurally colored systems through recursion

    CERN Document Server

    Amir, Ariel

    2012-01-01

    Interference phenomena are the source of some of the spectacular colors of animals and plants in nature. In some of these systems, the physical structure consists of an ordered array of layers with alternating high and low refractive indices. This periodicity leads to an optical band structure that is analogous to the electronic band structure encountered in semiconductor physics; namely, specific bands of wavelengths (the stop bands) are perfectly reflected. Here, we present a minimal model for optical band structure in a periodic multilayer and solve it using recursion relations. We present experimental data for various beetles, whose optical structure resembles the proposed model. The stop bands emerge in the limit of an infinite number of layers by finding the fixed point of the recursive relations. In order for these to converge, an infinitesimal amount of absorption needs to be present, reminiscent of the regularization procedures commonly used in physics calculations. Thus, using only the phenomenon of...

  10. Band structure of the heavily-electron-doped FeAs-based Ba(Fe,Co)2As2 superconductor suppresses antiferromagnetic correlations.

    Science.gov (United States)

    Sudayama, T; Wakisaka, Y; Takubo, K; Morinaga, R; Sato, T J; Arita, M; Namatame, H; Taniguchi, M; Mizokawa, T

    2010-04-30

    In the heavily-electron-doped regime of the Ba(Fe,Co)2As2 superconductor, three hole bands at the zone center are observed and two of them reach the Fermi level. The larger hole pocket at the zone center is apparently nested with the smaller electron pocket around the zone corner. However, the (pi,0) Fermi surface reconstruction reported for the hole-doped case is absent in the heavily-electron-doped case. This observation shows that the apparent Fermi surface nesting alone is not enough to enhance the antiferromagnetic correlation as well as the superconducting transition temperature.

  11. The valence band electronic structure of rhombohedral-like and tetragonal-like BiFeO{sub 3} thin films from hard X-ray photoelectron spectroscopy and first-principles theory

    Energy Technology Data Exchange (ETDEWEB)

    Mazumdar, Dipanjan [Department of Physics, Southern Illinois University, Carbondale, IL 62901 (United States); Knut, R. [Department of Physics and Astronomy, Uppsala University, Uppsala (Sweden); JILA and Department of Physics, University of Colorado Boulder and NIST, Boulder, CO 80309 (United States); Thöle, F. [Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, Zürich (Switzerland); Gorgoi, M. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, BESSY II, Berlin (Germany); Faleev, Sergei; Mryasov, O.N.; Shelke, Vilas [Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487 (United States); Ederer, C.; Spaldin, N.A. [Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, Zürich (Switzerland); Gupta, A. [Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487 (United States); Karis, O., E-mail: olof.karis@physics.uu.se [Department of Physics and Astronomy, Uppsala University, Uppsala (Sweden)

    2016-04-15

    Highlights: • Hard X-ray PES was used to study the bulk electronic structure of BiFeO{sub 3}. • Rhombohedral and tetragonal BiFeO{sub 3} exhibit dissimilar valence band structures. • Fe 3d states are largely unchanged between the rhombohedral and tetragonal structures. • Modified hybridization between Bi 6p and O 2p states in the two different structures. • Electronic structure calculations corroborates the experimental results. - Abstract: We investigate the electronic structure of rhombohedral-like (R) and tetragonal-like (T) BiFeO{sub 3} thin films using high energy X-ray photoelectron spectroscopy and first-principles electronic structure calculations. By exploiting the relative elemental cross sections to selectively probe the elemental composition of the valence band, we identify a strong Bi 6p contribution at the top of the valence band in both phases, overlapping in energy range with the O 2p states; this assignment is confirmed by our electronic structure calculations. We find that the measured occupied Bi 6p signal lies closer to the top of the valence band in the T phase than in the R phase, which we attribute, using our electronic structure calculations, to lower Bi–O hybridization in the T phase. We note, however, that our calculations of the corresponding densities of states underestimate the difference between the phases, suggesting that matrix element effects resulting from the different effective symmetries also contribute. Our results shed light on the chemical nature of the stereochemically active Bi lone pairs, which are responsible for the large ferroelectric polarization of BiFeO{sub 3}.

  12. Valence band electronic structure of Nb2Pd1.2Se5 and Nb2Pd0.95S5 superconductors

    Science.gov (United States)

    Lohani, H.; Mishra, P.; Goyal, R.; Awana, V. P. S.; Sekhar, B. R.

    2017-03-01

    We present a comparative study of our valence band photoemission results on Nb2Pd1.2Se5 and Nb2Pd0.95S5 superconductors which are supported by our DFT based electronic structure calculations. We observe that the VB spectra of both the compounds are qualitatively similar, except for some slight differences in the binding energy positions of all the features. This could be due to the unequal electronegativities of Se and S atom. The calculated density of states (DOS) reveals that the VB features are mainly composed of Pd-Se/S hybridized states. The nature of DOS originating from the distinctly coordinated Pd atoms is different. Further, various Pd-4d and Nb-4d states crossing the Fermi level (Ef) signifies the multiband character of these compounds. In addition, we find a temperature dependent pseudogap in Nb2Pd0.95S5 which is absent in Nb2Pd1.2Se5.

  13. Band-structure engineering in conjugated 2D polymers.

    Science.gov (United States)

    Gutzler, Rico

    2016-10-26

    Conjugated polymers find widespread application in (opto)electronic devices, sensing, and as catalysts. Their common one-dimensional structure can be extended into the second dimension to create conjugated planar sheets of covalently linked molecules. Extending π-conjugation into the second dimension unlocks a new class of semiconductive polymers which as a consequence of their unique electronic properties can find usability in numerous applications. In this article the theoretical band structures of a set of conjugated 2D polymers are compared and information on the important characteristics band gap and valence/conduction band dispersion is extracted. The great variance in these characteristics within the investigated set suggests 2D polymers as exciting materials in which band-structure engineering can be used to tailor sheet-like organic materials with desired electronic properties.

  14. Atomic structure of amorphous shear bands in boron carbide.

    Science.gov (United States)

    Reddy, K Madhav; Liu, P; Hirata, A; Fujita, T; Chen, M W

    2013-01-01

    Amorphous shear bands are the main deformation and failure mode of super-hard boron carbide subjected to shock loading and high pressures at room temperature. Nevertheless, the formation mechanisms of the amorphous shear bands remain a long-standing scientific curiosity mainly because of the lack of experimental structure information of the disordered shear bands, comprising light elements of carbon and boron only. Here we report the atomic structure of the amorphous shear bands in boron carbide characterized by state-of-the-art aberration-corrected transmission electron microscopy. Distorted icosahedra, displaced from the crystalline matrix, were observed in nano-sized amorphous bands that produce dislocation-like local shear strains. These experimental results provide direct experimental evidence that the formation of amorphous shear bands in boron carbide results from the disassembly of the icosahedra, driven by shear stresses.

  15. The complex band structure for armchair graphene nanoribbons

    Institute of Scientific and Technical Information of China (English)

    Zhang Liu-Jun; Xia Tong-Sheng

    2010-01-01

    Using a tight binding transfer matrix method, we calculate the complex band structure of armchair graphene nanoribbons. The real part of the complex band structure calculated by the transfer matrix method fits well with the bulk band structure calculated by a Hermitian matrix. The complex band structure gives extra information on carrier's decay behaviour. The imaginary loop connects the conduction and valence band, and can profoundly affect the characteristics of nanoscale electronic device made with graphene nanoribbons. In this work, the complex band structure calculation includes not only the first nearest neighbour interaction, but also the effects of edge bond relaxation and the third nearest neighbour interaction. The band gap is classified into three classes. Due to the edge bond relaxation and the third nearest neighbour interaction term, it opens a band gap for N= 3M-1. The band gap is almost unchanged for N = 3M + 1, but decreased for N = 3M. The maximum imaginary wave vector length provides additional information about the electrical characteristics of graphene nmaoribbons, and is also classified into three classes.

  16. Effective band structure of random alloys.

    Science.gov (United States)

    Popescu, Voicu; Zunger, Alex

    2010-06-11

    Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)). Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), e.g., effective masses or van Hove singularities. Here we use large supercells with randomly distributed A and B atoms, whereby many different local environments are allowed to coexist, and transform the eigenstates into an effective band structure (EBS) in the primitive cell using a spectral decomposition. The resulting EBS reveals the extent to which band characteristics are preserved or lost at different compositions, band indices, and k(→) points, showing in (In,Ga)N the rapid disintegration of the valence band Bloch character and in Ga(N,P) the appearance of a pinned impurity band.

  17. Crystal and electronic structures and high-pressure behavior of AgSO4, a unique narrow band gap antiferromagnetic semiconductor: LDA(+U) picture.

    Science.gov (United States)

    Derzsi, Mariana; Stasiewicz, Juliusz; Grochala, Wojciech

    2011-09-01

    We demonstrate that DFT calculations performed with the local density approximation (LDA) allow for significantly better reproduction of lattice constants, the unit cell volume and the density of Ag(II)SO(4) than those done with generalized gradient approximation (GGA). The LDA+U scheme, which accounts for electronic correlation effects, enables the accurate prediction of the magnetic superexchange constant of this strongly correlated material and its band gap at the Fermi level. The character of the band gap places the compound on the borderline between a Mott insulator and a charge transfer insulator. The size of the band gap (0.82 eV) indicates that AgSO(4) is a ferrimagnetic semiconductor, and possibly an attractive material for spintronics. A bulk modulus of 27.0 GPa and a compressibility of 0.037 GPa(-1) were determined for AgSO(4) from the third-order Birch-Murnaghan isothermal equation of state up to 20 GPa. Several polymorphic types compete with the ambient pressure P-1 phase as the external pressure is increased. The P-1 phase is predicted to resist pressure-induced metallization up to at least 20 GPa.

  18. Unfolding the band structure of GaAsBi

    Science.gov (United States)

    Maspero, R.; Sweeney, S. J.; Florescu, Marian

    2017-02-01

    Typical supercell approaches used to investigate the electronic properties of GaAs(1-x)Bi(x) produce highly accurate, but folded, band structures. Using a highly optimized algorithm, we unfold the band structure to an approximate E≤ft(\\mathbf{k}\\right) relation associated with an effective Brillouin zone. The dispersion relations we generate correlate strongly with experimental results, confirming that a regime of band gap energy greater than the spin-orbit-splitting energy is reached at around 10% bismuth fraction. We also demonstrate the effectiveness of the unfolding algorithm throughout the Brillouin zone (BZ), which is key to enabling transition rate calculations, such as Auger recombination rates. Finally, we show the effect of disorder on the effective masses and identify approximate values for the effective mass of the conduction band and valence bands for bismuth concentrations from 0-12%.

  19. Band structure analysis in SiGe nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Amato, Michele [' Centro S3' , CNR-Istituto Nanoscienze, via Campi 213/A, 41100 Modena (Italy); Dipartimento di Scienze e Metodi dell' Ingegneria, Universita di Modena e Reggio Emilia, via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia (Italy); Palummo, Maurizia [European Theoretical Spectroscopy Facility (ETSF) (Italy); CNR-INFM-SMC, Dipartimento di Fisica, Universita di Roma, ' Tor Vergata' , via della Ricerca Scientifica 1, 00133 Roma (Italy); Ossicini, Stefano, E-mail: stefano.ossicini@unimore.it [' Centro S3' , CNR-Istituto Nanoscienze, via Campi 213/A, 41100 Modena (Italy) and Dipartimento di Scienze e Metodi dell' Ingegneria, Universita di Modena e Reggio Emilia, via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia (Italy) and European Theoretical Spectroscopy Facility - ETSF (Italy) and Centro Interdipartimentale ' En and Tech' , Universita di Modena e Reggio Emilia, via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia (Italy)

    2012-06-05

    One of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modification of the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological applications.

  20. One-Dimensional Anisotropic Band Gap Structure

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    The band gap structure of one-dimensional anisotropic photonic crystal has been studied by means of the transfer matrix formalism. From the analytic expressions and numeric calculations we see some general characteristics of the band gap structure of anisotropic photonic crystals, each band separates into two branches and the two branches react to polarization sensitively. In the practical case of oblique incidence, gaps move towards high frequency when the angle of incidence increases. Under some special conditions, the two branches become degenerate again.

  1. Many-electron bands in transition metal compounds

    NARCIS (Netherlands)

    Stoyanova, A.; de Graaf, C.; Broer, R.; Simos, TE; Maroulis, G

    2007-01-01

    A new method is presented for generating correlated many-electron bands for localized excited states, hole states and added-electron states in extended systems with strong electron correlation effects. The method allows for a rigorous treatment of the local electronic response that accompanies the e

  2. Electronic structure and band alignments of ZnTe/CrTe(0 0 1), CdSe/CrTe(0 0 1) and CdTe/CrTe(0 0 1) interfaces

    Indian Academy of Sciences (India)

    F Ahmadian; R Zare

    2011-08-01

    All-electron full potential calculations based on spin density functional theory were performed to study cubic zincblende (ZB) and hexagonal NiAs structures of bulk CrTe and ZnTe/CrTe(0 0 1), CdTe/CrTe(0 0 1) and CdSe/CrTe(0 0 1) interfaces. The lattice mismatch effect in ZB CrTe and magnetic properties of CrTe in the ideal ZB CrTe structure were investigated. The band alignment properties of the ZnTe/CrTe(0 0 1), CdTe/CrTe(0 0 1) and CdSe/CrTe(0 0 1) interfaces were computed and a rather large minority valence band offset of about 1.09 eV was observed in ZnTe/CrTe(0 0 1) heterojunction. Also in the CdTe/CrTe(0 0 1) and CdSe/CrTe(0 0 1) interfaces, the conduction band minimum of minority spin in CrTe was above the conduction band minimum of CdTe and CdSe and so the majority spin electrons could be directly injected to both semiconductors, indicating the possibility of highly efficient spin injection into the CdSe and CdTe semiconductors.

  3. Electronic structure and band gap of zinc spinel oxides beyond LDA: ZnAl{sub 2}O{sub 4}, ZnGa{sub 2}O{sub 4} and ZnIn{sub 2}O{sub 4}

    Energy Technology Data Exchange (ETDEWEB)

    Dixit, H; Saniz, R; Lamoen, D; Partoens, B [CMT-group and EMAT, Departement Fysica, Universiteit Antwerpen Groenenborgerlaan 171, B-2020 Antwerpen (Belgium); Tandon, N [Instituut voor Kern- en Stralingsfysica, K U Leuven Celestijnenlaan 200D, B-3001 Leuven (Belgium); Cottenier, S; Van Speybroeck, V; Waroquier, M, E-mail: Hemant.Dixit@ua.ac.be [Center for Molecular Modeling, Ghent University Technologiepark 903, 9052 Zwijnaarde (Belgium)

    2011-06-15

    We examine the electronic structure of the family of ternary zinc spinel oxides ZnX{sub 2}O{sub 4} (X=Al, Ga and In). The band gap of ZnAl{sub 2}O{sub 4} calculated using density functional theory (DFT) is 4.25 eV and is overestimated compared with the experimental value of 3.8-3.9 eV. The DFT band gap of ZnGa{sub 2}O{sub 4} is 2.82 eV and is underestimated compared with the experimental value of 4.4-5.0 eV. Since DFT typically underestimates the band gap in the oxide system, the experimental measurements for ZnAl{sub 2}O{sub 4} probably require a correction. We use two first-principles techniques capable of describing accurately the excited states of semiconductors, namely the GW approximation and the modified Becke-Johnson (MBJ) potential approximation, to calculate the band gap of ZnX{sub 2}O{sub 4}. The GW and MBJ band gaps are in good agreement with each other. In the case of ZnAl{sub 2}O{sub 4}, the predicted band gap values are >6 eV, i.e. {approx}2 eV larger than the only reported experimental value. We expect future experimental work to confirm our results. Our calculations of the electron effective masses and the second band gap indicate that these compounds are very good candidates to act as transparent conducting host materials.

  4. Atomic arrangement and electron band structure of Si(1 1 1)-ß-√3 x √3-Bi reconstruction modified by alkali-metal adsorption: ab initio study.

    Science.gov (United States)

    Eremeev, S V; Chukurov, E N; Gruznev, D V; Zotov, A V; Saranin, A A

    2015-08-05

    Using ab initio calculations, atomic structure and electronic properties of Si(1 1 1)[Formula: see text]-Bi surface modified by adsorption of 1/3 monolayer of alkali metals, Li, Na, K, Rb and Cs, have been explored. Upon adsorption of all metals, a similar atomic structure develops at the surface where twisted chained Bi trimers are arranged into a honeycomb network and alkali metal atoms occupy the [Formula: see text] sites in the center of each honeycomb unit. Among other structural characteristics, the greatest variation concerns the relative heights at which alkali metals reside with respect to Bi-trimer layer. Except for Li, the other metals reside higher than Bi layer and their heights increase with atomic number. All adsorbed surface structures display similar electron band structures of which the most essential feature is metallic surface-state band with a giant spin splitting. This electronic property allows one to consider the Si(1 1 1)[Formula: see text]-Bi surfaces modified by alkali metal adsorption as a set of material systems showing promise for spintronic applications.

  5. Atomic arrangement and electron band structure of Si(1 1 1)-ß-\\sqrt{3}\\times \\sqrt{3} -Bi reconstruction modified by alkali-metal adsorption: ab initio study

    Science.gov (United States)

    Eremeev, S. V.; Chukurov, E. N.; Gruznev, D. V.; Zotov, A. V.; Saranin, A. A.

    2015-08-01

    Using ab initio calculations, atomic structure and electronic properties of Si(1 1 1)\\sqrt{3}× \\sqrt{3} -Bi surface modified by adsorption of 1/3 monolayer of alkali metals, Li, Na, K, Rb and Cs, have been explored. Upon adsorption of all metals, a similar atomic structure develops at the surface where twisted chained Bi trimers are arranged into a honeycomb network and alkali metal atoms occupy the {{T}4} sites in the center of each honeycomb unit. Among other structural characteristics, the greatest variation concerns the relative heights at which alkali metals reside with respect to Bi-trimer layer. Except for Li, the other metals reside higher than Bi layer and their heights increase with atomic number. All adsorbed surface structures display similar electron band structures of which the most essential feature is metallic surface-state band with a giant spin splitting. This electronic property allows one to consider the Si(1 1 1)\\sqrt{3}× \\sqrt{3} -Bi surfaces modified by alkali metal adsorption as a set of material systems showing promise for spintronic applications.

  6. Electronic structure of layered quaternary chalcogenide materials for band-gap engineering: The example of Cs2MIIM3IVQ8

    Science.gov (United States)

    Besse, Rafael; Sabino, Fernando P.; Da Silva, Juarez L. F.

    2016-04-01

    Quaternary chalcogenide materials offer a wide variety of chemical and physical properties, and hence, those compounds have been widely studied for several technological applications. Recently, experimental studies have found that the chalcogenide Cs2MIIM3IVQ8 family (MII = Mg , Zn , Cd , Hg , MIV = Ge , Sn and Q = S , Se , Te ), which includes 24 compounds, yields a wide range of band gaps, namely, from 1.07 to 3.4 eV, and hence, they have attracted great interest. To obtain an improved atomistic understanding of the role of the cations and anions on the physical properties, we performed a first-principles investigation of the 24 Cs2MIIM3IVQ8 compounds employing density functional theory within semilocal and hybrid exchange-correlation energy functionals and the addition of van der Waals corrections to improve the description of the weakly interacting layers. Our lattice parameters are in good agreement with the available experimental data (i.e., 11 compounds), and the equilibrium volume increases linearly by increasing the atomic number of the chalcogen, which can be explained by the increased atomic radius of the chalcogen atoms from S to Te . We found that van der Waals corrections play a crucial role in the lattice parameter in the stacking direction of the Cs2MIIM3IVQ8 layers, while the binding energy per unit area has similar magnitude as obtained for different layered materials. We obtained that the band gaps follow a linear relation as a function of the unit cell volume, which can be explained by the atomic size of the chalcogen atom and the relative position of the Q p states within the band structure. The fundamental and optical band gaps differ by less than 0.1 eV. The band gaps obtained with the hybrid functional are in good agreement with the available experimental data. Furthermore, we found from the Bader analysis, that the Coulomb interations among the cations and anions play a crucial role on the energetic properties.

  7. Surface band-gap narrowing in quantized electron accumulation layers.

    Science.gov (United States)

    King, P D C; Veal, T D; McConville, C F; Zúñiga-Pérez, J; Muñoz-Sanjosé, V; Hopkinson, M; Rienks, E D L; Jensen, M Fuglsang; Hofmann, Ph

    2010-06-25

    An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.

  8. Quasiparticle Band Structure of BaS

    Institute of Scientific and Technical Information of China (English)

    LU Tie-Yu; CHEN De-Yan; HUANG Mei-Chun

    2006-01-01

    @@ We calculate the band structure of BaS using the local density approximation and the GW approximation (GWA),i.e. in combination of the Green function G and the screened Coulomb interaction W. The Ba 4d states are treated as valence states. We find that BaS is a direct band-gap semiconductor. The result shows that the GWA band gap (Eg-Gw = 3.921 eV) agrees excellently with the experimental result (Eg-EXPT = 3.88 eV or 3.9eV).

  9. Electronic structure of PCBM

    Institute of Scientific and Technical Information of China (English)

    Sheng Chun-Qi; Wang Peng; Shen Ying; Li Yan-Jun; Zhang Wen-Hua; Xu Fa-Qiang; Zhu Jun-Fa; Li Hong-Nian; Lai Guo-Qiao

    2012-01-01

    We have studied the electronic structure of [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) using synchrotron radiation photoelectron spectroscopy (PES) measurements and first-principles calculations.The PES spectrum of the entire occupied valence band is reported,which exhibits abundant spectral features from the Fermi level to ~ 24 eV binding energy. All the spectral features are broadened as compared with the cases of C60. The reasons for the broadening are analysed by comparing the experimental data with the calculated energy levels and density of states.Special attention is paid to the analysis of the C60 highest occupied molecular orbital (HOMO)-1 derived states,which can play a crucial role in the bonding at the interfaces of PCBM/polymer blenders or PCBM/electrodes.Besides the well-known energy level splitting of the C60 backbone caused by the lowered symmetry,C 2p states from the side chain mix or hybridize with the molecular orbitals of parent C60.The contribution of the O 2p states can substantially modify the PES spectrum.

  10. Bulk band structure of Bi2Te3

    DEFF Research Database (Denmark)

    Michiardi, Matteo; Aguilera, Irene; Bianchi, Marco;

    2014-01-01

    -electron full-potential linearized augmented-plane-wave (FLAPW) formalism, fully taking into account spin-orbit coupling. Quasiparticle effects produce significant changes in the band structure of Bi2Te3 when compared to LDA. Experimental and calculated results are compared in the spectral regions where...... distinct differences between the LDA and GW results are present. Overall a superior agreement with GW is found, highlighting the importance of many-body effects in the band structure of this family of topological insulators....

  11. Maximizing band gaps in plate structures

    DEFF Research Database (Denmark)

    Halkjær, Søren; Sigmund, Ole; Jensen, Jakob Søndergaard

    2006-01-01

    Band gaps, i.e., frequency ranges in which waves cannot propagate, can be found in elastic structures for which there is a certain periodic modulation of the material properties or structure. In this paper, we maximize the band gap size for bending waves in a Mindlin plate. We analyze an infinite...... periodic plate using Bloch theory, which conveniently reduces the maximization problem to that of a single base cell. Secondly, we construct a finite periodic plate using a number of the optimized base cells in a postprocessed version. The dynamic properties of the finite plate are investigated...

  12. Nonequilibrium band structure of nano-devices

    Science.gov (United States)

    Hackenbuchner, S.; Sabathil, M.; Majewski, J. A.; Zandler, G.; Vogl, P.; Beham, E.; Zrenner, A.; Lugli, P.

    2002-03-01

    A method is developed for calculating, in a consistent manner, the realistic electronic structure of three-dimensional (3-D) heterostructure quantum devices under bias and its current density close to equilibrium. The nonequilibrium electronic structure is characterized by local Fermi levels that are calculated self-consistently. We have applied this scheme to predict asymmetric Stark shifts and tunneling of confined electrons and holes in single-dot GaAs/InGaAs photodiodes.

  13. Gigahertz-band electronically scanned antennas

    Science.gov (United States)

    Bei, Nikolai A.

    2000-12-01

    Foundation and principles of radio lenses construction of centimeter and millimeter wave ranges with controlled refracting index, combining the quality of phased array antennas with optical devices are stated. Possibilities of the electronically scanning with wide-angle sector and high gain are maintained. Construction principles of scanning antennas with controlled lenses, combining the quality of phased array antennas with optical devices, are stated. Possibilities of electronically scanning with broad angle sector and high gain are maintained. Some examples of construction of antennas millimeter range of waves are listed here.

  14. Volume and surface photoemission from tungsten. I. Calculation of band structure and emission spectra

    DEFF Research Database (Denmark)

    Christensen, N. Egede; Feuerbacher, B.

    1974-01-01

    The electronic energy-band structure of tungsten has been calculated by means of the relativistic-augmented-plane-wave method. A series of mutually related potentials are constructed by varying the electronic configuration and the amount of Slater exchange included. The best band structure...

  15. Production of S-band Accelerating Structures

    CERN Document Server

    Piel, C; Vogel, H; Vom Stein, P

    2004-01-01

    ACCEL currently produces accelerating structures for several scientific laboratories. Multi-cell cavities at S-band frequencies are required for the projects CLIC-driver-linac, DLS and ASP pre-injector linac and the MAMI-C microtron. Based on those projects differences and similarities in design, production technologies and requirements will be addressed.

  16. Electronic Band Structure and New Magneto-transport Properties in p-type Semiconductor Medium-infrared HgTe / CdTe Superlattice

    Science.gov (United States)

    Nafidi, Ab.; EL Abidi, A.; El Kaaouachi, A.; Nafidi, Ah.

    2005-06-01

    We report here the band structure and new magneto-transport results for HgTe (56 Å) / CdTe (30 Å) superlattice grown by molecular beam epitaxy (MBE). The angular dependence of the transverse magnetoresistance follows the two-dimensional (2D) behaviour. At low temperature, the sample exhibits p type conductivity with a concentration of 1.84×1012 cm-2 and a Hall mobility of 8200 cm2/Vs. The observed Shubnikov-de Haas effect gives a carrier density of 1.80×1012 cm-2. The superlattice heavy holes dominate the conduction in plane with an effective mass of 0.297 m0 and Fermi energy (2D) of 14 meV. In intrinsic regime, the measured gap Eg = 190 meV agree well with calculated Eg(Γ, 300 K) =178 meV. The formalism used here predicts that the system is semiconductor, for our HgTe to CdTe thickness ratio d1/d2 = 1,87, when d2 < 140 Å. In our case, d2=30 Å and Eg (Γ, 4.2 K) = 111 meV. In spite of it, the sample exhibits the features typical of a p type semiconductor and is a medium-infrared detector (7 μm< λ< 11 μm).

  17. Theory of Auger-electron and appearance-potential spectroscopy for interacting valence-band electrons

    Science.gov (United States)

    Nolting, W.; Geipel, G.; Ertl, K.

    1991-12-01

    A theory of Auger-electron spectroscopy (AES) and appearance-potential spectroscopy (APS) is presented for interacting electrons in a nondegenerate energy band, described within the framework of the Hubbard model. Both types of spectroscopy are based on the same two-particle spectral density. A diagrammatic vertex-correction method (Matsubara formalism) is used to express this function in terms of the one-particle spectral density. The latter is approximately determined for arbitrary temperature T, arbitrary coupling strength U/W (U, the intra-atomic Coulomb matrix element; W, the width of the ``free'' Bloch band), and arbitrary band occupations n (0QDOS) in relation to the Bloch density of states (BDOS), where, however, spontaneous magnetic order is excluded, irrespective of the band filling n. The AE (AP) spectra consist of only one structure a few eV wide (``bandlike'') which is strongly n dependent, but only slightly T dependent, being rather well approximated by a simple self-convolution of the occupied (unoccupied) QDOS. For strongly correlated electrons the Bloch band splits into two quasiparticle subbands. This leads for n1. For sufficiently strong correlations U/W additional satellites appear that refer to situations where the two excited quasiparticles (quasiholes) propagate as tightly bound pairs through the lattice without being scattered by other charge carriers. As soon as the satellite splits off from the bandlike part of the spectrum, it takes almost the full spectral weight, conveying the impression of an ``atomiclike'' AE (AP) line shape. The satellite has almost exactly the structure of the free BDOS. If the particle density n as well as the hole density 2-n exceed certain critical values determined by U/W and the BDOS ρ0(E), spontaneous ferromagnetism becomes possible in the strongly correlated electron band. The magnetic phase transition gives rise to a distinctive T dependence for the QDOS and hence also for the AE and AP line shapes

  18. ELECTRONIC BAND STRUCTURE OF THE ORDERED Zn0.5Cd0.5Se ALLOY CALCULATED BY THE SEMI-EMPIRICAL TIGHT-BINDING METHOD CONSIDERING SECOND-NEAREST NEIGHBOR

    Directory of Open Access Journals (Sweden)

    Juan Carlos Salcedo-Reyes

    2008-09-01

    Full Text Available Usually, semiconductor ternary alloys are studied via a pseudo-binary approach in which the semiconductoris described like a crystalline array were the cation/anion sub-lattice consist of a random distribution of thecationic/anionic atoms. However, in the case of reported III-V and II-VI artificial structures, in which anordering of either the cations or the anions of the respective fcc sub-lattice is involved, a pseudo-binaryapproach can no longer be employed, an atomistic point of view, which takes into account the localstructure, must be used to study the electronic and optical properties of these artificial semiconductoralloys. In particular, the ordered Zn0.5Cd0.5Se alloy has to be described as a crystal with the simple-tetragonalBravais lattice with a composition equal to the zincblende random ternary alloy. The change of symmetryproperties of the tetragonal alloy, in relation to the cubic alloy, results mainly in two effects: i reduction ofthe banned gap, and ii crystal field cleavage of the valence band maximum. In this work, the electronicband structure of the ordered Zn0.5Cd0.5Se alloy is calculated using a second nearest neighbor semi-empiricaltight binding method. Also, it is compared with the electronic band structure obtained by FP-LAPW (fullpotentiallinearized augmented-plane wave method.

  19. Midfrequency band dynamics of large space structures

    Science.gov (United States)

    Coppolino, Robert N.; Adams, Douglas S.; Levine, Marie B.

    2004-09-01

    High and low intensity dynamic environments experienced by a spacecraft during launch and on-orbit operations, respectively, induce structural loads and motions, which are difficult to reliably predict. Structural dynamics in low- and mid-frequency bands are sensitive to component interface uncertainty and non-linearity as evidenced in laboratory testing and flight operations. Analytical tools for prediction of linear system response are not necessarily adequate for reliable prediction of mid-frequency band dynamics and analysis of measured laboratory and flight data. A new MATLAB toolbox, designed to address the key challenges of mid-frequency band dynamics, is introduced in this paper. Finite-element models of major subassemblies are defined following rational frequency-wavelength guidelines. For computational efficiency, these subassemblies are described as linear, component mode models. The complete structural system model is composed of component mode subassemblies and linear or non-linear joint descriptions. Computation and display of structural dynamic responses are accomplished employing well-established, stable numerical methods, modern signal processing procedures and descriptive graphical tools. Parametric sensitivity and Monte-Carlo based system identification tools are used to reconcile models with experimental data and investigate the effects of uncertainties. Models and dynamic responses are exported for employment in applications, such as detailed structural integrity and mechanical-optical-control performance analyses.

  20. Electronic crosstalk correction for terra long wave infrared photovoltaic bands

    Science.gov (United States)

    Sun, Junqiang; Madhavan, Sriharsha; Xiong, Xiaoxiong; Wang, Menghua

    2014-11-01

    The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS). The first MODIS instrument was launched in December 1999 on-board the Terra spacecraft. MODIS has 36 bands, among which 27-30 are Long Wave Infrared (LWIR) PhotoVoltaic (PV) bands covering a wavelength range from 6.72 μm to 9.73 μm. It has been found that there is severe contamination in Terra band 27 from other three bands due to crosstalk of signals among them. The crosstalk effect induces strong striping in the Earth View (EV) images and causes large long-term drift in the EV Brightness Temperature (BT) in the band. An algorithm using a linear approximation derived from on-orbit lunar observations has been developed to correct the crosstalk effect for band 27. It was demonstrated that the crosstalk correction can substantially reduce the striping in the EV images and significantly remove the long-term drift in the EV BT. In this paper, it is shown that other three LWIR PV bands are also contaminated by the crosstalk of signals among themselves. The effect induces strong striping artifacts and large long-term drifts in these bands as similarly observed in band 27. The crosstalk correction algorithm previously developed is applied to correct the crosstalk effect. It is demonstrated that the crosstalk correction successfully reduces the striping in the EV images and removes long-term drifts in the EV BT in bands 28-30 as was done similarly for band 27. The crosstalk correction algorithm can thus substantially improve both the image quality and radiometric accuracy of the LWIR PV bands Level 1B (L1B) products. The algorithm can be applied to other MODIS bands and/or other remote sensors that exhibit an electronic crosstalk effect.

  1. Terra MODIS Band 27 Electronic Crosstalk Effect and Its Removal

    Science.gov (United States)

    Sun, Junqiang; Xiong, Xiaoxiong; Madhavan, Sriharsha; Wenny, Brian

    2012-01-01

    The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the NASA Earth Observing System (EOS). The first MODIS instrument was launched in December, 1999 on-board the Terra spacecraft. MODIS has 36 bands, covering a wavelength range from 0.4 micron to 14.4 micron. MODIS band 27 (6.72 micron) is a water vapor band, which is designed to be insensitive to Earth surface features. In recent Earth View (EV) images of Terra band 27, surface feature contamination is clearly seen and striping has become very pronounced. In this paper, it is shown that band 27 is impacted by electronic crosstalk from bands 28-30. An algorithm using a linear approximation is developed to correct the crosstalk effect. The crosstalk coefficients are derived from Terra MODIS lunar observations. They show that the crosstalk is strongly detector dependent and the crosstalk pattern has changed dramatically since launch. The crosstalk contributions are positive to the instrument response of band 27 early in the mission but became negative and much larger in magnitude at later stages of the mission for most detectors of the band. The algorithm is applied to both Black Body (BB) calibration and MODIS L1B products. With the crosstalk effect removed, the calibration coefficients of Terra MODIS band 27 derived from the BB show that the detector differences become smaller. With the algorithm applied to MODIS L1B products, the Earth surface features are significantly removed and the striping is substantially reduced in the images of the band. The approach developed in this report for removal of the electronic crosstalk effect can be applied to other MODIS bands if similar crosstalk behaviors occur.

  2. Electronic properties of cubic TaC{sub x}N{sub 1-x}: A comparative study using self-consistent cluster and ab initio band-structure calculations and X-ray spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Lavrentyev, A.A.; Gabrelian, B.V.; Vorzhev, V.B.; Nikiforov, I.Ya. [Department of Physics, Don State Technical University, Gagarin Sq. 1, Rostov-on-Don (Russian Federation); Khyzhun, O.Yu. [Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA-03142 Kyiv (Ukraine)], E-mail: khyzhun@ipms.kiev.ua

    2009-03-20

    To investigate the influence of substitution of carbon atoms for nitrogen atoms in the cubic TaC{sub x}N{sub 1-x} carbonitrides, total and partial densities of states were calculated for TaC, TaC{sub 0.5}N{sub 0.5} and TaN compounds (NaCl structure) using the self-consistent cluster (with the FEFF8 code) and ab initio band-structure augmented plane wave + local orbitals (APW + LO) methods. In the present work a rather good agreement of the theoretical FEFF8 and APW + LO data for electronic properties of the TaC{sub x}N{sub 1-x} system under consideration was obtained. The results indicate that a strong hybridization of the Ta 5d- and C(N) 2p-like states is characteristic for the valence band of the TaC{sub x}N{sub 1-x} carbonitrides. When going from TaC to TaN through the TaC{sub 0.5}N{sub 0.5} carbonitride, the main maxima of curves representing total and partial Ta 5d densities of states shift in the direction opposite to the position of the Fermi level. In the above sequence of compounds, an increase of occupation of the near-Fermi sub-band formed by contributions of Ta 5d(t{sub 2g}) states has been detected. The theoretical FEFF8 and APW + LO results for the electronic structure of the TaC{sub x}N{sub 1-x} carbonitrides were found to be in excellent agreement with the experimental data derived in the present work employing X-ray photoelectron, emission and absorption spectroscopy methods for cubic TaC{sub 0.98}, TaC{sub 0.52}N{sub 0.49} and TaN{sub 0.97} compounds.

  3. Complex band structure of topological insulator Bi2Se3

    Science.gov (United States)

    Betancourt, J.; Li, S.; Dang, X.; Burton, J. D.; Tsymbal, E. Y.; Velev, J. P.

    2016-10-01

    Topological insulators are very interesting from a fundamental point of view, and their unique properties may be useful for electronic and spintronic device applications. From the point of view of applications it is important to understand the decay behavior of carriers injected in the band gap of the topological insulator, which is determined by its complex band structure (CBS). Using first-principles calculations, we investigate the dispersion and symmetry of the complex bands of Bi2Se3 family of three-dimensional topological insulators. We compare the CBS of a band insulator and a topological insulator and follow the CBS evolution in both when the spin-orbit interaction is turned on. We find significant differences in the CBS linked to the topological band structure. In particular, our results demonstrate that the evanescent states in Bi2Se3 are non-trivially complex, i.e. contain both the real and imaginary contributions. This explains quantitatively the oscillatory behavior of the band gap obtained from Bi2Se3 (0 0 0 1) slab calculations.

  4. Electronic structure of hcp transition metals

    DEFF Research Database (Denmark)

    Jepsen, O.; Andersen, O. Krogh; Mackintosh, A. R.

    1975-01-01

    Using the linear muffin-tin-orbital method described in the previous paper, we have calculated the electronic structures of the hcp transition metals, Zr, Hf, Ru, and Os. We show how the band structures of these metals may be synthesized from the sp and d bands, and illustrate the effects...... of hybridization, relativistic band shifts, and spin-orbit coupling by the example of Os. By making use of parameters derived from the muffin-tin potential, we discuss trends in the positions and widths of the energy bands, especially the d bands, as a function of the location in the periodic table. The densities...... of states of the four metals are presented, and the calculated heat capacities compared with experiment. The Fermi surfaces of both Ru and Os are found to be in excellent quantitative agreement with de Haas-van Alphen measurements, indicating that the calculated d-band position is misplaced by less than 10...

  5. Complex banded structures in directional solidification processes.

    Science.gov (United States)

    Korzhenevskii, A L; Rozas, R E; Horbach, J

    2016-01-27

    A combination of theory and numerical simulation is used to investigate impurity superstructures that form in rapid directional solidification (RDS) processes in the presence of a temperature gradient and a pulling velocity with an oscillatory component. Based on a capillary wave model, we show that the RDS processes are associated with a rich morphology of banded structures, including frequency locking and the transition to chaos.

  6. The band-gap enhanced photovoltaic structure

    Science.gov (United States)

    Tessler, Nir

    2016-05-01

    We critically examine the recently suggested structure that was postulated to potentially add 50% to the photo-conversion efficiency of organic solar cells. We find that the structure could be realized using stepwise increase in the gap as long as the steps are not above 0.1 eV. We also show that the charge extraction is not compromised due to an interplay between the contact's space charge and the energy level modification, which result in a flat energy band at the extracting contact.

  7. Engineering flat electronic bands in quasiperiodic and fractal loop geometries

    Energy Technology Data Exchange (ETDEWEB)

    Nandy, Atanu, E-mail: atanunandy1989@gmail.com; Chakrabarti, Arunava, E-mail: arunava_chakrabarti@yahoo.co.in

    2015-11-06

    Exact construction of one electron eigenstates with flat, non-dispersive bands, and localized over clusters of various sizes is reported for a class of quasi-one-dimensional looped networks. Quasiperiodic Fibonacci and Berker fractal geometries are embedded in the arms of the loop threaded by a uniform magnetic flux. We work out an analytical scheme to unravel the localized single particle states pinned at various atomic sites or over clusters of them. The magnetic field is varied to control, in a subtle way, the extent of localization and the location of the flat band states in energy space. In addition to this we show that an appropriate tuning of the field can lead to a re-entrant behavior of the effective mass of the electron in a band, with a periodic flip in its sign. - Highlights: • Exact construction of eigenstates with flat and dispersive bands is reported. • Competition between translational order and growth of aperiodicity is discussed. • The effect of magnetic field on the location of flat band states is shown. • Flux tunable re-entrant behavior of the effective mass of electron is studied.

  8. Electronic Structure of Gadolinium Calcium Oxoborate

    Energy Technology Data Exchange (ETDEWEB)

    Nelson, A; Adams, J; Schaffers, K

    2004-07-01

    Gadolinium calcium oxoborate (GdCOB) is a nonlinear optical material that belongs to the calcium--rare-earth (R) oxoborate family, with general composition Ca{sub 4}RO(BO{sub 3}){sub 3} (R{sup 3+} = La, Sm, Gd, Lu, Y). X-ray photoemission was applied to study the valence band electronic structure and surface chemistry of this material. High resolution photoemission measurements on the valence band electronic structure and Gd 3d and 4d, Ca 2p, B 1s and O 1s core lines were used to evaluate the surface and near surface chemistry. These results provide measurements of the valence band electronic structure and surface chemistry of this rare-earth oxoborate.

  9. Modification of electronic band structure in mL + nL (m = 1, 2; n = 1-5) free-stacking graphene

    Science.gov (United States)

    Ji, Jianting; He, Rui; Jie, Yinghao; Zhang, Anmin; Ma, Xiaoli; Pan, Linjing; Wang, Le; Zhang, Liyuan; Zhang, Qing-Ming

    2016-10-01

    In this paper, we studied stacked mL + nL graphene layers using Raman scattering spectroscopy. Our results indicate that the 2D band from stacked graphene can be considered as a superposition of those from the constituent nL and mL graphene layers, and a blueshift in the 2D band is observed when n or m = 1. The blueshift increases with the number of stacked layers and can be well understood by the reduction of Fermi velocity in the single layer graphene, as studied in the 1L + 1L (or twisted bilayer) case. As the number of stacked layers changes from 1 to 5, the Fermi velocity in the single layer graphene reduces to about 85% of its initial value. This study shows a convenient way to realize the modification of the Fermi velocity in free-stacking graphene and is of significance to the applications of graphene-based heterostructures.

  10. An L-Band Polarized Electron PWT Photoinjector for the International Linear Collider (ILC)

    CERN Document Server

    Yu, David; Chen Ping; Lundquist, Martin; Luo, Yan; Smirnov, Alexei Yu

    2005-01-01

    A multi-cell, standing-wave, L-band, p-mode, plane-wave-transformer (PWT) photoinjector with an integrated photocathode in a novel linac structure is proposed by DULY Research Inc. as a polarized electron source. The PWT photoinjector is capable of operation in ultra high vacuum and moderate field gradient. Expected performance of an L-band polarized electron PWT injector operating under the parameters for the International Linear Collider is presented. The projected normalized transverse rms emittance is an order of magnitude lower than that produced with a polarized electron dc gun followed by subharmonic bunchers.

  11. Electronic structure calculations on helical conducting polymers.

    Science.gov (United States)

    Ripoll, Juan D; Serna, Andrei; Guerra, Doris; Restrepo, Albeiro

    2010-10-21

    We present a study of the electronic structure and derived properties of polyfurane (PFu), polypyrrol (PPy), and polythiophene (PTh). Two spatial arrangements are considered: trans chain (tc-PFu, tc-PPy, tc-PTh) and cis α-helical (α-PFu, α-PPy, α-PTh). Even at the small sizes considered here, helical conformations appear to be stable. Band gaps of pure, undoped oligomers fall into the semiconductor range. Density of states (DOS) analysis suggest dense valence and conduction bands. Bond length alternation analysis predicts almost complete delocalization of the π clouds in all spatial arrangements. Doping with electron donors or electron-withdrawing impurities reduces all band gaps close to the metallic regime in addition to increasing the DOS for the valence and conduction bands.

  12. Hubbard-U band-structure methods

    DEFF Research Database (Denmark)

    Albers, R.C.; Christensen, Niels Egede; Svane, Axel

    2009-01-01

    -body techniques such as using dynamical mean-field theory. We review the physics underlying these approaches and discuss their strengths and weaknesses in terms of the larger issues of electronic structure that they involve. In particular, we argue that the common assumptions made to justify such calculations...

  13. Valence band structure of binary chalcogenide vitreous semiconductors by high-resolution XPS

    Energy Technology Data Exchange (ETDEWEB)

    Kozyukhin, S., E-mail: sergkoz@igic.ras.ru [Russian Academy of Science, Institute of General and Inorganic Chemistry (Russian Federation); Golovchak, R. [Lviv Scientific Research Institute of Materials of SRC ' Carat' (Ukraine); Kovalskiy, A. [Lehigh University, Department of Materials Science and Engineering (United States); Shpotyuk, O. [Lviv Scientific Research Institute of Materials of SRC ' Carat' (Ukraine); Jain, H. [Lehigh University, Department of Materials Science and Engineering (United States)

    2011-04-15

    High-resolution X-ray photoelectron spectroscopy (XPS) is used to study regularities in the formation of valence band electronic structure in binary As{sub x}Se{sub 100-x}, As{sub x}S{sub 100-x}, Ge{sub x}Se{sub 100-x} and Ge{sub x}S{sub 100-x} chalcogenide vitreous semiconductors. It is shown that the highest occupied energetic states in the valence band of these materials are formed by lone pair electrons of chalcogen atoms, which play dominant role in the formation of valence band electronic structure of chalcogen-rich glasses. A well-expressed contribution from chalcogen bonding p electrons and more deep s orbitals are also recorded in the experimental valence band XPS spectra. Compositional dependences of the observed bands are qualitatively analyzed from structural and compositional points of view.

