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1

Hybrid density functional theory band structure engineering in hematite

We present a hybrid density functional theory (DFT) study of doping effects in ?-Fe2O3, hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure ?-Fe2O3 crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.

Pozun, Zachary D.; Henkelman, Graeme

2011-06-01

2

Energy band structure and density of states for BaBiBO4 nonlinear optical crystal

International Nuclear Information System (INIS)

We present results of the band structure and density of states (DOS) for BaBiBO4 nonlinear optical single crystals, using full potential linear augmented plane wave (FP-LAPW) method. We have for the first time, performed the experimental investigations of the electronic density of the states. We have found that this crystal is a semiconductor with an indirect energy band gap. Theoretical results of the DOS spectra are in good accord with the X-ray photoelectron spectroscopy measurements with respect to peak positioning. The band structure can be thought as arising from a superposition of the BO3 and BiO5 clusters possessing substantially different ionicities

3

Range-Separation Density-Fitting Band Structure Calculation with Gaussian Auxiliary Function

In this paper, we discuss a range-separation density-fitting method for obtaining the electronic band structure. The formalism is based on Gaussian orbital basis functions. The Hartree term and the nuclear attractive term are divided into long- and short-range contributions by using the error and complementary error functions, respectively. The long-range Hartree term is evaluated through a density-fitting procedure based on Gaussian auxiliary functions, where the net charge neutrality of electrons and atomic nuclei in the unit cell is ensured by Lagrange multipliers. We implemented a first-principles program code based on the methodology, and demonstrate calculation results, using semi-local density functional theory (DFT) functionals, the Hartree–Fock, and hybrid DFT methods. We also discuss the efficiency and accuracy of the methodology.

Shimazaki, Tomomi; Kosugi, Taichi; Nakajima, Takahito

2014-05-01

4

Valence and conduction band edges charge densities in ZnS compound with zinc-blende structure

International Nuclear Information System (INIS)

The electronic valence and conduction charge densities at the ?-dot and X k-points are calculated as a function of position in the unit cell for ZnS in the zinc-blende structure using wave functions derived from empirical pseudopotential band-structure calculations. Detailed plots of the charge density along the [111] direction and in the (110) plane are presented and discussed for the total valence bands and the first and second conduction ones.

5

Crystal structure of Ca (Al H4 )2 predicted from density-functional band-structure calculations

Calcium alanate [Ca(AlH4)2] is a candidate material for high-density reversible solid-state hydrogen storage that thus far has been scarcely studied. This paper presents a scheme for solving the crystal structure of a compound based on only a few model structures from similar compounds and employs this to predict the crystal structure and electronic structure of Ca(AlH4)2 . By deliberately breaking the symmetry of the model structures down to P1 , local minima may be avoided, and thus the number of required input models is smaller. Density-functional band-structure calculations within the generalized gradient approximation were used in the structural minimizations. The most stable structure was based on Ca(BaF4)2 and was in the orthorhombic space group Pbca with lattice constants a=1337 , b=928 , and c=891pm . The electronic density of states reveals an insulator with a band gap of around 4eV .

Løvvik, O. M.

2005-04-01

6

Band-structure-corrected local density approximation study of semiconductor quantum dots and wires

This paper presents results of ab initio accuracy thousand atom calculations of colloidal quantum dots and wires using the charge patching method. We have used density functional theory under local density approximation (LDA), and we have corrected the LDA bulk band structures by modifying the nonlocal pseudopotentials, so that their effective masses agree with experimental values. We have systematically studied the electronic states of group III-V (GaAs, InAs, InP, GaN, AlN, and InN) and group II-VI (CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, and ZnO) systems. We have also calculated the electron-hole Coulomb interactions in these systems. We report the exciton energies as functions of the quantum dot sizes and quantum wire diameters for all the above materials. We found generally good agreements between our calculated results and experimental measurements. For CdSe and InP, the currently calculated results agree well with the previously calculated results using semiempirical pseudopotentials. The ratios of band-gap-increases between quantum wires and dots are material-dependent, but a majority of them are close to 0.586, as predicted by the simple effective-mass model. Finally, the size dependence of 1Se-1Pe transition energies of CdSe quantum dots agrees well with the experiment. Our results can be used as benchmarks for future experiments and calculations.

Li, Jingbo; Wang, Lin-Wang

2005-09-01

7

A new method (NM) for filtering three-dimensional reconstructed densities is proposed. The algorithm is tested with simulated spectra and employed to study the electronic structure of the rare-earth compound LaB6. For this system, momentum densities are reconstructed from theoretical and experimental two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) spectra. The experimental results are in good agreement with the band structure calcula...

Kontrym-sznajd, G.; Samsel-czekala, M.; Biasini, M.; Kubo, Y.

2004-01-01

8

Wigner random banded matrices with sparse structure local spectral density of states

Random banded matrices with linearly increasing diagonal elements are recently considered as an attractive model for complex nuclei and atoms. Apart from early papers by Wigner \\cite{Wig} there were no analytical studies on the subject. In this letter we present analytical and numerical results for local spectral density of states (LDOS) for more general case of matrices with a sparsity inside the band. The crossover from the semicircle form of LDOS to that given by the Breit-Wigner formula is studied in detail.