  14. Electronic structures of TiO2-TCNE, -TCNQ, and -2,6-TCNAQ surface complexes studied by ionization potential measurements and DFT calculations: Mechanism of the shift of interfacial charge-transfer bands

    Science.gov (United States)

    Fujisawa, Jun-ichi; Hanaya, Minoru

    2016-06-01

    Interfacial charge-transfer (ICT) transitions between inorganic semiconductors and π-conjugated molecules allow direct charge separation without loss of energy. This feature is potentially useful for efficient photovoltaic conversions. Charge-transferred complexes of TiO2 nanoparticles with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its analogues (TCNX) show strong ICT absorption in the visible region. The ICT band was reported to be significantly red-shifted with extension of the π-conjugated system of TCNX. In order to clarify the mechanism of the red-shift, in this work, we systematically study electronic structures of the TiO2-TCNX surface complexes (TCNX; TCNE, TCNQ, 2,6-TCNAQ) by ionization potential measurements and density functional theory (DFT) calculations.

  15. Hydrogen production by tuning the photonic band gap with the electronic band gap of TiO₂.

    Science.gov (United States)

    Waterhouse, G I N; Wahab, A K; Al-Oufi, M; Jovic, V; Anjum, D H; Sun-Waterhouse, D; Llorca, J; Idriss, H

    2013-10-10

    Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

  16. A Method to Formulate the Unit Cell for Density Functional Theory (DFT) Calculations of the Electronic Band Structure of Heterostructures of Two-dimensional Nanosheets

    Science.gov (United States)

    2015-04-01

    distribution is unlimited. i CONTENTS Page Introduction 1 Two-dimensional Material Geometry and Analogs with Close-packed Systems 1 Matching...distribution is unlimited. 1 INTRODUCTION Two-dimensional (2D) material heterostructures offer novel and compelling electronic and optical...methods have undoubtedly been created for matching lattice constants of dissimilar nanomaterials , very few are actually covered explicitly in literature

  17. From lattice Hamiltonians to tunable band structures by lithographic design

    Science.gov (United States)

    Tadjine, Athmane; Allan, Guy; Delerue, Christophe

    2016-08-01

    Recently, new materials exhibiting exotic band structures characterized by Dirac cones, nontrivial flat bands, and band crossing points have been proposed on the basis of effective two-dimensional lattice Hamiltonians. Here, we show using atomistic tight-binding calculations that these theoretical predictions could be experimentally realized in the conduction band of superlattices nanolithographed in III-V and II-VI semiconductor ultrathin films. The lithographed patterns consist of periodic lattices of etched cylindrical holes that form potential barriers for the electrons in the quantum well. In the case of honeycomb lattices, the conduction minibands of the resulting artificial graphene host several Dirac cones and nontrivial flat bands. Similar features, but organized in different ways, in energy or in k -space are found in kagome, distorted honeycomb, and Lieb superlattices. Dirac cones extending over tens of meV could be obtained in superlattices with reasonable sizes of the lithographic patterns, for instance in InAs/AlSb heterostructures. Bilayer artificial graphene could be also realized by lithography of a double quantum-well heterostructure. These new materials should be interesting for the experimental exploration of Dirac-based quantum systems, for both fundamental and applied physics.

  18. Electronic Bands Behaviour at Sinusoidal Potential Presence of Incommensurate Crystals

    OpenAIRE

    Vlokh R.; Vlokh O.; Lukiyanets B.

    2004-01-01

    On the basis of solving the Schrodinger and Mathieu equations, for the case of crystal field perturbed by one-dimensional sinusoidal potential of the modulated phase in uniaxial ferroelectrics, it has been shown that the positions of electronic levels are sensitive to the ratio of periods of the crystal field and the perturbation potential. Considering the energy states as prototypes of bands normalized by perturbation, one can come to the conclusion that the level of the states is the same a...

  19. Band structure of superdeformed bands in odd-A Hg nuclei

    Institute of Scientific and Technical Information of China (English)

    陈星渠; 邢正

    1997-01-01

    Through particle-rotor model, band structure of superdeformed bands in odd-A Hg nuclei is analysed. An overall and excellent agreement between the calculated and observed kinematic and dynamic moments of inertia is obtained. The electromagnetic transition properties of SD bands can be used to identify the configuration with certainty.

  20. Electronic materials with a wide band gap: recent developments

    Directory of Open Access Journals (Sweden)

    Detlef Klimm

    2014-09-01

    Full Text Available The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap Eg = 0.66 eV after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (Eg = 1.12 eV. This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider Eg were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and β-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity.

  1. Calculation of band structure in (101)-biaxially strained Si

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    The structure model used for calculation was defined according to Vegard’s rule and Hooke’s law. Calculations were performed on the electronic structures of(101)-biaxially strained Si on relaxed Si1-X GeX alloy with Ge fraction ranging from X = 0 to 0.4 in steps of 0.1 by CASTEP approach. It was found that [±100] and [00±1] valleys(-4) splitting from the [0±10] valley(-2) constitute the conduction b0and(CB) edge,that valence band(VB) edge degeneracy is partially lifted and that the electron mass is un-altered under strain while the hole mass decreases in the [100] and [010] directions. In addition,the fitted dependences of CB splitting energy,VB splitting energy and indirect bandgap on X are all linear.

  2. Thermodynamic consequence of the new attribution of bands in the electronic absorption spectrum of electron donor-iodine-solvent systems

    Science.gov (United States)

    Abramov, Sergey P.

    1999-06-01

    The subject review pays attention to the peculiarities in behaviour of bands in the electronic absorption spectra of electron donor-iodine-solvent systems, the appearance of which is associated with the intermolecular interaction of molecular iodine with electron donor organic molecules. The new concept of the bands’ attribution to the isomeric equilibrium molecular charge-transfer complexes (CTCs) of CTC-I and CTC-II types is considered. The features of possible phase transitions in the solid state are discussed on the basis of the thermodynamic properties and electronic structures of the CTC-I and CTC-II in electron donor-iodine-solvent systems. The stabilisation of the CTC-II structure with the temperature lowering coincided in many cases with the electrons’ localisation in the solid state structures having charge-transfer bonds.

  3. A Theoretical Structure of High School Concert Band Performance

    Science.gov (United States)

    Bergee, Martin J.

    2015-01-01

    This study used exploratory (EFA) and confirmatory factor analysis (CFA) to verify a theoretical structure for high school concert band performance and to test that structure for viability, generality, and invariance. A total of 101 university students enrolled in two different bands rated two high school band performances (a "first"…

  4. Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Li, Z.Q.; Henriksen, E.A.; Jiang, Z.; Hao, Zhao; Martin, Michael C.; Kim, P.; Stormer, H.L.; Basov, Dimitri N.

    2008-12-10

    We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observe a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the next-nearest-neighbor interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.

  5. Particle simulation of dual-band coaxial relativistic backward-wave oscillator with single annular electron beam structure%单电子束双波段同轴相对论返波管粒子模拟

    Institute of Scientific and Technical Information of China (English)

    唐永福; 蒙林; 李海龙; 张斐娜

    2013-01-01

    设计了一种能在C波段和X波段实现稳定双频输出的带有非对称谐振反射腔的单电子束同轴相对论返波振荡器.采用耦合阻抗跃变型慢波结构,使用粒子PIC模拟软件进行了粒子模拟研究.模拟结果显示:轴向电场在系统中的分布得到改进,电子束的能散得到改善.在电子束电压511 kV,电流8.95 kA,引导磁场0.73T的条件下,双频器件实现了8.09 GHz和9.91 GHz的双波段频率稳定输出,平均功率为1.0 GW,波束互作用效率为21.9%,效率高于空心双波段返波管及其他双波段器件.器件辐射功率的拍频为1.82GHz,拍波更为明显和稳定.模拟研究中同时发现,随着慢波结构之间漂移段的变化,双频频率都呈现一种准周期的变化.%A C-band and X-band dual-frequency coaxial relativistic backward-wave oscillator (CRBWO) with an asymmetric resonant reflector and a single annular electron beam structure is designed and investigated in this paper. The improved slow wave structure (SWS) with stepwise variable coupling impedance is employed, and the particle-in-cell (PIC) simulation code is used to investigate the device. The PIC simulation results indicate that the axial electric field in the SWS is enhanced and the energy scatter of the relativistic electron beam (REB) is improved. With an electron beam of 511 kV and 8. 95 kA and an axial magnetic field of 0. 73 T, an average power of 1. 0 GW with power conversion efficiency of 21. 9% is obtained and the two dominant frequencies are 8. 09 GHz and 9. 91 GHz. The efficiency is higher than dual-band non-coaxial RBWO and other dual-band high-power microwave (HPM) generators. A more clear and stable beat radiation microwave power with beat frequency of 1. 82 GHz is acquired, and the dual frequencies both demonstrate periodic-like dependence on the length of the tapered waveguide between the two SWS sections.

  6. Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

    Science.gov (United States)

    Jobst, Johannes; van der Torren, Alexander J. H.; Krasovskii, Eugene E.; Balgley, Jesse; Dean, Cory R.; Tromp, Rudolf M.; van der Molen, Sense Jan

    2016-11-01

    High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the `chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.

  7. DFPT approach to the temperature dependence of electronic band energies

    Science.gov (United States)

    Boulanger, Paul; Cote, Michel; Gonze, Xavier

    2010-03-01

    The energy bands of semiconductors exhibit significant shifts and broadening with temperature at constant volume. This is an effect of the direct renormalization of band energies due to electron-phonon interactions. In search of an efficient linear response DFT approach to this effect, beyond semi-empirical approximation or frozen- phonon DFT, we have implemented formulas derived by Allen and Heine [J. Phys. C 9, 2305 (1976)] inside the ABINIT package. We have found that such formulas need a great number of bands, O(1000), to properly converge the thermal corrections of deep potential well atoms, i.e. elements of the first row. This leads to heavy computational costs even for simple systems like diamond. The DFPT formalism can be used to circumvent entirely the need for conduction bands by computing the first-order wave-functions using the self-consistent Sternheimer equation. We will compare the results of both formalism demonstrating that the DFPT approach reproduces the correct converged results of the formulas of Allen and Heine.

  8. A unified perspective of complex band structure: interpretations, formulations, and applications

    Science.gov (United States)

    Reuter, Matthew G.

    2017-02-01

    Complex band structure generalizes conventional band structure by also considering wavevectors with complex components. In this way, complex band structure describes both the bulk-propagating states from conventional band structure and the evanescent states that grow or decay from one unit cell to the next. Even though these latter states are excluded by translational symmetry, they become important when translational symmetry is broken via, for example, a surface or impurity. Many studies over the last 80 years have directly or indirectly developed complex band structure for an impressive range of applications, but very few discuss its fundamentals or compare its various results. In this work we build upon these previous efforts to expose the physical foundation of complex band structure, which mathematically implies its existence. We find that a material’s static and dynamic electronic structure are both completely described by complex band structure. Furthermore, we show that complex band structure reflects the minimal, intrinsic information contained in the material’s Hamiltonian. These realizations then provide a context for comparing and unifying the different formulations and applications of complex band structure that have been reported over the years. Ultimately, this discussion introduces the idea of examining the amount of information contained in a material’s Hamiltonian so that we can find and exploit the minimal information necessary for understanding a material’s properties.

  9. The electronic structures of solids

    CERN Document Server

    Coles, B R

    2013-01-01

    The Electronic Structures of Solids aims to provide students of solid state physics with the essential concepts they will need in considering properties of solids that depend on their electronic structures and idea of the electronic character of particular materials and groups of materials. The book first discusses the electronic structure of atoms, including hydrogen atom and many-electron atom. The text also underscores bonding between atoms and electrons in metals. Discussions focus on bonding energies and structures in the solid elements, eigenstates of free-electron gas, and electrical co

  10. Electronic structure of Fe-based superconductors

    Indian Academy of Sciences (India)

    Kalobaran Maiti

    2015-06-01

    Fe-based superconductors have drawn much attention during the last decade due to the presence of superconductivity in materials containing the magnetic element, Fe, and the coexistence of superconductivity and magnetism. Extensive study of the electronic structure of these systems suggested the dominant role of states in their electronic properties, which is significantly different from the cuprate superconductors. In this article, some of our studies of the electronic structure of these fascinating systems employing high-resolution photoemission spectroscopy is reviewed. The combined effect of electron correlation and covalency reveals an interesting scenario in their electronic structure. The contribution of ligand states at the Fermi level is found to be much more significant than indicated in earlier studies. Temperature evolution of the energy bands reveals the signature of transition akin to Lifshitz transition in these systems.

  11. Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State.

    Science.gov (United States)

    Lim, A; Foulkes, W M C; Horsfield, A P; Mason, D R; Schleife, A; Draeger, E W; Correa, A A

    2016-01-29

    We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.

  12. Band structures of 4f and 5f materials studied by angle-resolved photoelectron spectroscopy

    Science.gov (United States)

    Fujimori, Shin-ichi

    2016-04-01

    Recent remarkable progress in angle-resolved photoelectron spectroscopy (ARPES) has enabled the direct observation of the band structures of 4f and 5f materials. In particular, ARPES with various light sources such as lasers (hν ∼ 7~\\text{eV} ) or high-energy synchrotron radiations (hν ≳ 400~\\text{eV} ) has shed light on the bulk band structures of strongly correlated materials with energy scales of a few millielectronvolts to several electronvolts. The purpose of this paper is to summarize the behaviors of 4f and 5f band structures of various rare-earth and actinide materials observed by modern ARPES techniques, and understand how they can be described using various theoretical frameworks. For 4f-electron materials, ARPES studies of \\text{Ce}M\\text{I}{{\\text{n}}5} (M=\\text{Rh} , \\text{Ir} , and \\text{Co} ) and \\text{YbR}{{\\text{h}}2}\\text{S}{{\\text{i}}2} with various incident photon energies are summarized. We demonstrate that their 4f electronic structures are essentially described within the framework of the periodic Anderson model, and that the band-structure calculation based on the local density approximation cannot explain their low-energy electronic structures. Meanwhile, electronic structures of 5f materials exhibit wide varieties ranging from itinerant to localized states. For itinerant \\text{U}~5f compounds such as \\text{UFeG}{{\\text{a}}5} , their electronic structures can be well-described by the band-structure calculation assuming that all \\text{U}~5f electrons are itinerant. In contrast, the band structures of localized \\text{U}~5f compounds such as \\text{UP}{{\\text{d}}3} and \\text{U}{{\\text{O}}2} are essentially explained by the localized model that treats \\text{U}~5f electrons as localized core states. In regards to heavy fermion \\text{U} -based compounds such as the hidden-order compound \\text{UR}{{\\text{u}}2}\\text{S}{{\\text{i}}2} , their electronic structures exhibit complex behaviors. Their overall band structures

  13. An electronic structure perspective of graphene interfaces

    Science.gov (United States)

    Schultz, Brian J.; Dennis, Robert V.; Lee, Vincent; Banerjee, Sarbajit

    2014-03-01

    The unusual electronic structure of graphene characterized by linear energy dispersion of bands adjacent to the Fermi level underpins its remarkable transport properties. However, for practical device integration, graphene will need to be interfaced with other materials: 2D layered structures, metals (as ad-atoms, nanoparticles, extended surfaces, and patterned metamaterial geometries), dielectrics, organics, or hybrid structures that in turn are constituted from various inorganic or organic components. The structural complexity at these nanoscale interfaces holds much promise for manifestation of novel emergent phenomena and provides a means to modulate the electronic structure of graphene. In this feature article, we review the modifications to the electronic structure of graphene induced upon interfacing with disparate types of materials with an emphasis on iterative learnings from theoretical calculations and electronic spectroscopy (X-ray absorption fine structure (XAFS) spectroscopy, scanning transmission X-ray microscopy (STXM), angle-resolved photoemission spectroscopy (ARPES), and X-ray magnetic circular dichroism (XMCD)). We discuss approaches for engineering and modulating a bandgap in graphene through interfacial hybridization, outline experimental methods for examining electronic structure at interfaces, and overview device implications of engineered interfaces. A unified view of how geometric and electronic structure are correlated at interfaces will provide a rational means for designing heterostructures exhibiting emergent physical phenomena with implications for plasmonics, photonics, spintronics, and engineered polymer and metal matrix composites.

  14. Complex structures of dense lithium: Electronic origin

    Science.gov (United States)

    Degtyareva, V. F.

    2016-11-01

    Lithium—the lightest alkali metal exhibits unexpected structures and electronic behavior at high pressures. Like the heavier alkali metals, Li is bcc at ambient pressure and transforms first to fcc (at 7.5 GPa). The post-fcc high-pressure form Li-cI 16 (at 40-60 GPa) is similar to Na-cI 16 and related to more complex structures of heavy alkalis Rb-oC52 and Cs- oC84. The other high pressure phases for Li (oC88, oC40, oC24) observed at pressures up to 130 GPa are found only in Li. The different route of Li high-pressure structures correlates with its special electronic configuration containing the only 3 electrons (at 1s and 2s levels). Crystal structures for Li are analyzed within the model of Fermi sphere-Brillouin zone interactions. Stability of post-fcc structures for Li are supported by the Hume-Rothery arguments when new diffraction plains appear close to the Fermi level producing pseudogaps near the Fermi level and decreasing the crystal energy. The filling of Brillouin-Jones zones by electron states for a given structure defines the physical properties as optical reflectivity, electrical resistivity and superconductivity. To understand the complexity of structural and physical properties of Li above 60 GPa it is necessary to assume the valence electron band overlap with the core electrons and increase the valence electron count under compression.

  15. Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe

    Science.gov (United States)

    Watson, Matthew D.; Backes, Steffen; Haghighirad, Amir A.; Hoesch, Moritz; Kim, Timur K.; Coldea, Amalia I.; Valentí, Roser

    2017-02-01

    We use angle-resolved photoemission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of "Hubbard-like bands" at high binding energies consisting of incoherent many-body excitations originating from Fe 3 d states in addition to the renormalized quasiparticle bands near the Fermi level. Many high-energy features of the observed ARPES data can be accounted for when incorporating the effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron-based superconductors.

  16. Electronic structure and magnetism in actinide compounds

    Energy Technology Data Exchange (ETDEWEB)

    Durakiewicz, T. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)]. E-mail: tomasz@lanl.gov; Joyce, J.J. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Lander, G.H. [JRC, Institute of Transuranium Elements, Postfach 2340, 76125 Karlsruhe (Germany); Olson, C.G. [Ames Laboratory, Iowa State University, Ames, Iowa 5011 (United States); Butterfield, M.T. [Lawrence Livermoore National Laboratory, Livermoore, CA 94550 (United States); Guziewicz, E. [Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw (Poland); Batista, C.D. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Arko, A.J. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Morales, L. [Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Mattenberger, K. [Laboratorium fur Festkorperphysik, ETH, CH-8093, Zurich (Switzerland); Vogt, O. [Laboratorium fur Festkorperphysik, ETH, CH-8093, Zurich (Switzerland)

    2006-05-01

    A close relationship between electronic structure and magnetic properties is observed in actinide compounds. The exact nature of this relationship is under investigation. We present examples of a direct link between electronic structure and ordered magnetic moment and/or magnetization. Specifically, results obtained for cubic U, Np and Pu compounds and quasi-2D U compounds are be presented. In the case of cubic compounds, a direct relationship between binding energy of valence band features and magnetic moment will be discussed. A Stoner-like mechanism and simple mean-field explanation is proposed for ferromagnetic UTe.

  17. Tuning the electronic band-gap of fluorinated 3C-silicon carbide nanowires

    Science.gov (United States)

    Miranda Durán, Álvaro; Trejo Baños, Alejandro; Pérez, Luis Antonio; Cruz Irisson, Miguel

    The possibility of control and modulation of the electronic properties of silicon carbide nanowires (SiCNWs) by varying the wire diameter is well known. SiCNWs are particularly interesting and technologically important, due to its electrical and mechanical properties, allowing the development of materials with specific electronic features for the design of stable and robust electronic devices. Tuning the band gap by chemical surface passivation constitutes a way for the modification of the electronic band gap of these nanowires. We present, the structural and electronic properties of fluorinated SiCNWs, grown along the [111] crystallographic direction, which are investigated by first principles. We consider nanowires with six diameters, varying from 0.35 nm to 2.13 nm, and eight random covering schemes including fully hydrogen- and fluorine terminated ones. Gibbs free energy of formation and electronic properties were calculated for the different surface functionalization schemes and diameters considered. The results indicate that the stability and band gap of SiCNWs can be tuned by surface passivation with fluorine atoms This work was supported by CONACYT infrastructure project 252749 and UNAM-DGAPA-PAPIIT IN106714. A.M. would like to thank for financial support from CONACyT-Retención. Computing resources from proyect SC15-1-IR-27 of DGTIC-UNAM are acknowledged.

  18. RF design of X-band RF deflector for femtosecond diagnostics of LCLS electron beam

    Science.gov (United States)

    Dolgashev, Valery A.; Wang, Juwen

    2012-12-01

    We designed a successful constant impedance traveling wave X-band rf deflector for electron beam diagnostics at the 14 GeV SLAC Linac Coherent Light Source (LCLS). This is the first practical deflector built with a waveguide coupler. The 1-meter rf deflector produces 24 MeV peak transverse kick when powered with 20 MW of 11.424 GHz rf. The design is based on our experience with high gradient X-band accelerating structures. Several deflectors of this design have been built at SLAC and are currently in use. Here we describe the design and distinguishing features of this device.

  19. First-principle study of energy band structure of armchair graphene nanoribbons

    Science.gov (United States)

    Ma, Fei; Guo, Zhankui; Xu, Kewei; Chu, Paul K.

    2012-07-01

    First-principle calculation is carried out to study the energy band structure of armchair graphene nanoribbons (AGNRs). Hydrogen passivation is found to be crucial to convert the indirect band gaps into direct ones as a result of enhanced interactions between electrons and nuclei at the edge boundaries, as evidenced from the shortened bond length as well as the increased differential charge density. Ribbon width usually leads to the oscillatory variation of band gaps due to quantum confinement no matter hydrogen passivated or not. Mechanical strain may change the crystal symmetry, reduce the overlapping integral of C-C atoms, and hence modify the band gap further, which depends on the specific ribbon width sensitively. In practical applications, those effects will be hybridized to determine the energy band structure and subsequently the electronic properties of graphene. The results can provide insights into the design of carbon-based devices.

  20. Bi-directional evolutionary optimization for photonic band gap structures

    Energy Technology Data Exchange (ETDEWEB)

    Meng, Fei [Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001 (Australia); School of Civil Engineering, Central South University, Changsha 410075 (China); Huang, Xiaodong, E-mail: huang.xiaodong@rmit.edu.au [Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001 (Australia); Key Laboratory of Advanced Technology for Vehicle Body Design & Manufacture, Hunan University, Changsha, 410082 (China); Jia, Baohua [Centre for Micro-Photonics, Faculty of Engineering & Industrial Science, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122 (Australia)

    2015-12-01

    Toward an efficient and easy-implement optimization for photonic band gap structures, this paper extends the bi-directional evolutionary structural optimization (BESO) method for maximizing photonic band gaps. Photonic crystals are assumed to be periodically composed of two dielectric materials with the different permittivity. Based on the finite element analysis and sensitivity analysis, BESO starts from a simple initial design without any band gap and gradually re-distributes dielectric materials within the unit cell so that the resulting photonic crystal possesses a maximum band gap between two specified adjacent bands. Numerical examples demonstrated the proposed optimization algorithm can successfully obtain the band gaps from the first to the tenth band for both transverse magnetic and electric polarizations. Some optimized photonic crystals exhibit novel patterns markedly different from traditional designs of photonic crystals.

  1. Convergence of electronic bands for high performance bulk thermoelectrics.

    Science.gov (United States)

    Pei, Yanzhong; Shi, Xiaoya; LaLonde, Aaron; Wang, Heng; Chen, Lidong; Snyder, G Jeffrey

    2011-05-01

    Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems-such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity, which has led to the investigation of a variety of complex material systems. In this vein, it is well known that a high valley degeneracy (typically ≤6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe(1-x)Se(x) alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity.

  2. Electronic energy band structures of carbon nanotubes with spin-orbit coupling interaction%自旋轨道耦合作用对碳纳米管电子能带结构的影响

    Institute of Scientific and Technical Information of China (English)

    杨杰; 董全力; 江兆潭; 张杰

    2011-01-01

    Based on the symmetry adapted tight-binding model, the electronic energy band structures of single wall carbon nanotubes are calculated by considering the spin-orbit coupling interaction. The energy gaps at the Dirac point for the armchair nanotubes are formed due to the spin-orbit coupling interaction and the curvature effect. For the zigzag and chiral carbon nanotubes,the energy band splittings for the lowest unoccupied states and the highest occupied states are also formed by the spin-orbit coupling interaction. The energy splittings are not only dependedent on the diameter and the chiral angle of the carbon nanotubes, but also a symmetric with respect to the Fermi energy level. According to the chiral index (n, m) , different tube behaviors are grouped into three families. The numeral results are in good agreement with the experimental results.%本文考虑自旋轨道耦合作用的情况下,采用紧束缚近似螺旋对称模型计算了单壁碳纳米管的电子能带结构.研究发现:对于Armchair型单壁碳纳米管,自旋轨道耦合作用和弯曲效应共同导致了费米面Dirac点附近电子能带结构的能隙;对于Zigzag型和手性单壁碳纳米管,自旋轨道耦合作用使得电子最高占据态和最低未占据态产生能级劈裂,能级劈裂的大小不但与碳纳米管的直径和手性角密切相关,而且相对于费米面是不对称的;根据指数(n,m)可以将Zigzag型和手性单壁碳纳米管分为金属性碳纳米管(V=0)、第一类半导体性碳纳米管(v

  3. Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2

    KAUST Repository

    Waterhouse, G. I. N.

    2013-10-10

    Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

  4. Electronic structure studies of topological materials

    Science.gov (United States)

    Zhou, Shuyun

    Three-dimensional (3D) Dirac fermions are a new class of topological quantum materials. In 3D Dirac semimetals, the conduction and valence bands touch each other at discrete points in the momentum space and show linear dispersions along all momentum directions, forming 3D Dirac cones which are protected by the crystal symmetry. Here I will present our recent studies of the electronic structures of novel materials which host 3D Dirac fermions by using angle-resolved photoemission spectroscopy.

  5. Band structures in the nematic elastomers phononic crystals

    Science.gov (United States)

    Yang, Shuai; Liu, Ying; Liang, Tianshu

    2017-02-01

    As one kind of new intelligent materials, nematic elastomers (NEs) represent an exciting physical system that combines the local orientational symmetry breaking and the entropic rubber elasticity, producing a number of unique physical phenomena. In this paper, the potential application of NEs in the band tuning is explored. The band structures in two kinds of NE phononic crystals (PCs) are investigated. Through changing NE intrinsic parameters, the influence of the porosity, director rotation and relaxation on the band structures in NE PCs are analyzed. This work is a meaningful try for application of NEs in acoustic field and proposes a new intelligent strategy in band turning.

  6. Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

    Science.gov (United States)

    Vargas, W. E.; Hernández-Jiménez, M.; Libby, E.; Azofeifa, D. E.; Solis, Á.; Barboza-Aguilar, C.

    2015-09-01

    Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarab's cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.

  7. Band structures in Sierpinski triangle fractal porous phononic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Kai; Liu, Ying, E-mail: yliu5@bjtu.edu.cn; Liang, Tianshu

    2016-10-01

    In this paper, the band structures in Sierpinski triangle fractal porous phononic crystals (FPPCs) are studied with the aim to clarify the effect of fractal hierarchy on the band structures. Firstly, one kind of FPPCs based on Sierpinski triangle routine is proposed. Then the influence of the porosity on the elastic wave dispersion in Sierpinski triangle FPPCs is investigated. The sensitivity of the band structures to the fractal hierarchy is discussed in detail. The results show that the increase of the hierarchy increases the sensitivity of ABG (Absolute band gap) central frequency to the porosity. But further increase of the fractal hierarchy weakens this sensitivity. On the same hierarchy, wider ABGs could be opened in Sierpinski equilateral triangle FPPC; whilst, a lower ABG could be opened at lower porosity in Sierpinski right-angled isosceles FPPCs. These results will provide a meaningful guidance in tuning band structures in porous phononic crystals by fractal design.

  8. Structural properties and electronic structure of some ternary d-electron and f-electron intermetallics

    Science.gov (United States)

    Slebarski, A.; Orzechowska, M.; Wrona, A.; Szade, J.; Jezierski, A.

    2000-02-01

    We report on structural measurements on and electronic structure investigations of the alloyed compounds ZrNiSn, TiNiSn, CeNiSn and CeRhSb. We present measurements of lattice parameters as a function of temperature and analysis of a (T ) and its relation to icons/Journals/Common/chi" ALT="chi" ALIGN="TOP"/> T , icons/Journals/Common/chi" ALT="chi" ALIGN="TOP"/> being the magnetic susceptibility. We observed a linear dependence of a (T ) versus icons/Journals/Common/chi" ALT="chi" ALIGN="TOP"/> T for Zr, Ti and Ce alloys (for orthorhombic Ce alloys, the lattice parameters a , b and c scale linearly with icons/Journals/Common/chi" ALT="chi" ALIGN="TOP"/> T ). The x-ray photoelectron and ultraviolet photoemission spectra are further compared to the density of states, obtained from band-structure calculations.

  9. Band structure approach to the resonant x-ray scattering

    OpenAIRE

    Elfimov, I. S.; Skorikov, N. A.; Anisimov, V. I.; Sawatzky, G.A.

    2001-01-01

    We study the resonance behaviour of the forbidden 600 and 222 x-ray Bragg peaks in Ge using LDA band structure methods. These Bragg peaks remain forbidden in the resonant dipole scattering approximation even taking into account the non local nature of the band states. However they become allowed at resonance if the eigenstates of the unoccupied conduction band involve a hybridization of p like and d like atomic states. We show that the energy dependence of the resonant behaviour, including th...

  10. ELECTRONIC STRUCTURE OF CLUSTER ASSEMBLED Al12C (Si) SOLID

    Institute of Scientific and Technical Information of China (English)

    QUAN HONG-JUN; GONG XIN-GAO

    2000-01-01

    The electronic structures of the cluster-assembled solid Al12C (Si) are studied by the ab initio method. We find that Al12C (Si) can solidify into a van der Waals solid. The electronic band structures show very weak dispersion. The main features in the electronic structure of cluster are retained in the solid, and an energy gap up to about 1.5 eV is observed for Al12C and Al12Si solids.

  11. Electronic structure interpolation via atomic orbitals.

    Science.gov (United States)

    Chen, Mohan; Guo, G-C; He, Lixin

    2011-08-17

    We present an efficient scheme for accurate electronic structure interpolation based on systematically improvable optimized atomic orbitals. The atomic orbitals are generated by minimizing the spillage value between the atomic basis calculations and the converged plane wave basis calculations on some coarse k-point grid. They are then used to calculate the band structure of the full Brillouin zone using the linear combination of atomic orbitals algorithms. We find that usually 16-25 orbitals per atom can give an accuracy of about 10 meV compared to the full ab initio calculations, and the accuracy can be systematically improved by using more atomic orbitals. The scheme is easy to implement and robust, and works equally well for metallic systems and systems with complicated band structures. Furthermore, the atomic orbitals have much better transferability than Shirley's basis and Wannier functions, which is very useful for perturbation calculations.

  12. Crystal structure and band gap of AlGaAsN

    Science.gov (United States)

    Munich, D. P.; Pierret, R. F.

    1987-09-01

    Quantum dielectric theory is applied to the quaternary alloy Al xGa 1- xAs 1- yN y to predict its electronic properties as a function of Al and N mole fractions. Results are presented for the expected crystal structure, minimum electron energy band gap, and direction in k-space of the band gap minimum for all x and y values. The results suggest that, for a proper choice of x and y, Al xGa 1- xAs 1- yN y could exhibit certain advantages over Al xGa 1- xAs when utilized in field-effect transistor structures.

  13. Structure of nearly degenerate dipole bands in {sup 108}Ag

    Energy Technology Data Exchange (ETDEWEB)

    Sethi, J. [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Palit, R., E-mail: palit@tifr.res.in [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Saha, S.; Trivedi, T. [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Bhat, G.H.; Sheikh, J.A. [Department of Physics, University of Kashmir, Srinagar 190 006 (India); Datta, P. [Ananda Mohan College, Kolkata 700009 (India); Carroll, J.J. [US Army Research Laboratory, Adelphi, MD 20783 (United States); Chattopadhyay, S. [Saha Institute of Nuclear Physics, Kolkata 700064 (India); Donthi, R. [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Garg, U. [University of Notre Dame, Notre Dame, IN 46556 (United States); Jadhav, S.; Jain, H.C. [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Karamian, S. [Joint Institute for Nuclear Research, Dubna 141980 (Russian Federation); Kumar, S. [University of Delhi, Delhi 110007 (India); Litz, M.S. [US Army Research Laboratory, Adelphi, MD 20783 (United States); Mehta, D. [Panjab University, Chandigarh 160014 (India); Naidu, B.S. [Tata Institute of Fundamental Research, Colaba, Mumbai 400 005 (India); Naik, Z. [Sambalpur University, Sambalpur 143005 (India); Sihotra, S. [Panjab University, Chandigarh 160014 (India); and others

    2013-08-09

    The high spin negative parity states of {sup 108}Ag have been investigated with the {sup 11}B + {sup 100}Mo reaction at 39 MeV beam energy using the INGA facility at TIFR, Mumbai. From the γ–γ coincidence analysis, an excited negative parity band has been established and found to be nearly degenerate with the ground state band. The spin and parity of the levels are assigned using angular correlation and polarization measurements. This pair of degenerate bands in {sup 108}Ag is studied using the recently developed microscopic triaxial projected shell model approach. The observed energy levels and the ratio of the electromagnetic transition probabilities of these bands in this isotope are well reproduced by the present model. Further, it is shown that the partner band has a different quasiparticle structure as compared to the yrast band.

  14. Unfolding the band structure of non-crystalline photonic band gap materials.

    Science.gov (United States)

    Tsitrin, Samuel; Williamson, Eric Paul; Amoah, Timothy; Nahal, Geev; Chan, Ho Leung; Florescu, Marian; Man, Weining

    2015-08-20

    Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain.

  15. Influence of structure defects on optical and electronic properties of icosahedral boron rich solids

    CERN Document Server

    Schmechel, R

    1999-01-01

    doped beta-rhombohedral boron by Kramers-Kronig-Analysis gives information on the main transport processes. Beside hopping conduction of localized electrons, band conduction of delocalized electrons were found. While holes in the valence band are the delocalized charge carriers in boron carbide, in vanadium doped beta-rhombohedral boron delocalized electrons in an extrinsic impurity band are suggested. Boron and boron rich solids are known to have a high concentration on intrinsic structural imperfections. From known structure data of real crystals and known band structure calculations of perfect ideal crystals a correlation between intrinsic structure defect concentration and electron deficit in the valence band is concluded. This correlation forms the basis for the following theses: 1. The electron deficit in the valence band of a perfect crystal is the driving force for the intrinsic structure defects in a real crystal. 2. The small electron deficit becomes compensated by the structure defects - this expla...

  16. Quantum chemistry, band structures and polymers

    Science.gov (United States)

    André, Jean-Marie

    2012-06-01

    A short review of the long way from the first calculations on polyenes after the second world war to the recent electronic devices like Organic Light Emitting Diodes or Photovoltaic Cells is given. It shows how quantum chemical methods on one side and synthesis or experiments have (or should have) interacted as incentives to new methods and technologies.

  17. Band Structure in Yang-Mills Theories

    CERN Document Server

    Bachas, Constantin

    2016-01-01

    We show how Yang-Mills theory on $S^3\\times R$ can exhibit a spectrum with continuous bands if coupled either to a topological 3-form gauge field, or to a dynamical axion with heavy Peccei-Quinn scale. The basic mechanism consists in associating winding histories to a bosonic zero mode whose role is to convert a circle in configuration space into a helix. The zero mode is, respectively, the holonomy of the 3-form field or the axion momentum. In these models different theta sectors coexist but are not mixed by local operators. Our analysis sheds light on, and extends Seiberg's proposal for modifying the topological sums in quantum field theories. It refutes a recent claim that $B+L$ violation at LHC is unsuppressed.

  18. Study on band gap structure of Fibonacci quantum superlattices by using the transfer matrix method

    Science.gov (United States)

    Ferrando, V.; Castro-Palacio, J. C.; Marí, B.; Monsoriu, J. A.

    2014-02-01

    The scattering properties of particles in a one-dimensional Fibonacci sequence based potential have been analyzed by means of the Transfer Matrix Method. The electronic band gaps are examined comparatively with those obtained using the corresponding periodic potentials. The reflection coefficient shows self-similar properties for the Fibonacci superlattices. Moreover, by using the generalized Bragg's condition, the band gaps positions are derived from the golden mean involved in the design of the superlattice structure.

  19. High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4.

    Science.gov (United States)

    Errandonea, Daniel; Muñoz, Alfonso; Rodríguez-Hernández, Placida; Gomis, Oscar; Achary, S Nagabhusan; Popescu, Catalin; Patwe, Sadeque J; Tyagi, Avesh K

    2016-05-16

    The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.

  20. Band structure characteristics of T-square fractal phononic crystals

    Institute of Scientific and Technical Information of China (English)

    Liu Xiao-Jian; Fan You-Hua

    2013-01-01

    The T-square fractal two-dimensional phononic crystal model is presented in this article.A comprehensive study is performed for the Bragg scattering and locally resonant fractal phononic crystal.We find that the band structures of the fractal and non-fractal phononic crystals at the same filling ratio are quite different through using the finite element method.The fractal design has an important impact on the band structures of the two-dimensional phononic crystals.

  1. Band Gap Properties of Magnetoelectroelastic Grid Structures with Initial Stress

    Institute of Scientific and Technical Information of China (English)

    WANG Yi-Ze; LI Feng-Ming

    2012-01-01

    The propagation of elastic waves in magnetoelectroelastic grid structures is studied.Band gap properties are presented and the effects of the magnetoelectroelastic coupling and initial stress are considered. Numerical calculations are performed using the plane-wave expansion method.The results show that the band gap width can be tuned by the initial stress.It is hoped that our results will be helpful for designing acoustic filters with magnetoelectroelastic materials and grid structures.

  2. Electronic crosstalk in Terra MODIS thermal emissive bands

    Science.gov (United States)

    Sun, Junqiang; Madhavan, Sriharsha; Xiong, Xiaoxiong; Wang, Menghua

    2015-09-01

    The MODerate-resolution Imaging Spectroradiometer (MODIS) is a legacy Earth remote sensing instrument in the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS). The first MODIS instrument was launched in December 1999 on board the Terra spacecraft. MODIS has 36 bands, among which bands 20-25 and bands 27-36 are thermal emissive bands covering a wavelength range from 3.7μm to 14.2μm. It has been found that there are severe contaminations in Terra bands 27-30 (6.7 μm - 9.73 μm) due to crosstalk of signals among themselves. The crosstalk effect induces strong striping artifacts in the Earth View (EV) images and causes large long-term drifts in the EV brightness temperature (BT) in these bands. An algorithm using a linear approximation derived from on-orbit lunar observations has been developed to correct the crosstalk effect for them. It was demonstrated that the crosstalk correction can substantially reduce the striping noise in the EV images and significantly remove the long-term drifts in the EV BT in the Long Wave InfraRed (LWIR) water vapor channels (bands 27-28). In this paper, the crosstalk correction algorithm previously developed is applied to correct the crosstalk effect in the remaining LWIR bands 29 and 30. The crosstalk correction successfully reduces the striping artifact in the EV images and removes long-term drifts in the EV BT in bands 29-30 as was done similarly for bands 27-28. The crosstalk correction algorithm can thus substantially improve both the image quality and the radiometric accuracy of the Level 1B (L1B) products of the LWIR PV bands, bands 27-30. From this study it is also understood that other Terra MODIS thermal emissive bands are contaminated by the crosstalk effect and that the algorithm can be applied to these bands for crosstalk correction.

  3. Electronic Power Conditioner for Ku-band Travelling Wave Tube

    Science.gov (United States)

    Kowstubha, Palle; Krishnaveni, K.; Ramesh Reddy, K.

    2016-07-01

    A highly sophisticated regulated power supply is known as electronic power conditioner (EPC) is required to energise travelling wave tubes (TWTs), which are used as RF signal amplifiers in satellite payloads. The assembly consisting of TWT and EPC together is known as travelling wave tube amplifier (TWTA). EPC is used to provide isolated and conditioned voltage rails with tight regulation to various electrodes of TWT and makes its RF performance independent of solar bus variations which are caused due to varying conditions of eclipse and sunlit. The payload mass and their power consumption is mainly due to the existence of TWTAs that represent about 35 % of total mass and about 70-90 % (based on the type of satellite application) of overall dc power consumption. This situation ensures a continuous improvement in the design of TWTAs and their associated EPCs to realize more efficient and light products. Critical technologies involved in EPCs are design and configuration, closed loop regulation, component and material selection, energy limiting of high voltage (HV) outputs and potting of HV card etc. This work addresses some of these critical technologies evolved in realizing and testing the state of art of EPC and it focuses on the design of HV supply with a HV and high power capability, up to 6 kV and 170 WRF, respectively required for a space TWTA. Finally, an experimental prototype of EPC with a dc power of 320 W provides different voltages required by Ku-band TWT in open loop configuration.

  4. Optimum design of band-gap beam structures

    DEFF Research Database (Denmark)

    Olhoff, Niels; Niu, Bin; Cheng, Gengdong

    2012-01-01

    -sectional area. To study the band-gap for travelling waves, a repeated inner segment of the optimized beams is analyzed using Floquet theory and the waveguide finite element (WFE) method. Finally, the frequency response is computed for the optimized beams when these are subjected to an external time......The design of band-gap structures receives increasing attention for many applications in mitigation of undesirable vibration and noise emission levels. A band-gap structure usually consists of a periodic distribution of elastic materials or segments, where the propagation of waves is impeded...

  5. Automated effective band structures for defective and mismatched supercells

    Science.gov (United States)

    Brommer, Peter; Quigley, David

    2014-12-01

    In plane-wave density functional theory codes, defects and incommensurate structures are usually represented in supercells. However, interpretation of E versus k band structures is most effective within the primitive cell, where comparison to ideal structures and spectroscopy experiments are most natural. Popescu and Zunger recently described a method to derive effective band structures (EBS) from supercell calculations in the context of random alloys. In this paper, we present bs_sc2pc, an implementation of this method in the CASTEP code, which generates an EBS using the structural data of the supercell and the underlying primitive cell with symmetry considerations handled automatically. We demonstrate the functionality of our implementation in three test cases illustrating the efficacy of this scheme for capturing the effect of vacancies, substitutions and lattice mismatch on effective primitive cell band structures.

  6. The superatom states of fullerenes and their hybridization into the nearly free electron bands of fullerites.

    Science.gov (United States)

    Zhao, Jin; Feng, Min; Yang, Jinlong; Petek, Hrvoje

    2009-04-28

    Motivated by the discovery of the superatom states of C60 molecules, we investigate the factors that influence their energy and wave function hybridization into nearly free electron bands in molecular solids. As the n = 3 solutions of the radial Schrodinger equation of the central attractive potential consisting of the short-range C atom core and the long-range collective screening potentials, respectively, located on the icosahedral C60 molecule shell and within its hollow core, superatom states are distinguished by their atom-like orbitals corresponding to different orbital angular momentum states (l = 0, 1, 2,...). Because they are less tightly bound than the pi orbitals, that is, the n = 2 states, which are often exploited in the intermolecular electron transport in aromatic organic molecule semiconductors, superatom orbitals hybridize more extensively among aggregated molecules to form bands with nearly free electron dispersion. The prospect of exploiting the strong intermolecular coupling to achieve metal-like conduction in applications such as molecular electronics may be attained by lowering the energy of superatom states from 3.5 eV for single chemisorbed C60 molecules to below the Fermi level; therefore, we study how the superatom state energies depend on factors such as their aggregation into 1D-3D solids, cage size, and exo- and endohedral doping by metal atoms. We find, indeed, that if the ionization potential of endohedral atom, such as copper, is sufficiently large, superatom states can form the conduction band in the middle of the gap between the HOMO and LUMO of the parent C60 molecule. Through a plane-wave density functional theory study, we provide insights for a new paradigm for intermolecular electronic interaction beyond the conventional one among the sp(n) hybridized orbitals of the organic molecular solids that could lead to design of novel molecular materials and quantum structures with extraordinary optical and electronic properties.

  7. The C-Band accelerating structures for SPARC photoinjector energy upgrade

    Science.gov (United States)

    Alesini, D.; Boni, R.; Di Pirro, G.; Di Raddo, R.; Ferrario, M.; Gallo, A.; Lollo, V.; Marcellini, F.; Palumbo, L.; Spizzo, V.; Mostacci, A.; Campogiani, G.; Persichelli, S.; Enomoto, A.; Higo, T.; Kakihara, K.; Kamitani, T.; Matsumoto, S.; Sugimura, T.; Yokoyama, K.; Verdú-Andrés, S.

    2013-05-01

    The use of C-Band structures for electron acceleration and production of high quality beams has been proposed and adopted in several linac projects all over the world. The two main projects that adopted such type of structures are the Japanese Free Electron Laser (FEL) project in Spring-8 and the SwissFEL project at Paul Scherrer Institute (PSI). Also the energy upgrade of the SPARC photo-injector at LNF-INFN (Italy) from 150 to more than 240 MeV will be done by replacing a low gradient S-Band accelerating structure with two C-band structures. The structures are Traveling Wave (TW) and Constant Impedance (CI), have symmetric axial input couplers and have been optimized to work with a SLED RF input pulse. The paper presents the design criteria of the structures, the realization procedure and the low and high power RF test results on a prototype. The high power tests have been carried out by the Frascati INFN Laboratories in close collaboration with the Japanese Laboratory KEK. Experimental results confirmed the feasibility of the operation of the prototype at 50 MV/m with about 10-6 breakdowns per pulse per meter. Such high gradients have not been reached before in C-Band systems and demonstrated the possibility to use C-band accelerators, if needed, at such high field level. The results of the internal inspection of the structure after the high power test are also presented.