Fyodorov, Yu V; Izrailev, F M; Casati, G

1995-01-01

9

International Nuclear Information System (INIS)

After constructing a stress and strain model, the valence bands of in-plane biaxial tensile strained Si is calculated by k · p method. In the paper we calculate the accurate anisotropy valance bands and the splitting energy between light and heavy hole bands. The results show that the valance bands are highly distorted, and the anisotropy is more obvious. To obtain the density of states (DOS) effective mass, which is a very important parameter for device modeling, a DOS effective mass model of biaxial tensile strained Si is constructed based on the valance band calculation. This model can be directly used in the device model of metal—oxide semiconductor field effect transistor (MOSFET). It also a provides valuable reference for biaxial tensile strained silicon MOSFET design. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

10

The UHF-(70.5 cm wavelength) and S-band (10 cm wavelength) radar at NASA/Wallops Island Research Facility in Virginia, U.S.A. have been used to relate lightning activity with altitude and with the reflectivity structure of thunderstorms. Two centers of lightning flash density were found; one between 6 and 8 km altitude and another between 11 and 15 km. Previously announced in STAR as N83-31206

Mazur, V.; Gerlach, J. C.; Rust, W. D.

1984-01-01

11

Harmonic vibrational frequencies and vibronic intensities in the first S 0?S 1 ( ??) absorption band of free-base porphin (H 2 P) are investigated by hybrid density functional theory (DFT) with the standard B3LYP functional. The S 0-S 1 transition probability is calculated using time-dependent DFT with account of Franck-Condon (FC) and Herzberg-Teller (HT) contributions to the electric-dipole transition moments including displacements along all 108 vibrational modes. Two weak wide bands observed in the gas phase absorption spectra of the H 2 P molecule at 626 and 576 nm are interpreted as the 0-0 band of the X 1 A g?1B 3u transition and the 0-1 band with largest contributions from the ?10(a)=1610 cm -1 and ?19(b)=1600 cm -1 modes, respectively, in agreement with previous tentative assignments. Both bands are induced by the HT mechanism, while the FC contributions are negligible. A number of fine structure bands, including combination of two vibrational quanta, are obtained and compared with available spectra from supersonic jet and Shpolskij matrices. Both absorption and fluorescence spectra are interpreted on ground of the linear coupling model and a good fulfillment of the mirror-symmetry rule.

Minaev, Boris; Wang, Yan-Hua; Wang, Chuan-Kui; Luo, Yi; Ågren, Hans

2006-10-01

12

International Nuclear Information System (INIS)

Some aspects of the predictions of energy band theory for both the rare earths and actinides and their comparison with experiment are discussed. Recent developments in assessing eigenfunction behavior are emphasized. 5 figures

13

Hybrid density functional theory study of Cu(In1?xGaxSe2 band structure for solar cell application

Directory of Open Access Journals (Sweden)

Full Text Available Cu(In1?xGaxSe2 (CIGS alloy based thin film photovoltaic solar cells have attracted more and more attention due to its large optical absorption coefficient, long term stability, low cost and high efficiency. However, the previous theoretical investigation of this material with first principle calculation cannot fulfill the requirement of experimental development, especially the accurate description of band structure and density of states. In this work, we use first principle calculation based on hybrid density functional theory to investigate the feature of CIGS, with B3LYP applied in the CuIn1?xGaxSe2 stimulation of the band structure and density of states. We report the simulation of the lattice parameter, band gap and chemical composition. The band gaps of CuGaSe2, CuIn0.25Ga0.75Se2, CuIn0.5Ga0.5Se2, CuIn0.75Ga0.25Se2 and CuInSe2 are obtained as 1.568 eV, 1.445 eV, 1.416 eV, 1.275 eV and 1.205 eV according to our calculation, which agree well with the available experimental values. The band structure of CIGS is also in accordance with the current theory.

Xu-Dong Chen

2014-08-01

14

Hybrid density functional theory study of Cu(In1-xGax)Se2 band structure for solar cell application

Cu(In1-xGax)Se2 (CIGS) alloy based thin film photovoltaic solar cells have attracted more and more attention due to its large optical absorption coefficient, long term stability, low cost and high efficiency. However, the previous theoretical investigation of this material with first principle calculation cannot fulfill the requirement of experimental development, especially the accurate description of band structure and density of states. In this work, we use first principle calculation based on hybrid density functional theory to investigate the feature of CIGS, with B3LYP applied in the CuIn1-xGaxSe2 stimulation of the band structure and density of states. We report the simulation of the lattice parameter, band gap and chemical composition. The band gaps of CuGaSe2, CuIn0.25Ga0.75Se2, CuIn0.5Ga0.5Se2, CuIn0.75Ga0.25Se2 and CuInSe2 are obtained as 1.568 eV, 1.445 eV, 1.416 eV, 1.275 eV and 1.205 eV according to our calculation, which agree well with the available experimental values. The band structure of CIGS is also in accordance with the current theory.

Chen, Xu-Dong; Chen, Lin; Sun, Qing-Qing; Zhou, Peng; Zhang, David Wei

2014-08-01

15

Energy Technology Data Exchange (ETDEWEB)

Highlights: ? ZrGa{sub 2} and ZrGa{sub 3} crystals structure was analyzed. ? FP-LAPW method was used to solve the Kohn Sham DFT equations within the framework of the WIEN2K code. ? Electronic band structures are reported. ? The studied crystals exhibit potential optoelectronic applications. -- Abstract: Using FP-LAPW Method we have performed calculations of the band structure of the ZrGa{sub 2} and ZrGa{sub 3} crystals. The all-electron full potential linearized augmented plane wave method was used to solve the Kohn Sham DFT equations. We have explored different approximations using three kinds of exchange-correlation potentials on the electronic structure and we concluded that there is insignificant influence on the band structure and the density of states. It is clear that there exists a difference in the band dispersion with one move from ZrGa{sub 2} to ZrGa{sub 3} that is attributed to the fact that ZrGa{sub 2} has four formula per unit cell (Z = 4) while ZrGa{sub 3} has two formula per unit cell (Z = 2). Despite some similarity in the crystallochemistry of ZrGa{sub 2} to ZrGa{sub 3} some differences are observed in the band structure dispersion. There is a strong hybridization between the states. The interaction of charges between Zr and Ga atoms is due to the strong hybridization, and the covalent bond arises due to the degree of hybridization. Hence, there is a strong covalent bonding between these atoms. We have obtained a space electron charge density distribution in the average unit cell by calculations of the electron charge density distribution. The space electronic charge density contour distribution is illustrated in (1 0 0) and (1 1 0) planes.

Reshak, A.H., E-mail: maalidph@yahoo.co.uk [Institute of Complex systems, FFPW, CENAKVA, University of South Bohemia in CB, 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); Lakshminarayana, G., E-mail: glnphysics@rediffmail.com [Materials Science and Technology Division (MST-7), Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Ebothe, J. [Laboratoire de Recherche en Nanosciences, E.A. 4682, Université de Reims, 21, rue Clément Ader, 51685 Reims cedex 02 (France); Fedorchuk, A.O. [Lviv National University of Veterinary Medicine and Biotechnologies, Department of Inorganic and Organic Chemistry, Lviv (Ukraine); Fedyna, M.F. [National University of Forestry and Wood Technology of Ukraine Chuprynky Str., 103, 79057 Lviv (Ukraine); Kamarudin, H. [School of Material Engineering, Malaysia University of Perlis, P.O Box 77, d/a Pejabat Pos Besar, 01007 Kangar, Perlis (Malaysia); Mandracci, P. [Politecnico di Torino, Department of Applied Science and Technology, corso Duca degli Abruzzi 24, 10129, Torino (Italy); Auluck, S. [Council of Scientific and Industrial Research - National Physical Laboratory Dr. K.S. Krishnan Marg, New Delhi 110012 (India)

2013-04-15

16

Hybrid density functional theory study of Cu(In1?xGax)Se2 band structure for solar cell application

Cu(In1?xGax)Se2 (CIGS) alloy based thin film photovoltaic solar cells have attracted more and more attention due to its large optical absorption coefficient, long term stability, low cost and high efficiency. However, the previous theoretical investigation of this material with first principle calculation cannot fulfill the requirement of experimental development, especially the accurate description of band structure and density of states. In this work, we use first principle calculation ba...

Xu-Dong Chen; Lin Chen; Qing-Qing Sun; Peng Zhou; David Wei Zhang

2014-01-01

17

The electronic structure of single-crystal WO3 and Na0.67WO3 (a sodium-tungsten bronze) has been measured using soft x-ray absorption and resonant soft x-ray emission oxygen K-edge spectroscopies. The spectral features show clear differences in energy and intensity between WO3 and Na0.67WO3. The x-ray emission spectrum of metallic Na0.67WO3 terminates in a distinct Fermi edge. The rigid-band model fails to explain the electronic structure of Na0.67WO3 in terms of a simple addition of electrons to the conduction band of WO3. Instead, Na bonding and Na 3s-O 2p hybridization need to be considered for the sodium-tungsten bronze, along with occupation of the bottom of the conduction band. Furthermore, the anisotropy in the band structure of monoclinic ?-WO3 revealed by the experimental spectra with orbital-resolved geometry is explained via density functional theory calculations. For ?-WO3 itself, good agreement is found between the experimental O K-edge spectra and the theoretical partial density of states of O 2p orbitals. Indirect and direct bandgaps of insulating WO3 are determined from extrapolating separations between spectral leading edges and accounting for the core-hole energy shift in the absorption process. The O 2p non-bonding states show upward band dispersion as a function of incident photon energy for both compounds, which is explained using the calculated band structure and experimental geometry.