  8. Electrons and photons in periodic structures

    DEFF Research Database (Denmark)

    Pedersen, Jesper Goor

    This thesis concerns various theoretical proposals for engineering dispersion relations of photons and electrons for particular applications. The common concept is the use of a periodic modulation to induce new phenomena on length scales comparable with the periodicity of the modulation. In parti......This thesis concerns various theoretical proposals for engineering dispersion relations of photons and electrons for particular applications. The common concept is the use of a periodic modulation to induce new phenomena on length scales comparable with the periodicity of the modulation...... periodic modulation of an electron gas leads to the emergence of localized defect states with energies within the band gap, where no propagating modes exist. Secondly, the divergence of the photonic density of states near a photonic band gap leads to strongly modified light-matter interactions, which has...... the way for graphene transistors. Photonic band gaps can be engineered using structures with a periodic modulations of the refractive index, commonly referred to as photonic crystal. We discuss the application of photonic crystals to slow light phenomena, where advantage is taken of the divergence...

  9. Structural mechanisms of formation of adiabatic shear bands

    Directory of Open Access Journals (Sweden)

    Mikhail Sokovikov

    2016-10-01

    Full Text Available The paper focuses on the experimental and theoretical study of plastic deformation instability and localization in materials subjected to dynamic loading and high-velocity perforation. We investigate the behavior of samples dynamically loaded during Hopkinson-Kolsky pressure bar tests in a regime close to simple shear conditions. Experiments were carried out using samples of a special shape and appropriate test rigging, which allowed us to realize a plane strain state. Also, the shear-compression specimens proposed in were investigated. The lateral surface of the samples was investigated in a real-time mode with the aid of a high-speed infra-red camera CEDIP Silver 450M. The temperature field distribution obtained at different time made it possible to trace the evolution of plastic strain localization. Use of a transmission electron microscope for studying the surface of samples showed that in the regions of strain localization there are parts taking the shape of bands and honeycomb structure in the deformed layer. The process of target perforation involving plug formation and ejection was investigated using a high-speed infra-red camera. A specially designed ballistic set-up for studying perforation was used to test samples in different impulse loading regimes followed by plastic flow instability and plug ejection. Changes in the velocity of the rear surface at different time of plug ejection were analyzed by Doppler interferometry techniques. The microstructure of tested samples was analyzed using an optical interferometer-profilometer and a scanning electron microscope. The subsequent processing of 3D deformation relief data enabled estimation of the distribution of plastic strain gradients at different time of plug formation and ejection. It has been found that in strain localization areas the subgrains are elongated taking the shape of bands and undergo fragmentation leading to the formation of super-microcrystalline structure, in which the

  10. Evidence of Eu2+ 4f electrons in the valence band spectra of EuTiO3 and EuZrO3

    Science.gov (United States)

    Kolodiazhnyi, T.; Valant, M.; Williams, J. R.; Bugnet, M.; Botton, G. A.; Ohashi, N.; Sakka, Y.

    2012-10-01

    We report on optical band gap and valence electronic structure of two Eu2+-based perovskites, EuTiO3 and EuZrO3 as revealed by diffuse optical scattering, electron energy loss spectroscopy, and valence-band x-ray photoelectron spectroscopy. The data show good agreement with the first-principles studies in which the top of the valence band structure is formed by the narrow Eu 4f7 electron band. The O 2p band shows the features similar to those of the Ba(Sr)TiO3 perovskites except that it is shifted to higher binding energies. Appearance of the Eu2+ 4f7 band is a reason for narrowing of the optical band gap in the title compounds as compared to their Sr-based analogues.

  11. Valence and conduction band structure of the quasi-two-dimensional semiconductor Sn S2

    Science.gov (United States)

    Racke, David A.; Neupane, Mahesh R.; Monti, Oliver L. A.

    2016-02-01

    We present the momentum-resolved photoemission spectroscopy of both the valence and the conduction band region in the quasi-two-dimensional van der Waals-layered indirect band gap semiconductor Sn S2 . Using a combination of angle-resolved ultraviolet photoemission and angle-resolved two-photon photoemission (AR-2PPE) spectroscopy, we characterize the band structure of bulk Sn S2 . Comparison with density functional theory calculations shows excellent quantitative agreement in the valence band region and reveals several localized bands that likely originate from defects such as sulfur vacancies. Evidence for a moderate density of defects is also observed by AR-2PPE in the conduction band region, leading to localized bands not present in the computational results. The energetic structure and dispersion of the conduction bands is captured well by the computational treatment, with some quantitative discrepancies remaining. Our results provide a broader understanding of the electronic structure of Sn S2 in particular and van der Waals-layered semiconductors in general.

  12. Ab initio theory for ultrafast magnetization dynamics with a dynamic band structure

    Science.gov (United States)

    Mueller, B. Y.; Haag, M.; Fähnle, M.

    2016-09-01

    Laser-induced modifications of magnetic materials on very small spatial dimensions and ultrashort timescales are a promising field for novel storage and spintronic devices. Therefore, the contribution of electron-electron spin-flip scattering to the ultrafast demagnetization of ferromagnets after an ultrashort laser excitation is investigated. In this work, the dynamical change of the band structure resulting from the change of the magnetization in time is taken into account on an ab initio level. We find a large influence of the dynamical band structure on the magnetization dynamics and we illustrate the thermalization and relaxation process after laser irradiation. Treating the dynamical band structure yields a demagnetization comparable to the experimental one.

  13. Pathway to Oxide Photovoltaics via Band-Structure Engineering of SnO

    Energy Technology Data Exchange (ETDEWEB)

    Peng, Haowei; Bikowski, Andre; Zakutayev, Andriy; Lany, Stephan

    2016-10-01

    All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. SnO has exceptional doping and transport properties among oxides, but suffers from a strongly indirect band gap. Here, we address this shortcoming by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, and Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct semiconductors are achievable, while the comparatively small effective masses are preserved in the alloys. Initial thin film synthesis and characterization support the feasibility of the approach.

  14. The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique

    Energy Technology Data Exchange (ETDEWEB)

    Sutherland, Kevin Jerome [Iowa State Univ., Ames, IA (United States)

    2001-01-01

    Photonic band gap (PBG) crystals are periodic dielectric structures that manipulate electromagnetic radiation in a manner similar to semiconductor devices manipulating electrons. Whereas a semiconductor material exhibits an electronic band gap in which electrons cannot exist, similarly, a photonic crystal containing a photonic band gap does not allow the propagation of specific frequencies of electromagnetic radiation. This phenomenon results from the destructive Bragg diffraction interference that a wave propagating at a specific frequency will experience because of the periodic change in dielectric permitivity. This gives rise to a variety of optical applications for improving the efficiency and effectiveness of opto-electronic devices. These applications are reviewed later. Several methods are currently used to fabricate photonic crystals, which are also discussed in detail. This research involves a layer-by-layer micro-transfer molding ({mu}TM) and stacking method to create three-dimensional FCC structures of epoxy or titania. The structures, once reduced significantly in size can be infiltrated with an organic gain media and stacked on a semiconductor to improve the efficiency of an electronically pumped light-emitting diode. Photonic band gap structures have been proven to effectively create a band gap for certain frequencies of electro-magnetic radiation in the microwave and near-infrared ranges. The objective of this research project was originally two-fold: to fabricate a three dimensional (3-D) structure of a size scaled to prohibit electromagnetic propagation within the visible wavelength range, and then to characterize that structure using laser dye emission spectra. As a master mold has not yet been developed for the micro transfer molding technique in the visible range, the research was limited to scaling down the length scale as much as possible with the current available technology and characterizing these structures with other methods.

  15. Terra MODIS band 27 electronic crosstalk: cause, impact, and mitigation

    Science.gov (United States)

    Sun, J.; Madhavan, S.; Wenny, B. N.; Xiong, X.

    2011-11-01

    MODIS-Terra is one of the key sensors in the suite of remote sensing instruments in the Earth Observing System (EOS). MODIS on the Terra platform was launched into orbit in December of 1999 and has successfully completed eleven plus years of operation. MODIS has 36 spectral channels with wavelengths varying from 0.4 μm to 14.4 μm. The native spatial resolutions for the reflective channels are 2 bands at 0.25 km, 5 bands at 0.5 km and 29 bands at 1km. However, the MODIS L1B product allows the high spatial resolution bands to be aggregated into 1km resolution. All the thermal channels in MODIS (i.e. 3.75μm - 14.24μm) have a native spatial resolution of 1 km. Over the eleven plus years of mission lifetime, the sensor degradation has been carefully monitored using various On-Board Calibrators (OBC). In particular, the thermal channels are monitored using the on-board Black-Body (BB) which is traceable to NIST standards. MODIS also has a unique feature for calibration reference in terms of lunar irradiance. The lunar observations are scheduled for MODIS periodically (at least 9 observations in a calendar year). Based on the lunar observations, it was found that there was a possible signal leak for band 27 from its neighboring bands located on the Long-Wave Infrared (LWIR) focal plane. Further investigations revealed a possible leak from bands 28, 29 and 30. The magnitude of the leak was trended and correction coefficients were derived. In this paper, we demonstrate the across-band signal leak in MODIS band 27, its potential impact on the retrieved Brightness temperature (B.T.). Also, the paper explores a correction methodology to relieve the artifacts due to the across-band signal leak. Finally, the improvement in the band 27 image quality is quantified.

  16. Strain localization band width evolution by electronic speckle pattern interferometry strain rate measurement

    Energy Technology Data Exchange (ETDEWEB)

    Guelorget, Bruno [Institut Charles Delaunay-LASMIS, Universite de technologie de Troyes, FRE CNRS 2848, 12 rue Marie Curie, B.P. 2060, 10010 Troyes Cedex (France)], E-mail: bruno.guelorget@utt.fr; Francois, Manuel; Montay, Guillaume [Institut Charles Delaunay-LASMIS, Universite de technologie de Troyes, FRE CNRS 2848, 12 rue Marie Curie, B.P. 2060, 10010 Troyes Cedex (France)

    2009-04-15

    In this paper, electronic speckle pattern interferometry strain rate measurements are used to quantify the width of the strain localization band, which occurs when a sheet specimen is submitted to tension. It is shown that the width of this band decreases with increasing strain. Just before fracture, this measured width is about five times wider than the shear band and the initial sheet thickness.

  17. Cherenkov oscillator operating at the second band gap of leakage waveguide structures

    Science.gov (United States)

    Jang, Kyu-Ha; Park, Seong Hee; Lee, Kitae; Jeong, Young Uk

    2016-10-01

    An electromagnetic wave source operating around second band gaps of metallic grating structures is presented. The considered metallic grating structures are not perfect periodic but inhomogeneously structured within a period to have a second band gap where the wavelength is equal to the period of the structures. The radiation mechanism by an electron beam in the structures is different from the well-known Smith-Purcell radiation occurring in perfect periodic grating structures. That is, the radiating wave has a single frequency and the radiation is unidirectional. When the energy of the electron beam is synchronized at the standing wave point in the dispersion curves, strong interaction happens and coherent radiation perpendicular to the grating surface is generated with relatively lower starting oscillation current.

  18. Cherenkov oscillator operating at the second band gap of leakage waveguide structures

    Directory of Open Access Journals (Sweden)

    Kyu-Ha Jang

    2016-10-01

    Full Text Available An electromagnetic wave source operating around second band gaps of metallic grating structures is presented. The considered metallic grating structures are not perfect periodic but inhomogeneously structured within a period to have a second band gap where the wavelength is equal to the period of the structures. The radiation mechanism by an electron beam in the structures is different from the well-known Smith-Purcell radiation occurring in perfect periodic grating structures. That is, the radiating wave has a single frequency and the radiation is unidirectional. When the energy of the electron beam is synchronized at the standing wave point in the dispersion curves, strong interaction happens and coherent radiation perpendicular to the grating surface is generated with relatively lower starting oscillation current.

  19. Electronic structure and equilibrium properties of hcp titanium and zirconium

    Indian Academy of Sciences (India)

    B P Panda

    2012-08-01

    The electronic structures of hexagonal-close-packed divalent titanium (3-d) and zirconium (4-d) transition metals are studied by using a non-local model potential method. From the present calculation of energy bands, Fermi energy, density of states and the electronic heat capacity of these two metals are determined and compared with the existing results in the literature.

  20. Electron impact excitation of the Meinel band system of N2/+/

    Science.gov (United States)

    Mandelbaum, D.; Feldman, P. D.

    1976-01-01

    The emission spectrum of the Meinel band system of N2(+) was obtained in the region between 0.72 and 1.6 micrometers by electron impact excitation of N2. The relative excitation function is presented for the (0, 0) band from 22 to 120 eV. Cross sections of the Meinel bands that fall in this region were measured relative to the (2, 0) Meinel band. The results obtained are in good agreement with previously published values below 1.1 micrometers except for the (0, 0) band which is 30% higher. The electronic transition moment at large internuclear distances is found to agree with theoretical estimates.

  1. Structural simplicity and complexity of compressed calcium: electronic origin.

    Science.gov (United States)

    Degtyareva, Valentina F

    2014-06-01

    A simple cubic structure with one atom in the unit cell found in compressed calcium is counterintuitive to the traditional view of a tendency towards densely packed structures with an increase in pressure. To understand this unusual transformation it is necessary to assume electron transfer from the outer core band to the valence band, and an increase of valence electron number for calcium from 2 to ∼ 3.5. This assumption is supported by the Fermi sphere-Brillouin zone interaction model that increases under compression. The recently found structure of Ca-VII with a tetragonal cell containing 32 atoms (tI32) is similar to that in the intermetallic compound In5Bi3 with 3.75 valence electrons per atom. Structural relations are analyzed in terms of electronic structure resemblance. Correlations of structure and physical properties of Ca are discussed.

  2. Millimeter-wave waveguiding using photonic band structures

    Science.gov (United States)

    Eliyahu, Danny; Sadovnik, Lev S.; Manasson, Vladimir A.

    2000-07-01

    Current trends in device miniaturization and integration, especially in the development of microwave monolithic integrated circuits, calls for flexible, arbitrarily shaped and curved interconnects. Standard dielectric waveguides and microstrip lines are subject to prohibitive losses and their functionality is limited because of their unflexible structures. The problem is addressed by confining the wave- guiding path in a substrate with a Photonic Band Gap structure in a manner that will result in the guided mode being localized within the band gap. Two devices implementing Photonic Band Structures for millimeter waves confinement are presented. The first waveguide is a linear defect in triangular lattice created in a silicon slab (TE mode). The structure consists of parallel air holes of circular cross sections. The silicon was laser drilled to create the 2D crystal. The second device consists of alumina rods arranged in a triangular lattice, surrounded by air and sandwiched between two parallel metal plates (TM mode). Electromagnetic wave (W-band) confinement was obtained in both devices for straight and bent waveguides. Three branch waveguides (intersecting line defects) was studied as well. Measurements confirmed the lowloss waveguide confinement property of the utilizing Photonic Band Gap structure. This structure can find applications in power combiner/splitter and other millimeter wave devices.

  3. Band Structure in the Doubly Magic Nucleus 56Ni

    Institute of Scientific and Technical Information of China (English)

    DONG Bao-Guo; GUO Hong-Chao

    2004-01-01

    @@ Band structures near yrast lines of the Z = N doubly magic nucleus 56Ni are investigated with the configurationdependent cranked Nilsson-Strutinsky approach. The observed deformed bands are confirmed as highly deformed and their properties are explained theoretically. The calculated transition quadrupole moments Qt, ~ 1.7 eb at low spin as well as the kinematic and dynamic moments of inertia J1) and J(2) for configurations of interest are found to be generally in good agreement with the observed results. Two terminating states at 20+ and 29- for the two observed bands and other terminations in 56Ni are also predicted. It is found that the configuration-dependent cranked Nilsson-Strutinsky approach is better in the description of nuclear properties and band structures at high spin than other models.

  4. Band structure and optical properties of LiKB4O7 single crystal

    NARCIS (Netherlands)

    Smok, P; Seinert, H; Kityk, [No Value; Berdowski, J

    2003-01-01

    The band structure (BS), electronic charge density distribution and linear optical properties of the LiKB4O7 (LKB4) single crystal are calculated using a self-consistent norm-conserving pseudo-potential method within the framework of the local density approximation theory. Dispersion of the imaginar

  5. Coupling between Fano and Bragg bands in photonic band structure of two-dimensional metallic photonic structures

    CERN Document Server

    Markos, Peter

    2016-01-01

    Frequency and transmission spectrum of two-dimensional array of metallic rods is investigated numerically. Based on the recent analysis of the band structure of two-dimensional photonic crystal with dielectric rods [P. Marko\\v{s}, Phys. Rev. A 92 043814 (2015)] we identify two types of bands in the frequency spectrum: Bragg (P) bands resulting from a periodicity and Fano (F) bands which arise from Fano resonances associated with each of the cylinders within the periodic structure. It is shown that the existence of Fano band in a certain frequency range is manifested by a Fano resonance in the transmittance. In particular, we re-examine the symmetry properties of the H- polarized band structure in the frequency range where the spectrum consists of the localized modes associated with the single scatterer resonances and we explore process of formation of Fano bands by identifying individual terms in the expansion of the LCAO states. We demonstrate how the interplay between the two scattering mechanisms affects p...

  6. Band structures and localization properties of aperiodic layered phononic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Yan Zhizhong, E-mail: zzyan@bit.edu.cn [Department of Applied Mathematics, Beijing Institute of Technology, Beijing 100081 (China); Zhang Chuanzeng [Department of Civil Engineering, University of Siegen, D-57078 Siegen (Germany)

    2012-03-15

    The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.

  7. Band structures and localization properties of aperiodic layered phononic crystals

    Science.gov (United States)

    Yan, Zhi-Zhong; Zhang, Chuanzeng

    2012-03-01

    The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.

  8. Transient W-band EPR study of sequential electron transfer in photosynthetic bacterial reaction centers

    Energy Technology Data Exchange (ETDEWEB)

    Tang, J.; Utschig, L.M.; Poluektov, O.; Thurnauer, M.C. [Argonne National Lab., IL (United States). Chemistry Div.

    1999-06-17

    The key reaction of photosynthetic solar energy conversion involves the photoexcitation of a primary donor (P) followed by rapid, sequential electron transfer to a series of acceptors resulting in charge separation. Electron-spin polarized (ESP) EPR spectra at W-band (95 GHz) were obtained for deuterated Fe-removed/Zn-substituted photosynthetic bacterial reaction centers (RCs) to investigate the influence of the rate of charge separation on the observed P{sup +}Q{sub A}{sup {minus}} charge separated state. Temperature dependent ESP EPR spectra for kinetically characterized Zn-substituted RCs from Rb. sphaeroides R-26 having different rates (k{sub Q}) of the electron transfer from the bacteriopheophytin to the quinone acceptor were obtained. The Zn-RCs exhibited either the native fast (200 ps){sup {minus}1} k{sub Q} or a slow (3--6 ns){sup {minus}1} k{sub Q} at 298 K as determined from transient optical measurements. Sequential electron-transfer polarization modeling of the polarized W-band EPR spectra obtained with these samples was used to address the reason for the differences in the electron-transfer rates. Here, the authors report the k{sub Q} rate constant, the temperature dependence of k{sub Q}, and the reorganization energy for the P{sup +}H{sup {minus}}Q{sub A} and P{sup +}HQ{sub A}{sup {minus}} electron-transfer step determined from SETP modeling of the experimental spectra. The reorganization energy for the electron-transfer process between P{sup +}H{sup {minus}}Q{sub A} and P{sup +}HQ{sub A}{sup {minus}}, and not structural changes in the donor or acceptor, was found to be the dominant factor that is altered during Fe-removal procedures.

  9. Orbital approach to the electronic structure of solids

    CERN Document Server

    Canadell, Enric; Iung, Christophe

    2012-01-01

    This book provides an intuitive yet sound understanding of how structure and properties of solids may be related. The natural link is provided by the band theory approach to the electronic structure of solids. The chemically insightful concept of orbital interaction and the essential machinery of band theory are used throughout the book to build links between the crystal and electronic structure of periodic systems. In such a way, it is shown how important tools for understandingproperties of solids like the density of states, the Fermi surface etc. can be qualitatively sketched and used to ei

  10. Coupler design for an L-band electron linac

    Institute of Scientific and Technical Information of China (English)

    ZHAO Wei; TANG Xiao; SHI Rong-Jian; HOU Mi

    2012-01-01

    The RF coupler is a key component for an accelerating structure which is the most important component for a linac.In order to feed microwave power into the accelerating cavities effectively,the coupler has to be well matched with the feeding waveguide.In this paper,an electron linac coupler was designed,constructed and tested.A numerical simulation method based on the Kyhl's method was employed to search for the optimal dimensions of the coupler.The frequency and the coupling coefficient as a function of the coupler dimensions were also calculated.The results fitted the Kyhl's method simulation results well and gave tolerances of the coupler.The coupler was brazed to the accelerating cavities and it was cold-tested and hot-tested.The experimental results were consistent with the numerical simulation results.

  11. The electronic structure of impurities in semiconductors

    CERN Multimedia

    Nylandsted larsen, A; Svane, A

    2002-01-01

    The electronic structure of isolated substitutional or interstitial impurities in group IV, IV-IV, and III-V compound semiconductors will be studied. Mössbauer spectroscopy will be used to investigate the incorporation of the implanted isotopes on the proper lattice sites. The data can be directly compared to theoretical calculations using the LMTO scheme. Deep level transient spectroscopy will be used to identify the band gap levels introduced by metallic impurities, mainly in Si~and~Si$ _{x}$Ge$_{1-x}$. \\\\ \\\\

  12. Surface structure and electronic properties of materials

    Science.gov (United States)

    Siekhaus, W. J.; Somorjai, G. A.

    1975-01-01

    A surface potential model is developed to explain dopant effects on chemical vapor deposition. Auger analysis of the interaction between allotropic forms of carbon and silicon films has shown Si-C formation for all forms by glassy carbon. LEED intensity measurements have been used to determine the mean square displacement of surface atoms of silicon single crystals, and electron loss spectroscopy has shown the effect of structure and impurities on surface states located within the band gap. A thin film of Al has been used to enhance film crystallinity at low temperature.

  13. Electronic structure of sulfanilamides

    Energy Technology Data Exchange (ETDEWEB)

    Grechishkin, V.S.; Grechishkina, R.V.; Starovoitova, O.V.

    1986-05-01

    At present, about 30,000 derivatives of sulfanilamide are known. The establishment of a relationship between the structure of these compounds and their bacteriostatic activity is an urgent problem. In the present work, this problem is solved by means of NQR and NMR spectroscopy. Since the content of the /sup 14/N nuclei in these molecules is not high, to run the NQR, they used the double resonance method. Some samples of the sulfanilamides were studied by direct pulsed NQR method. The high resolution NMR spectra were run in heavy water solution on a RS-60MA spectrometer. All the measurements were carried out at 120/sup 0/K in the solid phase. The results of the calculation of eQq/sub zz/ for the NH/sub 2/ groups in the sulfanilamide residue are listed. To interpret the results by the MO LCAO method in the Hueckel approximation on the EC-1022 computer by a special FORTRAN program, they calculated the charged rho on an atom in the amino group with parameters of hetero atoms and coupling constants.

  14. Band Structure Analysis of La0.7Sr0.3MnO3 Perovskite Manganite Using a Synchrotron

    Directory of Open Access Journals (Sweden)

    Hong-Sub Lee

    2015-01-01

    Full Text Available Oxide semiconductors and their application in next-generation devices have received a great deal of attention due to their various optical, electric, and magnetic properties. For various applications, an understanding of these properties and their mechanisms is also very important. Various characteristics of these oxides originate from the band structure. In this study, we introduce a band structure analysis technique using a soft X-ray energy source to study a La0.7Sr0.3MnO3 (LSMO oxide semiconductor. The band structure is formed by a valence band, conduction band, band gap, work function, and electron affinity. These can be determined from secondary electron cut-off, valence band spectrum, O 1s core electron, and O K-edge measurements using synchrotron radiation. A detailed analysis of the band structure of the LSMO perovskite manganite oxide semiconductor thin film was established using these techniques.

  15. A phenomenological model of electronic band structure in ferroelectric Pb(In{sub 1/2}Nb{sub 1/2})O{sub 3}-Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-PbTiO{sub 3} single crystals around the morphotropic phase boundary determined by temperature-dependent transmittance spectra

    Energy Technology Data Exchange (ETDEWEB)

    Zhu, J.J.; Li, W.W. [Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241 (China); Xu, G.S. [R and D Center of Synthetic Crystals, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800 (China); Jiang, K. [Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241 (China); Hu, Z.G., E-mail: zghu@ee.ecnu.edu.cn [Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241 (China); Chu, J.H. [Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241 (China)

    2011-10-15

    The optical properties of ferroelectric Pb(In{sub 1/2}Nb{sub 1/2})O{sub 3}-Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-PbTiO{sub 3} (PIN-PMN-PT) single crystals around the morphotropic phase boundary (MPB) have been investigated using ultraviolet-infrared transmittance spectra in the temperature range of 8-300 K. Based on the temperature-dependent spectral measurement of the band gap, we propose a phenomenological model of band structure vs. temperature to explain both the negative and positive band narrowing coefficient dE{sub gd}/dT in ferroelectric PIN-PMN-PT crystals around the MPB where multiple phases coexist. The peculiar positive coefficient only exists in the fragile multiphase region of the MPB, while the negative coefficient, caused by thermal expansion of the lattice and renormalization of the band structure by electron-phonon interaction, exists in the rhombohedral or tetragonal single-phase region as well as in the stationary multiphase region of the MPB. The origin of the positive coefficient is a long-range increasing fraction of coexistence from the monoclinic phase with small band gap to rhombohedral phase with large band gap at elevated temperature. In agreement with optical transmittance results of PMN-PT/PIN-PMN-PT, the model predicts that these unusual positive band narrowing coefficients may exist for all ferroelectrics around the MPB where the coexistence of phases lacks thermodynamic stability.

  16. Dual-Band Perfect Absorption by Breaking the Symmetry of Metamaterial Structure

    Science.gov (United States)

    Hai, Le Dinh; Qui, Vu Dinh; Dinh, Tiep Hong; Hai, Pham; Giang, Trinh Thị; Cuong, Tran Manh; Tung, Bui Son; Lam, Vu Dinh

    2017-02-01

    Since the first proposal of Landy et al. (Phys Rev Lett 100:207402, 2008), the metamaterial perfect absorber (MPA) has rapidly become one of the most crucial research trends. Recently, dual-band, multi-band and broadband MPA have been highly desirable in electronic applications. In this paper, we demonstrate and evaluate a MPA structure which can generate dual-band absorption operating at the microwave frequency by breaking the symmetry of structure. There is an agreement between simulation and experimental results. The results can be explained by using the equivalent LC circuit and the electric field distribution of this structure. In addition, various structures with different symmetry configurations were studied to gain greater insight into the absorption.

  17. Tunable band structure and effective mass of disordered chalcopyrite

    Science.gov (United States)

    Wang, Ze-Lian; Xie, Wen-Hui; Zhao, Yong-Hong

    2017-02-01

    The band structure and effective mass of disordered chalcopyrite photovoltaic materials Cu1- x Ag x Ga X 2 ( X = S, Se) are investigated by density functional theory. Special quasirandom structures are used to mimic local atomic disorders at Cu/Ag sites. A local density plus correction method is adopted to obtain correct semiconductor band gaps for all compounds. The bandgap anomaly can be seen for both sulfides and selenides, where the gap values of Ag compounds are larger than those of Cu compounds. Band gaps can be modulated from 1.63 to 1.78 eV for Cu1- x Ag x Ga Se 2, and from 2.33 to 2.64 eV for Cu1- x Ag x Ga S 2. The band gap minima and maxima occur at around x = 0:5 and x = 1, respectively, for both sulfides and selenides. In order to show the transport properties of Cu1- x Ag x Ga X 2, the effective mass is shown as a function of disordered Ag concentration. Finally, detailed band structures are shown to clarify the phonon momentum needed by the fundamental indirect-gap transitions. These results should be helpful in designing high-efficiency photovoltaic devices, with both better absorption and high mobility, by Ag-doping in CuGa X 2.

  18. S-band linac-based X-ray source with {pi}/2-mode electron linac

    Energy Technology Data Exchange (ETDEWEB)

    Deshpande, Abhay, E-mail: abhay@post.kek.jp [Department of Accelerator Science, School of High Energy Accelerator Science, Graduate University for Advanced Studies, Shonan International Village, Hayama, Miura, Kanagawa 240-0193 (Japan); Society for Applied Microwave Electronic Engineering and Research (SAMEER), R and D Laboratory of the Government of India, IIT Campus, Powai, Mumbai 400 076 (India); Araki, Sakae [High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801 (Japan); Dixit, Tanuja [Society for Applied Microwave Electronic Engineering and Research (SAMEER), R and D Laboratory of the Government of India, IIT Campus, Powai, Mumbai 400 076 (India); Fukuda, Masafumi [High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801 (Japan); Krishnan, R; Pethe, Sanjay [Society for Applied Microwave Electronic Engineering and Research (SAMEER), R and D Laboratory of the Government of India, IIT Campus, Powai, Mumbai 400 076 (India); Sakaue, Kazuyuki [Waseda University, Shinjuku-ku, Tokyo 169-8555 (Japan); Terunuma, Nobuhiro; Urakawa, Junji [High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801 (Japan); Washio, Masakazu [Waseda University, Shinjuku-ku, Tokyo 169-8555 (Japan)

    2011-05-01

    The activities with the compact X-ray source are attracting more attention, particularly for the applications of the source in medical fields. We propose the fabrication of a compact X-ray source using the SAMEER electron linear accelerator and the KEK laser undulator X-ray source (LUCX) technologies. The linac developed at SAMEER is a standing wave side-coupled S-band linac operating in the {pi}/2 mode. In the proposed system, a photocathode RF gun will inject bunches of electrons in the linac to accelerate and achieve a high-energy, low-emittance beam. This beam will then interact with the laser in the laser cavity to produce X-rays of a type well suited for various applications. The side-coupled structure will make the system more compact, and the {pi}/2 mode of operation will enable a high repetition rate operation, which will help to increase the X-ray yield.

  19. Coherent phonon spectroscopy characterization of electronic bands at buried semiconductor heterointerfaces

    Energy Technology Data Exchange (ETDEWEB)

    Ishioka, Kunie, E-mail: ishioka.kunie@nims.go.jp [Nano Characterization Unit, National Institute for Materials Science, Tsukuba 305-0047 (Japan); Brixius, Kristina; Beyer, Andreas; Stolz, Wolfgang; Volz, Kerstin; Höfer, Ulrich [Faculty of Physics and Materials Sciences Center, Philipps-Universität Marburg, 35032 Marburg (Germany); Rustagi, Avinash; Stanton, Christopher J. [Department of Physics, University of Florida, Gainesville, Florida 32611 (United States); Petek, Hrvoje [Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (United States)

    2016-02-01

    We demonstrate an all-optical approach to probe electronic band structure at buried interfaces involving polar semiconductors. Femtosecond optical pulses excite coherent phonons in epitaxial GaP films grown on Si(001) substrate. We find that the coherent phonon amplitude critically depends on the film growth conditions, specifically in the presence of antiphase domains, which are independently characterized by transmission electron microscopy. We determine the Fermi levels at the buried interface of GaP/Si from the coherent phonon amplitudes and demonstrate that the internal electric fields are created in the nominally undoped GaP films as well as the Si substrates, possibly due to the carrier trapping at the antiphase boundaries and/or at the interface.

  20. Tunable band structures of polycrystalline graphene by external and mismatch strains

    Institute of Scientific and Technical Information of China (English)

    Jiang-Tao Wu; Xing-Hua Shi; Yu-Jie Wei

    2012-01-01

    Lacking a band gap largely limits the application of graphene in electronic devices.Previous study shows that grain boundaries (GBs) in polycrystalline graphene can dramatically alter the electrical properties of graphene.Here,we investigate the band structure of polycrystalline graphene tuned by externally imposed strains and intrinsic mismatch strains at the GB by density functional theory (DFT) calculations.We found that graphene with symmetrical GBs typically has zero band gap even with large uniaxial and biaxial strain.However,some particular asymmetrical GBs can open a band gap in graphene and their band structures can be substantially tuned by external strains.A maximum band gap about 0.19 eV was observed in matched-armchair GB (5,5) | (3,7) with a misorientation of θ =13° when the applied uniaxial strain increases to 9%.Although mismatch strain is inevitable in asymmetrical GBs,it has a small influence on the band gap of polycrystalline graphene.

  1. Electronic Structure of Silicon Nanowires Matrix from Ab Initio Calculations.

    Science.gov (United States)

    Monastyrskii, Liubomyr S; Boyko, Yaroslav V; Sokolovskii, Bogdan S; Potashnyk, Vasylyna Ya

    2016-12-01

    An investigation of the model of porous silicon in the form of periodic set of silicon nanowires has been carried out. The electronic energy structure was studied using a first-principle band method-the method of pseudopotentials (ultrasoft potentials in the basis of plane waves) and linearized mode of the method of combined pseudopotentials. Due to the use of hybrid exchange-correlation potentials (B3LYP), the quantitative agreement of the calculated value of band gap in the bulk material with experimental data is achieved. The obtained results show that passivation of dangling bonds with hydrogen atoms leads to substantial transformation of electronic energy structure. At complete passivation of the dangling silicon bonds by hydrogen atoms, the band gap value takes the magnitude which substantially exceeds that for bulk silicon. The incomplete passivation gives rise to opposite effect when the band gap value decreases down the semimetallic range.

  2. First Principles Study of Band Structure and Band Gap Engineering in Graphene for Device Applications

    Science.gov (United States)

    2015-03-20

    successfully to realise the full applications of graphene? What is the current status of the graphene based devices or Electronics ? How the graphene...gap value has increased to 1.5eV. It is reflected in the density of states (Fig.20c). The nitrogen atoms are at a distance of 5Ȧ. And there is a...completely modified.The calculation of surface doping of graphene with S is repeated with 96 atom simulation cell. The band gap value is 0.7 eV. The value of

  3. Complex band structures of transition metal dichalcogenide monolayers with spin-orbit coupling effects

    Science.gov (United States)

    Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd

    2016-09-01

    Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M  =  Mo, W; X  =  S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.

  4. Molecular electronic-structure theory

    CERN Document Server

    Helgaker, Trygve; Jorgensen, Poul

    2013-01-01

    Ab initio quantum chemistry is increasingly paired with computational methods to solve intractable problems in chemistry and molecular physics. Now in a paperback edition, this comprehensive and technical work covers all the important aspects of modern molecular electronic-structure theory, clearly explaining quantum-mechanical methods and applications to molecular equilibrium structure, atomization energies, and reaction enthalpies. Extensive numerical examples illustrate each method described. An excellent resource for researchers in quantum chemistry and anyone interested in the theory and its applications.

  5. Disorder enabled band structure engineering of a topological insulator surface

    Science.gov (United States)

    Xu, Yishuai; Chiu, Janet; Miao, Lin; He, Haowei; Alpichshev, Zhanybek; Kapitulnik, A.; Biswas, Rudro R.; Wray, L. Andrew

    2017-02-01

    Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.

  6. Design for maximum band-gaps in beam structures

    DEFF Research Database (Denmark)

    Olhoff, Niels; Niu, Bin; Cheng, Gengdong

    2012-01-01

    This paper aims to extend earlier optimum design results for transversely vibrating Bernoulli-Euler beams by determining new optimum band-gap beam structures for (i) different combinations of classical boundary conditions, (ii) much larger values of the orders n and n-1 of adjacent upper and lower...

  7. Analysis of photonic band-gap structures in stratified medium

    DEFF Research Database (Denmark)

    Tong, Ming-Sze; Yinchao, Chen; Lu, Yilong;

    2005-01-01

    Purpose - To demonstrate the flexibility and advantages of a non-uniform pseudo-spectral time domain (nu-PSTD) method through studies of the wave propagation characteristics on photonic band-gap (PBG) structures in stratified medium Design/methodology/approach - A nu-PSTD method is proposed...

  8. Doping-dependent quasiparticle band structure in cuprate superconductors

    NARCIS (Netherlands)

    Eder, R; Ohta, Y.; Sawatzky, G.A

    1997-01-01

    We present an exact diagonalization study of the single-particle spectral function in the so-called t-t'-t ''-J model in two dimensions. As a key result, we find that hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasi

  9. A periodic structure for electronic signals and demonstration of electronic lasing

    Science.gov (United States)

    Doiron, Serge; Giller, Christophe; Beaudoin, Normand; Haché, Alain

    2008-11-01

    A sequence of electronic phase shifters and partial reflectors/transmitters is shown to form a true periodic scattering structure for electronic signals, complete with stop bands, interference, and dispersion effects. An experimental demonstration at 1kHz is made with supporting theoretical results. As with conventional periodic media, sharp defect modes in the forbidden band arise when the periodicity is broken. When gain is added to the structure, a type of lasing action is observed that in almost every aspect is similar to conventional lasers.

  10. X-Band Photonic Band-Gap Accelerator Structure Breakdown Experiment

    Energy Technology Data Exchange (ETDEWEB)

    Marsh, Roark A.; /MIT /MIT /NIFS, Gifu /JAERI, Kyoto /LLNL, Livermore; Shapiro, Michael A.; Temkin, Richard J.; /MIT; Dolgashev, Valery A.; Laurent, Lisa L.; Lewandowski, James R.; Yeremian, A.Dian; Tantawi, Sami G.; /SLAC

    2012-06-11

    In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV/m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV/m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV/m and a surface magnetic field of 890 kA/m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV/m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.

  11. Three new chalcohalides, Ba{sub 4}Ge{sub 2}PbS{sub 8}Br{sub 2}, Ba{sub 4}Ge{sub 2}PbSe{sub 8}Br{sub 2} and Ba{sub 4}Ge{sub 2}SnS{sub 8}Br{sub 2}: Syntheses, crystal structures, band gaps, and electronic structures

    Energy Technology Data Exchange (ETDEWEB)

    Lin, Zuohong; Feng, Kai; Tu, Heng; Kang, Lei [Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); University of the Chinese Academy of Sciences, Beijing 100049 (China); Lin, Zheshuai [Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Yao, Jiyong, E-mail: jyao@mail.ipc.ac.cn [Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Wu, Yicheng [Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China); Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 (China)

    2014-10-25

    Highlights: • Three new chalcohalides: Ba{sub 4}Ge{sub 2}PbS{sub 8}Br{sub 2}, Ba{sub 4}Ge{sub 2}PbSe{sub 8}Br{sub 2} and Ba{sub 4}Ge{sub 2}SnS{sub 8}Br{sub 2} have been synthesized. • The MQ{sub 5}Br octahedra and GeQ{sub 4} tetrahedra form a three-dimensional framework with Ba{sup 2+} in the channels. • Band Gaps and electronic structures of the three compounds were studied. - Abstract: Single crystals of three new chalcohalides: Ba{sub 4}Ge{sub 2}PbS{sub 8}Br{sub 2}, Ba{sub 4}Ge{sub 2}PbSe{sub 8}Br{sub 2} and Ba{sub 4}Ge{sub 2}SnS{sub 8}Br{sub 2} have been synthesized for the first time. These isostructural compounds crystallize in the orthorhombic space group Pnma. In the structure, the tetra-valent Ge atom is tetrahedrally coordinated with four Q (Q = S, Se) atoms, while the bi-valent M atom (M = Pb, Sn) is coordinated with an obviously distorted octahedron of five Q (Q = S, Se) atoms and one Br atom, showing the stereochemical activity of the ns{sup 2} lone pair electron. The MQ{sub 5}Br (M = Sn, Pb; Q = S, Se) distorted octahedra and the GeQ{sub 4} (Q = S, Se) tetrahedra are connected to each other to form a three-dimensional framework with channels occupied by Ba{sup 2+} cations. Based on UV–vis–NIR spectroscopy measurements and the electronic structure calculations, Ba{sub 4}Ge{sub 2}PbS{sub 8}Br{sub 2}, Ba{sub 4}Ge{sub 2}PbSe{sub 8}Br{sub 2} and Ba{sub 4}Ge{sub 2}SnS{sub 8}Br{sub 2} have indirect band gaps of 2.054, 1.952, and 2.066 eV respectively, which are mainly determined by the orbitals from the Ge, M and Q atoms (M = Pb, Sn; Q = S, Se)

  12. Study on Band Structure of YbB6 and Analysis of Its Optical Conductivity Spectrum

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    The electronic structure of YbB6 crystal was studied by means of density functional (GGA+U) method.The calculations were performed by FLAPW method.The high accurate band structure was achieved.The correlation between the feature of the band structure and the Yb-B6 bonding in YbB6 was analyzed.On this basis, some optical constants of YbB6 such as reflectivity, dielectric function, optical conductivity, and energy-loss function were calculated.The results are in good agreement with the experiments.The real part of the optical conductivity spectrum and the energy-loss function spectrum were analyzed in detail.The assignments of the spectra were carried out to correlate the spectral peaks with the interband electronic transitions, which justify the reasonable part of previous empirical assignments and renew the missed or incorrect ones.

  13. Photonic crystal digital alloys and their band structure properties.

    Science.gov (United States)

    Lee, Jeongkug; Kim, Dong-Uk; Jeon, Heonsu

    2011-09-26

    We investigated semi-disordered photonic crystals (PCs), digital alloys, and made thorough comparisons with their counterparts, random alloys. A set of diamond lattice PC digital alloys operating in a microwave regime were prepared by alternately stacking two kinds of sub-PC systems composed of alumina and silica spheres of the same size. Measured transmission spectra as well as calculated band structures revealed that when the digital alloy period is short, band-gaps of the digital alloys are practically the same as those of the random alloys. This study indicates that the concept of digital alloys holds for photons in PCs as well.

  14. Planar C-Band Antenna with Electronically Controllable Switched Beams

    Directory of Open Access Journals (Sweden)

    Mariano Barba

    2009-01-01

    Full Text Available The design, manufacturing, and measurements of a switchable-beam antenna at 3.5 GHz for WLL or Wimax base station antennas in planar technology are presented. This antenna performs a discrete beam scan of a 60∘ sector in azimuth and can be easily upgraded to 5 or more steps. The switching capabilities have been implemented by the inclusion of phase shifters based on PIN diodes in the feed network following a strategy that allows the reduction of the number of switches compared to a classic design. The measurements show that the design objectives have been achieved and encourage the application of the acquired experience in antennas for space applications, such as X-band SAR and Ku-band DBS.

  15. Electronic structure tuning via surface modification in semimetallic nanowires

    Science.gov (United States)

    Sanchez-Soares, Alfonso; O'Donnell, Conor; Greer, James C.

    2016-12-01

    Electronic structure properties of nanowires (NWs) with diameters of 1.5 and 3 nm based on semimetallic α -Sn are investigated by employing density functional theory and perturbative GW methods. We explore the dependence of electron affinity, band structure, and band-gap values with crystallographic orientation, NW cross-sectional size, and surface passivants of varying electronegativity. We consider four chemical terminations in our study: methyl (CH3), hydrogen (H ), hydroxyl (OH ), and fluorine (F ). Results suggest a high degree of elasticity of Sn-Sn bonds within the Sn NWs' cores with no significant structural variations for nanowires with different surface passivants. Direct band gaps at Brillouin-zone centers are found for most studied structures with quasiparticle corrected band-gap magnitudes ranging from 0.25 to 3.54 eV in 1.5-nm-diameter structures, indicating an exceptional range of properties for semimetal NWs below the semimetal-to-semiconductor transition. Band-gap variations induced by changes in surface passivants indicate the possibility of realizing semimetal-semiconductor interfaces in NWs with constant cross-section and crystallographic orientation, allowing the design of novel dopant-free NW-based electronic devices.

  16. Feasibility study of electron transfer quantum well infrared photodetectors for spectral tuning in the long-wave infrared band

    Energy Technology Data Exchange (ETDEWEB)

    Jolley, Greg; Dehdashti Akhavan, Nima; Umana-Membreno, Gilberto; Antoszewski, Jarek; Faraone, Lorenzo [School of Electrical, Electronic and Computer Engineering, University of Western Australia, Perth, Western Australia 6009 (Australia)

    2013-11-21

    An electron transfer quantum well infrared photodetector (QWIP) consisting of repeating units of two coupled quantum wells (QWs) is capable of exhibiting a two color voltage dependent spectral response. However, significant electron transfer between the coupled QWs is required for spectral tuning, which may require the application of relatively high electric fields. Also, the band structure of coupled quantum wells is more complicated in comparison to a regular quantum well and, therefore, it is not always obvious if an electron transfer QWIP can be designed such that it meets specific performance characteristics. This paper presents a feasibility study of the electron transfer QWIP and its suitability for spectral tuning. Self consistent calculations have been performed of the bandstructure and the electric field that results from electron population within the quantum wells, from which the optical characteristics have been obtained. The band structure, spectral response, and the resonant final state energy locations have been compared with standard QWIPs. It is shown that spectral tuning in the long-wave infrared band can be achieved over a wide wavelength range of several microns while maintaining a relatively narrow spectral response FWHM. However, the total absorption strength is more limited in comparison to a standard QWIP, since the higher QW doping densities require much higher electric fields for electron transfer.

  17. Electronic Crosstalk in Aqua MODIS Long-Wave Infrared Photovoltaic Bands

    OpenAIRE

    Junqiang Sun; Menghua Wang

    2016-01-01

    Recent investigations have discovered that Terra MODerate-resolution Imaging Spectroradiometer (MODIS) long-wave infrared (LWIR) photovoltaic (PV) bands, bands 27–30, have strong crosstalk among themselves. The linear model developed to test the electronic crosstalk effect was instrumental in the first discovery of the effect in Terra MODIS band 27, and through subsequent investigations the model and the correction algorithm were tested further and established to be correct. It was shown that...

  18. Electronic structure and magnetism of complex materials

    CERN Document Server

    Papaconstantopoulos, D A

    2003-01-01

    Recent developments in electronic structure theory have led to a new understanding of magnetic materials at the microscopic level. This enables a truly first-principles approach to investigations of technologically important magnetic materials. Among these advances have been practical schemes for handling non-collinear magnetic systems, including relativity, understanding of the origins and role of orbital magnetism within band structure formalisms, density functional approaches for magnons and low-lying spin excitations, understanding of the interplay of orbital, spin and lattice orderings in complex oxides, transport theories for layered systems, and the theory of magnetic interactions in doped semiconductors. The book covers these recent developments with review articles by some of the main originators of these advances.