Chen, B.; Laverock, J.; Piper, L. F. J.; Preston, A. R. H.; Cho, S. W.; DeMasi, A.; Smith, K. E.; Scanlon, D. O.; Watson, G. W.; Egdell, R. G.; Glans, P.-A.; Guo, J.-H.

2013-04-01

18

Electronic band structure of beryllium oxide

International Nuclear Information System (INIS)

The energy-momentum resolved valence band structure of beryllium oxide has been measured by electron momentum spectroscopy (EMS). Band dispersions, bandwidths and intervalence bandgap, electron momentum density (EMD) and density of occupied states have been extracted from the EMS data. The experimental results are compared with band structure calculations performed within the full potential linear muffin-tin orbital approximation. Our experimental bandwidths of 2.1 ± 0.2 and 4.8 ± 0.3 eV for the oxygen s and p bands, respectively, are in accord with theoretical predictions, as is the s-band EMD after background subtraction. Contrary to the calculations, however, the measured p-band EMD shows large intensity at the ? point. The measured full valence bandwidth of 19.4 ± 0.3 eV is at least 1.4 eV larger than the theory. The experiment also finds a significantly higher value for the p-to-s-band EMD ratio in a broad momentum range compared to the theory

19

Electronic band structure of beryllium oxide

The energy-momentum resolved valence band structure of beryllium oxide has been measured by electron momentum spectroscopy (EMS). Band dispersions, bandwidths and intervalence bandgap, electron momentum density (EMD) and density of occupied states have been extracted from the EMS data. The experimental results are compared with band structure calculations performed within the full potential linear muffin-tin orbital approximation. Our experimental bandwidths of 2.1 +- 0.2 and 4.8 +- 0.3 eV for the oxygen s and p bands, respectively, are in accord with theoretical predictions, as is the s-band EMD after background subtraction. Contrary to the calculations, however, the measured p-band EMD shows large intensity at the GAMMA point. The measured full valence bandwidth of 19.4 +- 0.3 eV is at least 1.4 eV larger than the theory. The experiment also finds a significantly higher value for the p-to-s-band EMD ratio in a broad momentum range compared to the theory.

Sashin, V A; Kheifets, A S; Ford, M J

2003-01-01

20

International Nuclear Information System (INIS)

States of 184Ir, excited through several (HI, xn) reactions were studied using in-beam ?-ray and conversion electron spectroscopy techniques. A new high-spin level scheme, comprising three different rotational bands is presented. The negative-parity ?I=1 ground-state band is the most complete example of the semidecoupled ?h9/2x?i13/2 structure, in a prolate deformed situation, known to date. The other two bands are the doubly decoupled ?h9/2x?1/2- [521] and a ?I=1, ?5/2+ [402]x?i13/2 based structure of the compressed type. (orig.)

21

Energy Technology Data Exchange (ETDEWEB)

We present a detailed angle-resolved photoemission spectroscopy (ARPES) investigation of the RTe3 family, which sets this system as an ideal"textbook" example for the formation of a nesting driven charge density wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDWinstabilities, from the opening of large gaps on the best nested parts of Fermi surface (up to 0.4 eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k space. An additional advantage of RTe3 is that theband structure can be very accurately described by a simple two dimensional tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure by comparing our ARPES measurements with the linear muffin-tinorbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k space, the evolution of the CDW wave vector with R, and the shape of the residual metallic pockets. Finally, we give an estimation of the CDWinteraction parameters and find that the change in the electronic density of states n (EF), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

Hussain, Zahid; Brouet, Veronique; Yang, Wanli; Zhou, Xingjiang; Hussain, Zahid; Moore, R.G.; He, R.; Lu, D. H.; Shen, Z.X.; Laverock, J.; Dugdale, S.B.; Ru, N.; Fisher, R.

2008-01-16

22

Band Structure and Effective Masses of ZnMgO

We analyze the influence of the Mg concentration on several important properties of the band structure of ZnMgO alloys in wurtzite structure using ab initio calculations. For this purpose, the band structure for finite concentrations is defined in terms of the Bloch spectral density, which can be calculated within the coherent potential approximation. We investigate the concentration dependence of the band gap and the crystal-field splitting of the valence bands. The effecti...

Franz, Christian; Giar, Marcel; Heinemann, Markus; Czerner, Michael; Heiliger, Christian

2013-01-01

23

International Nuclear Information System (INIS)

The 122Sn(11B,4n) fusion-evaporation reaction at Elab=60 MeV was used to populate excited states in 129Cs, and the deexcitations were investigated using in-beam ?-ray spectroscopic techniques. The level scheme of 129Cs is established up to ?8 MeV excitation energy and 47/2 h spin. The observed band structures are interpreted for their configurations in the framework of cranking model calculations and systematic of the neighboring 55Cs isotopes. A negative-parity ?I=1 coupled band has been assigned the ?h11/2 x ?(h11/2)2 configuration as solution of the tilted-axis cranking, which coexists with the ?h11/2 yrast band resulting from the principal-axis cranking. A new band has been identified as a ?-vibrational band built on the ?h11/2 orbital. A pair of strongly coupled positive-parity bands exhibiting similar features have been assigned different unpaired three-quasiparticle configurations involving the ?h11/2 x ?h11/2 component. The previously identified unfavored signature partners of the ?d5/2 and ?g7/2 bands are reassigned as ? vibrations of the core coupled to the ?g7/2 single-particle configuration, and the favored signature of the ?d5/2 band, respectively.

24

International Nuclear Information System (INIS)

Heavily boron-doped p-type diamond is a key material for developing diamond-based applications in various fields. We obtain information about changes in the valence band edge of homoepitaxial boron-doped diamond films around and below Mott's critical density for a metal-non-metal transition. For a boron concentration of about 2-3 x 1020 cm-3, where this transition is expected to occur, a metal-like behaviour is observed at room temperature (RT) with the Fermi-level lying 0.1 eV above the valence band edge. For a boron concentration about 4 x 1019 cm-3, which is well below the critical density, the valence band edge is quite different from that of lightly doped samples. It is proposed as an explanation for these experimental differences that the excited-states of acceptors overlap. (Abstract Copyright [2009], Wiley Periodicals, Inc.)

25

Elementary energy bands in the band structure of the narrow-band-gap semiconductor CdSb

Based upon the example of a narrow band gap covalent CdSb crystal (D2h16) , it is shown that a spatial electron density distribution in the unit cell is related to the Zak’s elementary energy bands creating the valence band and to the corresponding Wyckoff positions. A role of particular electronic states in the creation of the elementary energy bands was analyzed by conducting ab initio band structure calculations of CdSb. The investigations of the topology of the elementrary energy bands allows one to predict the nature of chemical bondings in rhombic crystals.

Bercha, D. M.; Slipukhina, I. V.; Sznajder, M.; Rushchanskii, K. Z.

2004-12-01

26

Excited states in the 99Rh nucleus were populated using the fusion-evaporation reaction 75As(28Si,2p2n) at {{E}_{lab}}=120\\;MeV and the de-excitations were investigated through in-beam ?-ray spectroscopic techniques using the INGA spectrometer consisting of 18 clover detectors. The observed band structures are discussed in the framework of tilted axis cranking shell-model calculations. Level structures at low energies are identified as resulting from the rotational bands based on the \\pi {{p}_{1/2}} and \\pi {{g}_{9/2}} configurations. The \\Delta I = 1 coupled bands are observed at higher excitation energies and have been interpreted as based on the \\pi {{g}_{9/2}}\\otimes \

Kumar, S.; Singh, V.; Singh, K.; Sihotra, S.; Singh, N.; Goswamy, J.; Malik, S. S.; Ragnarsson, I.; Trivedi, T.; Singh, R. P.; Muralithar, S.; Kumar, R.; Bhowmik, R. K.; Palit, R.; Bharti, A.; Mehta, D.

2014-10-01

27

Energy band structure of spin-1 condensates in optical lattices

International Nuclear Information System (INIS)

The energy band structure of spin-1 condensates with repulsive spin-independent and either ferromagnetic or antiferromagnetic spin-dependent interactions in one-dimensional (1D) periodic optical lattices is discussed. Within the two-mode approximation, Bloch bands of spin-1 condensates are presented. The results show that the Bloch bands exhibit a complex structure as the atom density of mF = 0 hyperfine state increases: bands splitting, reversion, intersection and loop structure are excited subsequently. The complex band structure should be related to the tunneling and spin-mixing dynamics. (general)

28

Band Structure of AlSb Nanoclusters

Directory of Open Access Journals (Sweden)

Full Text Available *Ab initio* restricted Hartree-Fock method within the framework of large unit cell formalism is used to investigate the band structure of AlSb nanoclusters. Large unit cells of 8, 16, 54, 64 and 128 atoms are used to analyze the cohesive energy, energy gap, valence bandwidth, and the density of states of the energy levels for AlSb nanoclusters. The results of the present work revealed that the band structure of AlSb nanoclusters differs significantly from that of the bulk AlSb crystal. Also, it is found that the valence band width and the cohesive energy (absolute value increase as the AlSb crystal size increases, and they tend to stabilize for nanoclusters of more than 50 atoms.

Haider I. Isa

2012-08-01

29

Quasiparticle band structure of antiferromagnetic Eu Te

International Nuclear Information System (INIS)

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 TN 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) spin exchange processes between (5 d-6 s) conduction band electrons and localized 4 f moments. (author)

30

Susceptibilities for first principles band structures

We present a parallel implementation of a new method for calculating the unenhanced susceptibility proposed by us recently. Our implementation uses the first principles LMTO band structure within the tight binding approach as input to calculate the joint density of states. The susceptibility is then obtained by integrating over the product of the joint density of states and a Lindhard function. Our program, which has a simple friendly user interface, runs on the PC with a quadputer board, a Meiko Surface running CSTools powered either by T800 or i860 compute boards and the Intel iPSC/860 hypercube in Daresbury. Our method incorporates the troublesome matrix elements naturally and our results on Pd and Ni show that the decrease in ?( q) as we go away from the Brillouin zone centre is due mainly to the matrix elements rather than to the band energies.