  19. QUANTITATIVE ANALYSIS OF BANDED STRUCTURES IN DUAL-PHASE STEELS

    Directory of Open Access Journals (Sweden)

    Benoit Krebs

    2011-05-01

    Full Text Available Dual-Phase (DP steels are composed of martensite islands dispersed in a ductile ferrite matrix, which provides a good balance between strength and ductility. Current processing conditions (continuous casting followed by hot and cold rolling generate 'banded structures' i.e., irregular, parallel and alternating bands of ferrite and martensite, which are detrimental to mechanical properties and especially for in-use properties. We present an original and simple method to quantify the intensity and wavelength of these bands. This method, based on the analysis of covariance function of binary images, is firstly tested on model images. It is compared with ASTM E-1268 standard and appears to be more robust. Then it is applied on real DP steel microstructures and proves to be sufficiently sensitive to discriminate samples resulting from different thermo-mechanical routes.

  20. Extended Hückel theory for carbon nanotubes: band structure and transport properties.

    Science.gov (United States)

    Zienert, Andreas; Schuster, Jörg; Gessner, Thomas

    2013-05-02

    Extended Hückel theory (EHT) is a well established method for the description of the electronic structure of molecules and solids. In this article, we present a set of extended Hückel parameters for carbon nanotubes (CNTs), obtained by fitting the ab initio band structure of the (6,0) CNT. The new parameters are highly transferable to different types of CNTs. To demonstrate the versatility of the approach, we perform self-consistent EHT-based electron transport calculations for finite length CNTs with metal electrodes.

  1. Electron scattering from the octupole band in /sup 238/U

    Energy Technology Data Exchange (ETDEWEB)

    Hirsch, A.; Creswell, C.; Bertozzi, W.; Heisenberg, J.; Hynes, M.V.; Kowalski, S.; Miska, H.; Norum, B.; Rad, F.N.; Sargent, C.P.; Sasanuma, T.; Turchinetz, W.

    1978-03-06

    A simple model for nuclear surface vibrations in permanently deformed nuclei does well in reproducing electron scattering cross sections of rotational levels built on a K/sup ..pi../= 0/sup -/ intrinsic octupole vibration in /sup 238/U.

  2. Electronic structure of heterocyclic ring chain polymers

    NARCIS (Netherlands)

    Brocks, Geert; Tol, Arie

    1999-01-01

    The band gaps, ionization potentials and electron affinities of conjugated chain polymers comprising heterocyclic aromatic rings are studied systematically as a function of atomic substitutions with N, O and S using first principles density functional calculations.

  3. Structural and electronic properties of carbon nanotubes under hydrostatic pressures

    Institute of Scientific and Technical Information of China (English)

    Zhang Ying; Cao Jue-Xian; Yang Wei

    2008-01-01

    We studied the structural and electronic properties of carbon nanotubes under hydrostatic pressures based on molecular dynamics simulations and first principles band structure calculations.It is found that carbon nanotubes experience a hard-to-soft transition as external pressure increases.The bulk modulus of soft phase is two orders of magnitude smaller than that of hard phase.The band structure calculations show that band gap of (10,0) nanotube increases with the increase of pressure at low pressures. Above a critical pressure (5.70GPa),band gap of (10,0) nanotube drops rapidly and becomes zero at 6.62GPa. Moreover,the calculated charge density shows that a large pressure can induce an sp2-to-sp3 bonding transition,which is confirmed by recent experiments on deformed carbon nanotubes.

  4. Band structure of fcc-C60 solid state crystal study

    Directory of Open Access Journals (Sweden)

    S Javanbakht

    2009-09-01

    Full Text Available We studied the architecture of the C60 cluster to drive its atomic positions which can be seen at room temperature. We then used the obtained carbon positions as a basis set for the fcc structure to construct the fcc-C60 compound. Self consistent calculations were performed based on the density functional theory (DFT utilizing the accurate WIEN2K code to solve the single-particle Kohen-Sham equation within the augmented plane waves plus local orbital (APW+lo method. The cohesive energy has been found to be 1.537 eV for the fcc-C60 . The calculated small cohesive energy that results from the weak Van der Waals-London interactions among a C60 cluster with its nearest neighbors is in good agreement with experiment. The electron densities of states (DOSs were calculated for a C60 macromolecule as well as the fcc-C60 compound and the results were compared with each other. The band gap from DOS calculations has been found to be 0.7 eV. Band structures were also calculated within the generalized gradient approximation (GGA. The band structure calculation results in 1.04 eV for the direct band gap. Two kinds of σ and π bonds were determined in the band structure. Our results are in good agreement with experiment and pseudopotential calculations.

  5. Studies on electronic structure of GaN(0001) surface

    CERN Document Server

    Xie Chang Kun; Xu Fa Qiang; Deng Rui; Liu Feng; Yibulaxin, K

    2002-01-01

    An electronic structure investigation on GaN(0001) is reported. The authors employ a full-potential linearized augmented plane-wave (FPLAPW) approach to calculate the partial density of state, which is in agreement with previous experimental results. The effects of the Ga3d semi-core levels on the electronic structure of GaN are discussed. The valence-electronic structure of the wurtzite GaN(0001) surface is investigated using synchrotron radiation excited angle-resolved photoemission spectroscopy. The bulk bands dispersion along GAMMA A direction in the Brillouin zones is measured using normal-emission spectra by changing photon-energy. The band structure derived from authors' experimental data is compared well with the results of authors' FPLAPW calculation. Furthermore, off-normal emission spectra are also measured along the GAMMA K and GAMMA M directions. Two surface states are identified, and their dispersions are characterized

  6. Correlated Band Structure of a Transition Metal Oxide ZnO Obtained from a Many-Body Wave Function Theory

    Science.gov (United States)

    Ochi, Masayuki; Arita, Ryotaro; Tsuneyuki, Shinji

    2017-01-01

    Obtaining accurate band structures of correlated solids has been one of the most important and challenging problems in first-principles electronic structure calculation. There have been promising recent active developments of wave function theory for condensed matter, but its application to band-structure calculation remains computationally expensive. In this Letter, we report the first application of the biorthogonal transcorrelated (BITC) method: self-consistent, free from adjustable parameters, and systematically improvable many-body wave function theory, to solid-state calculations with d electrons: wurtzite ZnO. We find that the BITC band structure better reproduces the experimental values of the gaps between the bands with different characters than several other conventional methods. This study paves the way for reliable first-principles calculations of the properties of strongly correlated materials.

  7. Band Structure and Quantum Confined Stark Effect in InN/GaN superlattices

    DEFF Research Database (Denmark)

    Gorczyca, I.; Suski, T.; Christensen, Niels Egede;

    2012-01-01

    for the gap error. The calculated band gap shows a strong decrease with the thickness (m) of the InN well. In superlattices containing a single layer of InN (m = 1) the band gap increases weakly with the GaN barrier thickness n, reaching a saturation value around 2 eV. In superlattices with n = m and n > 5......InN/GaN superlattices offer an important way of band gap engineering in the blue-green range of the spectrum. This approach represents a more controlled method than the band gap tuning in quantum well systems by application of InGaN alloys. The electronic structures of short-period wurtzite InN/GaN......(0001) superlattices are investigated, and the variation of the band gap with the thicknesses of the well and the barrier is discussed. Superlattices of the form mInN/nGaN with n ≥ m are simulated using band structure calculations in the Local Density Approximation with a semiempirical correction...

  8. Development of X-band accelerating structures for high gradients

    Institute of Scientific and Technical Information of China (English)

    S. Bini; M. G. Grimaldi; L. Romano; F. Ruffino; R. Parodi; V. Chimenti; A. Marcelli; L. Palumbo; B. Spataro; V. A. Dolgashev; S. Tantawi; A.D. Yeremian; Y. Higashi

    2012-01-01

    Short copper standing wave (SW) structures operating at an X-band frequency have been recently designed and manufactured at the Laboratori Nazionali di Frascati of the Istituto Nazionale di Fisica Nucleare (INFN) using the vacuum brazing technique.High power tests of the structures have been performed at the SLAC National Accelerator Laboratory.In this manuscript we report the results of these tests and the activity in progress to enhance the high gradient performance of the next generation of structures,particularly the technological characterization of high performance coatings obtained via molybdenum sputtering.

  9. Band structure of germanium carbides for direct bandgap silicon photonics

    Science.gov (United States)

    Stephenson, C. A.; O'Brien, W. A.; Penninger, M. W.; Schneider, W. F.; Gillett-Kunnath, M.; Zajicek, J.; Yu, K. M.; Kudrawiec, R.; Stillwell, R. A.; Wistey, M. A.

    2016-08-01

    Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge1-xCx (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge0.998C0.002 shows a bandgap reduction supporting these results. Growth of Ge0.998C0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III-V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.

  10. Electronic and thermoelectric properties of Mexican hat bands in van-der-Waals materials

    Science.gov (United States)

    Wickramaratne, Darshana; Zahid, Ferdows; Lake, Roger

    2015-03-01

    Mexican hat dispersions are relatively common in few-layer two-dimensional materials. In one to four monolayers of the group-III chalcogenides (GaS, GaSe, InS, InSe) and Bi2Se3 the valence band undergoes a band inversion from a parabolic to an inverted Mexican hat dispersion as the film thickness is reduced from bulk to a single monolayer. The band inversion is robust against changes in stacking order, omission or inclusion of spin-orbit coupling and the choice of functional. The Mexican hat dispersion results in a 1/√{ E} singularity in the two-dimensional density of states and a step-function turn on in the density of modes. The largest radius of the ring of states occurs for a single monolayer of each material. The dispersion with the largest radius coincides with the maximum power factor and ZT for a material at room temperature. Ab-initio electronic structure calculations are used with a Landauer approach to calculate the thermoelectric transport coefficients. Analytical models of the Mexican hat and the parabolic dispersions are used for comparison and analysis. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring this effect since the radius of the Mexican hat band edge increases linearly with vertical electric field. Support by the NSF and SRC-NRI Project 2204.001 (NSF-ECCS-1124733), FAME, one of six centers of STARnet, a SRC program sponsored by MARCO and DARPA and the use of XSEDE NSF Grant # OCI-1053575.

  11. Band Gap Opening Induced by the Structural Periodicity in Epitaxial Graphene Buffer Layer.

    Science.gov (United States)

    N Nair, Maya; Palacio, Irene; Celis, Arlensiú; Zobelli, Alberto; Gloter, Alexandre; Kubsky, Stefan; Turmaud, Jean-Philippe; Conrad, Matthew; Berger, Claire; de Heer, Walter; Conrad, Edward H; Taleb-Ibrahimi, Amina; Tejeda, Antonio

    2017-04-12

    The epitaxial graphene buffer layer on the Si face of hexagonal SiC shows a promising band gap, of which the precise origin remains to be understood. In this work, we correlate the electronic to the atomic structure of the buffer layer by combining angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and high-resolution scanning transmission electron microscopy (HR-STEM). We show that the band structure in the buffer has an electronic periodicity related to the structural periodicity observed in STM images and published X-ray diffraction. Our HR-STEM measurements show the bonding of the buffer layer to the SiC at specific locations separated by 1.5 nm. This is consistent with the quasi 6 × 6 periodic corrugation observed in the STM images. The distance between buffer C and SiC is 1.9 Å in the bonded regions and up to 2.8 Å in the decoupled regions, corresponding to a 0.9 Å corrugation of the buffer layer. The decoupled regions are sp(2) hybridized. Density functional tight binding (DFTB) calculations demonstrate the presence of a gap at the Dirac point everywhere in the buffer layer, even in the decoupled regions where the buffer layer has an atomic structure close to that of graphene. The surface periodicity also promotes band in the superperiodic Brillouin zone edges as seen by photoemission and confirmed by our calculations.

  12. Electronic structure investigation of novel superconductors

    Energy Technology Data Exchange (ETDEWEB)

    Buling, Anna

    2014-05-15

    The discovery of superconductivity in iron-based pnictides in 2008 gave rise to a high advance in the research of high-temperature superconductors. But up to now there is no generally admitted theory of the non-BCS mechanism of these superconductors. The electron and hole doped Ba122 (BaFe{sub 2}As{sub 2}) compounds investigated in this thesis are supposed to be suitable model systems for studying the electronic behavior in order to shed light on the superconducting mechanisms. The 3d-transition metal doped Ba122 compounds are investigated using the X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and X-ray magnetic circular dichroism (XMCD), while the completely hole doped K122 is observed using XPS. The experimental measurements are complemented by theoretical calculations. A further new class of superconductors is represented by the electride 12CaO*7Al{sub 2}O{sub 3}: Here superconductivity can be realized by electrons accommodated in the crystallographic sub-nanometer-sized cavities, while the mother compound is a wide band gap insulator. Electronic structure investigations, represented by XPS, XAS and resonant X-ray photoelectron spectroscopy (ResPES), carried out in this work, should help to illuminate this unconventional superconductivity and resolve a debate of competing models for explaining the existence of superconductivity in this compound.

  13. Photoelectron spectroscopy bulk and surface electronic structures

    CERN Document Server

    Suga, Shigemasa

    2014-01-01

    Photoelectron spectroscopy is now becoming more and more required to investigate electronic structures of various solid materials in the bulk, on surfaces as well as at buried interfaces. The energy resolution was much improved in the last decade down to 1 meV in the low photon energy region. Now this technique is available from a few eV up to 10 keV by use of lasers, electron cyclotron resonance lamps in addition to synchrotron radiation and X-ray tubes. High resolution angle resolved photoelectron spectroscopy (ARPES) is now widely applied to band mapping of materials. It attracts a wide attention from both fundamental science and material engineering. Studies of the dynamics of excited states are feasible by time of flight spectroscopy with fully utilizing the pulse structures of synchrotron radiation as well as lasers including the free electron lasers (FEL). Spin resolved studies also made dramatic progress by using higher efficiency spin detectors and two dimensional spin detectors. Polarization depend...

  14. Engineering Design of a Multipurpose X-band Accelerating Structure

    CERN Document Server

    Gudkov, Dmitry; Samoshkin, Alexander; Zennaro, Riccardo; Dehler, Micha; Raguin, Jean-Yves

    2010-01-01

    Both FEL projects, SwissFEL and Fermi-Elettra each require an X-band RF accelerating structure for optimal bunch compression at the respective injectors. As the CLIC project is pursuing a program for producing and testing the X-band high-gradient RF structures, a collaboration between PSI, Elettra and CERN has been established to build a multipurpose X-band accelerating structure. This paper focuses on its engineering design, which is based on the disked cells jointed together by diffusion bonding. Vacuum brazing and laser beam welding is used for auxiliary components. The accelerating structure consists of two coupler subassemblies, 73 disks and includes a wakefield monitor and diagnostic waveguides. The engineering study includes the external cooling system, consisting of two parallel cooling circuits and an RF tuning system, which allows phase advance tuning of the cell by deforming the outer wall. The engineering solution for the installation and sealing of the wake field monitor feed-through devices that...

  15. Crystal Structure and Band Gap Engineering in Polyoxometalate-Based Inorganic-Organic Hybrids.

    Science.gov (United States)

    Roy, Soumyabrata; Sarkar, Sumanta; Pan, Jaysree; Waghmare, Umesh V; Dhanya, R; Narayana, Chandrabhas; Peter, Sebastian C

    2016-04-04

    We have demonstrated engineering of the electronic band gap of the hybrid materials based on POMs (polyoxometalates), by controlling its structural complexity through variation in the conditions of synthesis. The pH- and temperature-dependent studies give a clear insight into how these experimental factors affect the overall hybrid structure and its properties. Our structural manipulations have been successful in effectively tuning the optical band gap and electronic band structure of this kind of hybrids, which can find many applications in the field of photovoltaic and semiconducting devices. We have also addressed a common crystallographic disorder observed in Keggin-ion (one type of heteropolyoxometalate [POMs])-based hybrid materials. Through a combination of crystallographic, spectroscopic, and theoretical analysis of four new POM-based hybrids synthesized with tactically varied reaction conditions, we trace the origin and nature of the disorder associated with it and the subtle local structural coordination involved in its core picture. While the crystallography yields a centrosymmetric structure with planar coordination of Si, our analysis with XPS, IR, and Raman spectroscopy reveals a tetrahedral coordination with broken inversion symmetry, corroborated by first-principles calculations.

  16. Transient mid-IR study of electron dynamics in TiO2 conduction band.

    Science.gov (United States)

    Sá, Jacinto; Friedli, Peter; Geiger, Richard; Lerch, Philippe; Rittmann-Frank, Mercedes H; Milne, Christopher J; Szlachetko, Jakub; Santomauro, Fabio G; van Bokhoven, Jeroen A; Chergui, Majed; Rossi, Michel J; Sigg, Hans

    2013-04-07

    The dynamics of TiO2 conduction band electrons were followed with a novel broadband synchrotron-based transient mid-IR spectroscopy setup. The lifetime of conduction band electrons was found to be dependent on the injection method used. Direct band gap excitation results in a lifetime of 2.5 ns, whereas indirect excitation at 532 nm via Ru-N719 dye followed by injection from the dye into TiO2 results in a lifetime of 5.9 ns.

  17. Parameterization and algebraic structure of 3-band orthogonal wavelet systems

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    In this paper, a complete parameterization for the 3-band compact wavelet systems is presented. Using the parametric result, a program of the filterbank design is completed, which can give not only the filterbanks but also the graphs of all possible scaling functions and their corresponding wavelets. Especially some symmetric wavelets with small supports are given. Finally an algebraic structure for this kind of wavelet systems is characterized.

  18. Photoconductivities from band states and a dissipative electron dynamics: Si(111) without and with adsorbed Ag clusters

    Energy Technology Data Exchange (ETDEWEB)

    Vazhappilly, Tijo [Departments of Chemistry and of Physics, Quantum Theory Project, University of Florida, Gainesville, Florida 32611 (United States); Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400 085 (India); Hembree, Robert H.; Micha, David A., E-mail: micha@qtp.ufl.edu [Departments of Chemistry and of Physics, Quantum Theory Project, University of Florida, Gainesville, Florida 32611 (United States)

    2016-01-14

    A new general computational procedure is presented to obtain photoconductivities starting from atomic structures, combining ab initio electronic energy band states with populations from density matrix theory, and implemented for a specific set of materials based on Si crystalline slabs and their nanostructured surfaces without and with adsorbed Ag clusters. The procedure accounts for charge mobility in semiconductors in photoexcited states, and specifically electron and hole photomobilities at Si(111) surfaces with and without adsorbed Ag clusters using ab initio energy bands and orbitals generated from a generalized gradient functional, however with excited energy levels modified to provide correct bandgaps. Photoexcited state populations for each band and carrier type were generated using steady state solution of a reduced density matrix which includes dissipative medium effects. The present calculations provide photoexcited electronic populations and photoinduced mobilities resulting from applied electric fields and obtained from the change of driven electron energies with their electronic momentum. Extensive results for Si slabs with 8 layers, without and with adsorbed Ag clusters, show that the metal adsorbates lead to substantial increases in the photomobility and photoconductivity of electrons and holes.

  19. k - dependent Jeff=1/2 band splitting and the electron-hole asymmetry in SrIrO3

    Science.gov (United States)

    Singh, Vijeta; Pulikkotil, J. J.

    2017-02-01

    The Ir ion in Srn+1 IrnO 3 n + 1 series of compounds is octahedrally coordinated. However, unlike Sr2IrO4 (n=1) and Sr3Ir2O7 (n=2) which are insulating due to spin-orbit induced Jeff splitting of the t2g bands, SrIrO3 (n= ∞) is conducting. To explore whether such a splitting is relevant in SrIrO3, and if so to what extent, we investigate the electronic structure of orthorhombic SrIrO3 using density functional theory. Calculations reveal that the crystal field split Ir t2 g bands in SrIrO3 are indeed split into Jeff=3/2 and and Jeff=1/2 states. However, the splitting is found to be strongly k - dependent with its magnitude determined by the Ir - O orbital hybridization. Besides, we find that the spin-orbit induced pseudo-gap, into which the Fermi energy is positioned, is composed of both light electron-like and heavy hole-like bands. These features in the band structure of SrIrO3 suggest that variations in the carrier concentration control the electronic transport properties in SrIrO3, which is consistent with the experiments.

  20. Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

    Indian Academy of Sciences (India)

    Navinder Singh

    2005-01-01

    A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)–phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron–hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron–phonon coupling, where the usual two-temperature model [1–4] may not be appropriate.

  1. Electron-Hole Counting Approach to Surface Atomic Structure

    Science.gov (United States)

    Chadi, D. J.

    The observed reconstructions of III-V semiconductor surfaces are shown to be consistent with constraints imposed by a simple "electron-hole" counting rule proposed by Pashley. The rule ensures that the predicted surfaces are nonmetallic, nonpolar, and at least, metastable since the compensation of the "donor" electrons leaves no occupied states in the upper part of the band gap which can easily induce other reconstructions. Applications of the method to the problem of surface structure and passivation are examined.

  2. Structural Dynamics of Electronic Systems

    Science.gov (United States)

    Suhir, E.

    2013-03-01

    The published work on analytical ("mathematical") and computer-aided, primarily finite-element-analysis (FEA) based, predictive modeling of the dynamic response of electronic systems to shocks and vibrations is reviewed. While understanding the physics of and the ability to predict the response of an electronic structure to dynamic loading has been always of significant importance in military, avionic, aeronautic, automotive and maritime electronics, during the last decade this problem has become especially important also in commercial, and, particularly, in portable electronics in connection with accelerated testing of various surface mount technology (SMT) systems on the board level. The emphasis of the review is on the nonlinear shock-excited vibrations of flexible printed circuit boards (PCBs) experiencing shock loading applied to their support contours during drop tests. At the end of the review we provide, as a suitable and useful illustration, the exact solution to a highly nonlinear problem of the dynamic response of a "flexible-and-heavy" PCB to an impact load applied to its support contour during drop testing.

  3. CdS and Cd-Free Buffer Layers on Solution Phase Grown Cu2ZnSn(SxSe1- x)4 :Band Alignments and Electronic Structure Determined with Femtosecond Ultraviolet Photoemission Spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Haight, Richard; Barkhouse, Aaron; Wang, Wei; Yu, Luo; Shao, Xiaoyan; Mitzi, David; Hiroi, Homare; Sugimoto, Hiroki

    2013-12-02

    The heterojunctions formed between solution phase grown Cu2ZnSn(SxSe1- x)4(CZTS,Se) and a number of important buffer materials including CdS, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission spectroscopy (fs-UPS) and photovoltage spectroscopy. With this approach we extract the magnitude and direction of the CZTS,Se band bending, locate the Fermi level within the band gaps of absorber and buffer and measure the absorber/buffer band offsets under flatband conditions. We will also discuss two-color pump/probe experiments in which the band bending in the buffer layer can be independently determined. Finally, studies of the bare CZTS,Se surface will be discussed including our observation of mid-gap Fermi level pinning and its relation to Voc limitations and bulk defects.

  4. A Chemical Understanding of the Band Convergence in Thermoelectric CoSb 3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

    Energy Technology Data Exchange (ETDEWEB)

    Hanus, Riley; Guo, Xingyu; Tang, Yinglu; Li, Guodong; Snyder, G. Jeffrey; Zeier, Wolfgang G.

    2017-01-13

    N-Type skutterudites, such as YbxCo4Sb12, have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conduction band closer to the primary conduction band and thus inducing band convergence.

  5. Electronic structure of semiconductor-metal-semiconductor heterostructures

    Science.gov (United States)

    Masri, Pierre

    For the first time, we present in this article a microscopic self-consistent theory of the electronic structure of semiconductor-metal-semiconductor (SMS) heterostructures. This is done within the framework of a tight-binding approximation. We use a one-band model and a simplified two-band model to describe metal and semiconductor bulk bands, respectively. Results are given for a material-symmetrical and interface-assymetrical SMS structure: this involves the same semiconductors, but different interface polarities (anion- and cation-like interfaces). These results include metal-like states (built-in metal band) and metal-induced semiconductor-like states. The relevance of the charge neutrality condition to this feature and to the determination of the position of the SMS Fermi level is discussed. We also emphasize the confining role of interfaces, with respect to semiconductor-like states, within the semiconductor gap.

  6. Hyperfine interactions and electronic band structure in Tb{sub 0.27}Dy{sub 0.73}(Fe{sub 1-x}Co{sub x}){sub 2} compounds

    Energy Technology Data Exchange (ETDEWEB)

    Bodnar, W.; Szklarska - Lukasik, M. [Faculty of Physics and Applied Computer Science, AGH, Al. Mickiewicza 30, 30-059 Krakow (Poland); Stoch, P. [Institute of Atomic Energy, 05-400 Swierk-Otwock (Poland); Faculty of Material Science and Ceramics, AGH, Al. Mickiewicza 30, 30-059 Krakow (Poland); Zachariasz, P. [Institute of Atomic Energy, 05-400 Swierk-Otwock (Poland); Pszczola, J., E-mail: pszczola@agh.edu.p [Faculty of Physics and Applied Computer Science, AGH, Al. Mickiewicza 30, 30-059 Krakow (Poland); Suwalski, J. [Institute of Atomic Energy, 05-400 Swierk-Otwock (Poland)

    2010-04-30

    The after-effects of Fe/Co substitution in the intermetallic series Tb{sub 0.27}Dy{sub 0.73}(Fe{sub 1-x}Co{sub x}){sub 2}, with a starting compound Tb{sub 0.27}Dy{sub 0.73}Fe{sub 2} known as Terfenol-D, were studied. Co substitution introduces a local area, at sub-nanoscale, with random Fe/Co neighbourhoods of the {sup 57}Fe atoms. {sup 57}Fe Moessbauer effect measurements for the intermetallic system Tb{sub 0.27}Dy{sub 0.73}(Fe{sub 1-x}Co{sub x}){sub 2} carried out at 4.2 K evidence an [100] easy axis of magnetization. Hyperfine interaction parameters: isomer shift, a magnetic hyperfine field and a quadrupole interaction parameter were obtained from the fitting procedure of the spectra, both for the local area and for the sample as bulk. As a result of Fe/Co substitution, a Slater-Pauling type curve for the average magnetic hyperfine field vs. Co content is observed. It is found that the magnetic hyperfine fields corresponding to the local area sorted out against Co contribution in the Fe/Co neighbourhoods also create a dependence similar to a Slater-Pauling type curve. Band structure calculations using the Full-Potential Linearized Augmented Plane Waves (FLAPW) method were performed. The experimentally determined magnetic hyperfine field correlates linearly with the weighted magnetic moment calculated per transition metal atom.

  7. Electronic structure of EuFe2As2.

    Science.gov (United States)

    Adhikary, Ganesh; Sahadev, Nishaina; Biswas, Deepnarayan; Bindu, R; Kumar, Neeraj; Thamizhavel, A; Dhar, S K; Maiti, Kalobaran

    2013-06-01

    Employing high resolution photoemission spectroscopy, we studied the temperature evolution of the electronic structure of EuFe2As2, a unique pnictide, where antiferromagnetism of the Eu layer survives within the superconducting phase due to 'FeAs' layers, achieved via substitution and/or pressure. High energy and angle resolution helped to reveal the signature of peak-dip features, having significant p orbital character and spin density wave transition induced band folding in the electronic structure. A significant spectral weight redistribution is observed below 20 K manifesting the influence of antiferromagnetic order on the conduction electrons.

  8. Structural and electronic properties of arsenic nitrogen monolayer

    Science.gov (United States)

    Liu, Pei; Nie, Yao-zhuang; Xia, Qing-lin; Guo, Guang-hua

    2017-03-01

    We present our first-principles calculations of a new two-dimensional material, arsenic nitrogen monolayer. The structural, electronic, and mechanical properties are investigated in detail by means of density functional theory computations. The calculated binding energy and the phonon spectra demonstrate that the AsN can form stable monolayer in puckered honeycomb structure. It is a semiconductor with indirect band gap of 0.73 eV, and displays highly anisotropic mechanical properties. Strain has obvious influence on the electronic properties of AsN monolayer. It is found that in the armchair direction, a moderate compression strain (-12%) can trigger an indirect to direct band gap transition and a tensile strain of 18% can make the AsN becoming a stable metal. In the zigzag direction, a rather smaller strain than armchair direction (12% for compression and 8% for stretch) can induce the indirect band gap to metal transition.

  9. Determining binding energies of valence-band electrons in insulators and semiconductors via lanthanide spectroscopy

    NARCIS (Netherlands)

    Dorenbos, P.

    2013-01-01

    Models and methods to determine the absolute binding energy of 4f-shell electrons in lanthanide dopants will be combined with data on the energy of electron transfer from the valence band to a lanthanide dopant. This work will show that it provides a powerful tool to determine the absolute binding e

  10. Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

    Science.gov (United States)

    Zhan, W.; Granerød, C. S.; Venkatachalapathy, V.; Johansen, K. M. H.; Jensen, I. J. T.; Kuznetsov, A. Yu; Prytz, Ø.

    2017-03-01

    Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

  11. Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure.

    Science.gov (United States)

    Simakov, Evgenya I; Arsenyev, Sergey A; Buechler, Cynthia E; Edwards, Randall L; Romero, William P; Conde, Manoel; Ha, Gwanghui; Power, John G; Wisniewski, Eric E; Jing, Chunguang

    2016-02-12

    We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.

  12. Electronic structure of α-oligothiophenes with various substituents

    Science.gov (United States)

    Vikramaditya, Talapunur; Saisudhakar, Mukka; Sumithra, Kanakamma

    2015-02-01

    Density functional theory is employed to investigate the effect of various substituents on the electronic structure of α-oligothiophenes. The effect of electron donating and withdrawing groups of oligothiophenes in the regio regular HT-HT form is studied. Depending on the type of substituent and the substitution pattern, large differences in the delocalization pattern are observed between the substituted and unsubstituted oligomers. It is found that the band gaps critically depend on the chemical structure and regioselectivity of the building blocks. For the 3-substitued systems, electron donating and electron withdrawing substituents are shown to decrease and increase band gaps respectively compared to unsubstituted systems. There are charge separation effects introduced as a result of lack of symmetry in some of the substituted oligothiophenes. A new strategy is explained to achieve low band gap materials by making use of the regioselective form with lesser symmetry. Push-pull substitution with an electron donor at one end of the conjugation and an acceptor the other end is also investigated. Comparisons of band gaps of the substituted oligothiophenes with the corresponding polymeric systems are also done.

  13. Electron confinement in thin metal films. Structure, morphology and interactions

    Energy Technology Data Exchange (ETDEWEB)

    Dil, J.H.

    2006-05-15

    This thesis investigates the interplay between reduced dimensionality, electronic structure, and interface effects in ultrathin metal layers (Pb, In, Al) on a variety of substrates (Si, Cu, graphite). These layers can be grown with such a perfection that electron confinement in the direction normal to the film leads to the occurrence of quantum well states in their valence bands. These quantum well states are studied in detail, and their behaviour with film thickness, on different substrates, and other parameters of growth are used here to characterise a variety of physical properties of such nanoscale systems. The sections of the thesis deal with a determination of quantum well state energies for a large data set on different systems, the interplay between film morphology and electronic structure, and the influence of substrate electronic structure on their band shape; finally, new ground is broken by demonstrating electron localization and correlation effects, and the possibility to measure the influence of electron-phonon coupling in bulk bands. (orig.)

  14. Familial band--shaped keratopathy and spheroidal degeneration. Clinical and electron microscopic study.

    Science.gov (United States)

    Kloucek, F

    1977-12-31

    Presumptive primary band-shaped keratopathy was described in a 35-year-old man and his 62-year-old paternal uncle. Lamellar keratoplasty was performed in one eye in each of these patients. The light and electron microscopic studies were carried out on both corneal specimens. Histologic stains for calcium were negative. Noncalcific band-shaped keratopathy was confirmed by electron microscopic findings too. Electron-dense globular deposits were found in the region of Bowman's membrane and superficial stroma. These may be characterized as a secondary form of spheroidal degeneration.

  15. Temperature dependence of band gaps in semiconductors: electron-phonon interaction

    Energy Technology Data Exchange (ETDEWEB)

    Kremer, Reinhard K.; Cardona, M.; Lauck, R. [MPI for Solid State Research, Stuttgart (Germany); Bhosale, J.; Ramdas, A.K. [Physics Dept., Purdue University, West Lafayette, IN (United States); Burger, A. [Fisk University, Dept. of Life and Physical Sciences, Nashville, TN (United States); Munoz, A. [MALTA Consolider Team, Dept. de Fisica Fundamental II, Universidad de La Laguna, Tenerife (Spain); Instituto de Materiales y Nanotecnologia, Universidad de La Laguna, Tenerife (Spain); Romero, A.H. [CINVESTAV, Dept. de Materiales, Unidad Queretaro, Mexico (Mexico); MPI fuer Mikrostrukturphysik, Halle an der Saale (Germany)

    2013-07-01

    We investigate the temperature dependence of the energy gap of several semiconductors with chalcopyrite structure and re-examine literature data and analyze own high-resolution reflectivity spectra in view of our new ab initio calculations of their phonon properties. This analysis leads us to distinguish between materials with d-electrons in the valence band (e.g. CuGaS{sub 2}, AgGaS{sub 2}) and those without d-electrons (e.g. ZnSnAs{sub 2}). The former exhibit a rather peculiar non-monotonic temperature dependence of the energy gap which, so far, has resisted cogent theoretical description. We demonstrate it can well be fitted by including two Bose-Einstein oscillators with weights of opposite sign leading to an increase at low-T and a decrease at higher T's. We find that the energy of the former correlates well with characteristic peaks in the phonon density of states associated with low-energy vibrations of the d-electron constituents.

  16. Band gaps, ionization potentials, and electron affinities of periodic electron systems via the adiabatic-connection fluctuation-dissipation theorem

    Science.gov (United States)

    Trushin, Egor; Betzinger, Markus; Blügel, Stefan; Görling, Andreas

    2016-08-01

    An approach to calculate fundamental band gaps, ionization energies, and electron affinities of periodic electron systems is explored. Starting from total energies obtained with the help of the adiabatic-connection fluctuation-dissipation (ACFD) theorem, these physical observables are calculated according to their basic definition by differences of the total energies of the N -, (N -1 ) -, and (N +1 ) -electron system. The response functions entering the ACFD theorem are approximated here by the direct random phase approximation (dRPA). For a set of prototypical semiconductors and insulators it is shown that even with this quite drastic approximation the resulting band gaps are very close to experiment and of a similar quality to those from the computationally more involved G W approximation. By going beyond the dRPA in the future the accuracy of the calculated band gaps may be significantly improved further.

  17. Determination of the band structure of LuNi{sub 2}B{sub 2}C

    Energy Technology Data Exchange (ETDEWEB)

    Bergk, B. [Hochfeld-Magnetlabor, Forschungszentrum Rossendorf, Dresden (Germany); Inst. fuer Festkoerperphysik, Technische Univ. Dresden (Germany); Bartkowiak, M.; Ignatchik, O. [Hochfeld-Magnetlabor, Forschungszentrum Rossendorf, Dresden (Germany); Jaeckel, M. [Inst. fuer Festkoerperphysik, Technische Univ. Dresden (Germany); Wosnitza, J.; Rosner, H.; Petzold, V. [MPI fuer chemische Physik fester Stoffe, Dresden (Germany); Canfield, P. [Iowa State Univ. of Science and Technology, Ames (United States). Ames Lab., Condensed Matter Physics

    2007-07-01

    We present de Haas-van Alphen (dHvA) investigations on the nonmagnetic borocarbide superconductor LuNi{sub 2}B{sub 2}C which have been performed by use of the torque method in high magnetic fields up to 32 T and at low temperatures down to 50 mK. The complex band structure is extracted from the quantum oscillations in the normal state. In comparison with full-potential-local-orbital calculations of the band structure we are able to assign the observed dHvA frequencies to the different bands. Temperature dependent dHvA investigations allowed the extraction of the effective band masses for the several Fermi-surface sheets. We observe an enhancement of the effective masses compared to the theoretical calculations which is due to electron-phonon interaction. Finally, we are able to examine the angular dependence of the electron-phonon coupling for the different Fermi-surface sheets. (orig.)

  18. Strain effect on graphene nanoribbon carrier statistic in the presence of non-parabolic band structure

    Science.gov (United States)

    Izuani Che Rosid, N. A.; Ahmadi, M. T.; Ismail, Razali

    2016-09-01

    The effect of tensile uniaxial strain on the non-parabolic electronic band structure of armchair graphene nanoribbon (AGNR) is investigated. In addition, the density of states and the carrier statistic based on the tight-binding Hamiltonian are modeled analytically. It is found that the property of AGNR in the non-parabolic band region is varied by the strain. The tunable energy band gap in AGNR upon strain at the minimum energy is described for each of n-AGNR families in the non-parabolic approximation. The behavior of AGNR in the presence of strain is attributed to the breakable AGNR electronic band structure, which varies the physical properties from its normality. The linear relation between the energy gap and the electrical properties is featured to further explain the characteristic of the deformed AGNR upon strain. Project supported by the Ministry of Higher Education (MOHE), Malaysia under the Fundamental Research Grant Scheme (FRGS) (Grant No.Q.J130000.7823.4F477). We also thank the Research Management Center (RMC) of Universiti Teknologi Malaysia (UTM) for providing an excellent research environment.

  19. Portable X-Band Linear Electron Accelerators for Radiographic Applications

    CERN Document Server

    Saverskiy, Aleksandr J; Hernandez, Michael; Mishin, Andrey V; Skowbo, Dave

    2005-01-01

    The MINAC series portable linear electron accelerator systems designed and manufactured at American Science and Engineering, Inc. High Energy Systems Division (AS&E HESD) are discussed in this paper. Each system can be configured as either an X-ray or electron beam source. The powerful 4 MeV and 6 MeV linacs powered by a 1,5 MW magnetron permit operation in a dose rate range from 100 R/min at 80 cm to 600 R/min at 80 cm. Each MINAC is a self-contained source with radiation leakage outside of the X-ray head less than 0,1% of the maximum dose. Along with these systems a 1 MeV ultra compact MINAC has been successfully tested. The unit is available with radiation leakage less then 0.01% and permits producing X-ray beam in an energy range (1…2) MeV at a high output dose rate. Design and experimental parameters are presented. The common and system specific features are also discussed.

  20. Molecular electronic-structure theory

    CERN Document Server

    Helgaker, Trygve; Olsen, Jeppe

    2014-01-01

    Ab initio quantum chemistry has emerged as an important tool in chemical research and is appliced to a wide variety of problems in chemistry and molecular physics. Recent developments of computational methods have enabled previously intractable chemical problems to be solved using rigorous quantum-mechanical methods. This is the first comprehensive, up-to-date and technical work to cover all the important aspects of modern molecular electronic-structure theory. Topics covered in the book include: * Second quantization with spin adaptation * Gaussian basis sets and molecular-integral evaluati

  1. First-principles studies of the electric-field effect on the band structure of trilayer graphenes

    Science.gov (United States)

    Wang, Yun-Peng; Li, Xiang-Guo; Cheng, Hai-Ping

    Electric-field effects on the electronic structure of trilayer graphene are investigated using the density functional theory in the generalized gradient approximation. Two different stacking orders, namely Bernal and rhombohedral, of trilayer graphene are considered. Our calculations reproduce the experimentally data on band gap opening in Bernal stacking and band overlap in rhombohedral trilayer graphene. In addition, we studied effects of charge doping using dual gate configurations. The size of band gap opening in Bernal trilayer graphene can be tuned by charge doping, and charge doping also causes an electron-hole asymmetry in the density of states. Furthermore, hole-doping can reopen a band gap in the band overlapping region of rhombohedral trilayer grapheme induced by electric fields, which contributes to an extra peak in the optical conductivity spectra. This work is supported by DOE # DE-FG02-02ER45995.

  2. Electronic structure of spin systems

    Energy Technology Data Exchange (ETDEWEB)

    Saha-Dasgupta, Tanusri

    2016-04-15

    Highlights: • We review the theoretical modeling of quantum spin systems. • We apply the Nth order muffin-tin orbital electronic structure method. • The method shows the importance of chemistry in the modeling. • CuTe{sub 2}O{sub 5} showed a 2-dimensional coupled spin dimer behavior. • Ti substituted Zn{sub 2}VO(PO{sub 4}){sub 2} showed spin gap behavior. - Abstract: Low-dimensional quantum spin systems, characterized by their unconventional magnetic properties, have attracted much attention. Synthesis of materials appropriate to various classes within these systems has made this field very attractive and a site of many activities. The experimental results like susceptibility data are fitted with the theoretical model to derive the underlying spin Hamiltonian. However, often such a fitting procedure which requires correct guess of the assumed spin Hamiltonian leads to ambiguity in deciding the representative model. In this review article, we will describe how electronic structure calculation within the framework of Nth order muffin-tin orbital (NMTO) based Wannier function technique can be utilized to identify the underlying spin model for a large number of such compounds. We will show examples from compounds belonging to vanadates and cuprates.

  3. Band Structure and Optical Properties of Kesterite Type Compounds: first principle calculations

    Science.gov (United States)

    Palaz, S.; Unver, H.; Ugur, G.; Mamedov, A. M.; Ozbay, E.

    2017-02-01

    In present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated.

  4. The dependence of the tunneling characteristic on the electronic energy bands and the carrier’s states of Graphene superlattice

    Science.gov (United States)

    Yang, C. H.; Shen, G. Z.; Ao, Z. M.; Xu, Y. W.

    2016-09-01

    Using the transfer matrix method, the carrier tunneling properties in graphene superlattice generated by the Thue-Morse sequence and Kolakoski sequence are investigated. The positions and strength of the transmission can be modulated by the barrier structures, the incident energy and angle, the height and width of the potential. These carriers tunneling characteristic can be understood from the energy band structures in the corresponding superlattice systems and the carrier’s states in well/barriers. The transmission peaks above the critical incident angle rely on the carrier’s resonance in the well regions. The structural diversity can modulate the electronic and transport properties, thus expanding its applications.

  5. Fermi surface and band structure of BiPd from ARPES studies

    Science.gov (United States)

    Lohani, H.; Mishra, P.; Gupta, Anurag; Awana, V. P. S.; Sekhar, B. R.

    2017-03-01

    We present a detailed electronic structure study of the non-centrosymmetric superconductor BiPd based on our angle resolved photoemission spectroscopy (ARPES) measurements and Density Functional Theory (DFT) based calculations. We observe a high intensity distribution on the Fermi surface (FS) of this compound resulting from various electron and hole like bands which are present in the vicinity of the Fermi energy (Ef). The near Ef states are primarily composed of Bi-6p with a little admixture of Pd-4dx2-y2/zy orbitals. There are various spin-orbit split bands involved in the crossing of Ef making a complex FS. The FS mainly consists of multi sheets of three dimensions which disfavor the nesting between different sheets of the FS. Our comprehensive study elucidates that BiPd could be a s-wave multiband superconductor.

  6. Manipulation of electronic structure in WSe2 monolayer by strain

    Science.gov (United States)

    Yang, Cong-xia; Zhao, Xu; Wei, Shu-yi

    2016-11-01

    In this paper, we study the electronic properties of WSe2 monolayer with biaxial tensile strain and compressive strain by using first principles based on the density function theory. Under the biaxial tensile strain, WSe2 monolayer retains direct band gap with increasing strain and the band gap of WSe2 continuously decreases with increasing strain, eventually turn to metal when strain is equal to or more than 13%. Under the biaxial compressive strain, WSe2 monolayer turns to indirect gap and the band gap continuously decreases with increasing strain, finally turn to metal when strain is up to -7%. The strain can reduce the band gap of the WSe2 monolayer regardless of the strain direction. By comparison, we can see that the tensile strain appears to be more effective in reducing the band gap of pristine WSe2 monolayer than the compressive strain from -5% to 5%. But the band gap turns to zero quickly from -6% to -7% under compressive strain, however for tensile strain from 5% to 13%, the band gap decreases slowly. Based on the further analysis of the projected charge density for WSe2 monolayer, the fundamental reason of the change of band structure under biaxial tensile strain is revealed.

  7. Excitation of the Werner bands of H2 by electron impact.

    Science.gov (United States)

    Stone, E. J.; Zipf, E. C.

    1972-01-01

    Absolute cross sections for the excitation of the Werner band system of molecular hydrogen have been measured from energy threshold to 300 eV for electron impact on molecular hydrogen. The bands were observed in emission in the wavelength region of 1100 to 1250 A. From a comparison of the measured cross sections with previously calculated transition probabilities, it is concluded that the Werner bands are suitable as the basis for relative spectral response calibration only when the bands are observed under sufficiently high resolution. The effect of the perturbation interaction between the B and C states of the hydrogen molecule was observed in the rotational intensity distribution of the Werner (3,7) and (3,6) bands.

  8. Wakefield Monitor Experiments with X-Band Accelerating Structures

    CERN Document Server

    Lillestøl, Reidar; Corsini, Roberto; Döbert, Steffen; Farabolini, Wilfrid; Malina, Lukas; Pfingstner, Juergen; Wuensch, Walter

    2015-01-01

    The accelerating structures for CLIC must be aligned with a precision of a few um with respect to the beam trajectory in order to mitigate emittance growth due to transverse wake fields. We report on first results from wake field monitor tests in an X-band structure, with a probe beam at the CLIC Test Facility. The monitors are currently installed in the CLIC Two-Beam Module. In order to fully demonstrate the feasibility of using wakefield monitors for CLIC, the precision of the monitors must be verified using a probe beam while simultaneously filling the structure with high power rf used to drive the accelerating mode. We outline plans to perform such a demonstration in the CLIC Test Facility.

  9. Collective Band Structures in Neutron-Rich 108Mo Nucleus

    Institute of Scientific and Technical Information of China (English)

    DING Huai-Bo; WANG Jian-Guo; XU Qiang; ZHU Sheng-Jiang; J. H. Hamilton; A. V. Ramayya; J. K. Hwang; Y. X. Luo; J. O. Rasmussen; I. Y. Lee; CHE Xing-Lai

    2007-01-01

    High spin states in the neutron-rich 108Mo nucleus are studied by measuring prompt γ-rays following the spontaneous fission of 252Cf with a Gammasphere detector array. The ground-state band is confirmed, and the one-phonon γ-vibrational band is updated with spin up to 12 h. A new collective band with the band head level at 1422.4 keV is suggested as a two-phonon γ-vibrational band. Another new band is proposed as a two-quasi-proton excitation band. Systematic characteristics of the collective bands are discussed.

  10. Theory of silicon superlattices - Electronic structure and enhanced mobility

    Science.gov (United States)

    Moriarty, J. A.; Krishnamurthy, S.

    1983-01-01

    A realistic tight-binding band-structure model of silicon superlattices is formulated and used to study systems of potential applied interest, including periodic layered Si-Si(1-x)Ge(x) heterostructures. The results suggest a possible new mechanism for achieving enhanced transverse carrier mobility in such structures: reduced transverse conductivity effective masses associated with the superlattice band structure. For electrons in 100-line-oriented superlattices, a reduced conductivity mass arises intrinsically from the lower symmetry of the superlattice and its unique effect on the indirect bulk silicon band gap. An order of magnitude estimate of the range of mobility enhancement expected from this mechanism appears to be consistent with preliminary experimental results on Si-Si(1-x)Ge(x) superlattices.

  11. Spectrophotometric method for optical band gap and electronic transitions determination of semiconductor materials

    Science.gov (United States)

    Sangiorgi, Nicola; Aversa, Lucrezia; Tatti, Roberta; Verucchi, Roberto; Sanson, Alessandra

    2017-02-01

    The optical band gap energy and the electronic processes involved are important parameters of a semiconductor material and it is therefore important to determine their correct values. Among the possible methods, the spectrophotometric is one of the most common. Several methods can be applied to determine the optical band gap energy and still now a defined consensus on the most suitable one has not been established. A highly diffused and accurate optical method is based on Tauc relationship, however to apply this equation is necessary to know the nature of the electronic transitions involved commonly related to the coefficient n. For this purpose, a spectrophotometric technique was used and we developed a graphical method for electronic transitions and band gap energy determination for samples in powder form. In particular, the n coefficient of Tauc equation was determined thorough mathematical elaboration of experimental results on TiO2 (anatase), ZnO, and SnO2. The results were used to calculate the band gap energy values and then compared with the information obtained by Ultraviolet Photoelectron Spectroscopy (UPS). This approach provides a quick and accurate method for band gap determination through n coefficient calculation. Moreover, this simple but reliable method can be used to evaluate the nature of electronic transition that occurs in a semiconductor material in powder form.

  12. Quasiparticle band structure for the Hubbard systems: Application to. alpha. -CeAl sub 2

    Energy Technology Data Exchange (ETDEWEB)

    Costa-Quintana, J.; Lopez-Aguilar, F. (Departamento de Fisica, Grupo de Electromagnetismo, Universidad Autonoma de Barcelona, Bellaterra, E-08193 Barcelona, Spain (ES)); Balle, S. (Departament de Fisica, Universitat de les Illes Balears, E-07071 Palma de Mallorca, Spain (ES)); Salvador, R. (Control Data Corporation, TALLAHASSEE, FL (USA) Supercomputer Computations Research Institute, Florida State University, Tallahassee, Florida 32306-4052 (USA))

    1990-04-01

    A self-energy formalism for determining the quasiparticle band structure of the Hubbard systems is deduced. The self-energy is obtained from the dynamically screened Coulomb interaction whose bare value is the correlation energy {ital U}. A method for integrating the Schroedingerlike equation with the self-energy operator is given. The method is applied to the cubic Laves phase of {alpha}-CeAl{sub 2} because it is a clear Hubbard system with a very complex electronic structure and, moreover, this system provides us with sufficient experimental data for testing our method.

  13. DFT Study of Effects of Potassium Doping on Band Structure of Crystalline Cuprous Azide

    Institute of Scientific and Technical Information of China (English)

    ZHU,Wei-Hua; ZHANG,Xiao-Wen; WEI,Tao; XIAO,He-Ming

    2008-01-01

    The structure and defect formation energies of the K-doped CuN3 were studied using density functional theory within the generalized gradient approximation. The results show that the K-doping breaks the azide symmetry and causes asymmetric atomic displacement. As the K-doping level increases, the band gap of the doped system gradually increases. The K impurity is easily incorporated into the crystal thermodynamically. The Cu vacancy is easily created thermodynamically and the K impurity can serve as nucleation centers for vacancy clustering. Finally the effects of K-doping concentrations on the sensitivity of CuN3 were understood based on electronic structures.

  14. Halogen versus halide electronic structure

    Institute of Scientific and Technical Information of China (English)

    Willem-Jan; van; Zeist; F.Matthias; Bickelhaupt

    2010-01-01

    Halide anions X-are known to show a decreasing proton affinity(PA),as X descends in the periodic table along series F,Cl,Br and I.But it is also well-known that,along this series,the halogen atom X becomes less electronegative(or more electropositive).This corresponds to an increasing energy of the valence np atomic orbital(AO) which,somewhat contradictorily,suggests that the electron donor capability and thus the PA of the halides should increase along the series F,Cl,Br,I.To reconcile these contradictory observations,we have carried out a detailed theoretical analysis of the electronic structure and bonding capability of the halide anions X-as well as the halogen radicals X-,using the molecular orbital(MO) models contained in Kohn-Sham density functional theory(DFT,at SAOP/TZ2P as well as OLYP/TZ2P levels) and ab initio theory(at the HF/TZ2P level).We also resolve an apparent intrinsic contradiction in Hartree-Fock theory between orbital-energy and PA trends.The results of our analyses are of direct relevance for understanding elementary organic reactions such as nucleophilic substitution(SN2) and base-induced elimination(E2) reactions.

  15. Experimental Study of Electronic Quantum Interference, Photonic Crystal Cavity, Photonic Band Edge Effects for Optical Amplification

    Science.gov (United States)

    2016-01-26

    performed. 2.0 INTRODUCTION Three dimensional (3D) photonic crystals and their optical properties have attracted a lot of attention in the past decade... physical phenomena. The band gap frequency of this system can be varied to tailor to the electronic transition levels of a gain medium such as InAs...quantum dot or an InGaAs quantum well. The band gap can be varied in addition to include either one or two electronic levels of a multi-level system

  16. Longitudinal spin relaxation of donor-bound electrons in direct band-gap semiconductors

    Science.gov (United States)

    Linpeng, Xiayu; Karin, Todd; Durnev, M. V.; Barbour, Russell; Glazov, M. M.; Sherman, E. Ya.; Watkins, S. P.; Seto, Satoru; Fu, Kai-Mei C.

    2016-09-01

    We measure the donor-bound electron longitudinal spin-relaxation time (T1) as a function of magnetic field (B ) in three high-purity direct band-gap semiconductors: GaAs, InP, and CdTe, observing a maximum T1 of 1.4, 0.4, and 1.2 ms, respectively. In GaAs and InP at low magnetic field, up to ˜2 T, the spin-relaxation mechanism is strongly density and temperature dependent and is attributed to the random precession of the electron spin in hyperfine fields caused by the lattice nuclear spins. In all three semiconductors at high magnetic field, we observe a power-law dependence T1∝B-ν with 3 ≲ν ≲4 . Our theory predicts that the direct spin-phonon interaction is important in all three materials in this regime in contrast to quantum dot structures. In addition, the "admixture" mechanism caused by Dresselhaus spin-orbit coupling combined with single-phonon processes has a comparable contribution in GaAs. We find excellent agreement between high-field theory and experiment for GaAs and CdTe with no free parameters, however a significant discrepancy exists for InP.

  17. Optical Properties of One-dimensional Three-component Photonic Band Gap Structure

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    Theoretical study of the optical properties of one-dimensional three-component photonic band gap structure, which is composed of three alternating dielectric layers of different refractive indices and thickness in a unit cell, is performed. This one-dimensional photonic band gap structure exhibits the transparency band and forbidden band. We find that there are several mini-bands of the allowed transmission to be created within the photonic band gap region of the structure if a defect designed specially is introduced inside the structure. This characteristic is very important for some practical applications.

  18. Electronic structure of a graphene superlattice with massive Dirac fermions

    Energy Technology Data Exchange (ETDEWEB)

    Lima, Jonas R. F., E-mail: jonas.iasd@gmail.com [Instituto de Ciencia de Materiales de Madrid (CSIC) - Cantoblanco, Madrid 28049 (Spain)

    2015-02-28

    We study the electronic and transport properties of a graphene-based superlattice theoretically by using an effective Dirac equation. The superlattice consists of a periodic potential applied on a single-layer graphene deposited on a substrate that opens an energy gap of 2Δ in its electronic structure. We find that extra Dirac points appear in the electronic band structure under certain conditions, so it is possible to close the gap between the conduction and valence minibands. We show that the energy gap E{sub g} can be tuned in the range 0 ≤ E{sub g} ≤ 2Δ by changing the periodic potential. We analyze the low energy electronic structure around the contact points and find that the effective Fermi velocity in very anisotropic and depends on the energy gap. We show that the extra Dirac points obtained here behave differently compared to previously studied systems.

  19. Structure of negative parity yrast bands in odd mass 125-131Ce nuclei

    Indian Academy of Sciences (India)

    Arun Bharti; Suram Singh; S K Khosa

    2010-04-01

    The negative parity yrast bands of neutron-deficient 125-131Ce nuclei are studied by using the projected shell model approach. Energy levels, transition energies and (1)/(2) ratios are calculated and compared with the available experimental data. The calculations reproduce the band-head spins of negative parity yrast bands and indicate the multi-quasiparticle structure for these bands.

  20. Temperature-dependent band structure of SrTiO3 interfaces

    Science.gov (United States)

    Raslan, Amany; Lafleur, Patrick; Atkinson, W. A.

    2017-02-01

    We build a theoretical model for the electronic properties of the two-dimensional (2D) electron gas that forms at the interface between insulating SrTiO3 and a number of polar cap layers, including LaTiO3, LaAlO3, and GdTiO3. The model treats conduction electrons within a tight-binding approximation and the dielectric polarization via a Landau-Devonshire free energy that incorporates strontium titanate's strongly nonlinear, nonlocal, and temperature-dependent dielectric response. The self-consistent band structure comprises a mix of quantum 2D states that are tightly bound to the interface and quasi-three-dimensional (3D) states that extend hundreds of unit cells into the SrTiO3 substrate. We find that there is a substantial shift of electrons away from the interface into the 3D tails as temperature is lowered from 300 K to 10 K. This shift is least important at high electron densities (˜1014cm-2 ) but becomes substantial at low densities; for example, the total electron density within 4 nm of the interface changes by a factor of two for 2D electron densities ˜1013cm-2 . We speculate that the quasi-3D tails form the low-density high-mobility component of the interfacial electron gas that is widely inferred from magnetoresistance measurements.

  1. First Principle Calculation for the Electronic Bands and Absorption of CdTe1-xSbx

    Institute of Scientific and Technical Information of China (English)

    WANG Long; HUANG Zheng; MA Huan-feng; QIANG Wei-rong; PAN Min

    2010-01-01

    The lattice parameters for the derivatives of cadmium telluride, CdTe1-xSbx, with the zinc blend crystal structure are calculated using the generalized gradient approximation method; which is based on the density functional theory (DFT). The effects of antimony (Sb) on the lattices, electric bands, electronic state density, absorption spectroscopy, and band gap between the valence band maximum (VBM) and the conduction band minimum (CBM) of CdTe1-xSbx are discussed. The results show that the antimonic atoms in the lattice are advantageous in promoting the hole concentration and conductivities of CdTe1-xSbx. The increase of the Sb content in CdTe1-xSbx reduces the interaction among Cd, Te, and Sb; resulting in a decreased binding energy within CdTe1-xSbx as well as an increase in the electronic gap. Also discussed are the mechanics for the lattice phase change of CdTe1-xSbx at x=0.5.

  2. Structural Evolution of a Warm Frontal Precipitation Band During GCPEx

    Science.gov (United States)

    Colle, Brian A.; Naeger, Aaron; Molthan, Andrew; Nesbitt, Stephen

    2015-01-01

    A warm frontal precipitation band developed over a few hours 50-100 km to the north of a surface warm front. The 3-km WRF was able to realistically simulate band development, although the model is somewhat too weak. Band genesis was associated with weak frontogenesis (deformation) in the presence of weak potential and conditional instability feeding into the band region, while it was closer to moist neutral within the band. As the band matured, frontogenesis increased, while the stability gradually increased in the banding region. Cloud top generating cells were prevalent, but not in WRF (too stable). The band decayed as the stability increased upstream and the frontogenesis (deformation) with the warm front weakened. The WRF may have been too weak and short-lived with the band because too stable and forcing too weak (some micro issues as well).

  3. Band gap bowing and electron localization of (GaxIn1-x)N

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Byounghak; Wang, Lin-Wang

    2006-05-09

    The band gap bowing and the electron localization ofGaxIn1-xN are calculated using both the local density approximation (LDA)and screened-exchange local density functional (sX-LDA) methods. Thecalculated sX-LDA band gaps are in good agreement with the experimentallyobserved values, with errors of -0.26 and 0.09 eV for bulk GaN and InN,respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN andInN, in order. In contrast to the gap itself, the band gap bowingparameter is found to be very similar in sX-LDA and LDA. We identify thelocalization of hole states in GaxIn1-xN alloys along In-N-In chains. Thepredicted localizationis stronger in sX-LDA.

  4. Dynamic Beam Shaping Using a Dual-Band Metasurface-Inspired Electronically Tunable Reflectarray Antenna

    CERN Document Server

    Tayebi, Amin; Paladhi, Pavel Roy; Udpa, Lalita; Udpa, Satish; Rothwell, Edward

    2015-01-01

    An electronically reconfigurable dual-band-reflectarray antenna is presented in this paper. The tunable unit cell, a ring loaded square patch with a single varactor diode connected across the gap between the ring and the patch, is modeled using both a full-wave solver and an equivalent circuit. The parameters of the equivalent circuit are calculated independently of the simulation and experiment using analysis techniques employed in frequency selective surfaces. The reflection phase of the proposed unit cell is shown to provide an excellent phase range of 335$^{\\circ}$ in F band and 340$^{\\circ}$ in S band. Results from the analysis are used to design and build a 10x10 element reflectarray antenna. The high tuning phase range of each element allows the fabricated reflectarray to demonstrate a very broad steering range of up to $\\pm$60$^{\\circ}$ in both frequency bands.

  5. Density changes in shear bands of a metallic glass determined by correlative analytical transmission electron microscopy

    Energy Technology Data Exchange (ETDEWEB)

    Rösner, Harald, E-mail: rosner@uni-muenster.de [Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster (Germany); Peterlechner, Martin [Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster (Germany); Kübel, Christian [Karlsruhe Institute of Technology (KIT), Institute of Nanotechnology (INT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen (Germany); Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen (Germany); Schmidt, Vitalij [Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster (Germany); Wilde, Gerhard [Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster (Germany); Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444 (China)

    2014-07-01

    Density changes between sheared zones and their surrounding amorphous matrix as a result of plastic deformation in a cold-rolled metallic glass (melt-spun Al{sub 88}Y{sub 7}Fe{sub 5}) were determined using high-angle annular dark-field (HAADF) detector intensities supplemented by electron-energy loss spectroscopy (EELS), energy-dispersive X-ray (EDX) and nano-beam diffraction analyses. Sheared zones or shear bands were observed as regions of bright or dark contrast arising from a higher or lower density relative to the matrix. Moreover, abrupt contrast changes from bright to dark and vice versa were found within individual shear bands. We associate the decrease in density mainly with an enhanced free volume in the shear bands and the increase in density with concomitant changes of the mass. This interpretation is further supported by changes in the zero loss and Plasmon signal originating from such sites. The limits of this new approach are discussed. - Highlights: • We describe a novel approach for measuring densities in shear bands of metallic glasses. • The linear relation of the dark-field intensity I/I{sub 0} and the mass thickness ρt was used. • Individual shear bands showed abrupt contrast changes from bright to dark and vice versa. • Density changes ranging from about −10% to +6% were found for such shear bands. • Mixtures of amorphous/medium range ordered domains were found within the shear bands.

  6. Strongly correlated flat-band systems: The route from Heisenberg spins to Hubbard electrons

    Science.gov (United States)

    Derzhko, Oleg; Richter, Johannes; Maksymenko, Mykola

    2015-05-01

    On a large class of lattices (such as the sawtooth chain, the kagome and the pyrochlore lattices), the quantum Heisenberg and the repulsive Hubbard models may host a completely dispersionless (flat) energy band in the single-particle spectrum. The flat-band states can be viewed as completely localized within a finite volume (trap) of the lattice and allow for construction of many-particle states, roughly speaking, by occupying the traps with particles. If the flat-band happens to be the lowest-energy one, the manifold of such many-body states will often determine the ground-state and low-temperature physics of the models at hand even in the presence of strong interactions. The localized nature of these many-body states makes possible the mapping of this subset of eigenstates onto a corresponding classical hard-core system. As a result, the ground-state and low-temperature properties of the strongly correlated flat-band systems can be analyzed in detail using concepts and tools of classical statistical mechanics (e.g., classical lattice-gas approach or percolation approach), in contrast to more challenging quantum many-body techniques usually necessary to examine strongly correlated quantum systems. In this review, we recapitulate the basic features of the flat-band spin systems and briefly summarize earlier studies in the field. The main emphasis is made on recent developments which include results for both spin and electron flat-band models. In particular, for flat-band spin systems, we highlight field-driven phase transitions for frustrated quantum Heisenberg antiferromagnets at low temperatures, chiral flat-band states, as well as the effect of a slight dispersion of a previously strictly flat-band due to nonideal lattice geometry. For electronic systems, we discuss the universal low-temperature behavior of several flat-band Hubbard models, the emergence of ground-state ferromagnetism in the square-lattice Tasaki-Hubbard model and the related Pauli

  7. A Brief Introduction to Band Structure in Three Dimensions

    CERN Document Server

    Iannucci, Peter

    2011-01-01

    Without our ability to model and manipulate the band structure of semiconducting materials, the modern digital computer would be impractically large, hot, and expensive. In the undergraduate QM curriculum, we studied the effect of spatially periodic potentials on the spectrum of a charged particle in one dimension. We would like to understand how to extend these methods to model actual crystalline materials. Along the way, we will explore the construction of periodic potentials in three dimensions, and we use this framework to relate the single-particle Hamiltonian to the potential contribution from each atom. We then construct a crude model system analogous to the semiconductor silicon, and demonstrate the appearance of level splitting and band gaps as the strength of the potential is varied, in accordance with our intuition from the one-dimensional case. We discuss refinements of the model to include many-particle effects, and finally we show how a careful choice of the potential function leads to good agre...

  8. Electronic structure of spontaneously strained graphene on hexagonal boron nitride

    Science.gov (United States)

    San-Jose, Pablo; Gutiérrez-Rubio, A.; Sturla, Mauricio; Guinea, Francisco

    2014-09-01

    Hexagonal boron nitride substrates have been shown to dramatically improve the electric properties of graphene. Recently, it has been observed that when the two honeycomb crystals are close to perfect alignment, strong lattice distortions develop in graphene due to the moiré adhesion landscape. Simultaneously, a gap opens at the Dirac point. Here, we derive a simple low-energy electronic model for graphene aligned with the substrate, taking into account spontaneous strains at equilibrium and pseudogauge fields. We carry out a detailed characterization of the modified band structure, gap, local and global density of states, and band topology in terms of physical parameters. We show that the overall electronic structure is strongly modified by the spontaneous strains.

  9. Electronic shell and supershell structure in graphene flakes

    CERN Document Server

    Manninen, M; Akola, J

    2008-01-01

    We use a simple tight-binding (TB) model to study electronic properties of free graphene flakes. Valence electrons of triangular graphene flakes show a shell and supershell structure which follows an analytical expression derived from the solution of the wave equation for triangular cavity. However, the solution has different selection rules for triangles with armchair and zigzag edges, and roughly 40000 atoms are needed to see clearly the first supershell oscillation. In the case of spherical flakes, the edge states of the zigzag regions dominate the shell structure which is thus sensitive to the flake diameter and center. A potential well that is made with external gates cannot have true bound states in graphene due to the zero energy band gap. However, it can cause strong resonances in the conduction band.

  10. Analysis of Kikuchi band contrast reversal in electron backscatter diffraction patterns of silicon.

    Science.gov (United States)

    Winkelmann, Aimo; Nolze, Gert

    2010-02-01

    We analyze the contrast reversal of Kikuchi bands that can be seen in electron backscatter diffraction (EBSD) patterns under specific experimental conditions. The observed effect can be reproduced using dynamical electron diffraction calculations. Two crucial contributions are identified to be at work: First, the incident beam creates a depth distribution of incoherently backscattered electrons which depends on the incidence angle of the beam. Second, the localized inelastic scattering in the outgoing path leads to pronounced anomalous absorption effects for electrons at grazing emission angles, as these electrons have to go through the largest amount of material. We use simple model depth distributions to account for the incident beam effect, and we assume an exit angle dependent effective crystal thickness in the dynamical electron diffraction calculations. Very good agreement is obtained with experimental observations for silicon at 20keV primary beam energy.

  11. A Compact UWB Band-Pass Filter Using Embedded Circular Slot Structures for Improved Upper Stop-band Performance

    DEFF Research Database (Denmark)

    Shen, Ming; Ren, Jian; Mikkelsen, Jan Hvolgaard;

    2016-01-01

    structures into the ring resonator. This is different from conventional designs using cascaded bandstop/low-pass filters for stop-band response suppression, which usually leads to big circuit sizes. And hence the proposed approach can reduce the circuit size significantly. A prototype filter with a compact...... size (13.6 mm×6.75 mm) has been implemented for experimental validation. The measured results show a −3 dB frequency band from 3.4 GHz to 11.7 GHz and > 20 dB upper stop-band suppression from 12.5 GHz to 20GHz....

  12. Electronic structure of (Ca0.85La0.15)FeAs2

    Science.gov (United States)

    Liu, Z.-H.; Kim, T. K.; Sala, A.; Ogino, H.; Shimoyama, J.; Büchner, B.; Borisenko, S. V.

    2015-02-01

    We report a comprehensive study of orbital character and tridimensional nature of the electronic structure of (Ca0.85La0.15)FeAs2 from recently discovered "112" family of Iron-based superconductors (IBS), with angle-resolved photoemission spectroscopy. We observed that the band structure is similar to that of "122" family, namely, there are three hole-like bands at the Brillouin zone (BZ) center and two electron-like bands at the BZ corner. The bands near the Fermi level (EF) are mainly derived from the Fe t2g orbitals. On the basis of our present and earlier studies, we classify IBS into the three types according to their crystal structures. We show that although the bands near EF mainly originate from Fe 3d electrons, they are significantly modified by the interaction between the superconducting slabs and the intermediate atoms.

  13. Structural and electronic properties of poly(vinyl alcohol) using density functional theory

    Energy Technology Data Exchange (ETDEWEB)

    Dabhi, Shweta, E-mail: shwetadabhi1190@gmail.com; Jha, Prafulla K., E-mail: shwetadabhi1190@gmail.com [Department of Physics, Maharaja Krishnakumasinhji Bhavnagar University, Bhavnagar-364001 (India)

    2014-04-24

    The first principles calculations have been carried out to investigate the structural, electronic band structure density of states along with the projected density of states for poly(vinyl alcohol). Our structural calculation suggests that the poly(vinyl alcohol) exhibits monoclinic structure. The calculated structural lattice parameters are in excellent agreement with available experimental values. The band structure calculations reveal that the direct and indirect band gaps are 5.55 eV and 5.363 eV respectively in accordance with experimental values.

  14. Electronic Crosstalk in Aqua MODIS Long-Wave Infrared Photovoltaic Bands

    Directory of Open Access Journals (Sweden)

    Junqiang Sun

    2016-09-01

    Full Text Available Recent investigations have discovered that Terra MODerate-resolution Imaging Spectroradiometer (MODIS long-wave infrared (LWIR photovoltaic (PV bands, bands 27–30, have strong crosstalk among themselves. The linear model developed to test the electronic crosstalk effect was instrumental in the first discovery of the effect in Terra MODIS band 27, and through subsequent investigations the model and the correction algorithm were tested further and established to be correct. It was shown that the correction algorithm successfully mitigated the anomalous features in the calibration coefficients as well as the severe striping and the long-term drift in the Earth view (EV retrievals for the affected Terra bands. Here, the examination into Aqua MODIS using the established methodology confirms the existence of significant crosstalk contamination in its four LWIR PV, although the finding shows the overall effect to be of lesser degree. The crosstalk effect is characterized and the crosstalk correction coefficients are derived for all four Aqua LWIR PV bands via analysis of signal contamination in the lunar imagery. Sudden changes in the crosstalk contamination are clearly seen, as also in the Terra counterparts in previous investigations. These sudden changes are consistent with the sudden jumps observed in the linear calibration coefficients for many years, thus this latest finding provides an explanation to the long-standing but unexplained anomalies in the calibration coefficients of the four Aqua LWIR bands. It is also shown that the crosstalk contamination for these bands are of similar level for the two MODIS instruments in the early mission that can lead to as much as 2 K increase in brightness temperature for the affected bands, thus demonstrating significant impact on the science results already started at the early going. As Aqua MODIS is a legacy sensor, the crosstalk correction to its LWIR PV bands will be important to remove the impact of

  15. Band structure systematics and symmetries in even-even nuclei

    Science.gov (United States)

    Bucurescu, D.; Cata-Danil, Gh.; Ivascu, M.; Ur, C. A.

    1993-07-01

    It is shown that the experimental in-band energy ratios for the even-even nuclei obey universal systematics similar to those observed by Mallmann for the quasiground band. Systematic correlations between energy ratios belonging to different bands are also found in certain cases. Finally, correlations between mixed energy ratios are shown to be useful in characterizing the evolution of the nulcear collectivity.

  16. Two-dimensional silica: Structural, mechanical properties, and strain-induced band gap tuning

    Energy Technology Data Exchange (ETDEWEB)

    Gao, Enlai; Xie, Bo [Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084 (China); Xu, Zhiping, E-mail: xuzp@tsinghua.edu.cn [Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084 (China); State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)

    2016-01-07

    Two-dimensional silica is of rising interests not only for its practical applications as insulating layers in nanoelectronics, but also as a model material to understand crystals and glasses. In this study, we examine structural and electronic properties of hexagonal and haeckelite phases of silica bilayers by performing first-principles calculations. We find that the corner-sharing SiO{sub 4} tetrahedrons in these two phases are locally similar. The robustness and resilience of these tetrahedrons under mechanical perturbation allow effective strain engineering of the electronic structures with band gaps covering a very wide range, from of that for insulators, to wide-, and even narrow-gap semiconductors. These findings suggest that the flexible 2D silica holds great promises in developing nanoelectronic devices with strain-tunable performance, and lay the ground for the understanding of crystalline and vitreous phases in 2D, where bilayer silica provides an ideal test-bed.

  17. Band-structural and Fourier-spectral properties of one-dimensional generalized Fibonacci lattices

    Science.gov (United States)

    Oh, G. Y.; Lee, M. H.

    1993-11-01

    We study the electronic and Fourier-spectral properties of one-dimensional generalized Fibonacci lattices generated by the stacking rule Sl+1=SnlSml-1 with positive integers n and m, where Sl is the lth generational binary sequence. After showing that, in the limit of the large potential strength, the energy spectrum of a lattice with certain specific n and m can be determined by the associated characteristic value τ(n,m), we investigate the relation between the electronic band structure and the Fourier spectrum. When the lattice possesses the Pisot-Vijayaraghavan (PV) property (i.e., when n+1>m), the Fourier spectrum is closely related to the electronic band structure; the location and the relative strength of the Fourier spectral peak is in agreement with the location and the relative width of the energy spectral gap. On the other hand, when the lattice possesses no PV property (i.e., when n+1nature of the lattice becomes clearer with the increase of p.

  18. Electronic structure of ternary hydrides based on light elements

    Energy Technology Data Exchange (ETDEWEB)

    Orgaz, E. [Departamento de Fisica y Quimica Teorica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, CP 04510 Coyoacan, Mexico, D.F. (Mexico)]. E-mail: orgaz@eros.pquim.unam.mx; Membrillo, A. [Departamento de Fisica y Quimica Teorica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, CP 04510 Coyoacan, Mexico, D.F. (Mexico); Castaneda, R. [Departamento de Fisica y Quimica Teorica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, CP 04510 Coyoacan, Mexico, D.F. (Mexico); Aburto, A. [Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, CP 04510 Coyoacan, Mexico, D.F. (Mexico)

    2005-12-08

    Ternary hydrides based on light elements are interesting owing to the high available energy density. In this work we focused into the electronic structure of a series of known systems having the general formula AMH{sub 4}(A=Li,Na,M=B,Al). We computed the energy bands and the total and partial density of states using the linear-augmented plane waves method. In this report, we discuss the chemical bonding in this series of complex hydrides.

  19. The two dimensional electron system as a nanoantenna in the microwave and terahertz bands

    Science.gov (United States)

    Iñarrea, Jesús

    2011-12-01

    We study the magnetoresistance of two-dimensional electron systems under several radiation sources of different frequencies for moderate power. We use the model of radiation-driven electron orbits extended to this regime. First, we consider the case of two different radiations and we find a regime of superposition or interference of harmonic motions, i.e., a modulated magnetoresistance response with pulses and beats. Finally, we consider a multiple photoexcitation case where we propose the two-dimensional electron system as a potential nanoantenna device or ultrasensitive detector for the microwave and terahertz bands. Thus, these results could be of special interest in nanophotonics and nanoelectronics.

  20. Electronic structure of herbicides: Atrazine and bromoxynil

    Science.gov (United States)

    Novak, Igor; Kovač, Branka

    2011-06-01

    The electronic structures of herbicides atrazine and bromoxynil have been investigated by UV photoelectron spectroscopy (UPS), quantum chemical calculations and comparison with X-ray diffraction, molecular docking and molecular dynamics studies. Their electronic and molecular structures are discussed in the context of their biological activity. This is the first report which correlates the molecular mechanism of biological activity of these herbicides with their experimentally determined electronic and molecular structures.

  1. The effect of spin-orbit coupling in band structure of few-layer graphene

    Energy Technology Data Exchange (ETDEWEB)

    Sahdan, Muhammad Fauzi, E-mail: sahdan89@yahoo.co.id; Darma, Yudi, E-mail: sahdan89@yahoo.co.id [Department of Physics, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung 40132 (Indonesia)

    2014-03-24

    Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. This can be happened due to spin-orbit coupling and time-reversal symmetry. Moreover, the edge current flows through their edge or surface depends on its spin orientation and also it is robust against non-magnetic impurities. Therefore, topological insulators are predicted to be useful ranging from spintronics to quantum computation. Graphene was first predicted to be the precursor of topological insulator by Kane-Mele. They developed a Hamiltonian model to describe the gap opening in graphene. In this work, we investigate the band structure of few-layer graphene by using this model with analytical approach. The results of our calculations show that the gap opening occurs at K and K’ point, not only in single layer, but also in bilayer and trilayer graphene.

  2. Direct observation of the band structure in bulk hexagonal boron nitride

    Science.gov (United States)

    Henck, Hugo; Pierucci, Debora; Fugallo, Giorgia; Avila, José; Cassabois, Guillaume; Dappe, Yannick J.; Silly, Mathieu G.; Chen, Chaoyu; Gil, Bernard; Gatti, Matteo; Sottile, Francesco; Sirotti, Fausto; Asensio, Maria C.; Ouerghi, Abdelkarim

    2017-02-01

    A promising route towards nanodevice applications relies on the association of graphene and transition metal dichalcogenides with hexagonal boron nitride (h -BN ). Due to its insulating nature, h -BN has emerged as a natural substrate and gate dielectric for graphene-based electronic devices. However, some fundamental properties of bulk h -BN remain obscure. For example, the band structure and the position of the Fermi level have not been experimentally resolved. Here, we report a direct observation of parabolic dispersions of h -BN crystals using high-resolution angle-resolved photoemission spectroscopy (ARPES). We find that h -BN exfoliation on epitaxial graphene enables overcoming the technical difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at constant energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual p -type character of typical h -BN .

  3. THE BAND STRUCTURE AND WORK FUNCTION OF TRANSPARENT CONDUCTING ALUMINUM AND MANGANESE CO-DOPED ZINC OXIDE FILMS

    Institute of Scientific and Technical Information of China (English)

    H.T. Cao; Z.L. Pei; X.B. Zhang; J. Gong; C. Sun; L.S. Wen

    2005-01-01

    Al and Mn co-doped-ZnO films have been prepared at room temperature by DC reactive magnetron sputtering technique. The optical absorption coefficient, apparent and fundamental band gap, and work function of the films have been investigated using optical spectroscopy, band structure analyses and ultraviolet photoelectron spectroscopy (UPS). ZnO films have direct allowed transition band structure, which has been confirmed by the character of the optical absorption coefficient. The apparent band gap has been found directly proportional to N2/3, showing that the effect of Burstein-Moss shift on the band gap variations dominates over the many-body effect. With only standard cleaning protocols, the work function of ZnO: (Al, Mn) and ZnO: Al films have been measured to be 4.26 and 4.21eV, respectively. The incorporation of Mn element into the matrix of ZnO, as a relatively deep donor, can remove some electrons from the conduction band and deplete the density of occupied states at the Fermi energy, which causes a loss in measured photoemission intensity and an increase in the surface work function. Based on the band gap and work function results, the energy band diagram of the ZnO: (Al, Mn)film near its surface is also given.

  4. Electronic and structural properties of the (1010) and (1120) ZnO surfaces.

    Science.gov (United States)

    Marana, N L; Longo, V M; Longo, E; Martins, J B L; Sambrano, J R

    2008-09-25

    The structural and electronic properties of ZnO (1010) and (1120) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (1010) as compared to the (1120) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (1010), and (1120) surfaces, respectively. The band structures for both surfaces are very similar.

  5. A Banding Structure in a Ni-Cu-Si Cast Alloy

    Institute of Scientific and Technical Information of China (English)

    Qi ZHENG; Yufeng ZHENG; Hongyu ZHANG; Xiaofeng SUN; Hengrong GUAN; Zhuangqi HU

    2008-01-01

    The solidified microstructure of a Ni-Cu-Si cast alloy has been investigated, and a kind of banding structure was observed. The results showed that, the banding structure was composed of coarser particles which were Ni3Si type of precipitates and similar to the fine particles precipitate uniformly distributed within matrix of Ni solid solution, in both crystal structure and composition. The formation of bandings was resulted from cast thermal stress and dislocation walls. It was found that the cracks propagated along these bandings in tensile test. The banding structure can be depressed by reducing the cast thermal stress, which can improve the Qtensile ductility.

  6. Enlargement of Photonic Band Gaps and Physical Picture of Photonic Band Structures

    Institute of Scientific and Technical Information of China (English)

    ZHANG Yan; SHI Jun-Jie

    2006-01-01

    @@ Light propagation in a one-dimensional photonic crystal (PC), consisting of alternative slabs with refractive indices (layer thicknesses) n1 (a) and n2 (b), is investigated. An important optimal parameter matching condition,n1a ≈ n2b, is obtained for the largest photonic band gap (PBG). Moreover, we find that the exact analytical solutions for the electric/magnetic field eigenmodes at the band edges are standing waves with odd or even symmetry about the centre of each layer. The electric/magnetic field eigenfunctions at the top and bottom of the nth band have n and n - 1 nodes in one period of PC, respectively. The PBG arises from the symmetric differences of the field eigenfunctions at the band edges.

  7. Origins of electronic band gap reduction in Cr/N codoped TiO2.

    Science.gov (United States)

    Parks Cheney, C; Vilmercati, P; Martin, E W; Chiodi, M; Gavioli, L; Regmi, M; Eres, G; Callcott, T A; Weitering, H H; Mannella, N

    2014-01-24

    Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

  8. Core levels, valence band structure and unoccupied states of clean InN surfaces

    Energy Technology Data Exchange (ETDEWEB)

    Himmerlich, Marcel; Eisenhardt, Anja; Schaefer, Juergen A.; Krischok, Stefan [Institut fuer Physik and Institut fuer Mikro- und Nanotechnologien, TU Ilmenau (Germany)

    2008-07-01

    In this study we used a surface analytics system directly connected to a MBE growth module to study the surface properties of thin InN films. The samples were prepared by plasma assisted molecular beam epitaxy on GaN/Al{sub 2}O{sub 3}(0001) templates and exhibited a 2 x 2 reconstruction after growth. The prepared samples were analysed by photoelectron spectroscopy as well as electron energy loss spectroscopy (EELS). For the occupied states, a very good agreement to available theoretical calculations is found. Although, the valence band maximum is located at 1.6 eV, indicating strong downward band bending of {proportional_to}0.9 eV, photoemission is detected up to E{sub F}. This indicates that the Fermi level is pinned above the conduction band minimum, as recently predicted. The spin-orbit splitting of the In 4d level at 17.8 eV could be resolved using He II radiation. Furthermore, from the fine structure of the secondary electron cascade peak we extract the energy of different unoccupied states 0 eV to 9 eV above the vacuum level. These measurements enable us to identify features in the InN EELS spectra, with a loss energy larger than 16 eV, as interband transitions from the In 4d level.

  9. Phase analysis on dual-phase steel using band slope of electron backscatter diffraction pattern.

    Science.gov (United States)

    Kang, Jun-Yun; Park, Seong-Jun; Moon, Man-Been

    2013-08-01

    A quantitative and automated phase analysis of dual-phase (DP) steel using electron backscatter diffraction (EBSD) was attempted. A ferrite-martensite DP microstructure was produced by intercritical annealing and quenching. An EBSD map of the microstructure was obtained and post-processed for phase discrimination. Band slope (BS), which was a measure of pattern quality, exhibited much stronger phase contrast than another conventional one, band contrast. Owing to high sensitivity to lattice defect and little orientation dependence, BS provided handiness in finding a threshold for phase discrimination. Its grain average gave a superior result on the discrimination and volume fraction measurement of the constituent phases in the DP steel.

  10. Photoinduced Reconstruction of Electronic Structure in Half-Metal CrO2

    Institute of Scientific and Technical Information of China (English)

    WU Xue-Wei; NIU Dong-Lin; LIU Xiao-Jun

    2007-01-01

    We investigate the photoinduced effects on the electronic structure for haft-metallic ferromagnet CrO2(Tc~390K),in which the conducting electrons are totally polarized,by using the LSDA+U method.A significant change is found for the band structure and the density of states (DOS) for CrO2 under photo-excitation,especially for the Cr 3dr2g band:disappearance of the spin-split band,suggesting collapse of the half-metallic state.We ascribe the change of electronic structure under photo-excitation to the wider one-electron band W via the strong hybridization of the down-spin Cr 3d and O 2p states.Furthermore we discuss the magnetic properties under photo-excitation.

  11. Atomic structure and electronic states of extended defects in silicon

    CERN Document Server

    Riedel, F; Schröter, W

    2002-01-01

    Defects in silicon like dislocations, grain boundaries, silicide precipitates, etc. are spatially extended and associated with a large number of electronic states in the band gap. Our knowledge on the relation between atomic structure and electronic states of these extended defects presently starts to grow by applying high-resolution electron microscopy (HRTEM) and deep level transient spectroscopy (DLTS) in combination with numerical simulations. While by means of HRTEM details of structure can be studied, DLTS has been shown to allow for a classification of extended defect states into bandlike and localized. Moreover, this method opens the perspective to distinguish between trap-like and recombination-like electrical activity. In this paper, we emphasize the particular role of nickel and copper silicide precipitates, since in their cases structural features could be successfully related to specific DLTS line characteristics. Rapid quenching from high diffusion temperatures prevents decoration of platelet-sh...

  12. Electronic and crystal structures of thermoelectric CaMgSi intermetallic compound

    Energy Technology Data Exchange (ETDEWEB)

    Miyazaki, Hidetoshi, E-mail: miyazaki@nitech.ac.jp [Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555 (Japan); Inukai, Manabu [Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555 (Japan); Soda, Kazuo [Department of Quantum Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603 (Japan); Miyazaki, Nobufumi; Adachi, Nozomu; Todaka, Yoshikazu [Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi 441-8580 (Japan); Nishino, Yoichi [Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555 (Japan)

    2016-01-15

    Highlights: • We report the electronic and crystal structures of the TiNiSi-type CaMgSi compound. • CaMgSi has a semiconductor-like electronic structure with a small band gap. • CaMgSi is a Mott-type insulator owing to strongly correlated electrons effect. - Abstract: We investigated the electronic and crystal structures of a new thermoelectric material, CaMgSi compound, by using synchrotron radiation photoemission spectroscopy (SR-PES), synchrotron radiation X-ray powder diffraction (SR-XRD) measurements, and electronic band structure calculation to understand the way leading to improvement in the thermoelectric properties of this material. Electronic band structure calculation of the CaMgSi compound using the crystal structure determined from SR-XRD measurement showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. In contrast to the predicted semi-metallic electronic structure, the SR-PES results showed a small semiconductor-like gap at the Fermi level. This result revealed that the CaMgSi compound is a Mott-type insulator owing to strongly correlated electrons effect in the Ca 3d and Mg 3p states being well hybridized with those in the Si 3p states. The observed electronic structure of the CaMgSi compound suggests that an optimal carrier doping exists to best control the n- and p-type thermoelectric properties and enhance the power factors.

  13. QUANTUM-MECHANICAL MODELING OF SPATIAL AND BAND STRUCTURE OF Y3AL5O12 SCINTILLATION CRYSTAL

    Directory of Open Access Journals (Sweden)

    I. I. Vrubel

    2016-05-01

    Full Text Available Spatial and electronic structures of a unit cell of yttrium-aluminum garnet have been studied. Quantum-mechanical model have been presented. Semi-empirical methods PM6 and PM7 have been used for geometry optimization of the crystal unit cell. Band structure has been calculated within density functional theory with the use of PBE exchange-correlation functional. Histograms of metal-oxygen distances for equilibrium geometry have been constructed. Comparison of the used methods has been carried out and recommendation about their applicability for such problems was given. The single-particle wave functions and energies have been calculated. The bandgap was estimated. The band structure was plotted. It was shown that the method gives reliable results for spatial and band structure of Y3Al5O12 scintillation crystal. The results of this work can be used for improvement of characteristics of garnet scintillation crystals.

  14. Electronic structure, structural and optical properties of thermally evaporated CdTe thin films

    OpenAIRE

    S Lalitha; Karazhanov, S. Zh.; Ravindran, P.; Senthilarasu, S.; Sathyamoorthy, R.; Janabergenov, J.

    2006-01-01

    Thin films of CdTe were deposited on glass substrates by thermal evaporation. From the XRD measurements itis found that the films are of zinc-blende-type structure. Transmittance, absorption, extinction, and refractive coefficients are measured. Electronic structure, band parameters and optical spectra of CdTe were calculated from ab initio studies within the LDA and LDA+U approximations. It is shown that LDA underestimates the band gap, energy levels of the Cd-4d states, s-d coupling and ban...

  15. Pattern reconfigurable antenna using electromagnetic band gap structure

    Science.gov (United States)

    Ismail, M. F.; Rahim, M. K. A.; Majid, H. A.; Hamid, M. R.; Yusoff, M. F. M.; Dewan, R.

    2017-01-01

    In this paper, a single rectangular patch antenna incorporated with an array of electromagnetic band gap (EBG) structures is proposed. The proposed antenna features radiation pattern agility by means of connecting the shorting pin vias to the EBG unit cells. The proposed design consists of 32 mm × 35.5 mm rectangular patch antenna and 10.4-mm-square mushroom-like EBG unit cells. The EBGs are placed at both sides of the antenna radiating patch and located on the thicker substrate of thickness, h. The copper tape which represents the PIN diode is used to control the connection between the EBG's via and the ground plane as reconfigurable mechanism of the antenna. The simulated result shows by switching the ON and OFF EBG structures in either sides or both, the directional radiation pattern can be tilted from 0 to +14°. The proposed antenna exhibits 7.2 dB realized gain at 2.42 GHz. The parametric study on EBG and antenna is also discussed.

  16. Structural and electronic properties of Diisopropylammonium bromide molecular ferroelectric crystal

    Science.gov (United States)

    Alsaad, A.; Qattan, I. A.; Ahmad, A. A.; Al-Aqtash, N.; Sabirianov, R. F.

    2015-10-01

    We report the results of ab-initio calculations based on Generalized Gradient Approximation (GGA) and hybrid functional (HSE06) of electronic band structure, density of states and partial density of states to get a deep insight into structural and electronic properties of P21 ferroelectric phase of Diisopropylammonium Bromide molecular crystal (DIPAB). We found that the optical band gap of the polar phase of DIPAB is ∼ 5 eV confirming it as a good dielectric. Examination of the density of states and partial density of states reveal that the valence band maximum is mainly composed of bromine 4p orbitals and the conduction band minimum is dominated by carbon 2p, carbon 2s, and nitrogen 2s orbitals. A unique aspect of P21 ferroelectric phase is the permanent dipole within the material. We found that P21 DIPAB has a spontaneous polarization of 22.64 consistent with recent findings which make it good candidate for the creation of ferroelectric tunneling junctions (FTJs) which have the potential to be used as memory devices.

  17. Coulomb correlation effects in the quasiparticle band structure of ferromagnetic rare-earth insulators

    Science.gov (United States)

    Nolting, W.; Borgiel, W.; Borstel, G.

    1988-05-01

    We present a method for calculating the temperature dependence of the electronic quasiparticle density of states (QDOS) of a ferromagnetic rare-earth insulator like EuO. Special attention is devoted to how the ``localized'' ferromagnetism manifests itself in x-ray photoemission and bremsstrahlung isochromat spectra. Our study includes the first six conduction bands of EuO (the first five are Eu 5d like, the sixth is mainly of Eu 6s character) as well as the rather flat 4f levels. The starting point is an extended d-f exchange model, the main parts of which are an exchange interaction between 4f moments and conduction electrons, a Coulomb repulsion between highly correlated 4f electrons, and a hybridization of 4f with conduction-band states. We use an exact T=0 relationship between spin-up quasiparticle energies and one-electron Bloch energies ɛm(k) for an optimal determination of the latter by performing a self-consistent, spin-polarized band-structure calculation based on density-functional theory. For finite temperatures the model is approximately solved by a many-body procedure. The QDOS exhibits a striking temperature dependence mainly due to the d-f exchange. Two 4f-like peaks appear in the spin-polarized QDOS, the low-energy one being occupied, the high-energy one being empty. The temperature dependence of the localized ferromagnetism appears in the QDOS as a temperature-dependent shift of spectral weight between the low- and the high-energy peak.