Crockford, D. J.; Yeung, W.

1993-04-01

31

Elementary energy bands in ab initio calculations of the YAlO3 and SbSI crystal band structure

The local density approximation has been implemented to determine the band structure of orthorhombic crystals YAlO3 (YAP) and SbSI. The topology of the valence band structures was analyzed. It has been demonstrated that Zak's elementary energy bands in the band structure are distinguishable on the basis of the empty-lattice approximation. The calculated electron density distribution of YAP and SbSI crystals is related to particular Wyckoff positions. Moreover, there is a direct correspondence between the obtained elementary energy bands and the aforementioned Wyckoff positions characterized by electron density distribution.

Bercha, D. M.; Rushchanskii, K. Z.; Sznajder, M.; Matkovskii, A.; Potera, P.

2002-11-01

32

The electronic structure of single-crystal WO3 and Na0.67WO3 (a sodium-tungsten bronze) has been measured using soft x-ray absorption and resonant soft x-ray emission oxygen K-edge spectroscopies. The spectral features show clear differences in energy and intensity between WO3 and Na0.67WO3. The x-ray emission spectrum of metallic Na0.67WO3 terminates in a distinct Fermi edge. The rigid-band model fails to explain the electronic structure of Na0.67WO3 in terms of a simple addition of electron...

Chen, B.; Laverock, J.; Piper, Lf; Preston, Ar; Cho, Sw; Demasi, A.; Smith, Ke; Scanlon, Do; Watson, Gw; Egdell, Rg; Glans, Pa; Guo, Jh

2013-01-01

33

Weak morphology dependent valence band structure of boron nitride

We report a hard X-ray photoelectron spectroscopy (HX-PES) investigation on valence band structure of Boron Nitrides (BN) having different morphologies, including nanosheets, nanotubes, and micro-sized particles. Very weak morphology/valence band structure dependence was observed. For each case, the B-N ?-band overlapping with ?-band between 0 to -12.5 eV and the s-band below -15 eV were identified. No obvious morphology-induced band shifts and intensity variations were observed. First-principles calculations based on density functional theory were performed and the results were compared with the experimental data. This theoretical analysis well explains the weak morphology dependent valence band spectra of BN nanomaterials obtained during HX-PES measurements.

Zhi, Chunyi; Ueda, Shigenori; Zeng, Haibo; Wang, Xuebin; Tian, Wei; Wang, Xi; Bando, Yoshio; Golberg, Dmitri

2013-08-01

34

Dynamical mean field band structure: LDA++ approach.

We discuss a general approach to the electronic structure of real materials which contains correlated d- or f- electrons. In accordance with the strength of average on-site Coulomb correlations parameter U over the total band width W, it is useful to distinguish three regime of strong, moderate and week correlations. In the case of strong interactions (U/W > 1 - ''rare-earth system``) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix N_mm^' the band structure problem is formulated as the functional problem on N_mm^' for an f-electrons and the standard LDA-functional for delocalized electrons. In the case of moderate correlations (U/W ~ 1 - ''metal-insulator regime, Kondo systems``) we start from the D_? dynamical mean field ITP-scheme of G. Kotliar et. al. and also make use of our multiband atomic Green function for constrained N_mm^' . Finally for the week interactions (U/W Bickers and D. Scalapino tested for the multiband case of 3d-metals.

Liechtenstein, A. I.; Katsnelson, M. I.

1997-03-01

35

Collective band structure in 66Zn

International Nuclear Information System (INIS)

The collective nature of states in 66Zn has been studied by carrying out a deformed configuration mixing shell model calculation in 1psub(3/2)0fsub(5/2)1psub(1/2)0gsub(9/2) model space. An effective interaction obtained for this space by Kuo has been used. The collective structure for 2 positive-parity bands and 5 negative-parity bands is identified. A qualitative understanding of the backbending at the J = 6+ state in the yrast positive-parity band is given in terms of the band crossing of the ground-state band and the more deformed excited band arising from 2p2h excitation to the gsub(9/2) orbit. Several high-spin members of the observed bands as well as in-band E2 transition strengths have been predicted. (orig.)