  18. Electronic structure of superlattices of graphene and hexagonal boron nitride

    KAUST Repository

    Kaloni, Thaneshwor P.

    2011-11-14

    We study the electronic structure of superlattices consisting of graphene and hexagonal boron nitride slabs, using ab initio density functional theory. We find that the system favors a short C–B bond length at the interface between the two component materials. A sizeable band gap at the Dirac point is opened for superlattices with single graphene layers but not for superlattices with graphene bilayers. The system is promising for applications in electronic devices such as field effect transistors and metal-oxide semiconductors.

  19. Projected shell model study of band structure of 90Nb

    Science.gov (United States)

    Kumar, Amit; Singh, Dhanvir; Gupta, Anuradha; Singh, Suram; Bharti, Arun

    2016-05-01

    A systematic study of two-quasiparticle bands of the odd-odd 90Nb nucleus is performed using the projected shell model approach. Yrast band with some other bands have been obtained and back-bending in moment of inertia has also been calculated and compared with the available experimental. On comparing the available experimental data, it is found that the treatment with PSM provides a satisfactory explanation of the available data.

  20. Electronic Structure of Single-Crystal Monolayer Graphene on Hydrogen-Terminated Germanium Surface

    Science.gov (United States)

    Ahn, Sung Joon; Lee, Jae-Hyun; Ahn, Joung Real; Whang, Dongmok

    2015-03-01

    Graphene, atomically flat 2-Dimensional layered nano material, has a lot of interesting characteristics from its unusual electronic structure. Almost properties of graphene are influenced by its crystallinity, therefore the uniform growth of single crystal graphene and layer control over the wafer scale areas remains a challenge in the fields of electronic, photonic and other devices based on graphene. Here, we report the method to make wafer scale single crystal monolayer graphene on hydrogen terminated germanium(110) surface and properties and electronic band structure of the graphene by using the tool of scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, electron transport measurement, electron diffraction and angle-resolved photoemission spectroscopy.

  1. Electronic Structure of Lithium Tetraborate

    Science.gov (United States)

    2010-06-01

    limited effort of Kuznetsov, et al. [27] in mapping the valence band states via XPS , several of the most recent lithium tetraborate research efforts... Gd2O3 and HfO2 thin films in Rare-Earth Doping of Advanced Materials for Photonic Applications," (2008), pp. D07. 14. U. S. Department of Energy...space unit cells, and thus small Brillouin zones, as in the case of monoclinic Gd2O3 (-402) [4]. The absence of any such dispersion in this borate

  2. Bounce resonance scattering of radiation belt electrons by H+ band EMIC waves

    Science.gov (United States)

    Cao, Xing; Ni, Binbin; Summers, Danny; Bortnik, Jacob; Tao, Xin; Shprits, Yuri Y.; Lou, Yuequn; Gu, Xudong; Fu, Song; Shi, Run; Xiang, Zheng; Wang, Qi

    2017-02-01

    We perform a detailed analysis of bounce-resonant pitch angle scattering of radiation belt electrons due to electromagnetic ion cyclotron (EMIC) waves. It is found that EMIC waves can resonate with near-equatorially mirroring electrons over a wide range of L shells and energies. H+ band EMIC waves efficiently scatter radiation belt electrons of energy >100 keV from near 90° pitch angles to lower pitch angles where the cyclotron resonance mechanism can take over to further diffuse electrons into the loss cone. Bounce-resonant electron pitch angle scattering rates show a strong dependence on L shell, wave normal angle distribution, and wave spectral properties. We find distinct quantitative differences between EMIC wave-induced bounce-resonant and cyclotron-resonant diffusion coefficients. Cyclotron-resonant electron scattering by EMIC waves has been well studied and found to be a potentially crucial electron scattering mechanism. The new investigation here demonstrates that bounce-resonant electron scattering may also be very important. We conclude that bounce resonance scattering by EMIC waves should be incorporated into future modeling efforts of radiation belt electron dynamics.

  3. Theoretical and experimental differential cross sections for electron impact excitation of the electronic bands of furfural

    Science.gov (United States)

    Jones, D. B.; Neves, R. F. C.; Lopes, M. C. A.; da Costa, R. F.; do N. Varella, M. T.; Bettega, M. H. F.; Lima, M. A. P.; García, G.; Limão-Vieira, P.; Brunger, M. J.

    2016-03-01

    We report results from a joint experimental and theoretical investigation into electron scattering from the important industrial species furfural (C5H4O2). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C5H4O2. The measurements were carried out at energies in the range 20-40 eV, and for scattered-electron angles between 10° and 90°. The energy resolution of those experiments was typically ˜80 meV. Corresponding Schwinger multichannel method with pseudo-potential calculations, for energies between 6-50 eV and with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were undertaken at the static exchange plus polarisation-level using a minimum orbital basis for single configuration interaction (MOB-SCI) approach. Agreement between the measured and calculated DCSs was qualitatively quite good, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOB-SCI. The role of multichannel coupling on the computed electronic-state DCSs is also explored in some detail.

  4. Experimental Work With Photonic Band Gap Fiber: Building A Laser Electron Accelerator

    Energy Technology Data Exchange (ETDEWEB)

    Lincoln, Melissa; Ischebeck, Rasmus; Nobel, Robert; Siemann, Robert; /SLAC

    2006-09-29

    In the laser acceleration project E-163 at the Stanford Linear Accelerator Center, work is being done toward building a traveling wave accelerator that uses as its accelerating structure a length of photonic band gap fiber. The small scale of the optical fiber allows radiation at optical wavelengths to be used to provide the necessary accelerating energy. Optical wavelength driving energy in a small structure yields higher accelerating fields. The existence of a speed-of-light accelerating mode in a photonic band gap fiber has been calculated previously [1]. This paper presents an overview of several of the experimental challenges posed in the development of the proposed photonic band gap fiber accelerator system.

  5. Electronic structure and tautomerism of thioamides

    Energy Technology Data Exchange (ETDEWEB)

    Novak, Igor, E-mail: inovak@csu.edu.au [Charles Sturt University, POB 883, Orange, NSW 2800 (Australia); Klasinc, Leo, E-mail: klasinc@irb.hr [Physical Chemistry Department, Ruđer Bošković Institute, HR-10002 Zagreb (Croatia); McGlynn, Sean P., E-mail: sean.mcglynn@chemgate.chem.lsu.edu [Louisiana State University, Baton Rouge, LA 70803 (United States)

    2016-05-15

    Highlights: • Electronic structure of thioamide group and its relation to Lewis basicity. • Tautomerism of the (thio)amide groups. • Substituent effects on the electronic structure of (thio)amide group. - Abstract: The electronic structures of several thioamides have been studied by UV photoelectron spectroscopy (UPS). The relative stabilities of keto–enol tautomers have been determined using high-level ab initio calculations and the results were used in the analysis of UPS spectra. The main features of electronic structure and tautomerism of thioamide derivatives are discussed. The predominant tautomers in the gas phase are of keto–(thio)keto form. The addition of cyclohexanone moiety to the thioamide group enhances the Lewis base character of the sulfur atom. The addition of phenyl group to the (thio)amide group significantly affects its electronic structure.

  6. Electronic structure of disordered alloys, surfaces and interfaces

    CERN Document Server

    Turek, Ilja; Kudrnovský, Josef; Šob, Mojmír; Weinberger, Peter

    1997-01-01

    At present, there is an increasing interest in the prediction of properties of classical and new materials such as substitutional alloys, their surfaces, and metallic or semiconductor multilayers. A detailed understanding based on a thus of the utmost importance for fu­ microscopic, parameter-free approach is ture developments in solid state physics and materials science. The interrela­ tion between electronic and structural properties at surfaces plays a key role for a microscopic understanding of phenomena as diverse as catalysis, corrosion, chemisorption and crystal growth. Remarkable progress has been made in the past 10-15 years in the understand­ ing of behavior of ideal crystals and their surfaces by relating their properties to the underlying electronic structure as determined from the first principles. Similar studies of complex systems like imperfect surfaces, interfaces, and mul­ tilayered structures seem to be accessible by now. Conventional band-structure methods, however, are of limited use ...

  7. Gate-Tunable Band Structure of the LaAlO3 -SrTi O3 Interface

    Science.gov (United States)

    Smink, A. E. M.; de Boer, J. C.; Stehno, M. P.; Brinkman, A.; van der Wiel, W. G.; Hilgenkamp, H.

    2017-03-01

    The two-dimensional electron system at the interface between LaAlO3 and SrTiO3 has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schrödinger-Poisson calculations and observe a Lifshitz transition at a density of 2.9 ×1 013cm-2 . Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex-oxide interfaces.

  8. Electron tomography of dislocation structures

    Energy Technology Data Exchange (ETDEWEB)

    Liu, G.S.; House, S.D.; Kacher, J. [Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green St., Urbana, IL 61801 (United States); Tanaka, M.; Higashida, K. [Department of Materials Science and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Robertson, I.M., E-mail: irobertson@wisc.edu [Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green St., Urbana, IL 61801 (United States); Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706 (United States)

    2014-01-15

    Recent developments in the application of electron tomography for characterizing microstructures in crystalline solids are described. The underlying principles for electron tomography are presented in the context of typical challenges in adapting the technique to crystalline systems and in using diffraction contrast imaging conditions. Methods for overcoming the limitations associated with the angular range, the number of acquired images, and uniformity of image contrast are introduced. In addition, a method for incorporating the real space coordinate system into the tomogram is presented. As the approach emphasizes development of experimental solutions to the challenges, the solutions developed and implemented are presented in the form of examples.

  9. Surface-plasmon enhanced photodetection at communication band based on hot electrons

    Energy Technology Data Exchange (ETDEWEB)

    Wu, Kai; Zhan, Yaohui, E-mail: yhzhan@suda.edu.cn, E-mail: xfli@suda.edu.cn; Wu, Shaolong; Deng, Jiajia; Li, Xiaofeng, E-mail: yhzhan@suda.edu.cn, E-mail: xfli@suda.edu.cn [College of Physics, Optoelectronics and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China and Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province and Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006 (China)

    2015-08-14

    Surface plasmons can squeeze light into a deep-subwavelength space and generate abundant hot electrons in the nearby metallic regions, enabling a new paradigm of photoconversion by the way of hot electron collection. Unlike the visible spectral range concerned in previous literatures, we focus on the communication band and design the infrared hot-electron photodetectors with plasmonic metal-insulator-metal configuration by using full-wave finite-element method. Titanium dioxide-silver Schottky interface is employed to boost the low-energy infrared photodetection. The photodetection sensitivity is strongly improved by enhancing the plasmonic excitation from a rationally engineered metallic grating, which enables a strong unidirectional photocurrent. With a five-step electrical simulation, the optimized device exhibits an unbiased responsivity of ∼0.1 mA/W and an ultra-narrow response band (FWHM = 4.66 meV), which promises to be a candidate as the compact photodetector operating in communication band.

  10. Differential cross sections for electron impact excitation of the electronic bands of phenol

    Energy Technology Data Exchange (ETDEWEB)

    Neves, R. F. C. [School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001 (Australia); Instituto Federal do Sul de Minas Gerais, Campus Poços de Caldas, Minas Gerais (Brazil); Departamento de Física, UFJF, Juiz de Fora, Minas Gerais (Brazil); Jones, D. B. [School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001 (Australia); Lopes, M. C. A.; Nixon, K. L. [Departamento de Física, UFJF, Juiz de Fora, Minas Gerais (Brazil); Silva, G. B. da [School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001 (Australia); Universidade Federal de Mato Grosso, Barra do Garças, Mato Grosso (Brazil); Duque, H. V. [School of Chemical and Physical Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001 (Australia); Departamento de Física, UFJF, Juiz de Fora, Minas Gerais (Brazil); Oliveira, E. M. de; Lima, M. A. P. [Instituto de Física “Gleb Wataghin,” Universidade Estadual de Campinas, 13083-859 Campinas, São Paulo (Brazil); Costa, R. F. da [Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-580 Santo André, São Paulo (Brazil); Varella, M. T. do N. [Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970 São Paulo (Brazil); Bettega, M. H. F. [Departamento de Física, Universidade Federal do Paraná, CP 19044, 81531-990 Curitiba, Paraná (Brazil); and others

    2015-03-14

    We report results from a joint theoretical and experimental investigation into electron scattering from the important organic species phenol (C{sub 6}H{sub 5}OH). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C{sub 6}H{sub 5}OH. The measurements were carried out at energies in the range 15–40 eV, and for scattered-electron angles between 10{sup ∘} and 90{sup ∘}. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potentials calculations, with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were conducted at the static exchange plus polarisation (SEP)-level using a minimum orbital basis for single configuration interaction (MOBSCI) approach. Agreement between the measured and calculated DCSs was typically fair, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOBSCI.

  11. The first UV absorption band for indole is not due to two simultaneous orthogonal electronic transitions differing in dipole moment.

    Science.gov (United States)

    Catalán, Javier

    2015-05-21

    The currently accepted model for the photophysics of indole assumes that the first UV absorption band encompasses two orthogonal electronic transitions ((1)Lb and (1)La), leading to two electronic states with a markedly different dipole moment. However, there is a body of evidence not explained by this model, which led us to develop a new photophysical model for indole. Based on the new model, the polarity of the electronic ground state (S0) in indoles is very similar to that of the first electronic excited state (S1) producing this structured emission; however, this excited state can lead to a highly dipolar excited state (S1') with largely structureless emission under the influence of the polarity of the medium, and also, very likely, of its viscosity. The molecular structure of the new excited state can be reversibly converted into the normal structure of the compound. Previous observations were confirmed by the absorption, emission, and excitation spectra for indole, as well as by its polarized emission and excitation spectra in various media. Thus, the polarized emission spectra for indole in glycerol at 283 K and 223 K showed the transition dipole moments for the emission from the first two excited states in a polar medium, S1 and S1', to differ by less than 20°.

  12. Band Structure and Fermi-Surface Properties of Ordered beta-Brass

    DEFF Research Database (Denmark)

    Skriver, Hans Lomholt; Christensen, N. E.

    1973-01-01

    The band structure of ordered β-brass (β′-CuZn) has been calculated throughout the Brillouin zone by the augmented-plane-wave method. The present band model differs from previous calculations with respect to the position and width of the Cu 3d band. The derived dielectric function ε2(ω) and the p...

  13. Electronic structure and electron energy-loss spectroscopy of ZrO2 zirconia

    Science.gov (United States)

    Dash, L. K.; Vast, Nathalie; Baranek, Philippe; Cheynet, Marie-Claude; Reining, Lucia

    2004-12-01

    The atomic and electronic structures of zirconia are calculated within density functional theory, and their evolution is analyzed as the crystal-field symmetry changes from tetrahedral [cubic (c-ZrO2) and tetragonal (t-ZrO2) phases] to octahedral (hypothetical rutile ZrO2 ), to a mixing of these symmetries (monoclinic phase, m-ZrO2 ). We find that the theoretical bulk modulus in c-ZrO2 is 30% larger than the experimental value, showing that the introduction of yttria in zirconia has a significant effect. Electronic structure fingerprints which characterize each phase from their electronic spectra are identified. We have carried out electron energy-loss spectroscopy experiments at low momentum transfer and compared these results to the theoretical spectra calculated within the random phase approximation. We show a dependence of the valence and 4p ( N2,3 edge) plasmons on the crystal structure, the dependence of the latter being brought into the spectra by local-field effects. Last, we attribute low energy excitations observed in EELS of m-ZrO2 to defect states 2eV above the top of the intrinsic valence band, and the EELS fundamental band gap value is reconciled with the 5.2 or 5.8eV gaps determined by vacuum ultraviolet spectroscopy.

  14. Electronic structure and lattice dynamics of orthorhombic BiGaO{sub 3}

    Energy Technology Data Exchange (ETDEWEB)

    Kaczkowski, J., E-mail: jakub_k@ifmpan.poznan.pl

    2014-11-15

    Highlights: • Electronic structure of the orthorhombic phase of BiGaO{sub 3} have been calculated within GGA, hybrid HSE and TB-mBJ functionals. • Structural parameters were obtained within GGA and HSE functionals. • The band gap of BiGaO{sub 3} is indirect with the value: 1.88 eV (GGA), 3.14 eV (HSE) and 3.00 eV (TB-mBJ). • Elastic and vibrational properties were calculated. • The phonon band structure shows a soft mode between Γ and Z direction. - Abstract: The electronic structure, structural, elastic and vibrational properties of the orthorhombic BiGaO{sub 3} were calculated by using the density functional theory. For the electronic structure calculations standard generalized gradient approximation, nonlocal hybrid Heyd–Scuseria–Ernzerhof and semilocal Tran–Blaha functionals were used. The standard and hybrid functionals were applied to obtain structural parameters. The electronic structure shows that BiGaO{sub 3} is a indirect band gap semiconductor. The values of the band gap are 1.88 eV, 3.14 eV and 3.00 eV within generalized gradient approximation, hybrid and semilocal functionals, respectively. The direct method have been used to obtain the vibrational properties. The phonon band structure shows a soft mode between Γ and Z direction. This soft mode could be responsible for structural phase transition.

  15. Probing the graphite band structure with resonant soft-x-ray fluorescence

    Energy Technology Data Exchange (ETDEWEB)

    Carlisle, J.A.; Shirley, E.L.; Hudson, E.A. [Lawrence Berkeley National Lab., CA (United States)] [and others

    1997-04-01

    Soft x-ray fluorescence (SXF) spectroscopy using synchrotron radiation offers several advantages over surface sensitive spectroscopies for probing the electronic structure of complex multi-elemental materials. Due to the long mean free path of photons in solids ({approximately}1000 {angstrom}), SXF is a bulk-sensitive probe. Also, since core levels are involved in absorption and emission, SXF is both element- and angular-momentum-selective. SXF measures the local partial density of states (DOS) projected onto each constituent element of the material. The chief limitation of SXF has been the low fluorescence yield for photon emission, particularly for light elements. However, third generation light sources, such as the Advanced Light Source (ALS), offer the high brightness that makes high-resolution SXF experiments practical. In the following the authors utilize this high brightness to demonstrate the capability of SXF to probe the band structure of a polycrystalline sample. In SXF, a valence emission spectrum results from transitions from valence band states to the core hole produced by the incident photons. In the non-resonant energy regime, the excitation energy is far above the core binding energy, and the absorption and emission events are uncoupled. The fluorescence spectrum resembles emission spectra acquired using energetic electrons, and is insensitive to the incident photon`s energy. In the resonant excitation energy regime, core electrons are excited by photons to unoccupied states just above the Fermi level (EF). The absorption and emission events are coupled, and this coupling manifests itself in several ways, depending in part on the localization of the empty electronic states in the material. Here the authors report spectral measurements from highly oriented pyrolytic graphite.

  16. Structural Features That Stabilize ZnO Clusters: An Electronic Structure Approach

    Directory of Open Access Journals (Sweden)

    Csaba E. Szakacs

    2013-05-01

    Full Text Available We show that a simple approach to building small computationally inexpensive clusters offers insights on specific structural motifs that stabilize the electronic structure of ZnO. All-electron calculations on ZniOi needle (i = 6, 9, 12, 15, and 18 and plate (i = 9 and 18 clusters within the density functional theory (DFT formalism show a higher stability for ZnO needles that increases with length. Puckering of the rings to achieve a more wurtzite-like structure destabilizes the needles, although this destabilization is reduced by going to infinite needles (calculated using periodic boundary conditions. Calculations of density of states (DOS curves and band gaps for finite clusters and infinite needles highlight opportunities for band-gap tuning through kinetic control of nanocrystal growth.

  17. True photonic band-gap mode-control in VCSEL structures

    DEFF Research Database (Denmark)

    Romstad, F.; Madsen, M.; Birkedal, Dan;

    2003-01-01

    Photonic band-gap mode confinement in novel nano-structured large area VCSEL structures is confirmed by the amplified spontaneous emission spectrum. Both guide and anti-guide VCSEL structures are experimentally characterised to verify the photonic band-gap effect....

  18. DFTB Parameters for the Periodic Table: Part 1, Electronic Structure.

    Science.gov (United States)

    Wahiduzzaman, Mohammad; Oliveira, Augusto F; Philipsen, Pier; Zhechkov, Lyuben; van Lenthe, Erik; Witek, Henryk A; Heine, Thomas

    2013-09-10

    A parametrization scheme for the electronic part of the density-functional based tight-binding (DFTB) method that covers the periodic table is presented. A semiautomatic parametrization scheme has been developed that uses Kohn-Sham energies and band structure curvatures of real and fictitious homoatomic crystal structures as reference data. A confinement potential is used to tighten the Kohn-Sham orbitals, which includes two free parameters that are used to optimize the performance of the method. The method is tested on more than 100 systems and shows excellent overall performance.

  19. Electronic structure characterization and bandgap engineeringofsolar hydrogen materials

    Energy Technology Data Exchange (ETDEWEB)

    Guo, Jinghua

    2007-11-01

    Bandgap, band edge positions as well as the overall band structure of semiconductors are of crucial importance in photoelectrochemical and photocatalytic applications. The energy position of the band edge level can be controlled by the electronegativity of the dopants, the pH of the solution (flatband potential variation of 60 mV per pH unit), as well as by quantum confinement effects. Accordingly, band edges and bandgap can be tailored to achieve specific electronic, optical or photocatalytic properties. Synchrotron radiation with photon energy at or below 1 keV is giving new insight into such areas as condensed matter physics and extreme ultraviolet optics technology. In the soft x-ray region, the question tends to be, what are the electrons doing as they migrated between the atoms. In this paper, I will present a number of soft x-ray spectroscopic study of nanostructured 3d metal compounds Fe{sub 2}O{sub 3} and ZnO.

  20. Electronic Structure of Doped Trans-Polyacetylene

    Institute of Scientific and Technical Information of China (English)

    2002-01-01

    The behavior of electronic structures of doped trans-polyacetylene is revealed by a simplemethod. (C24H26)+n is used to simulate p-type doped trans-polyacetylene at various doping concentrations.The electronic structure is calculated by CNDO/2 method. These calculations show that at low doping lev-el, the decrease of electronic energy compensates the increase of elastic energy, thus the bond alternationexists, and the charge carriers are solitons. When doping level is high, the increase of elastic energy islarger than the decrease of electronic energy, the bond alternation disappears, solitons no longer exist,and polyacetylene is in a metalic state.

  1. Self-consistent treatment of v-groove quantum wire band structure in no parabolic approximation

    Directory of Open Access Journals (Sweden)

    Crnjanski Jasna V.

    2004-01-01

    Full Text Available The self-consistent no parabolic calculation of a V-groove-quantum-wire (VQWR band structure is presented. A comparison with the parabolic flat-band model of VQWR shows that both, the self-consistency and the nonparabolicity shift sub band edges, in some cases even in the opposite directions. These shifts indicate that for an accurate description of inter sub band absorption, both effects have to be taken into the account.

  2. Cohesive band structure of carbon nanotubes for applications in quantum transport.

    Science.gov (United States)

    Arora, Vijay K; Bhattacharyya, Arkaprava

    2013-11-21

    An integrated cohesive band structure of carbon nanotubes (CNTs) applicable to all chirality directions (n, m), starting from the Dirac cone of a graphene nanolayer in k-space, is demarcated, in direct contrast to dissimilar chiral and achiral versions in the published literature. The electron wave state of a CNT is quantized into one-dimensional (1-D) nanostructure with a wrapping mode, satisfying the boundary conditions from one Dirac K-point to an equivalent neighboring one with an identical phase and returning to the same K point. The repetitive rotation for an identical configuration with added band index (n-m)mod3, yields one metallic (M) with zero bandgap corresponding to (n-m)mod3 = 0, semiconducting state SC1 with (n-m)mod3 = 1 and SC2 with (n-m)mod3 = 2. The band gap and effective mass of SC2 state are twice as large as those of SC1 state. A broad-spectrum expression signifying the linear dependence of the effective mass on the bandgap is obtained. Both the Fermi energy and the intrinsic velocity limiting the current to the saturation level is calculated as a function of the carrier concentration. Limitations of the parabolic approximation are pointed out. Several new features of the band structure are acquired in a seamlessly unified mode for all CNTs, making it suitable for all-encompassing applications. Applications of the theory to high-field transport are advocated with an example of a metallic CNT, in agreement with experimental observations. The mechanism behind the breakdown of the linear current-voltage relation of Ohm's law and the associated surge in resistance are explained on the basis of the nonequilibrium Arora's distribution function (NEADF). These results are important for the performance evaluation and characterization of a variety of applications on CNT in modern nanoscale circuits and devices.

  3. Evidence for an ultrafast breakdown of the BeO band structure due to swift argon and xenon ions.

    Science.gov (United States)

    Schiwietz, G; Czerski, K; Roth, M; Grande, P L; Koteski, V; Staufenbiel, F

    2010-10-29

    Auger-electron spectra associated with Be atoms in the pure metal lattice and in the stoichiometric oxide have been investigated for different incident charged particles. For fast incident electrons, for Ar7+ and Ar15+ ions as well as Xe15+ and Xe31+ ions at velocities of 6% to 10% the speed of light, there are strong differences in the corresponding spectral distributions of Be-K Auger lines. These differences are related to changes in the local electronic band structure of BeO on a femtosecond time scale after the passage of highly charged heavy ions.

  4. Analysis of the Band-Structure in (Ga, MnAs Epitaxial Layers by Optical Methods

    Directory of Open Access Journals (Sweden)

    O. Yastrubchak

    2012-03-01

    Full Text Available The ternary III-V semiconductor (Ga, MnAs has recently drawn a lot of attention as the model diluted ferromagnetic semiconductor, combining semiconducting properties with magnetism. (Ga, MnAs layers are usually gown by the low-temperature molecular-beam epitaxy (LT-MBE technique. Below a magnetic transition temperature, TC, substitutional Mn2+ ions are ferromagnetically ordered owing to interaction with spin-polarized holes. However, the character of electronic states near the Fermi energy and the electronic structure in ferromagnetic (Ga, MnAs are still a matter of controversy. The photoreflectance (PR spectroscopy was applied to study the band-structure evolution in (Ga, MnAs layers with increasing Mn content. We have investigated thick (800-700 nm and 230-300 nm (Ga, MnAs layers with Mn content in the wide range from 0.001 % to 6 % and, as a reference, undoped GaAs layer, grown by LT-MBE on semi-insulating (001 GaAs substrates. Our findings were interpreted in terms of the model, which assumes that the mobile holes residing in the valence band of ferromagnetic (Ga, MnAs and the Fermi level position determined by the concentration of valence-band holes. The ternary III-V semiconductor (Ga, MnAs has recently drawn a lot of attention as the model diluted ferromagnetic semiconductor, combining semiconducting properties with magnetism. (Ga, MnAs layers are usually gown by the low-temperature molecular-beam epitaxy (LT-MBE technique. Below a magnetic transition temperature, TC, substitutional Mn2+ ions are ferromagnetically ordered owing to interaction with spin-polarized holes. However, the character of electronic states near the Fermi energy and the electronic structure in ferromagnetic (Ga, MnAs are still a matter of controversy. The photoreflectance (PR spectroscopy was applied to study the band-structure evolution in (Ga, MnAs layers with increasing Mn content. We have investigated thick (800-700 nm and 230-300 nm (Ga

  5. Atomic and electronic structure of exfoliated black phosphorus

    Energy Technology Data Exchange (ETDEWEB)

    Wu, Ryan J.; Topsakal, Mehmet; Jeong, Jong Seok; Wentzcovitch, Renata M.; Mkhoyan, K. Andre, E-mail: mkhoyan@umn.edu [Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (United States); Low, Tony; Robbins, Matthew C.; Haratipour, Nazila; Koester, Steven J. [Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (United States)

    2015-11-15

    Black phosphorus, a layered two-dimensional crystal with tunable electronic properties and high hole mobility, is quickly emerging as a promising candidate for future electronic and photonic devices. Although theoretical studies using ab initio calculations have tried to predict its atomic and electronic structure, uncertainty in its fundamental properties due to a lack of clear experimental evidence continues to stymie our full understanding and application of this novel material. In this work, aberration-corrected scanning transmission electron microscopy and ab initio calculations are used to study the crystal structure of few-layer black phosphorus. Directly interpretable annular dark-field images provide a three-dimensional atomic-resolution view of this layered material in which its stacking order and all three lattice parameters can be unambiguously identified. In addition, electron energy-loss spectroscopy (EELS) is used to measure the conduction band density of states of black phosphorus, which agrees well with the results of density functional theory calculations performed for the experimentally determined crystal. Furthermore, experimental EELS measurements of interband transitions and surface plasmon excitations are also consistent with simulated results. Finally, the effects of oxidation on both the atomic and electronic structure of black phosphorus are analyzed to explain observed device degradation. The transformation of black phosphorus into amorphous PO{sub 3} or H{sub 3}PO{sub 3} during oxidation may ultimately be responsible for the degradation of devices exposed to atmosphere over time.

  6. A New Q-Band EPR Probe for Quantitative Studies of Even Electron Metalloproteins

    Science.gov (United States)

    Petasis, D. T.; Hendrich, M. P.

    1999-02-01

    Existing Q-band (35 GHz) EPR spectrometers employ cylindrical cavities for more intense microwave magnetic fields B1, but are so constructed that only one orientation between the external field B and B1is allowed, namely the B ⊥ B1orientation, thus limiting the use of the spectrometer to measurements on Kramers spin systems (odd electron systems). We have designed and built a Q-band microwave probe to detect EPR signals in even electron systems, which operates in the range 2 K ≤ T ≤ 300 K for studies of metalloprotein samples. The cylindrical microwave cavity operates in the TE011mode with cylindrical wall coupling to the waveguide, thus allowing all orientations of the external magnetic field B relative to the microwave field B1. Such orientations allow observation of EPR transitions in non-Kramers ions (even electron) which are either forbidden or significantly weaker for B ⊥ B1. Rotation of the external magnetic field also permits easy differentiation between spin systems from even and odd electron oxidation states. The cavity consists of a metallic helix and thin metallic end walls mounted on epoxy supports, which allows efficient penetration of the modulation field. The first quantitative EPR measurements from a metalloprotein (Hemerythrin) at 35 GHz with B1‖ B are presented.

  7. Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption.

    Science.gov (United States)

    Roy, Ahin; Amin, Kazi Rafsanjani; Tripathi, Shalini; Biswas, Sangram; Singh, Abhishek K; Bid, Aveek; Ravishankar, N

    2017-01-13

    Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO2 adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO2 reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO2 supports the appearance of a metallic state in NO2 adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in NO2 adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors.

  8. Phosphorene Nanoribbons: Electronic Structure and Electric Field Modulation

    Science.gov (United States)

    Soleimanikahnoj, Sina; Knezevic, Irena

    Phosphorene, a newcomer among the 2D van der Waals materials, has attracted the attention of many scientists due to its promising electronic properties. Monolayer phosphorene has a direct band gap of 2 eV located at the Gamma point of the Brillouin zone. Increasing the number of layers reduces the bandgap due to the van der Waals interaction. The direct nature of the bandgap makes phosphorene particularly favorable for electronic transport and optoelectronic applications. While multilayer phosphorene sheets have been studied, the electronic properties of their 1D counterparts are still unexplored. An accurate tight-binding model was recently proposed for multilayer phosphorene nanoribbons. Employing this model along with the non-equilibrium Green's function method, we calculate the band structure and electronic properties of phosphorene nanoribbons. We show that, depending on the edge termination, phosphorene nanoribbons can be metallic or semiconducting. Our analysis also shows that the electronic properties of phosphorene nanoribbons are highly tunable by in-plane and out-of-plane electric fields. In metallic ribbons, the conductance can be switched off by a threshold electric field, similar to field effect devices. Support by the NSF through the University of Wisconsin MRSEC Seed (NSF Award DMR-1121288).

  9. Design, realization and test of C-band accelerating structures for the SPARC_LAB linac energy upgrade

    Science.gov (United States)

    Alesini, D.; Bellaveglia, M.; Biagini, M. E.; Boni, R.; Brönnimann, M.; Cardelli, F.; Chimenti, P.; Clementi, R.; Di Pirro, G.; Di Raddo, R.; Ferrario, M.; Ficcadenti, L.; Gallo, A.; Kalt, R.; Lollo, V.; Palumbo, L.; Piersanti, L.; Schilcher, T.

    2016-11-01

    The energy upgrade of the SPARC_LAB photo-injector at LNF-INFN (Frascati, Italy) has been originally conceived replacing one low gradient (13 MV/m) 3 m long SLAC type S-band traveling wave (TW) section with two 1.4 m long C-band accelerating sections. Due to the higher gradients reached by such structures, a higher energy beam can be obtained within the same accelerator footprint length. The use of C-band structures for electron acceleration has been adopted in a few FEL linacs in the world, among others, the Japanese Free Electron Laser at SPring-8 and the SwissFEL at Paul Scherrer Institute (PSI). The C-band sections are traveling wave, constant impedance structures with symmetric input and output axial couplers. Their design has been optimized for the operation with a SLED RF pulse compressor. In this paper we briefly review their design criteria and we focus on the construction, tuning, low and high-power RF tests. We also illustrate the design and realization of the dedicated low level RF system that has been done in collaboration with PSI in the framework of the EU TIARA project. Preliminary experimental results appear to confirm the operation of such structures with accelerating gradients larger than 35 MV/m.

  10. Electronic Structure of the Actinide Metals

    DEFF Research Database (Denmark)

    Johansson, B.; Skriver, Hans Lomholt

    1982-01-01

    Some recent experimental photoelectron spectroscopic results for the actinide metals are reviewed and compared with the theoretical picture of the basic electronic structure that has been developed for the actinides during the last decade. In particular the experimental data confirm the change from...... itinerant to localized 5f electron behaviour calculated to take place between plutonium and americium. From experimental data it is shown that the screening of deep core-holes is due to 5f electrons for the lighter actinide elements and 6d electrons for the heavier elements. A simplified model for the full...... LMTO electronic structure calculations is introduced. In this model the spd and 5f electronic contributions are treated as separable entities. It is shown that the model reproduces quite well the results from the full treatment. The equilibrium volume, cohesive energy and bulk modulus are calculated...

  11. Epitaxial graphene electronic structure and transport

    Energy Technology Data Exchange (ETDEWEB)

    De Heer, Walt A; Berger, Claire; Wu Xiaosong; Sprinkle, Mike; Hu Yike; Ruan Ming; First, Phillip N [School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 (United States); Stroscio, Joseph A [Center for Nanoscale Science and Technology, NIST, Gaithersburg, MD 20899 (United States); Haddon, Robert [Center for Nanoscale Science and Engineering, Departments of Chemistry and Chemical and Environmental Engineering, University of California, Riverside, CA 92521 (United States); Piot, Benjamin; Faugeras, Clement; Potemski, Marek [LNCMI -CNRS, Grenoble, 38042 Cedex 9 (France); Moon, Jeong-Sun, E-mail: walt.deheer@physics.gateh.ed [HRL Laboratories LLC, Malibu, CA 90265 (United States)

    2010-09-22

    Since its inception in 2001, the science and technology of epitaxial graphene on hexagonal silicon carbide has matured into a major international effort and is poised to become the first carbon electronics platform. A historical perspective is presented and the unique electronic properties of single and multilayered epitaxial graphenes on electronics grade silicon carbide are reviewed. Early results on transport and the field effect in Si-face grown graphene monolayers provided proof-of-principle demonstrations. Besides monolayer epitaxial graphene, attention is given to C-face grown multilayer graphene, which consists of electronically decoupled graphene sheets. Production, structure and electronic structure are reviewed. The electronic properties, interrogated using a wide variety of surface, electrical and optical probes, are discussed. An overview is given of recent developments of several device prototypes including resistance standards based on epitaxial graphene quantum Hall devices and new ultrahigh frequency analogue epitaxial graphene amplifiers.

  12. Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

    Science.gov (United States)

    Wickramaratne, Darshana; Zahid, Ferdows; Lake, Roger K.

    2015-08-01

    The valence band of a variety of few-layer, two-dimensional materials consist of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a "Mexican hat" or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III-VI materials GaS, GaSe, InS, InSe, for Bi2Se3, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III-VI materials and Bi2Se3 is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.

  13. Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

    Energy Technology Data Exchange (ETDEWEB)

    Wickramaratne, Darshana, E-mail: darshanaw@engineering.ucsb.edu, E-mail: rlake@ece.ucr.edu; Lake, Roger K., E-mail: darshanaw@engineering.ucsb.edu, E-mail: rlake@ece.ucr.edu [Laboratory for Terahertz and Terascale Electronics, Department of Electrical and Computer Engineering, University of California, Riverside, California 92521 (United States); Zahid, Ferdows [Department of Physics and the Center of Theoretical and Computational Physics, The University of Hong Kong, Pokfulam Road, Hong Kong (China)

    2015-08-21

    The valence band of a variety of few-layer, two-dimensional materials consist of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a “Mexican hat” or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III–VI materials GaS, GaSe, InS, InSe, for Bi{sub 2}Se{sub 3}, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III–VI materials and Bi{sub 2}Se{sub 3} is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.

  14. First-principles studies of electric field effects on the electronic structure of trilayer graphene

    Science.gov (United States)

    Wang, Yun-Peng; Li, Xiang-Guo; Fry, James N.; Cheng, Hai-Ping

    2016-10-01

    A gate electric field is a powerful way to manipulate the physical properties of nanojunctions made of two-dimensional crystals. To simulate field effects on the electronic structure of trilayer graphene, we used density functional theory in combination with the effective screening medium method, which enables us to understand the field-dependent layer-layer interactions and the fundamental physics underlying band gap variations and the resulting band modifications. Two different graphene stacking orders, Bernal (or ABC) and rhombohedral (or ABA), were considered. In addition to confirming the experimentally observed band gap opening in ABC-stacked and the band overlap in ABA-stacked trilayer systems, our results reveal rich physics in these fascinating systems, where layer-layer couplings are present but some characteristics features of single-layer graphene are partially preserved. For ABC stacking, the electric-field-induced band gap size can be tuned by charge doping, while for ABA band the tunable quantity is the band overlap. Our calculations show that the electronic structures of the two stacking orders respond very differently to charge doping. We find that in the ABA stacking hole doping can reopen a band gap in the band-overlapping region, a phenomenon distinctly different from electron doping. The physical origins of the observed behaviors were fully analyzed, and we conclude that the dual-gate configuration greatly enhances the tunability of the trilayer systems.

  15. Electronic structure of EuN: Growth, spectroscopy, and theory

    DEFF Research Database (Denmark)

    Richter, J. H.; Ruck, B.J.; Simpson, M.

    2011-01-01

    We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth...... and the lowest-lying 8S multiplet. The Hubbard-I model is also in good agreement with purely atomic multiplet calculations for the Eu M-edge XAS. LSDA+U and DMFT calculations find a metallic ground state, while QSGW results predict a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption...... edge seen in optical transmittance at 0.9 eV, and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow-band-gap semiconductor that lies...

  16. Efficient calculation of inelastic vibration signals in electron transport: Beyond the wide-band approximation

    DEFF Research Database (Denmark)

    Lu, Jing Tao; Christensen, Rasmus Bjerregaard; Foti, Giuseppe;

    2014-01-01

    We extend the simple and efficient lowest order expansion (LOE) for inelastic electron tunneling spectroscopy (IETS) to include variations in the electronic structure on the scale of the vibration energies. This enables first-principles calculations of IETS line shapes for molecular junctions clo...

  17. The magneto-optical properties of semiconductors and the band structure of gallium nitride

    CERN Document Server

    Shields, P A

    2001-01-01

    the wells, and observing a resonant phenomenon from an undoped GaN/Al sub x Ga sub 1 sub - sub x N single heterojunction. The oscillating peak intensity was understood in terms of the changing screening efficiency of the two-dimensional electron gas. Finally, many-body effects within an electron-hole plasma in ln sub x Al sub y Ga sub 1 sub - sub x sub - sub y As quantum wells have been explored to study the effects of mass renormalisation at carrier densities typically present in laser devices. A peak in the mass renormalisation confirms the importance of excitonic correlations in the excited plasma. This thesis has applied magneto-optical techniques to enable a better understanding of the band structure of gallium nitride, particularly the complex behaviour expected as a result of strong valence band mixing. Effective hole masses are deduced from free excitonic-like transitions observed in magneto-reflectivity, to give a heavy A hole mass of 1.3 m sub o and the first experimental suggestion of a light B ban...

  18. Photonic band gap of one-dimensional periodic structure containing dispersive left-handed metamaterials

    Institute of Scientific and Technical Information of China (English)

    Zhanshan Wang; Tian Sang; Fengli Wang; Yonggang Wu; Lingyan Chen

    2008-01-01

    Band structures of one-dimensional(1D)photonic crystals(PCs)containing dispersive left-handed metamaterials are studied theoretically.The results show that the structure possesses a type of photonic band gap originating from total internal reflection(TIR).In contrast to photonic band gaps corresponding to zero average refractive index and zero phase.the TIR gap exhibits sharp angular effect and has no polarization effect.It should also be noted that band structures of transverse electric(TE) and transverse magnetic(TM) mode waves are exactly the same in the PCs we studied.

  19. Theoretical study on the band structure and optical properties of 4H-SiC

    Institute of Scientific and Technical Information of China (English)

    Xu Peng-Shou; Xie Chang-Kun; Pan Hai-Bin; Xu Fa-Qiang

    2004-01-01

    We have studied the band structure and optical properties of 4H-SiC by using a full potential linearized augmented plane waves (FPLAPW) method. The density of states (DOS) and band structure are presented. The imaginary part of the dielectric function has been obtained directly from the band structure calculation. With band gap correction, the real part of the dielectric function has been derived from the imaginary part by the Kramers-Kronig (KK) dispersion relationship. The values of reflectivity for normal incidence as a function of photon energy have also been calculated.We found the theoretical results are in good agreement with the experimental data.

  20. Block Tridiagonal Matrices in Electronic Structure Calculations

    DEFF Research Database (Denmark)

    Petersen, Dan Erik

    This thesis focuses on some of the numerical aspects of the treatment of the electronic structure problem, in particular that of determining the ground state electronic density for the non–equilibrium Green’s function formulation of two–probe systems and the calculation of transmission in the Lan...

  1. Structure of Wet Specimens in Electron Microscopy

    Science.gov (United States)

    Parsons, D. F.

    1974-01-01

    Discussed are past work and recent advances in the use of electron microscopes for viewing structures immersed in gas and liquid. Improved environmental chambers make it possible to examine wet specimens easily. (Author/RH)

  2. Computational Chemistry Using Modern Electronic Structure Methods

    Science.gov (United States)

    Bell, Stephen; Dines, Trevor J.; Chowdhry, Babur Z.; Withnall, Robert

    2007-01-01

    Various modern electronic structure methods are now days used to teach computational chemistry to undergraduate students. Such quantum calculations can now be easily used even for large size molecules.

  3. New bismuth borophosphate Bi{sub 4}BPO{sub 10}: Synthesis, crystal structure, optical and band structure analysis

    Energy Technology Data Exchange (ETDEWEB)

    Babitsky, Nicolay A.; Leshok, Darya Y.; Mikhaleva, Natalia S. [Siberian Federal University, 79 Svobodny Av, Krasnoyarsk, 660041 (Russian Federation); Kuzubov, Aleksandr A., E-mail: alexkuzubov@gmail.com [Siberian Federal University, 79 Svobodny Av, Krasnoyarsk, 660041 (Russian Federation); Institute of Physics SB RAS, Krasnoyarsk 660036 (Russian Federation); Zhereb, Vladimir P. [Siberian Federal University, 79 Svobodny Av, Krasnoyarsk, 660041 (Russian Federation); Kirik, Sergei D., E-mail: kiriksd@yandex.ru [Siberian Federal University, 79 Svobodny Av, Krasnoyarsk, 660041 (Russian Federation)

    2015-08-01

    New bismuth borophosphate Bi{sub 4}BPO{sub 10} was obtained by spontaneous crystallization from the melt of correspondent composition at 804 °C. Crystal structure with orthorhombic lattice parameters: a = 22.5731(3) Å, b = 14.0523(2) Å, c = 5.5149(1) Å, V = 1749.34(4), Z = 8, SG Pcab was determined by X-ray powder diffraction technique. The [Bi{sub 2}O{sub 2}]{sup 2+} -layers, which are typical for bismuth oxide compounds, transform into cationic endless strips of 4 bismuth atoms width directed along the c-axis in Bi{sub 4}BPO{sub 10}. The strips combining stacks are separated by flat triangle [BO{sub 3}]{sup 3−} -anions within stacks. Neighboring stacks are separated by tetrahedral [PO{sub 4}]{sup 3−}-anions and shifted relatively to each other. Bismuth atoms are placed in 5–7 vertex oxygen irregular polyhedra. Bi{sub 4}BPO{sub 10} is stable up to 812 °C, then melts according to the peritectic law. The absorption spectrum in the range 350–700 nm was obtained and the width of the forbidden band was estimated as 3.46 eV. The band electronic structure of Bi{sub 4}BPO{sub 10} was modeled using DFT approach. The calculated band gap (3.56 eV) is in good agreement with the experimentally obtained data. - Graphical abstract: Display Omitted - Highlights: • New bismuth borophosphate with composition Bi{sub 4}BPO{sub 10} was synthesized. • The crystal structure was determined by X-ray powder diffraction technique. • Bismuth-oxygen part [Bi{sub 4}O{sub 3}]{sup 6+} forms endless strips of 4 bismuth atoms width. • Electronic structure was modeled by DFT method. • The calculated band gap (3.56 eV) is very close to the experimental one (3.46 eV)

  4. Design and analysis of defected ground structure transformer for dual-band antenna

    Directory of Open Access Journals (Sweden)

    Wai-Wa Choi

    2014-12-01

    Full Text Available This study presents a novel dual-band antenna design methodology utilising a dual-frequency impedance transformer with defected ground structure (DGS. The proposed dual-frequency DGS impedance transformer generates a second resonant frequency from a conventional single-band antenna, resulting dual-band operation. Simulation studies illustrate that the adopted design achieves versatile configurations for arbitrary operating frequencies and diverse input impedance ranges in planar antenna structures. To experimentally verify the proposed design methodology, a dual-frequency DGS impedance transformer was implemented for a 2.4 GHz monopole antenna to obtain a 900/2400 MHz dual-band antenna. Measurement shows that the 10 dB return loss bandwidth in 900 MHz band is 34.4 MHz, whereas that in 2400 MHz band is wider than 530 MHz. Typical monopole radiation patterns are observed at both operating bands.