36

Unique electronic band structures of hydrogen-terminated silicon nanowires

International Nuclear Information System (INIS)

Band structure mutation from an indirect to a direct gap is a well-known character of small hydrogen-terminated and silicon nanowires (SiNWs), and suggests the possible emission of silicon. In contrast, we show that hydrogen-terminated SiNWs consistently present indirect band gaps even at an extremely small size, according to our calculations using density functional theory. Interestingly, the band gap of SiNWs shows a quasi-direct feature as the wire size increases, suggesting the possibility of using medium SiNWs in optoelectronic devices. This result also indicates that the electronic structures of SiNWs are strongly orientation dependent

37

Pseudopotential band structure of indium nitride

International Nuclear Information System (INIS)

The band structure, density of states and the imaginary part of the dielectric function have been calculated for indium nitride by a pseudopotential method. Comparison with published reflectance data permits the identification of principal optical transitions at the GAMMA,M,K, and H symmetry points and we have been able to correct previous transition assignments. The calculated long-wavelength refractive index of 2.88 +- 0.5 compares well with experiment. The material has a direct energy gap at the zone center and the spherically symmetric extrema are well described by scalar effective masses of 0.17m0, 0.5m0 (holes), and 0.12m0 (electrons)

38

Elementary energy bands in band structure calculations of some wide-bandgap crystals

Semiconducting Tl3AsS4 crystal was chosen as an example to show that its valence band is composed of the elementary energy bands. Their topology and symmetry obtained in the empty-lattice approximation is confirmed by ab initio band structure calculations. It was shown that these elementary energy bands correspond to the Wyckoff position c in a unit cell. Earlier predictions that the largest electron density distribution is focused in the vicinity of this position, similarly to the case of YAlO3 and SbSI crystals, is confirmed. A conclusion on the common topology and symmetry of the elementary energy bands in electronic and phonon spectra is presented.

Sznajder, M.; Bercha, D. M.; Rushchanskii, K. Z.

2004-01-01

39

Band structure parameters of the nitrides: The origin of the small band gap of InN

Using a band-structure method that includes the correction to the band gap error in the local density approximation (LDA), we study the chemical trends of the band gap variation in III-V semiconductors and predict that the band gap for InN is 0.8 +/- 0.1 eV, which is much smaller than previous experimental value of approx. 1.9 eV. The unusually small band gap for InN is explained in terms of the high electronegativity of nitrogen and consequently the small band gap deformation potential of InN. The possible origin of the measured large band gaps is discussed in terms of the non-parabolicity of the bands and the Moss-Burstein shift. Based on the error analysis of our LDA calculation and available experimental data we have compiled the recommended band structure parameters for wurtzite AlN, GaN and InN.

Wei, S H; Wei, Su-Huai; Carrier, Pierre

2004-01-01

40

Band structure parameters of the nitrides: The origin of the small band gap of InN

Using a band-structure method that includes the correction to the band gap error in the local density approximation (LDA), we study the chemical trends of the band gap variation in III-V semiconductors and predict that the band gap for InN is 0.8 +/- 0.1 eV, which is much smaller than previous experimental value of approx. 1.9 eV. The unusually small band gap for InN is explained in terms of the high electronegativity of nitrogen and consequently the small band gap deformati...

Wei, Su-huai; Carrier, Pierre

2004-01-01

41

Automated effective band structures for defective and mismatched supercells

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.

Brommer, Peter; Quigley, David

2014-12-01

42

Automated effective band structures for defective and mismatched supercells.

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. PMID:25388668

Brommer, Peter; Quigley, David

2014-12-01

43

The bulk band structure of Bi2Te3 has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot GW approximation within the all-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.

Michiardi, Matteo; Aguilera, Irene; Bianchi, Marco; de Carvalho, Vagner Eustáquio; Ladeira, Luiz Orlando; Teixeira, Nayara Gomes; Soares, Edmar Avellar; Friedrich, Christoph; Blügel, Stefan; Hofmann, Philip

2014-08-01

44

DEFF Research Database (Denmark)

The bulk band structure of Bi2Te3 has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot GW approximation within the all-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.

Michiardi, Matteo; Aguilera, Irene

2014-01-01

45

Band structure of boron doped carbon nanotubes

We present {\\it ab initio} and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive ``acceptor''-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon $\\pi$-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of acceptor and unoccupied carbon states above the Fermi edge.

Wirtz, L; Wirtz, Ludger; Rubio, Angel

2003-01-01

46

The surface band structure of W(112)

International Nuclear Information System (INIS)

Both the experimental and theoretical band structure of the W(112) surface are presented, with the theoretical band structure calculated by the film-LAPW (linearized augmented plane waves) method. The results permit one to compare the W(112) and Mo(112) band structures and illuminate the similarities. It is found that for the W(112) surface the main photoemission features combine contributions from both the surface and bulk, as has been previously noted for Mo(112). The main differences between the electronic structures of the furrowed W(112) and Mo(112) surfaces are seen in the width of occupied bands. The differences are attributed to the extent of localization of valence 4d and 5s electrons in Mo and 5d and 6s electrons in W. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

47

Phononic band gap structures as optimal designs

DEFF Research Database (Denmark)

In this paper we use topology optimization to design phononic band gap structures. We consider 2D structures subjected to periodic loading and obtain the distribution of two materials with high contrast in material properties that gives the minimal vibrational response of the structure. Both in-plane and out-of-plane vibrations are considered.