  5. Structure of dipole bands in doubly odd 102Ag

    Science.gov (United States)

    Singh, V.; Sihotra, S.; Malik, S. S.; Bhat, G. H.; Palit, R.; Sheikh, J. A.; Kumar, S.; Singh, N.; Singh, K.; Goswamy, J.; Sethi, J.; Saha, S.; Trivedi, T.; Mehta, D.

    2016-10-01

    Excited states in the transitional doubly odd 102Ag nucleus were populated in the 75As(31P,p 3 n ) fusion-evaporation reaction using the 125 MeV incident 31P beam. The subsequent deexcitations were investigated through in-beam γ -ray spectroscopic techniques using the Indian National Gamma Array spectrometer equipped with 21 clover Ge detectors. The level scheme in 102Ag has been established up to excitation energy ˜6.5 MeV and angular momentum 19 ℏ . The earlier reported level scheme is considerably extended and modified to result in a pair of nearly degenerate negative-parity dipole bands. Lifetime measurements for the states of these two dipole bands have been performed by using the Doppler-shift attenuation method. The two nearly degenerate bands exhibit different features with regard to kinetic moment of inertia, and the reduced transition probabilities B (M 1 ) and B (E 2 ) , which do not favor these to be chiral partners. These bands are discussed in the framework of the hybrid version of tilted-axis cranking (tac) model calculations and assigned the π g9 /2⊗ν h11 /2 and π g9 /2⊗ν h11 /2(d5/2/g7 /2) 2 configurations. The tac model calculations are extended to the nearly degenerate bands observed in the heavier doubly odd Ag-108104 isotopes.

  6. Structural and electronic effects resulting from metal-flavin ligation.

    Science.gov (United States)

    Clarke, M J; Dowling, M G; Garafalo, A R; Brennan, T F

    1980-04-25

    The synthesis and physical properties of a model metalloflavin complex, [(10-methylisoalloxazine)-(NH3)4Ru](PF6)2 . 2H2O are reported. The structure of this stable, enantiomeric compound was elucidated by x-ray diffraction methods with a final unweighted R value of 0.054. Crystals belong to the triclinic space group P1 with unit cell dimensions: a = 9.631, b = 10.618, c = 13.216 A; alpha = 113.86, beta = 100.19, gamma = 94.12 degrees. Chelation of the metal ion to the flavin occurs at the N(5) and O(4) positions, with a short Ru-N(5) bond distance of 1.979 A. Steric and electronic factors induce a 9.9 degrees bend in the isoalloxazine ring system and a significant lengthening of the C(4a)-N(5) bond. The flavin absorption bands shift significantly toward lower energy on complexation and a new band occurs at 617 nm. These absorptions are pH-dependent and pK alpha values for the complex are 0.6 and 7.4. The spectra of this complex exhibit similarities to that of metallosemiquinone species and arguments are made that a significant amount of electron density is donated to the pi-system of the flavin. Proton NMR studies suggest enhanced electron density at the N(10) position, probably occurring through backbonding interactions. Cyclic voltametry studies are also consistent with substantial metal to ligand pi-electron donation, since it is significantly more difficult to reduce the coordinated flavin relative to the free ligand under the same conditions. Moreover, the complexed flavin accepts electrons in 1-electron rather than 2-electron steps. Spectroelectrochemical studies on the 1-electron reduced complex indicate a similarity with other M(II)-F1 species.

  7. Determining the band gap and mean kinetic energy of atoms from reflection electron energy loss spectra

    Energy Technology Data Exchange (ETDEWEB)

    Vos, M. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Marmitt, G. G. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Instituto de Fisica da Universidade Federal do Rio Grande do Sul, Avenida Bento Goncalves 9500, 91501-970 Porto Alegre, RS (Brazil); Finkelstein, Y. [Nuclear Research Center — Negev, Beer-Sheva 84190 (Israel); Moreh, R. [Physics Department, Ben-Gurion University of the Negev, Beer-Sheva 84105 (Israel)

    2015-09-14

    Reflection electron energy loss spectra from some insulating materials (CaCO{sub 3}, Li{sub 2}CO{sub 3}, and SiO{sub 2}) taken at relatively high incoming electron energies (5–40 keV) are analyzed. Here, one is bulk sensitive and a well-defined onset of inelastic excitations is observed from which one can infer the value of the band gap. An estimate of the band gap was obtained by fitting the spectra with a procedure that includes the recoil shift and recoil broadening affecting these measurements. The width of the elastic peak is directly connected to the mean kinetic energy of the atom in the material (Doppler broadening). The experimentally obtained mean kinetic energies of the O, C, Li, Ca, and Si atoms are compared with the calculated ones, and good agreement is found, especially if the effect of multiple scattering is taken into account. It is demonstrated experimentally that the onset of the inelastic excitation is also affected by Doppler broadening. Aided by this understanding, we can obtain a good fit of the elastic peak and the onset of inelastic excitations. For SiO{sub 2}, good agreement is obtained with the well-established value of the band gap (8.9 eV) only if it is assumed that the intensity near the edge scales as (E − E{sub gap}){sup 1.5}. For CaCO{sub 3}, the band gap obtained here (7 eV) is about 1 eV larger than the previous experimental value, whereas the value for Li{sub 2}CO{sub 3} (7.5 eV) is the first experimental estimate.

  8. Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Yaqin [State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054 (China); Tang, Wu, E-mail: tang@uestc.edu.cn [State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054 (China); Zhang, Lan [State Key Laboratory for Mechanical Behavior of Materials, Xi' an Jiaotong University, Xi' an 710049 (China); Zhao, Junliang [Shanghai Juntech Co. Ltd., 1378 Xingxian Road, Shanghai 201815 (China)

    2014-08-28

    Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32 eV to 3.45 eV with the increasing carrier density from 2.0 × 10{sup 20} cm{sup −3} to 3.24 × 10{sup 20} cm{sup −3}. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices. - Highlights: • The effects of electron concentration on optical band gap were analyzed. • The measured optical band gap corresponded well with the calculated ones. • The Burstein–Moss (BM) and band gap renormalization (BGR) effects were considered. • Nonparabolic conduction band parameters were used in theoretical analysis. • The BM effect was offset by the BGR effect in both conduction band and valence band.

  9. Electron states in curved quantum structures with varying radius

    DEFF Research Database (Denmark)

    Gravesen, Jens; Willatzen, Morten

    2008-01-01

    The influence of size and shape is investigated for quantum-dot electronic states and intra-band oscillator strengths adapting a method originally due to Stevenson. The present work solves the one-band envelope-function problem for conduction-band eigenstates in the framework of k⋅p theory using ...

  10. ELECTRONIC-STRUCTURE OF LA2-XSRXNIO4 STUDIED BY PHOTOEMISSION AND INVERSE-PHOTOEMISSION SPECTROSCOPY

    NARCIS (Netherlands)

    EISAKI, H; UCHIDA, S; MIZOKAWA, T; NAMATAME, H; FUJIMORI, A; VANELP, J; KUIPER, P; SAWATZKY, GA; HOSOYA, S; KATAYAMAYOSHIDA, H

    1992-01-01

    The electronic structure of La2-xSrxNiO4 is studied by use of photoemission spectroscopy, bremsstrahlung-isochromat spectroscopy (BIS), and electron-energy-loss spectroscopy. Quantitative analyses are made on the valence-band and Ni 2p core-level photoemission spectra through configuration-interacti

  11. Band structures in silicene on monolayer gallium phosphide substrate

    Science.gov (United States)

    Ren, Miaojuan; Li, Mingming; Zhang, Changwen; Yuan, Min; Li, Ping; Li, Feng; Ji, Weixiao; Chen, Xinlian

    2016-07-01

    Opening a sizable band gap in the zero-gap silicene is a key issue for its application in nanoelectronics. We design new 2D silicene and GaP heterobilayer (Si/GaP HBL) composed of silicene and monolayer (ML) GaP. Based on first-principles calculations, we find that the interaction energies are in the range of -295.5 to -297.5 meV per unit cell, indicating a weak interaction between silicene and gallium phosphide (GaP) monolayer. The band gap changes ranging from 0.06 to 0.44 eV in hybrid HBLs. An unexpected indirect-direct band gap crossover is also observed in HBLs, dependent on the stacking pattern. These provide a possible way to design effective FETs out of silicene on GaP monolayer.

  12. Implementation of electronic crosstalk correction for terra MODIS PV LWIR bands

    Science.gov (United States)

    Geng, Xu; Madhavan, Sriharsha; Chen, Na; Xiong, Xiaoxiong

    2015-09-01

    The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the fleet of NASA's Earth Observing Systems (EOS) in space. Terra MODIS has completed 15 years of operation far exceeding its design lifetime of 6 years. The MODIS Level 1B (L1B) processing is the first in the process chain for deriving various higher level science products. These products are used mainly in understanding the geophysical changes occurring in the Earth's land, ocean, and atmosphere. The L1B code is designed to carefully calibrate the responses of all the detectors of the 36 spectral bands of MODIS and provide accurate L1B radiances (also reflectances in the case of Reflective Solar Bands). To fulfill this purpose, Look Up Tables (LUTs), that contain calibration coefficients derived from both on-board calibrators and Earth-view characterized responses, are used in the L1B processing. In this paper, we present the implementation mechanism of the electronic crosstalk correction in the Photo Voltaic (PV) Long Wave InfraRed (LWIR) bands (Bands 27-30). The crosstalk correction involves two vital components. First, a crosstalk correction modular is implemented in the L1B code to correct the on-board Blackbody and Earth-View (EV) digital number (dn) responses using a linear correction model. Second, the correction coefficients, derived from the EV observations, are supplied in the form of LUTs. Further, the LUTs contain time stamps reflecting to the change in the coefficients assessed using the Noise Equivalent difference Temperature (NEdT) trending. With the algorithms applied in the MODIS L1B processing it is demonstrated that these corrections indeed restore the radiometric balance for each of the affected bands and substantially reduce the striping noise in the processed images.

  13. Effect of electronic acceptor segments on photophysical properties of low-band-gap ambipolar polymers.

    Science.gov (United States)

    Li, Yuanzuo; Cui, Jingang; Zhao, Jianing; Liu, Jinglin; Song, Peng; Ma, Fengcai

    2013-01-01

    Stimulated by a recent experimental report, charge transfer and photophysical properties of donor-acceptor ambipolar polymer were studied with the quantum chemistry calculation and the developed 3D charge difference density method. The effects of electronic acceptor strength on the structure, energy levels, electron density distribution, ionization potentials, and electron affinities were also obtained to estimate the transporting ability of hole and electron. With the developed 3D charge difference density, one visualizes the charge transfer process, distinguishes the role of molecular units, and finds the relationship between the role of DPP and excitation energy for the three polymers during photo-excitation.

  14. The spin polarized band structure of strained thin films of gadolinium

    Energy Technology Data Exchange (ETDEWEB)

    Waldfried, C.; Dowben, P.A. [Univ. of Nebraska, Lincoln, NE (United States); Vescovo, E. [Brookhaven National Lab., Upton, NY (United States). National Synchrotron Light Source

    1998-12-31

    The magnetic properties of strained thin films of gadolinium are characterized by a wave vector and thickness dependence of the exchange splitting. The spin-resolved band structure has been mapped by spin polarized photoemission, and provides considerable insight into the relationship between magnetism of local moment systems, and band structure.

  15. Comparative studies in method for stratigraphical structure measurement of ice cores: Identification of cloudy bands

    Institute of Scientific and Technical Information of China (English)

    Morimasa Takata; Hitoshi Shoji; Atsushi Miyamoto; Kimiko Shimohara

    2003-01-01

    Cloudy bands are typical stratigraphic structure in deep ice core.Detailed recording of cloudy bands is important for dating of ice core since pair of series cloudy band and clear layer is corresponds to annual layer and it sometimes corresponds to volcanic ash layer.We developed two type scanners, transmitted light method and laser tomograph method for the stratigraphic study.Measurements were carried out for NGRIP deep ice core, which containing many cloudy bands, using the two type scanners and digital camera.We discussed about the possibility of identification of cloudy bands by each method and about advantage and disadvantage of measurements and their results.

  16. Band structures of TiO2 doped with N, C and B*

    OpenAIRE

    2006-01-01

    This study on the band structures and charge densities of nitrogen (N)-, carbon (C)- and boron (B)-doped titanium dioxide (TiO2) by first-principles simulation with the CASTEP code (Segall et al., 2002) showed that the three 2p bands of impurity atom are located above the valence-band maximum and below the Ti 3d bands, and that along with the decreasing of impurity atomic number, the fluctuations become more intensive. We cannot observe obvious band-gap narrowing in our result. Therefore, the...

  17. Fast and accurate approximate quasiparticle band structure calculations of ZnO, CdO, and MgO polymorphs

    Science.gov (United States)

    Ataide, C. A.; Pelá, R. R.; Marques, M.; Teles, L. K.; Furthmüller, J.; Bechstedt, F.

    2017-01-01

    We investigate ZnO, CdO, and MgO oxides crystallizing in rocksalt, wurtzite, and zincblende structures. Whereas in MgO calculations, the conventional LDA-1/2 method is employed through a self-energy potential (VS), the shallow d bands in ZnO and CdO are treated through an increased amplitude (A ) of VS to modulate the self-energy of the d states to place them in the quasiparticle position. The LDA+A -1/2 scheme is applied to calculate band structures and electronic density of states of ZnO and CdO. We compare the results with those of more sophisticated quasiparticle calculations and experiments. We demonstrate that this new LDA+A -1/2 method reaches accuracy comparable to state-of-the-art methods, opening a door to study more complex systems containing shallow core electrons to the prize of LDA studies.

  18. Application of the new LDA+GTB method for the band structure calculation of n-type cuprates

    Energy Technology Data Exchange (ETDEWEB)

    Korshunov, M.M. [L.V. Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation)]. E-mail: mkor@iph.krasn.ru; Ovchinnikov, S.G. [L.V. Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation); Gavrichkov, V.A. [L.V. Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences, 660036 Krasnoyarsk (Russian Federation); Nekrasov, I.A. [Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170 (Russian Federation); Pchelkina, Z.V. [Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170 (Russian Federation); Anisimov, V.I. [Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170 (Russian Federation)

    2006-05-01

    A novel hybrid scheme is proposed and applied for band structure calculations of undoped n-type cuprate Nd{sub 2}CuO{sub 4}. The ab initio LDA calculation is used to obtain single electron and Coulomb parameters of the multiband Hubbard-type model. In strong correlation regime the electronic structure within this model is calculated by the generalized tight-binding (GTB) method, that combines the exact diagonalization of the model Hamiltonian for a small cluster with perturbation treatment of the intercluster hopping and interactions. For Nd{sub 2}CuO{sub 4}, this scheme results in charge transfer insulator with value of the gap and band dispersion in agreement to the experimental data.

  19. Effects of FeSb6 octahedral deformations on the electronic structure of LaFe4Sb12

    KAUST Repository

    Pulikkotil, Jiji Thomas Joseph

    2011-09-01

    First-principles density functional based electronic structure calculations are performed in order to clarify the influence of FeSb6 octahedral deformations on the structural and electronic structure properties of LaFe 4Sb12. Our results show that octahedral tiltings correlate with the band dispersions and, consequently, the band masses. While total energy variation points at an enhanced role of lattice anharmonicity, flat bands emerge from a redistribution of the electronic states. © 2011 Elsevier B.V. All rights reserved.

  20. Crystal structure re-investigation in wide band gap CIGSe compounds

    Energy Technology Data Exchange (ETDEWEB)

    Souilah, M. [Institut des Materiaux Jean Rouxel, Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 03 (France)], E-mail: marc.souilah@cnrs-imn.fr; Rocquefelte, X.; Lafond, A.; Guillot-Deudon, C. [Institut des Materiaux Jean Rouxel, Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 03 (France); Morniroli, J.-P. [Laboratoire de Metallurgie Physique et Genie des Materiaux, UMR CNRS 8517, USTL and ENSCL, Bat C6, Cite Scientifique, 59655 Villeneuve d' Ascq Cedex (France); Kessler, J. [Institut des Materiaux Jean Rouxel, Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 03 (France)

    2009-02-02

    There is agreement in the literature that Cu(In{sub 1} {sub -} {sub x}Ga{sub x})Se{sub 2} (CIGSe) absorber used in solar cells has an optimum composition (x {approx} 0.3) corresponding to a band gap (1.1-1.2 eV) far below the theoretical value giving the maximum (1.4-1.5 eV). This paper presents a re-investigation of the crystal structure of bulk CIGSe compounds for both stoichiometric and Cu-poor compositions. Regardless of the gallium content, all the stoichiometric compounds are found to adopt the well-known chalcopyrite structure (space group I-42d) while a modification of the structure is evidenced for the high Ga-content Cu-poor compounds. The X-ray diffraction analyses demonstrate that the crystal structure of Cu{sub 0.743}In{sub 0.543} Ga{sub 0.543}Se{sub 2} is derived from that of the stannite structure (space-group I-42m). Ab-initio calculations show a strong dependence of the electronic structure near the Fermi level with the copper content. Such modifications are expected to significantly change the optical properties of Cu-poor CIGSe materials.

  1. Handbook of the band structure of elemental solids from Z = 1 to Z = 112

    CERN Document Server

    Papaconstantopoulos, Dimitris A

    2015-01-01

    This handbook presents electronic structure data and tabulations of Slater-Koster parameters for the whole periodic table. This second edition presents data sets for all elements up to Z = 112, Copernicium, whereas the first edition contained only 53 elements. In this new edition, results are given for the equation of state of the elements together with the parameters of a Birch fit, so that the reader can regenerate the results and derive additional information, such as Pressure-Volume relations and variation of Bulk Modulus with Pressure. For each element, in addition to the equation of state, the energy bands, densities of states, and a set of tight-binding parameters is provided. For a majority of elements, the tight-binding parameters are presented for both a two- and three-center approximation. For the hcp structure, new three-center tight-binding results are given. Other new material in this edition include: energy bands and densities of states of all rare-earth metals, a discussion of the McMillan-Gas...

  2. Electronic Structure of New Superconducting Perovskite MgCNi3

    Institute of Scientific and Technical Information of China (English)

    Li CHEN; Hua LI; Liangmo MEI

    2004-01-01

    The electronic structures of new superconducting perovskite MgCNis and related compounds MgCNi2T (T=Co, Fe,and Cu) have been studied using MS-Xα method. In MgCNi3, the main peak of density of states is located below the Fermi level and dominated by Ni d. From the results of total energy calculations, it was found that the number of Ni valence electron decreases faster for the Fe-doped case than that for the Co-doped case. The valence state of Ni changes from +1.43 in MgCNi2Co to +3.02 in MgCNi2Fe. It was confirmed that Co and Fe dopants in MgCNi3 behave as a source of d-band holes and the suppression of superconductivity occurs faster for the Fe-doped case than that for the Co-doped case. In order to explain the fact that Co and Fe dopants in MgCNi3 behave as a source of d-band holes rather than magnetic scattering centers that quench superconductivity, we have also investigated the effects of electron (Cu) doping on the superconductivity and found that both electron (Cu) doping and hole (Co, Fe)doping quench superconductivity exist. Comparing with the hole (Co) doping, there was no much difference between Cu and Co doping. This suggests that Co and Fe doping do not actas magnetic impurity.

  3. The determination of the electron-phonon interaction from tunneling data in the two-band superconductor MgB2

    CERN Document Server

    Daghero, D; Ummarino, G A; Dolgov, O V; Kortus, J; Golubov, A A; Shulga, S V

    2004-01-01

    We calculate the tunneling density of states (DOS) of MgB2 for different tunneling directions, by directly solving the real-axis, two-band Eliashberg equations (EE). Then we show that the numeric inversion of the standard single-band EE, if applied to the DOS of the two-band superconductor MgB2, may lead to wrong estimates of the strength of certain phonon branches (e.g. the E_2g) in the extracted electron-phonon spectral function alpha^(2)F(omega). The fine structures produced by the two-band interaction turn out to be clearly observable only for tunneling along the ab planes in high-quality single crystals. The results are compared to recent experimental data.

  4. Effect of acicular ferrite on banded structures in low-carbon microalloyed steel

    Institute of Scientific and Technical Information of China (English)

    Lei Shi; Ze-sheng Yan; Yong-chang Liu; Xu Yang; Cheng Zhang; Hui-jun Li

    2014-01-01

    The effect of acicular ferrite (AF) on banded structures in low-carbon microalloyed steel with Mn segregation during both iso-thermal transformation and continuous cooling processes was studied by dilatometry and microscopic observation. With respect to the iso-thermal transformation process, the specimen isothermed at 550°C consisted of AF in Mn-poor bands and martensite in Mn-rich bands, whereas the specimen isothermed at 450°C exhibited two different morphologies of AF that appeared as bands. At a continuous cooling rate in the range of 4 to 50°C/s, a mixture of AF and martensite formed in both segregated bands, and the volume fraction of martensite in Mn-rich bands was always higher than that in Mn-poor bands. An increased cooling rate resulted in a decrease in the difference of martensite volume fraction between Mn-rich and Mn-poor bands and thereby leaded to less distinct microstructural banding. The results show that Mn segregation and cooling rate strongly affect the formation of AF-containing banded structures. The formation mechanism of microstructural banding was also discussed.

  5. Study on relationships of electromagnetic band structures and left/right handed structures

    Institute of Scientific and Technical Information of China (English)

    GAO Chu; CHEN ZhiNing; WANG YunYi; YANG Ning

    2007-01-01

    Two types of dual periodic circuits are introduced. The distributions of passbands and stopbands are generated from their dispersion relationships. Based on the study, Brillouin diagrams of three representative special cases are drawn; S parameters of these three cases are simulated by Aglient ADS; the S parameters of one of the three cases are verified by an experiment. The phase characteristics are compared with those generated from the dispersion relationship. The theoretical analysis and the experimental verification show that both types of the periodic structures can behave as electromagnetic band gap (EBG) structures, right-handed structures (RHS), and left-handed structures (LHS), when they operate at different frequency ranges. Thus, the possibility of a physical structure showing these three different characteristics at different frequency ranges is proven.

  6. Electronic origin of structural transition in 122 Fe based superconductors

    Science.gov (United States)

    Ghosh, Haranath; Sen, Smritijit; Ghosh, Abyay

    2017-03-01

    Direct quantitative correlations between the orbital order and orthorhombicity is achieved in a number of Fe-based superconductors of 122 family. The former (orbital order) is calculated from first principles simulations using experimentally determined doping and temperature dependent structural parameters while the latter (the orthorhombicity) is taken from already established experimental studies; when normalized, both the above quantities quantitatively corresponds to each other in terms of their doping as well as temperature variations. This proves that the structural transition in Fe-based materials is electronic in nature due to orbital ordering. An universal correlations among various structural parameters and electronic structure are also obtained. Most remarkable among them is the mapping of two Fe-Fe distances in the low temperature orthorhombic phase, with the band energies Edxz, Edyz of Fe at the high symmetry points of the Brillouin zone. The fractional co-ordinate zAs of As which essentially determines anion height is inversely (directly) proportional to Fe-As bond distances (with exceptions of K doped BaFe2As2) for hole (electron) doped materials as a function of doping. On the other hand, Fe-As bond-distance is found to be inversely (directly) proportional to the density of states at the Fermi level for hole (electron) doped systems. Implications of these results to current issues of Fe based superconductivity are discussed.

  7. The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique

    Energy Technology Data Exchange (ETDEWEB)

    Sutherland, Kevin Jerome [Iowa State Univ., Ames, IA (United States)

    2001-06-27

    Over the last ten years, photonic band gap (PBG) theory and technology have become an important area of research because of the numerous possible applications ranging from high-efficiency laser diodes to optical circuitry. This research concentrates on reducing the length scale in the fabrication of layered photonic band gap structures and developing procedures to improve processing consistency. Various procedures and materials have been used in the fabrication of layered PBG structures. This research focused on an economical micro transfer molding approach to create the final PBG structure. A poly dimethylsiloxane (PDMS) rubber mold was created from a silicon substrate. It was filled with epoxy and built layer-by-layer to create a 3-D epoxy structure. This structure was infiltrated with nanoparticle titania or a titania sol-gel, then fired to remove the polymer mold, leaving a monolithic ceramic inverse of the epoxy structure. The final result was a lattice of titania rolds that resembles a face-centered tetragonal structure. The original intent of this research was to miniaturize this process to a bar size small enough to create a photonic band gap for wavelengths of visible electro-magnetic radiation. The factor limiting progress was the absence of a silicon master mold of small enough dimensions. The Iowa State Microelectronics Research Center fabricated samples with periodicities of 2.5 and 1.0 microns with the existing technology, but a sample was needed on the order of 0.3 microns or less. A 0.4 micron sample was received from Sandia National Laboratory, which was made through an electron beam lithography process, but it contained several defects. The results of the work are primarily from the 2.5 and 1.0 micron samples. Most of the work focused on changing processing variables in order to optimize the infiltration procedure for the best results. Several critical parameters were identified, ranging from the ambient conditions to the specifics of the

  8. First principle study of structural, electronic and magnetic properties of silicon doped zigzag boron nitride nanoribbon

    Science.gov (United States)

    Bahadur, Amar; Verma, Mohan L.; Mishra, Madhukar

    2015-04-01

    Using first principle calculation, we investigate the structural, electronic and magnetic properties of silicon doped zigzag boron nitride nanoribbon (ZBNNR). Our results show that the shift in position of silicon doping with respect to the ribbon edge causes change in the structural geometry, electronic structure and magnetization of ZBNNR. The band gap of silicon doped ZBNNR is found to become narrower as compared to that of perfect ZBNNR. We find that band gap and magnetic moment of ZBNNR can be tuned by substitutional silicon doping position and doping concentration.

  9. Band Structure and Optical Properties of Ordered AuCu3

    DEFF Research Database (Denmark)

    Skriver, Hans Lomholt; Lengkeek, H. P.

    1979-01-01

    The optical spectra of ordered AuCu3 have been measured at low temperatures by a direct ellipsometric technique. We find several structural elements above the absorption edge as well as in the infrared. The measured spectra are interpreted in terms of the interband absorption calculated from an ab...... initio band structure obtained by the relativistic linear muffin-tin orbitals method. The band calculation reveals that ordered AuCu3 has distinct copper and gold d bands positioned in and hybridizing with an s band common to copper and gold. The calculated state density is found to be in good agreement...

  10. Inter-spin distance determination using L-band (1-2 GHz) non-adiabatic rapid sweep electron paramagnetic resonance (NARS EPR)

    Science.gov (United States)

    Kittell, Aaron W.; Hustedt, Eric J.; Hyde, James S.

    2012-08-01

    Site-directed spin-labeling electron paramagnetic resonance (SDSL EPR) provides insight into the local structure and motion of a spin probe strategically attached to a molecule. When a second spin is introduced to the system, macromolecular information can be obtained through measurement of inter-spin distances either by continuous wave (CW) or pulsed electron double resonance (ELDOR) techniques. If both methodologies are considered, inter-spin distances of 8-80 Å can be experimentally determined. However, there exists a region at the upper limit of the conventional X-band (9.5 GHz) CW technique and the lower limit of the four-pulse double electron-electron resonance (DEER) experiment where neither method is particularly reliable. The work presented here utilizes L-band (1.9 GHz) in combination with non-adiabatic rapid sweep (NARS) EPR to address this opportunity by increasing the upper limit of the CW technique. Because L-band linewidths are three to seven times narrower than those at X-band, dipolar broadenings that are small relative to the X-band inhomogeneous linewidth become observable, but the signal loss, due to the frequency dependence of the Boltzmann factor, has made L-band especially challenging. NARS has been shown to increase sensitivity by a factor of five, and overcomes much of this loss, making L-band distance determination more feasible [1]. Two different systems are presented, and distances of 18-30 Å have been experimentally determined at physiologically relevant temperatures. Measurements are in excellent agreement with a helical model and values determined by DEER.

  11. Influence of nonparabolicity on electronic structure of quantum cascade laser

    Science.gov (United States)

    Vuković, Nikola; Milanović, Vitomir; Radovanović, Jelena

    2014-06-01

    We analyze the influence of nonparabolicity on the bound electronic states in the conduction-band of quantum wells in external electric field. Numerical results, obtained by transfer matrix method are presented for active region of GaAs/Al0.3Ga0.7As quantum cascade laser. The structure was initially optimized by genetic algorithm, using Kane's model of nonparabolicity, with emission wavelength set to λ≈15.1 μm. However, our numerical results indicate the change in lasing wavelength to 14.04 μm when using a more comprehensive description of nonparabolicity.

  12. Electronic structure and improper electric polarization of samarium orthoferrite

    Science.gov (United States)

    Triguk, V. V.; Makoed, I. I.; Ravinski, A. F.

    2016-12-01

    The band structure and distributions of the electron and spin densities of samarium orthoferrite have been calculated within the framework of the first-principles density functional theory in the LSDA + U approximation taking into account the collinear antiferromagnetic ordering of the magnetic moments of iron and samarium cations. The possibility of inducing a ferroelectric state at temperatures below the antiferromagnetic ordering temperature of the magnetic sublattice formed by samarium cations has been considered using the results of the group-theoretical analysis. In the high-temperature range, the formation of regions with a spontaneous electric polarization is possible in the presence of additional factors that reduce the symmetry of the crystal.

  13. Design and analysis of 5045 S-band klystron DC electron gun

    CERN Document Server

    Rehman, Abdul; Zhou, Z

    2015-01-01

    The design and performance analysis of DC electron gun for 5045 S-band klystron has been worked out using SLAC beam trajectory program (EGUN) and Computer Simulation Technology Particle Studio (CST-PS), Codes. Specifications of electron gun were focused on beam; current, perveance, size and emittance. Optimized beam; current, perveance, diameter and emittance were 414.00A, 2.00uP, 26.82 mm and 103.10 pi mm mrad, respectively. Furthermore, the optimized characteristic parameters of the gun were also calculated and compared with the simulated and experimental values which were in agreement. Accuracy of simulation was verified by comparison of emitted beam current which has error of zero percent.

  14. Ku Band Hemispherical Fully Electronic Antenna for Aircraft in Flight Entertainment

    Directory of Open Access Journals (Sweden)

    Alfredo Catalani

    2009-01-01

    Full Text Available The results obtained in the frame of the ESA activity “Advanced Antenna Concepts For Aircraft In Flight Entertainment” are presented. The aim of the activity consists in designing an active antenna able to guarantee the Ku band link between an aircraft and a geostationary satellite in order to provide in flight entertainment services. The transmit-receive antenna generates a single narrow beam to be steered electronically in a half sphere remaining compliant with respect to stringent requirements in terms of pattern shape, polarization alignment, EIRP, G/T, and using customized electronic devices. At the same time, the proposed solution should be competitive in terms of cost and complexity.

  15. Energy band alignment and electronic states of amorphous carbon surfaces in vacuo and in aqueous environment

    Energy Technology Data Exchange (ETDEWEB)

    Caro, Miguel A., E-mail: mcaroba@gmail.com [Department of Electrical Engineering and Automation, Aalto University, Espoo (Finland); Department of Applied Physics, COMP Centre of Excellence in Computational Nanoscience, Aalto University, Espoo (Finland); Määttä, Jukka [Department of Chemistry, Aalto University, Espoo (Finland); Lopez-Acevedo, Olga [Department of Applied Physics, COMP Centre of Excellence in Computational Nanoscience, Aalto University, Espoo (Finland); Laurila, Tomi [Department of Electrical Engineering and Automation, Aalto University, Espoo (Finland)

    2015-01-21

    In this paper, we obtain the energy band positions of amorphous carbon (a–C) surfaces in vacuum and in aqueous environment. The calculations are performed using a combination of (i) classical molecular dynamics (MD), (ii) Kohn-Sham density functional theory with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, and (iii) the screened-exchange hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). PBE allows an accurate generation of a-C and the evaluation of the local electrostatic potential in the a-C/water system, HSE yields an improved description of energetic positions which is critical in this case, and classical MD enables a computationally affordable description of water. Our explicit calculation shows that, both in vacuo and in aqueous environment, the a-C electronic states available in the region comprised between the H{sub 2}/H{sub 2}O and O{sub 2}/H{sub 2}O levels of water correspond to both occupied and unoccupied states within the a-C pseudogap region. These are localized states associated to sp{sup 2} sites in a-C. The band realignment induces a shift of approximately 300 meV of the a-C energy band positions with respect to the redox levels of water.

  16. Band structure engineering of graphene by a local gate defined periodic potential

    Science.gov (United States)

    Forsythe, Carlos; Maher, Patrick; Scarabelli, Diego; Dean, Cory; Kim, Philip

    Recent improvements in 2-dimensional (2D) material layering have resulted in enhanced device quality and created pathways for new device architectures. We fabricate periodic arrays from a patterned local back gate and a uniform top gate on hBN encapsulated graphene channels. The symmetry and lattice size of the periodic potential can be determined by state-of-art electron beam lithography and etching, achieving a lattice constant of 35 nm. The strength of the electric potential modulation can be controlled through applied voltage on the patterned gate. We observe signatures of superlattice modulation near the main Dirac peak in the density dependent resistance measurement at zero magnetic field. Current studies focus on the exploration of Hofstadter fractal band structures under magnetic fields. Our nano-patterned engineered superlattices on graphene hold great promise for wider applications.

  17. Band structures of TiO2 doped with N, C and B

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    This study on the band structures and charge densities of nitrogen (N)-, carbon (C)- and boron (B)-doped titanium dioxide (TiO2) by first-principles simulation with the CASTEP code (Segall et al., 2002) showed that the three 2p bands of impurity atom are located above the valence-band maximum and below the Ti 3d bands, and that along with the decreasing of impurity atomic number, the fluctuations become more intensive. We cannot observe obvious band-gap narrowing in our result.Therefore, the cause of absorption in visible light might be the isolated impurity atom 2p states in band-gap rather than the band-gap narrowing.

  18. Near-edge band structures and band gaps of Cu-based semiconductors predicted by the modified Becke-Johnson potential plus an on-site Coulomb U

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Yubo; Zhang, Jiawei; Wang, Youwei [State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 (China); Gao, Weiwei; Abtew, Tesfaye A. [Department of Physics, University at Buffalo, SUNY, Buffalo, New York 14260 (United States); Zhang, Peihong, E-mail: pzhang3@buffalo.edu, E-mail: wqzhang@mail.sic.ac.cn [Department of Physics, University at Buffalo, SUNY, Buffalo, New York 14260 (United States); Beijing Computational Science Research Center, Beijing 100084 (China); Zhang, Wenqing, E-mail: pzhang3@buffalo.edu, E-mail: wqzhang@mail.sic.ac.cn [State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 (China); School of Chemistry and Chemical Engineering and Sate Key Laboratory of Coordination Chemistry, Nanjing University, Jiangsu 210093 (China)

    2013-11-14

    Diamond-like Cu-based multinary semiconductors are a rich family of materials that hold promise in a wide range of applications. Unfortunately, accurate theoretical understanding of the electronic properties of these materials is hindered by the involvement of Cu d electrons. Density functional theory (DFT) based calculations using the local density approximation or generalized gradient approximation often give qualitative wrong electronic properties of these materials, especially for narrow-gap systems. The modified Becke-Johnson (mBJ) method has been shown to be a promising alternative to more elaborate theory such as the GW approximation for fast materials screening and predictions. However, straightforward applications of the mBJ method to these materials still encounter significant difficulties because of the insufficient treatment of the localized d electrons. We show that combining the promise of mBJ potential and the spirit of the well-established DFT + U method leads to a much improved description of the electronic structures, including the most challenging narrow-gap systems. A survey of the band gaps of about 20 Cu-based semiconductors calculated using the mBJ + U method shows that the results agree with reliable values to within ±0.2 eV.

  19. Nanoscale probing of electronic band gap and topography of VO2 thin film surfaces by scanning tunneling microscopy

    Science.gov (United States)

    Yin, W.; Wolf, S.; Ko, C.; Ramanathan, S.; Reinke, P.

    2011-01-01

    The metal-insulator transition (MIT) in vanadium dioxide in the vicinity of room temperature makes it one of the most interesting materials for novel switching device applications. It is therefore essential to have a fundamental understanding of the VO2 surface when it is incorporated into multilayer structures or nanodevices. This study focuses on the surface modification of VO2 in response to the thermal treatment during phase transition. Vacuum annealing at temperatures in the vicinity of the MIT triggers a partial reduction in the surface, and thus initiates a chemical phase transition. Scanning tunneling microscopy and spectroscopy are used to investigate the electronic properties and surface structure of the VO2 thin film on (0001) sapphire substrates. Band gap maps with a high spatial resolution and single point spectroscopy I-V curves are measured as the sample is cycled through the MIT, and thus provide a direct observation of the surface phase transition at the nanoscale. The VO2 surface exhibits a homogeneous insulating behavior with a typical band gap of ˜0.5 eV at room temperature, and the surface becomes more metallic and spatially inhomogeneous in conductivity during MIT, and wide range of surface oxides can be identified. The surface still remains partially metallic after cooling down from a long period anneal, and such irreversible surface electrical change is attributed to the loss of oxygen. The location of metallic islands after thermal cycling is strongly coupled to the topography of the film, and relaxation processes and continued modification of the spatial distribution of the metallic regions are recognized on a longer timescale. The impact of film morphology, strain, surface chemistry, and structural phase transition on the electronic characteristics of VO2 surfaces are discussed.

  20. Ultra-broad band and dual-band highly efficient polarization conversion based on the three-layered chiral structure

    Science.gov (United States)

    Xu, Kai-kai; Xiao, Zhong-yin; Tang, Jing-yao; Liu, De-jun; Wang, Zi-hua

    2016-07-01

    In the paper, a novel three-layered chiral structure is proposed and investigated, which consists of a split-ring resonator sandwiched between two layers of sub-wavelength gratings. This designed structure can achieve simultaneously asymmetric transmission with an extremely broad bandwidth and high amplitude as well as multi-band 90° polarization rotator with very low dispersion. Numerical simulations adopted two kinds of softwares with different algorithms demonstrate that asymmetric parameter can reach a maximum of 0.99 and over than 0.8 from 4.6 to 16.8 GHz, which exhibit magnitude and bandwidth improvement over previous chiral metamaterials in microwave bands (S, C, X and Ku bands). Specifically, the reason of high amplitude is analyzed in detail based on the Fabry-perot like resonance. Subsequently, the highly efficient polarization conversion with very low dispersion between two orthogonal linearly polarized waves is also analyzed by the optical activity and ellipticity. Finally, the electric fields are also investigated and further demonstrate the correctness of the simulated and calculated results.

  1. Energy band structure tailoring of vertically aligned InAs/GaAsSb quantum dot structure for intermediate-band solar cell application by thermal annealing process.

    Science.gov (United States)

    Liu, Wei-Sheng; Chu, Ting-Fu; Huang, Tien-Hao

    2014-12-15

    This study presents an band-alignment tailoring of a vertically aligned InAs/GaAs(Sb) quantum dot (QD) structure and the extension of the carrier lifetime therein by rapid thermal annealing (RTA). Arrhenius analysis indicates a larger activation energy and thermal stability that results from the suppression of In-Ga intermixing and preservation of the QD heterostructure in an annealed vertically aligned InAs/GaAsSb QD structure. Power-dependent and time-resolved photoluminescence were utilized to demonstrate the extended carrier lifetime from 4.7 to 9.4 ns and elucidate the mechanisms of the antimony aggregation resulting in a band-alignment tailoring from straddling to staggered gap after the RTA process. The significant extension in the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.

  2. Band structure of Heusler compounds studied by photoemission and tunneling spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Arbelo Jorge, Elena

    2011-07-01

    Heusler compounds are key materials for spintronic applications. They have attracted a lot of interest due to their half-metallic properties predicted by band structure calculations. The aim of this work is to evaluate experimentally the validity of the predictions of half metallicity by band structure calculations for two specific Heusler compounds, Co{sub 2}FeAl{sub 0.3}Si{sub 0.7} and Co{sub 2}MnGa. Two different spectroscopy methods for the analysis of the electronic properties were used: Angular Resolved Ultraviolet Photoemission Spectroscopy (ARUPS) and Tunneling Spectroscopy. Heusler compounds are prepared as thin films by RF-sputtering in an ultra high vacuum system. For the characterization of the samples, bulk and surface crystallographic and magnetic properties of Co{sub 2}FeAl{sub 0.3}Si{sub 0.7} and Co{sub 2}MnGa are studied. X-ray and electron diffraction reveal a bulk and surface crossover between two different types of sublattice order (from B2 to L2{sub 1}) with increasing annealing temperature. X-ray magnetic circular dichroism results show that the magnetic properties in the surface and bulk are identical, although the magnetic moments obtained are 5 % below from the theoretically predicted. By ARUPS evidence for the validity of the predicted total bulk density of states (DOS) was demonstrated for both Heusler compounds. Additional ARUPS intensity contributions close to the Fermi energy indicates the presence of a specific surface DOS. Moreover, it is demonstrated that the crystallographic order, controlled by annealing, plays an important role on broadening effects of DOS features. Improving order resulted in better defined ARUPS features. Tunneling magnetoresistance measurements of Co{sub 2}FeAl{sub 0.3}Si{sub 0.7} and Co{sub 2}MnGa based MTJ's result in a Co{sub 2}FeAl{sub 0.3}Si{sub 0.7} spin polarization of 44 %, which is the highest experimentally obtained value for this compound, although it is lower than the 100 % predicted. For Co

  3. Band structure of TiO sub 2 -doped yttria-stabilized zirconia probed by soft-x-ray spectroscopy

    CERN Document Server

    Higuchi, T; Kobayashi, K; Yamaguchi, S; Fukushima, A; Shin, S

    2003-01-01

    The electronic structure of TiO sub 2 -doped yttria-stabilized zirconia (YSZ) has been studied by soft-X-ray emission spectroscopy (SXES) and X-ray absorption spectroscopy (XAS). The valence band is mainly composed of the O 2p state. The O 1s XAS spectrum exhibits the existence of the Ti 3d unoccupied state under the Zr 4d conduction band. The intensity of the Ti 3d unoccupied state increases with increasing TiO sub 2 concentration. The energy separation between the top of the valence band and the bottom of the Ti 3d unoccupied state is in accord with the energy gap, as expected from dc-polarization and total conductivity measurements. (author)

  4. Chaos and band structure in a three-dimensional optical lattice.

    Science.gov (United States)

    Boretz, Yingyue; Reichl, L E

    2015-04-01

    Classical chaos is known to affect wave propagation because it signifies the presence of broken symmetries. The effect of chaos has been observed experimentally for matter waves, electromagnetic waves, and acoustic waves. When these three types of waves propagate through a spatially periodic medium, the allowed propagation energies form bands. For energies in the band gaps, no wave propagation is possible. We show that optical lattices provide a well-defined system that allows a study of the effect of chaos on band structure. We have determined the band structure of a body-centered-cubic optical lattice for all theoretically possible couplings, and we find that the band structure for those lattices realizable in the laboratory differs significantly from that expected for the bands in an "empty" body-centered-cubic crystal. However, as coupling is increased, the lattice becomes increasingly chaotic and it becomes possible to produce band structure that has behavior qualitatively similar to the "empty" body-centered-cubic band structure, although with fewer degeneracies.

  5. Electronic structure and polarizability of metallic nanoshells

    Science.gov (United States)

    Prodan, E.; Nordlander, P.

    2002-01-01

    An efficient method for the calculation of the electronic structure of metallic nanoshells is developed. The method is applied to a large nanoshell (of 10 nm in diameter) containing more than 2.5×10 4 conduction electrons. The calculations show that the density of states of the nanoshell is relatively bulk-like. The frequency dependent polarizability is calculated and shown to display strong confinement effects and features similar to what is predicted by semi-classical electrodynamic theory.

  6. Tandem-structured, hot electron based photovoltaic cell with double Schottky barriers.

    Science.gov (United States)

    Lee, Young Keun; Lee, Hyosun; Park, Jeong Young

    2014-04-03

    We demonstrate a tandem-structured, hot electron based photovoltaic cell with double Schottky barriers. The tandem-structured, hot electron based photovoltaic cell is composed of two metal/semiconductor interfaces. Two types of tandem cells were fabricated using TiO2/Au/Si and TiO2/Au/TiO2, and photocurrent enhancement was detected. The double Schottky barriers lead to an additional pathway for harvesting hot electrons, which is enhanced through multiple reflections between the two barriers with different energy ranges. In addition, light absorption is improved by the band-to-band excitation of both semiconductors with different band gaps. Short-circuit current and energy conversion efficiency of the tandem-structured TiO2/Au/Si increased by 86% and 70%, respectively, compared with Au/Si metal/semiconductor nanodiodes, showing an overall solar energy conversion efficiency of 5.3%.

  7. Synthesis, physical properties and band structure of non-magnetic Y3AlC

    Science.gov (United States)

    Ghule, S. S.; Garde, C. S.; Ramakrishnan, S.; Singh, S.; Rajarajan, A. K.; Laad, Meena

    2016-10-01

    Y3AlC has been synthesized by arc melting and subsequent annealing. Rietveld analysis of the powder x-ray diffraction (XRD) data confirms cubic Pm-3m structure. Electrical resistivity (ρ) of Y3AlC exhibits metallic behaviour. No sign of superconductivity is observed down to the lowest measurement temperatures of 4.2 K in ρ, and 2 K in magnetic susceptibility (χ) and specific heat (Cp) measurements. The value of the electronic specific heat coefficient γ is 1.36 mJ/K2 mol from which the density of states (DOS) at the Fermi energy (EF) is obtained as 0.57 states/eV.unit cell. The value of Debye temperature θD is estimated to be 315 K. Electronic band structure calculations of Y3AlC reveal a pseudo-gap in the DOS at EF leading to a small value of 0.5 states/eV unit cell which matches quite well with that obtained from γ. Non-zero value of the DOS indicates metallic behaviour as confirmed by our ρ data. Covalent and ionic bonding seem to co-exist with metallic bonding in Y3AlC as indicated by van Arkel- Ketelaar triangle for Zintl-like systems.

  8. Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating.

    Science.gov (United States)

    Knutson, Jeremy L; Martin, James D; Mitzi, David B

    2005-06-27

    Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.

  9. Detailed Study of the TE band structure of two dimensional metallic photonic crystals with square symmetry

    Science.gov (United States)

    Sedghi, Aliasghar; Valiaghaie, Soma; Soufiani, Ahad Rounaghi

    2014-10-01

    By virtue of the efficiency of the Dirichlet-to-Neumann map method, we have calculated, for H-polarization (TE mode), the band structure of 2D photonic crystals with a square lattice composed of metallic rods embedded in an air background. The rod in the unit cell is chosen to be circular in shape. Here, from a practical point of view, in order to obtain maximum band gaps, we have studied the band structure as a function of the size of the rods. We have also studied the flat bands appearing in the band structures and have shown that for frequencies around the surface plasmon frequency, the modes are highly localized at the interface between the metallic rods and the air background.

  10. Obtaining the band structure of a complicated photonic crystal by linear operations

    Institute of Scientific and Technical Information of China (English)

    吴良; 叶卓; 何赛灵

    2003-01-01

    Absolute band gaps can be created by lifting the degeneracy in the bands of a photonic crystal.To calculate the band structure of a complicated photonic crystal generated by e.g.symmetry breaking,general forms of all possible linear operations are presented in terms of matrices and a procedure to combine these operations is given.Other forms of linear operations(such as the addition,subtraction,and translation transforms) are also presented to obtain an explicit expression for the Fourier coefficient of the dielectric function in the plane-wave expansion method.With the present method,band structures for various complicated photonic crystals(related through these linear operations) can be obtained easily and quickly.As a numerical example,a large absolute band gap for a complicated photonic crystal structure of GaAs is found in the high region of normalized frequency.

  11. Detailed study of the TE band structure of two dimensional metallic photonic crystals with square symmetry

    Energy Technology Data Exchange (ETDEWEB)

    Sedghi, Aliasghar [Islamic Azad University, Shabestar (Iran, Islamic Republic of); Valiaghaie, Soma [Islamic Azad University, Sanandaj (Iran, Islamic Republic of); Soufiani, Ahad Rounaghi [Islamic Azad University, Sufian (Iran, Islamic Republic of)

    2014-10-15

    By virtue of the efficiency of the Dirichlet-to-Neumann map method, we have calculated, for H-polarization (TE mode), the band structure of 2D photonic crystals with a square lattice composed of metallic rods embedded in an air background. The rod in the unit cell is chosen to be circular in shape. Here, from a practical point of view, in order to obtain maximum band gaps, we have studied the band structure as a function of the size of the rods. We have also studied the flat bands appearing in the band structures and have shown that for frequencies around the surface plasmon frequency, the modes are highly localized at the interface between the metallic rods and the air background.

  12. Photonic Band Gaps in Two-Dimensional Crystals with Fractal Structure

    Institute of Scientific and Technical Information of China (English)

    刘征; 徐建军; 林志方

    2003-01-01

    We simulate the changes of the photonic band structure of the crystal in two dimensions with a quasi-fractal structure when it is fined to a fractal. The result shows that when the dielectric distribution is fined, the photonic band structure will be compressed on the whole and the ground photonic band gap (PBG) closed while the next PBGs shrunk, in conjunction with their position declining in the frequency spectrum. Furthermore, the PBGs in the high zone are much more sensitive than those in low zones.

  13. Analysis of photonic band-gap (PBG) structures using the FDTD method

    DEFF Research Database (Denmark)

    Tong, M.S.; Cheng, M.; Lu, Y.L.

    2004-01-01

    In this paper, a number of photonic band-gap (PBG) structures, which are formed by periodic circuit elements printed oil transmission-line circuits, are studied by using a well-known numerical method, the finite-difference time-domain (FDTD) method. The results validate the band-stop filter...

  14. Electronic Properties in a Hierarchical Multilayer Structure

    Institute of Scientific and Technical Information of China (English)

    ZHU Chen-Ping; XIONG Shi-Jie

    2001-01-01

    We investigate electronic properties of a hierarchical multilayer structure consisting of stacking of barriers and wells. The structure is formed in a sequence of generations, each of which is constructed with the same pattern but with the previous generation as the basic building blocks. We calculate the transmission spectrum which shows the multifractal behavior for systems with large generation index. From the analysis of the average resistivity and the multifractal structure of the wavefunctions, we show that there exist different types of states exhibiting extended, localized and intermediate characteristics. The degree of localization is sensitive to the variation of the structural parameters.Suggestion of the possible experimental realization is discussed.

  15. 41 CFR 101-26.508 - Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band).

    Science.gov (United States)

    2010-07-01

    ... Programs § 101-26.508 Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate... 41 Public Contracts and Property Management 2 2010-07-01 2010-07-01 true Electronic data processing (EDP) tape and instrumentation tape (wide and intermediate band). 101-26.508 Section...

  16. Observation of banded spherulites and lamellar structures by atomic force microscopy

    Institute of Scientific and Technical Information of China (English)

    姜勇; 罗艳红; 范泽夫; 王霞瑜; 徐军; 郭宝华; 李林

    2003-01-01

    Lamellar structures of banded spherulites of poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends are observed using tapping mode atomic force microscopy (AFM). The surface of the PCL/PVC banded spherulites presents to be concentric periodic ups and downs. The period of the bands corresponds to the extinction rings under the polarized optical microscopy observation. The lamellae with edge-on orientation in the ridges and the flat-on lamellae in the valleys of the banded spherulites are observed clearly. The twisting between the edge-on and flat-on lamellae is also observed.

  17. Density functional calculation of equilibrium geometry and electronic structure of pyrite

    Institute of Scientific and Technical Information of China (English)

    邱冠周; 肖奇; 胡岳华; 徐竞

    2001-01-01

    The equilibrium geometry and electronic structure of pyrite has been studied using self-consistent density-functional theory within the local density approximation (LDA). The optimum bulk geometry is in good agreement with crystallographic data. The calculated band structure and density of states in the region around the Fermi energy show that valence-band maximum (VBM) is at X (100), and the conduction-band minimum (CBM) is at G (000). The indirect and direct band gaps are 0.6eV and 0.74eV, respectively. The calculated contour map of difference of charge density shows excess charge in nonbonding d electron states on the Fe sites. The density increases between sulfur nuclei and between iron and sulfur nuclei qualitatively reveal that S-S bond and Fe-S bond are covalent binding.

  18. The electronic structure of free aluminum clusters: metallicity and plasmons.

    Science.gov (United States)

    Andersson, Tomas; Zhang, Chaofan; Tchaplyguine, Maxim; Svensson, Svante; Mårtensson, Nils; Björneholm, Olle

    2012-05-28

    The electronic structure of free aluminum clusters with ∼3-4 nm radius has been investigated using synchrotron radiation-based photoelectron and Auger electron spectroscopy. A beam of free clusters has been produced using a gas-aggregation source. The 2p core level and the valence band have been probed. Photoelectron energy-loss features corresponding to both bulk and surface plasmon excitation following photoionization of the 2p level have been observed, and the excitation energies have been derived. In contrast to some expectations, the loss features have been detected at energies very close to those of the macroscopic solid. The results are discussed from the point of view of metallic properties in nanoparticles with a finite number of constituent atoms.

  19. The electronic structure of free aluminum clusters: Metallicity and plasmons

    Science.gov (United States)

    Andersson, Tomas; Zhang, Chaofan; Tchaplyguine, Maxim; Svensson, Svante; Mârtensson, Nils; Björneholm, Olle

    2012-05-01

    The electronic structure of free aluminum clusters with ˜3-4 nm radius has been investigated using synchrotron radiation-based photoelectron and Auger electron spectroscopy. A beam of free clusters has been produced using a gas-aggregation source. The 2p core level and the valence band have been probed. Photoelectron energy-loss features corresponding to both bulk and surface plasmon excitation following photoionization of the 2p level have been observed, and the excitation energies have been derived. In contrast to some expectations, the loss features have been detected at energies very close to those of the macroscopic solid. The results are discussed from the point of view of metallic properties in nanoparticles with a finite number of constituent atoms.

  20. Ab initio electronic structure and optical conductivity of bismuth tellurohalides

    CERN Document Server

    Schwalbe, Sebastian; Starke, Ronald; Schober, Giulio A H; Kortus, Jens

    2016-01-01

    We investigate the electronic structure, dielectric and optical properties of bismuth tellurohalides BiTeX (X = I, Cl, Br) by means of all-electron density functional theory. In particular, we present the ab initio conductivities and dielectric tensors calculated over a wide frequency range, and compare our results with the recent measurements by Akrap et al. , Makhnev et al. , and Rusinov et al. . We show how the low-frequency branch of the optical conductivity can be used to identify characteristic intra- and interband transitions between the Rashba spin-split bands in all three bismuth tellurohalides. We further calculate the refractive indices and dielectric constants, which in turn are systematically compared to previous predictions and measurements. We expect that our quantitative analysis will contribute to the general assessment of bulk Rashba materials for their potential use in spintronics devices.

  1. Acoustic band pinning in the phononic crystal plates of anti-symmetric structure

    Institute of Scientific and Technical Information of China (English)

    Cai Chen; Zhu Xue-Feng; Chen Qian; Yuan Ying; Liang Bin; Cheng Jian-Chun

    2011-01-01

    Acoustic bands are studied numerically for a Lamb wave propagating in an anti-symmetric structure of a onedimensional periodic plate by using the method of supercell plane-wave expansion.The results show that all the bands are pinned in pairs at the Brillouin zone boundary as long as the anti-symmetry remains and acoustic band gaps (ABGs) only appear between certain bands.In order to reveal the relationship between the band pinning and the anti-symmetry,the method of eigenmode analysis is introduced to calculate the displacement fields of different plate structures.Further,the method of harmony response analysis is employed to calculate the reference spectra to verify the accuracy of numerical calculations of acoustic band map,and both the locations and widths of ABGs in the acoustic band map are in good agreement with those of the reference spectra.The investigations show that the pinning effect is very sensitive to the anti-symmetry of periodic plates,and by introducing different types of breakages,more ABGs or narrow pass bands will appear,which is meaningful in band gap engineering.

  2. Renormalization of spin polarized itinerant electron bands in the normal state of a model ferromagnetic superconductor

    Institute of Scientific and Technical Information of China (English)

    Ma Lei; Huang Ai-Qun; Li Jun

    2011-01-01

    This paper studies the normal state properties of itinerant electrons in a toy model, which is constructed according to the model for coexisting ferromagnetism and superconductivity proposed by Suhl [Suhl H 2001 Phys. Rev. Lett. 87 167007]. In this theory with ferromagnetic ordering based on localized spins, the exchange interaction J between conduction electrons and localized spin is taken as the pairing glue for s-wave superconductivity. It shows that this J term will first renormalize the normal state single conduction electron structures substantially. It finds dramatically enhanced or suppressed magnetization of itinerant electrons for positive or negative J. Singlet Cooper pairing can be ruled out due to strong spin polarisation in the J > 0 case while a narrow window for s-wave superconductivity is opened around some ferromagnetic J.

  3. Electronic structure of nanograin barium titanate ceramics

    Institute of Scientific and Technical Information of China (English)

    DENG Xiangyun; WANG Xiaohui; LI Dejun; LI Longtu

    2007-01-01

    The density of states and band structure of 20 nm barium titanate(BaTiO3,BT)ceramics are investigated by first-principles calculation.The full potential linearized augmented plane wave(FLAPW)method is used and the exchange correlation effects are treated by the generalized gradient approximation(GGA).The results show that there is substantial hybridization between the Ti 3d and O 2p states in 20 nm BT ceramics and the interaction between barium and oxygen is typically ionic.

  4. Electronic structure of multi-walled carbon fullerenes

    Science.gov (United States)

    Doore, Keith; Cook, Matthew; Clausen, Eric; Lukashev, Pavel V.; Kidd, Tim E.; Stollenwerk, Andrew J.

    2017-02-01

    Despite an enormous amount of research on carbon based nanostructures, relatively little is known about the electronic structure of multi-walled carbon fullerenes, also known as carbon onions. In part, this is due to the very high computational expense involved in estimating electronic structure of large molecules. At the same time, experimentally, the exact crystal structure of the carbon onion is usually unknown, and therefore one relies on qualitative arguments only. In this work we present the results of a computational study on a series of multi-walled fullerenes and compare their electronic structures to experimental data. Experimentally, the carbon onions were fabricated using ultrasonic agitation of isopropanol alcohol and deposited onto the surface of highly ordered pyrolytic graphite using a drop cast method. Scanning tunneling microscopy images indicate that the carbon onions produced using this technique are ellipsoidal with dimensions on the order of 10 nm. The majority of differential tunneling spectra acquired on individual carbon onions are similar to that of graphite with the addition of molecular-like peaks, indicating that these particles span the transition between molecules and bulk crystals. A smaller, yet sizable number exhibited a semiconducting gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. These results are compared with the electronic structure of different carbon onion configurations calculated using first-principles. Similar to the experimental results, the majority of these configurations are metallic with a minority behaving as semiconductors. Analysis of the configurations investigated here reveals that each carbon onion exhibiting an energy band gap consisted only of non-metallic fullerene layers, indicating that the interlayer interaction is not significant enough to affect the total density of states in these structures.

  5. Structure of conduction electrons on polysilanes

    Energy Technology Data Exchange (ETDEWEB)

    Ichikawa, Tsuneki [Hokkaido Univ., Sapporo (Japan); Kumagai, Jun

    1998-10-01

    The orbital structures of conduction electrons on permethylated oligosilane, Si{sub 2n}(CH{sub 3}){sub 2n+2}(n = 2 - 8), and poly(cyclohexylmethylsilane) have been determined by the electron spin-echo envelope modulation signals of the radical anions of these silanes in a deuterated rigid matrix at 77 K. The conduction electron on permethylated oligosilane is delocalized over the entire main chain, whereas that on poly(cyclohexylmethylsilane) is localized on a part of the main chain composed of about six Si atoms. Quantum-chemical calculations suggest that Anderson localization due to fluctuation of {sigma} conjugation by conformational disorder of the main chain is responsible for the localization of both the conduction electron and the hole. (author)

  6. Real-space finite-difference calculation method of generalized Bloch wave functions and complex band structures with reduced computational cost.

    Science.gov (United States)

    Tsukamoto, Shigeru; Hirose, Kikuji; Blügel, Stefan

    2014-07-01

    Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

  7. The electronic structure of antiferromagnetic chromium

    DEFF Research Database (Denmark)

    Skriver, Hans Lomholt

    1981-01-01

    The author has used the local spin density formalism to perform self-consistent calculations of the electronic structure of chromium in the non-magnetic and commensurate antiferromagnetic phases, as a function of the lattice parameter. A change of a few per cent in the atomic radius brings...

  8. Electron conductance in curved quantum structures

    DEFF Research Database (Denmark)

    Willatzen, Morten; Gravesen, Jens

    2010-01-01

    A differential-geometry analysis is employed to investigate the transmission of electrons through a curved quantum-wire structure. Although the problem is a three-dimensional spatial problem, the Schrodinger equation can be separated into three general coordinates. Hence, the proposed method...

  9. The Electronic Structure Effect in Heterogeneous Catalysis

    DEFF Research Database (Denmark)

    Nilsson, A.; Pettersson, L. G. M.; Hammer, Bjørk

    2005-01-01

    Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal...... surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure....

  10. Experimental determination of excitonic band structures of single-walled carbon nanotubes using circular dichroism spectra

    Science.gov (United States)

    Wei, Xiaojun; Tanaka, Takeshi; Yomogida, Yohei; Sato, Naomichi; Saito, Riichiro; Kataura, Hiromichi

    2016-10-01

    Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species.

  11. Temperature dependence of the electronic structure of semiconductors and insulators

    Energy Technology Data Exchange (ETDEWEB)

    Poncé, S., E-mail: samuel.pon@gmail.com; Gillet, Y.; Laflamme Janssen, J.; Gonze, X. [European Theoretical Spectroscopy Facility and Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Chemin des étoiles 8, bte L07.03.01, B-1348 Louvain-la-neuve (Belgium); Marini, A. [Consiglio Nazionale delle Ricerche (CNR), Via Salaria Km 29.3, CP 10, 00016 Monterotondo Stazione (Italy); Verstraete, M. [European Theoretical Spectroscopy Facility and Physique des matériaux et nanostructures, Université de Liège, Allée du 6 Août 17, B-4000 Liège (Belgium)

    2015-09-14

    The renormalization of electronic eigenenergies due to electron-phonon coupling (temperature dependence and zero-point motion effect) is sizable in many materials with light atoms. This effect, often neglected in ab initio calculations, can be computed using the perturbation-based Allen-Heine-Cardona theory in the adiabatic or non-adiabatic harmonic approximation. After a short description of the recent progresses in this field and a brief overview of the theory, we focus on the issue of phonon wavevector sampling convergence, until now poorly understood. Indeed, the renormalization is obtained numerically through a slowly converging q-point integration. For non-zero Born effective charges, we show that a divergence appears in the electron-phonon matrix elements at q → Γ, leading to a divergence of the adiabatic renormalization at band extrema. This problem is exacerbated by the slow convergence of Born effective charges with electronic wavevector sampling, which leaves residual Born effective charges in ab initio calculations on materials that are physically devoid of such charges. Here, we propose a solution that improves this convergence. However, for materials where Born effective charges are physically non-zero, the divergence of the renormalization indicates a breakdown of the adiabatic harmonic approximation, which we assess here by switching to the non-adiabatic harmonic approximation. Also, we study the convergence behavior of the renormalization and develop reliable extrapolation schemes to obtain the converged results. Finally, the adiabatic and non-adiabatic theories, with corrections for the slow Born effective charge convergence problem (and the associated divergence) are applied to the study of five semiconductors and insulators: α-AlN, β-AlN, BN, diamond, and silicon. For these five materials, we present the zero-point renormalization, temperature dependence, phonon-induced lifetime broadening, and the renormalized electronic band structure.

  12. Magnon band structure and magnon density in one-dimensional magnonic crystals

    Science.gov (United States)

    Qiu, Rong-ke; Huang, Te; Zhang, Zhi-dong

    2014-11-01

    By using Callen's Green's function method and the Tyablikov and Anderson-Callen decoupling approximations, we systematically study the magnon band structure and magnon density perpendicular to the superlattice plane of one-dimensional magnonic crystals, with a superlattice consisting of two magnetic layers with ferromagnetic (FM) or antiferromagnetic (AFM) interlayer exchange coupling. The effects of temperature, interlayer coupling, anisotropy and external magnetic field on the magnon-energy band and magnon density in the Kx-direction are investigated in three situations: a) the magnon band of magnetic superlattices with FM interlayer coupling, b) separate and c) overlapping magnon bands of magnetic superlattices with AFM interlayer coupling. In the present work, a quantum approach is developed to study the magnon band structure and magnon density of magnonic crystals and the results are beneficial for the design of magnonic-crystal waveguides or gigahertz-range spin-wave filters.

  13. Band Structure Calculation of Si and Ge by Non-Local Empirical Pseudo-Potential Technique

    Institute of Scientific and Technical Information of China (English)

    CHEN Yong; RAVAIOLI Umberto

    2005-01-01

    In this paper, the princ iple of spatial nonlocal empirical pseudopotential and its detailed calculation procedure is presented. Consequently, this technique is employed to calculate the band structuresof Silicon and Germaniun. By comparing the results with photoemission experimental data, the validity and accuracy of this calculation are fully conformed for valence or conductance band,respectively. Thus it can be concluded that the spin-orbit Hamiltonian will only affect the energy band gap and another conductance or valence band structure. Therefore, this nonlocal approach without spin-orbit part is adequate for the device simulation of only one carrier transport such as metal oxide semiconductor field effect transistors (MOSFET)'s, and it can significantly reduce the complication of band structure calculation.

  14. 第一性原理计算A、B位掺杂对NaNbO3电子结构的影响%Influences of A and B site dopings on the electronic band structures of NaNbO3 from first-principle calculations

    Institute of Scientific and Technical Information of China (English)

    蓝奔月; 史海峰

    2014-01-01

    NaNbO3 is known as a perovskite oxide that exhibits photocatalytic activity for water splitting and the reduction of CO2.Unfortunately,it only absorbs ultraviolet light owing to its wide band gap (Eg~ 3.40 eV).In order to shift its optical absorption from UV region to visible light region,the influ-ences of A site (Ag+)doping and B site (Sb5+,V5+)doping on the electronic structures and optical ab-sorption properties of NaNbO3 have been calculated by first-principle calculations based on density func-tional theory (DFT).As for Ag+doping,the O 2p states are mixed with Ag 4d states so as to move the top of valence band upward.After Sb5+ or V5+ doping,the Nb 4d states are mixed with Sb 5s5p states or V 3 d states,which results in the downward shift of the bottom of conduction band.Based on the cal-culated results,the possible mechanisms for realizing the red shift of absorption edge are proposed, which would be significance of developing doped NaNbO3 photocatalytic materials under visible light.%钙钛矿结构的 NaNbO3具有光催化分解水和降解有机物的性能,但其禁带宽度为3.40 eV,因此仅具有紫外光响应.为了实现其可见光响应,本文通过密度泛函理论计算探讨了 A 位(Ag+)掺杂和 B 位(Sb5+、V5+)掺杂对 NaNbO3的电子结构和光吸收性能的影响.结果表明,掺杂 Ag+后,O 2p轨道和 Ag 4d轨道杂化进而导致价带位置上移.掺杂 Sb5+后,Nb 4d轨道和 Sb 5s5p 轨道杂化,导致导带位置下移.掺杂V5+后,Nb 4d轨道和 V 3d轨道杂化,也导致导带位置下移.基于上述结果,提出了通过 A、B位掺杂 NaN-bO3实现其光吸收红移的机制,本研究对通过掺杂调控 NaNbO3光催化材料的光吸收方面有一定的指导意义.

  15. Pressure-induced transition from localized electron toward band antiferromagnetism in LaMnO(3).

    Science.gov (United States)

    Zhou, J-S; Goodenough, J B

    2002-08-19

    The temperature dependence of the ac susceptibility under pressure has been used to track the Néel temperature T(N) of the Mott insulators LaMnO3, CaMnO3, and YCrO3. Bloch's rule relating T(N) to volume V, viz., alpha=dlog(T(N)/dlog(V=-3.3, is obeyed in YCrO3 and CaMnO3; it fails in LaMnO3. This breakdown is interpreted to be due to a sharp increase in the factor [U(-1)+(2Delta)(-1)] entering the superexchange perturbation formula. A first-order change at 7 kbar indicates that the transition from localized-electron to band magnetism is not smooth.

  16. Temperature dependence of Q-band electron paramagnetic resonance spectra of nitrosyl heme proteins

    Energy Technology Data Exchange (ETDEWEB)

    Flores, Marco; Wajnberg, Eliane; Bemski, George

    1997-11-01

    The Q-band (35 GHz) electron paramagnetic resonance (EPR) spectra of nitrosyl hemoglobin (Hb N O) and nitrosyl myoglobin (Mb NO) were studied as a function of temperature between 19 K and 200 K. The spectra of both heme proteins show classes of variations as a function of temperature. The first one has previously been associated with the existence of two paramagnetic species, one with rhombic and the other with axial symmetry. The second one manifests itself in changes in the g-factors and linewidths of each species. These changes are correlated with the conformational substates model and associate the variations of g-values with changes in the angle of the N(his)-Fe-N (NO) bond in the rhombic species and with changes in the distance between Fe and N of the proximal (F8) histidine in the axial species. (author) 24 refs., 6 figs.

  17. Observation of electron excitation into silicon conduction band by slow-ion surface neutralization

    CERN Document Server

    Shchemelinin, S

    2016-01-01

    Bare reverse biased silicon photodiodes were exposed to 3eV He+, Ne+, Ar+, N2+, N+ and H2O+ ions. In all cases an increase of the reverse current through the diode was observed. This effect and its dependence on the ionization energy of the incident ions and on other factors are qualitatively explained in the framework of Auger-type surface neutralization theory. Amplification of the ion-induced charge was observed with an avalanche photodiode under high applied bias. The observed effect can be considered as ion-induced internal potential electron emission into the conduction band of silicon. To the best of our knowledge, no experimental evidence of such effect was previously reported. Possible applications are discussed.

  18. Band structure engineering of anatase TiO{sub 2} by metal-assisted P-O coupling

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Jiajun; Meng, Qiangqiang [Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 (China); Huang, Jing [Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 (China); School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei, Anhui 230601 (China); Li, Qunxiang, E-mail: liqun@ustc.edu.cn; Yang, Jinlong [Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 (China); Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026 (China)

    2014-05-07

    In this work, we demonstrate that the metal-assisted P-O coupling is an effective approach to improve the photoelectrochemical properties of TiO{sub 2}. The (Sc + P) and (In + P) codoping effects on electronic structures and photocatalytic activities of anatase TiO{sub 2} are examined by performing hybrid density functional theory calculations. It is found that the coupling of P dopant with the second-nearest neighboring O atom assisted by acceptor metals (Sc/In) leads to the fully occupied and delocalized intermediate bands within the band gap of anatase TiO{sub 2}, which is driven by the P-O antibonding states (π*). This metal-assisted P-O coupling can prevent the recombination of photogenerated electron-hole pairs and effectively reduce the band gap of TiO{sub 2}. Moreover, the band edge alignments in (Sc + P) and (In + P) codoped anatase TiO{sub 2} are desirable for water-splitting. The calculated optical absorption curves indicate that (Sc + P) and (In + P) codoping in anatase TiO{sub 2} can also effectively enhance the visible light absorption.

  19. Band structure engineering of anatase TiO2 by metal-assisted P-O coupling.

    Science.gov (United States)

    Wang, Jiajun; Meng, Qiangqiang; Huang, Jing; Li, Qunxiang; Yang, Jinlong

    2014-05-07

    In this work, we demonstrate that the metal-assisted P-O coupling is an effective approach to improve the photoelectrochemical properties of TiO2. The (Sc + P) and (In + P) codoping effects on electronic structures and photocatalytic activities of anatase TiO2 are examined by performing hybrid density functional theory calculations. It is found that the coupling of P dopant with the second-nearest neighboring O atom assisted by acceptor metals (Sc/In) leads to the fully occupied and delocalized intermediate bands within the band gap of anatase TiO2, which is driven by the P-O antibonding states (π*). This metal-assisted P-O coupling can prevent the recombination of photogenerated electron-hole pairs and effectively reduce the band gap of TiO2. Moreover, the band edge alignments in (Sc + P) and (In + P) codoped anatase TiO2 are desirable for water-splitting. The calculated optical absorption curves indicate that (Sc + P) and (In + P) codoping in anatase TiO2 can also effectively enhance the visible light absorption.

  20. B3LYP, BLYP and PBE DFT band structures of the nucleotide base stacks

    Science.gov (United States)

    Szekeres, Zs; Bogár, F.; Ladik, J.

    DFT crystal orbital (band structure) calculations have been performed for the nucleotide base stacks of cytosine, thymine, adenine, and guanine arranged in DNA B geometry. The band structures obtained with PBE, BLYP, and B3LYP functionals are presented and compared to other related experimental and theoretical results. The influence of the quality of the basis set on the fundamental gap values was also investigated using Clementi's double ζ, 6-31G and 6-31G* basis sets.

  1. Theoretical study of band structure of odd-mass 115,117I isotopes

    Science.gov (United States)

    Singh, Dhanvir; Kumar, Amit; Sharma, Chetan; Singh, Suram; Bharti, Arun

    2016-05-01

    By using the microscopic approach of Projected Shell Model (PSM), negative-parity band structures of odd mass neutron-rich 115,117I nuclei have been studied with the deformed single-particle states generated by the standard Nilsson potential. For these isotopes, the band structures have been analyzed in terms of quasi-particles configurations. The phenomenon of back bending in moment of inertia is also studied in the present work.

  2. Electrons in feldspar II: A consideration of the influence of conduction band-tail states on luminescence processes

    DEFF Research Database (Denmark)

    Poolton, H.R.J.; Ozanyan, K.B.; Wallinga, J.

    2002-01-01

    Most natural feldspars contain many charged impurities, and display a range of bond angles, distributed about the ideal. These effects can lead to complications in the structure of the conduction band, giving rise to a tail of energy states (below the high-mobility conduction band) through which ...

  3. Electronic Structure and Luminescence of Quasi-Freestanding MoS2 Nanopatches on Au(111)

    Science.gov (United States)

    2016-01-01

    Monolayers of transition metal dichalcogenides are interesting materials for optoelectronic devices due to their direct electronic band gaps in the visible spectral range. Here, we grow single layers of MoS2 on Au(111) and find that nanometer-sized patches exhibit an electronic structure similar to their freestanding analogue. We ascribe the electronic decoupling from the Au substrate to the incorporation of vacancy islands underneath the intact MoS2 layer. Excitation of the patches by electrons from the tip of a scanning tunneling microscope leads to luminescence of the MoS2 junction and reflects the one-electron band structure of the quasi-freestanding layer. PMID:27459588

  4. Dual-Band Terahertz Left-Handed Metamaterial with Fishnet Structure

    Institute of Scientific and Technical Information of China (English)

    DU Qiu-Jiao; LIU Jin-Song; WANG Ke-Jia; YI Xu-Nong; YANG Hong-Wu

    2011-01-01

    We present the design of a dual-band left-handed metamaterial with fishnet structure in the terahertz regime. Its left-handed properties are described by the retrieved effective electromagnetic parameters. We introduce an equivalent circuit which offers a theoretical explanation for the left-handed behavior of the dual-band fishnet metamaterial, and investigate its losses receiving higher figure of merit. The design is beneficial to the development of frequency agile and broadband THz materials and devices. The dual-band fishnet metamaterial can be extended to infrared and optical frequency ranges by regulating the structural parameters.

  5. Cell and band structures in cold rolled polycrystalline copper

    DEFF Research Database (Denmark)

    Ananthan, V.S.; Leffers, Torben; Hansen, Niels

    1991-01-01

    The effect of plastic strain on the deformation microstructure has been investigated in polycrystalline copper rolled at room temperature to 5, 10, 20, and 30% reduction in thickness equivalent strain 0.06-0.42). Results from transmission electron microscopy (TEM) observations show that dense...... on {111}. Finally, the evolution of the deformation microstructure in copper is compared with that observed in other face centred cubic metals, especially aluminium....

  6. Realization of Band-Notch UWB Monopole Antenna Using AMC Structure

    Directory of Open Access Journals (Sweden)

    Pradeep Kumar

    2013-06-01

    Full Text Available This article presents the design, simulation and testing of an Ultra Wide Band (UWB planar monopole antenna with WLAN band-notch characteristic. The proposed antenna consists, the combination of planar monopole antenna with partial ground and a pair of AMC structures. The AMC structure used for the design is mushroom-like. Design equation of EBG parameters is also proposed for FR4 substrate using transmission line model. Using proposed equations, Mushroom-like EBG structure is integrated along the feed line of a monopole antenna for WLAN (5 GHz – 6 GHz band rejection. TheCurrent distribution and equivalent circuit model of antenna is used to explain band-notch characteristic of EBG resonator. The proposed antenna is fabricated on an FR4 substrate with a thickness of 1.6 mmand εr = 4.4. The measured VSWR characteristic is less than 2 for complete UWB band except for WLAN band i.e. 5 GHz – 6 GHz. The gain of the proposed structure is around 2 dBi – 6.7 dBi for complete UWBband except for WLAN band where it is reduced to -4 dBi. The measured radiation pattern of proposed antenna is omnidirectional along H plane and bidirectional in E plane. A nearly constant group delaywith variations < 2ns, except for the notched bandwidth makes proposed antenna suitable for UWB application.

  7. Electronic Energy Band and Transport Properties in Monolayer Graphene with Periodically Modulated Magnetic Vector Potential and Electrostatic Potential

    Institute of Scientific and Technical Information of China (English)

    刘正方; 伍清萍; 刘念华

    2012-01-01

    We investigated the electronic energy band and transport features of graphene superlattice with periodically modulated magnetic vector potential and electrostatic potential. It is found that both parallel magnetic vector potential and electrostatic potential can decisively shift Dirac point in a different way, which may be an efficient way to achieve electron or hole filter. We a/so find that applying modulated parallel and anti-parallel magnetic vector potential to the electrons can efficiently change electronic states between pass and stop states, which can be useful in designing electron or hole switches and lead to large magneto-resistance.

  8. The band structure of VO2 measured by angle-resolved photoemission

    Science.gov (United States)

    Moreschini, Luca; Chang, Young Jun; Innocenti, Davide; Walter, Andrew L.; Kim, Young Su; Gaines, Geoffrey; Bostwick, Aaron; Denlinger, Jonathan; Rotenberg, Eli

    2011-03-01

    The origin of the 340K metal-insulator transition (MIT) in VO2 is still under debate. the main reason is that no direct experimental verifications of the electronic structure of VO2 exist up to this point. The quality of the available single crystals is not sufficient for ARPES measurements, so that photoemission is limited to angle-integrated mode. New opportunities are offered by oxide films, on which data of equal or even higher quality have been reported (Saeki et al., PRB 2009). WIth the in situ pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source we have grown VO2(001) films on a TiO2 substrate and measured the Fermi surface of the metallic phase. These results will permit a direct comparison with the existing band calculations and open the way to the study of the MIT as a function, e.g., of film thickness or electron doping with Cr. Work supported by U.S. DOE (DE-AC02-05CH11231 for ALS), the Max Planck Society, and the Swiss National Science Foundation (PBELP2-125484).

  9. Electronic Structure of Germanium Nanocrystal Films Probed with Synchrotron Radiation

    Energy Technology Data Exchange (ETDEWEB)

    Bostedt, C

    2002-05-01

    The fundamental structure--property relationship of semiconductor quantum dots has been investigated. For deposited germanium nanocrystals strong quantum confinement effects have been determined with synchrotron radiation based x-ray absorption and photoemission techniques. The nanocrystals are condensed out of the gas phase with a narrow size distribution and subsequently deposited in situ onto various substrates. The particles are crystalline in the cubic phase with a structurally disordered surface shell and the resulting film morphology depends strongly on the substrate material and condition. The disordered surface region has an impact on the overall electronic structure of the particles. In a size-dependent study, the conduction and valence band edge of germanium nanocrystals have been measured for the first time and compared to the bulk crystal. The band edges move to higher energies as the particle size is decreased, consistent with quantum confinement theory. To obtain a more accurate analysis of confinement effects in the empty states, a novel analysis method utilizing an effective particle size for the x-ray absorption experiment, which allows a deconvolution of absorption edge broadening effects, has been introduced. Comparison of the present study to earlier studies on silicon reveals that germanium exhibits stronger quantum confinement effects than silicon. Below a critical particle size of 2.3 {+-} 0.7 nm, the band gap of germanium becomes larger than that of silicon--even if it is the opposite for bulk materials. This result agrees phenomenologically with effective mass and tight binding theories but contradicts the findings of recent pseudopotential calculations. The discrepancy between theory and experiments is attributed to the differences in the theoretical models and experimental systems. The experimentally observed structural disorder of the particle surface has to be included in the theoretical models.

  10. X-ray and photoelectron spectroscopy of the structure, reactivity, and electronic structure of semiconductor nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Hamad, Kimberly Sue [Univ. of California, Berkeley, CA (United States)

    2000-01-01

    Semiconductor nanocrystals are a system which has been the focus of interest due to their size dependent properties and their possible use in technological applications. Many chemical and physical properties vary systematically with the size of the nanocrystal and thus their study enables the investigation of scaling laws. Due to the increasing surface to volume ratio as size is decreased, the surfaces of nanocrystals are expected to have a large influence on their electronic, thermodynamic, and chemical behavior. In spite of their importance, nanocrystal surfaces are still relatively uncharacterized in terms of their structure, electronic properties, bonding, and reactivity. Investigation of nanocrystal surfaces is currently limited by what techniques to use, and which methods are suitable for nanocrystals is still being determined. This work presents experiments using x-ray and electronic spectroscopies to explore the structure, reactivity, and electronic properties of semiconductor (CdSe, InAs) nanocrystals and how they vary with size. Specifically, x-ray absorption near edge spectroscopy (XANES) in conjunction with multiple scattering simulations affords information about the structural disorder present at the surface of the nanocrystal. X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) probe the electronic structure in terms of hole screening, and also give information about band lineups when the nanocrystal is placed in electric contact with a substrate. XPS of the core levels of the nanocrystal as a function of photo-oxidation time yields kinetic data on the oxidation reaction occurring at the surface of the nanocrystal.

  11. X-ray and photoelectron spectroscopy of the structure, reactivity, and electronic structure of semiconductor nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Hamad, K.S.

    2000-05-01

    Semiconductor nanocrystals are a system which has been the focus of interest due to their size dependent properties and their possible use in technological applications. Many chemical and physical properties vary systematically with the size of the nanocrystal and thus their study enables the investigation of scaling laws. Due to the increasing surface to volume ratio as size is decreased, the surfaces of nanocrystals are expected to have a large influence on their electronic, thermodynamic, and chemical behavior. In spite of their importance, nanocrystal surfaces are still relatively uncharacterized in terms of their structure, electronic properties, bonding, and reactivity. Investigation of nanocrystal surfaces is currently limited by what techniques to use, and which methods are suitable for nanocrystals is still being determined. This work presents experiments using x-ray and electronic spectroscopies to explore the structure, reactivity, and electronic properties of semiconductor (CdSe, InAs) nanocrystals and how they vary with size. Specifically, x-ray absorption near edge spectroscopy (XANES) in conjunction with multiple scattering simulations affords information about the structural disorder present at the surface of the nanocrystal. X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) probe the electronic structure in terms of hole screening, and also give information about band lineups when the nanocrystal is placed in electric contact with a substrate. XPS of the core levels of the nanocrystal as a function of photo-oxidation time yields kinetic data on the oxidation reaction occurring at the surface of the nanocrystal.

  12. Electronic structure of nitride-based quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Winkelnkemper, Momme

    2008-11-07

    In the present work the electronic and optical properties of In{sub x}Ga{sub 1-x}N/GaN and GaN/AlN QDs are studied by means of eight-band k.p theory. Experimental results are interpreted in detail using the theoretical results. The k.p model for the QD electronicstructure calculations accounts for strain, piezo- and pyroelectric effects, spin-orbit and crystal-field splitting, and is implemented for arbitrarily shaped QDs on a finite differences grid. Few-particle corrections are included using the self-consistent Hartree method. Band parameters for the wurtzite and zinc-blende phases of GaN, AlN, and InN are derived from first-principle G{sub 0}W{sub 0} band-structure calculations. Reliable values are also provided for parameters that have not been determined experimentally yet. The electronic properties of nitride QDs are dominated by the large built-in piezo- and pyroelectric fields, which lead to a pronounced red-shift of excitonic transition energies and extremely long radiative lifetimes in large GaN/AlN QDs. In In{sub x}Ga{sub 1-x}N/GaN QDs these fields induce a pronounced dependence of the radiative excitonic lifetimes on the exact QD shape and composition. It is demonstrated that the resulting variations of the radiative lifetimes in an inhomogeneous QD ensemble are the origin of the multi-exponential luminescence decay frequently observed in time-resolved ensemble measurements on In{sub x}Ga{sub 1-x}N/GaN QDs. A polarization mechanism in nitride QDs based on strain-induced valence-band mixing effects is discovered. Due to the valence-band structure of wurtzite group-III nitrides and the specific strain situation in c-plane QDs, the confined hole states are formed predominantly by the two highest valence bands. In particular, the hole ground state (h{sub 0} {identical_to} h{sub A}) is formed by the A band, and the first excited hole state (h{sub 1} {identical_to} h{sub B}) by the B band. It is shown that the interband transitions involving h{sub A} or h

  13. Low-frequency photonic band structures in graphene-like triangular metallic lattice

    Science.gov (United States)

    Wang, Kang

    2016-11-01

    We study the low frequency photonic band structures in triangular metallic lattice, displaying Dirac points in the frequency spectrum, and constructed upon the lowest order regular polygonal tiles. We show that, in spite of the unfavourable geometrical conditions intrinsic to the structure symmetry, the lowest frequency photonic bands are formed by resonance modes sustained by local structure patterns, with the corresponding electric fields following a triangular distribution at low structure filling rate and a honeycomb distribution at high filling rate. For both cases, the lowest photonic bands, and thus the plasma gap, can be described in the framework of a tight binding model, and analysed in terms of local resonance modes and their mutual correlations. At high filling rate, the Dirac points and their movement following the structure deformation are described in the same framework, in relation with local structure patterns and their variations, as well as the particularity of the metallic lattice that enhances the topological anisotropy.

  14. Photonic band gaps in materials with triply periodic surfaces and related tubular structures

    NARCIS (Netherlands)

    Michielsen, K; Kole, JS

    2003-01-01

    We calculate the photonic band gap of triply periodic bicontinuous cubic structures and of tubular structures constructed from the skeletal graphs of triply periodic minimal surfaces. The effect of the symmetry and topology of the periodic dielectric structures on the existence and the characteristi

  15. Atomic and electronic structure of twin growth defects in magnetite.

    Science.gov (United States)

    Gilks, Daniel; Nedelkoski, Zlatko; Lari, Leonardo; Kuerbanjiang, Balati; Matsuzaki, Kosuke; Susaki, Tomofumi; Kepaptsoglou, Demie; Ramasse, Quentin; Evans, Richard; McKenna, Keith; Lazarov, Vlado K

    2016-02-15

    We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains.

  16. [Structured electronic consultation letter for shoulder disorders].

    Science.gov (United States)

    Paloneva, Juha; Oikari, Marjo; Ylinen, Jari; Ingalsuo, Minna; Ilkka, Kunnamo; Ilkka, Kiviranta

    2012-01-01

    Referral to a specialist has a significant influence on management of the patient and costs associated with the treatments. However, development and research of the process by which patients are referred has been almost neglected. Expectations considering the purpose, contents, and timing of the referral of the consulting physician and the consultant do not always meet. A structured, electronic consultation letter was developed to respond this need. Functionality and interactivity are the key elements of the referral, including (1) an electronic referral letter to a specialist, (2) interactive education in clinical examination and management of shoulder disorders, and (3) an instrument of clinical examination and documentation of shoulder disorders.

  17. Valence one-electron and shake-up ionisation bands of polycyclic aromatic hydrocarbons. IV. The dibenzanthracene species

    Energy Technology Data Exchange (ETDEWEB)

    Deleuze, Michael S. [Theoretische Chemie, Departement SBG, Universiteit Hasselt, Agoralaan, Gebouw D, B-3590 Diepenbeek (Belgium)], E-mail: michael.deleuze@uhasselt.be

    2006-10-26

    A comprehensive study of the He (I) ultra-violet photoelectron spectra of the 1.2,3.4; 1.2,5.6 and 1.2,7.8 isomers of dibenzanthracene up to the double ionisation threshold at {approx}18 eV is presented with the aid of one-particle Green's Function calculations performed using the outer-valence Green's Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with basis sets of improving quality. Suited extrapolations of the ADC(3) results for the one-electron energies characterising the {pi}-band system ({epsilon} {sub b} < 10 eV) to Dunning's correlation consistent basis set of triple zeta quality (cc-pVTZ) enable theoretical insights into HeI measurements which approach chemical accuracy (1 kcal/mol or 43.4 meV). In contrast, a confrontation of simulated spectral envelopes with high-resolution He I photoelectron spectra indicates that polycyclic aromatic molecules with sterically overcrowded bay regions are more susceptible to undergo vibronic coupling complications at the {sigma}-ionisation onset. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital (or one-electron) picture of ionisation at the ADC(3)/6-31G levels. The extent of shake-up bands is correspondingly related to topological, structural and magnetic criteria of aromaticity. Comparison is made with calculations of the lowest doublet-doublet excitation energies of the related radical cations, by means of time-dependent density functional theory (TDDFT)

  18. Pressure effects on crystal and electronic structure of bismuth tellurohalides

    Science.gov (United States)

    Rusinov, I. P.; Menshchikova, T. V.; Sklyadneva, I. Yu; Heid, R.; Bohnen, K.-P.; Chulkov, E. V.

    2016-11-01

    We study the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the surface states exhibit a Bychkov-Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal-TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect. We have also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound.

  19. Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures

    Science.gov (United States)

    Senkovskiy, B. V.; Usachev, D. Yu.; Fedorov, A. V.; Shelyakov, A. V.; Adamchuk, V. K.

    2012-08-01

    The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti50Ni50 and Ti50Ni25Cu25 alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti50Ni50 - x Cu x system ( x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.

  20. Photonic band structure of ZnO photonic crystal slab laser

    CERN Document Server

    Yamilov, A; Cao, H

    2005-01-01

    We recently reported on the first realization of ultraviolet photonic crystal laser based on zinc oxide [Appl. Phys. Lett. {\\bf 85}, 3657 (2004)]. Here we present the details of structural design and its optimization. We develop a computational super-cell technique, that allows a straightforward calculation of the photonic band structure of ZnO photonic crystal slab on sapphire substrate. We find that despite of small index contrast between the substrate and the photonic layer, the low order eigenmodes have predominantly transverse-electric (TE) or transverse-magnetic (TM) polarization. Because emission from ZnO thin film shows strong TE preference, we are able to limit our consideration to TE bands, spectrum of which can possess a complete photonic band gap with an appropriate choice of structure parameters. We demonstrate that the geometry of the system may be optimized so that a sizable band gap is achieved.