Jensen, Jakob SØndergaard; Sigmund, Ole

2003-01-01

48

Bulk Band Structure of Bi$_2$Te$_3$

The bulk band structure of Bi$_2$Te$_3$ has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot $GW$ approximation within the all-electron full-potential linearized augmented-plane-wave (FLAPW) formalism, fully taking into account spin-orbit coupling. Quasiparticle effects produce significant changes ...

Michiardi, Matteo; Aguilera, Irene; Bianchi, Marco; Carvalho, Vagner Eusta?quio; Ladeira, Luiz Orlando; Teixeira, Nayara Gomes; Soares, Edmar Avellar; Friedrich, Chiristoph; Blu?gel, Stefan; Hofmann, Philip

2014-01-01

49

Band-Structure of Thallium by the LMTO Method

DEFF Research Database (Denmark)

The relativistic band structure of thallium has been calculated using the linear muffin-tin orbital (LMTO) method. The positions and extents of the bands were found to follow the Wigner-Seitz rule approximately, and the origin of the dispersion of the bands was established from the canonical s and p bands for the HCP structure. Energy bands have been evaluated both with and without spin-orbit coupling which is particularly large in thallium. Energy bands close to the Fermi level were found to be mainly 6p like in character. The 6s states lay below the 6p bands and were separated from them by an energy gap. The 6d and 7s bands were found to be far above the Fermi level and the 5d states were found to be far below it. Fermi surface properties and the electronic specific heat are computed and compared with experiment. The joint density of states has also been computed and is in reasonable agreement with experimental optical properties.

Holtham, P. M.; Jan, J. P.

1977-01-01

50

The dependence of graphene Raman D-band on carrier density.

Raman spectroscopy has been an integral part of graphene research and can provide information about graphene structure, electronic characteristics, and electron-phonon interactions. In this study, the characteristics of the graphene Raman D-band, which vary with carrier density, are studied in detail, including the frequency, full width half-maximum, and intensity. We find the Raman D-band frequency increases for hole doping and decreases for electron doping. The Raman D-band intensity increases when the Fermi level approaches half of the excitation energy and is higher in the case of electron doping than that of hole doping. These variations can be explained by electron-phonon interaction theory and quantum interference between different Raman pathways in graphene. The intensity ratio of Raman D- and G-band, which is important for defects characterization in graphene, shows a strong dependence on carrier density. PMID:24283411

Liu, Junku; Li, Qunqing; Zou, Yuan; Qian, Qingkai; Jin, Yuanhao; Li, Guanhong; Jiang, Kaili; Fan, Shoushan

2013-01-01

51

Measurement of valence band structure in arbitrary dielectric films

International Nuclear Information System (INIS)

A new way of measuring the band structure of various dielectric materials using the secondary electron emission from Auger neutralization of ions is introduced. The first example of this measurement scheme is the magnesium oxide (MgO) films with respect to the application of the films in the display industries. The density of state in the valence bands of MgO film and MgO film with a functional layer (FL) deposited over a dielectric surface reveals that the density peak of film with a FL is considerably less than that of film, thereby indicating a better performance of MgO film with functional layer in display devices. The second example of the measurement is the boron-zinc oxide (BZO) films with respect to the application of the films to the development of solar cells. The measurement of density of state in BZO film suggests that a high concentration of boron impurity in BZO films may enhance the transition of electrons and holes through the band gap from the valence to the conduction band in zinc oxide crystals; thereby improving the conductivity of the film. Secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials.

52

Multiple band structures of 131Cs

International Nuclear Information System (INIS)

Excited states in 131Cs were investigated through in-beam ?-ray spectroscopic techniques following its population in the 124Sn(11B, 4n) fusion-evaporation reaction at a beam energy of 46 MeV. The previously known level scheme has been substantially extended up to ?9 MeV excitation energy and 49/2(?/2?) spin with the addition of seven new band structures. The present level scheme consisting of 15 bands exhibits a variety of collective features in this nucleus at intermediate spin. The excitation energies of the observed levels in different bands and the corresponding ratios of transition strengths, i.e., B(M1)/B(E2), have been compared with the results of projected deformed Hartree-Fock calculations based on various quasiparticle configurations. A strongly coupled band has been reassigned a high-K three-quasiparticle ?h11/2 x ?(h11/2d3/2) configuration based on the properties of this band and that of its new coupled side band. The configurations of these bands are also discussed in the framework of tilted-axis cranking model calculations and the systematics of the odd-A Cs isotopes. Additional three energetically closely placed coupled bands have been assigned different unpaired three-quasiparticle configurations. ?-vibrational bands coupled to the ?h11/2 and ?g7/2 single-particle configurations have been reported in this nucleus. Observation of new E1 transitions linkingn of new E1 transitions linking the opposite-parity ?h11/2 and ?d5/2 bands provides fingerprints of possible octupole correlations

53

Band structure of CdTe under high pressure

International Nuclear Information System (INIS)

The band structures and density of states of cadmium telluride (CdTe) under various pressures ranging from normal to 4.5 Mbar are obtained. The electronic band structure at normal pressure of CdTe (ZnS structure) is analyzed and the direct band gap value is found to be 1.654 eV. CdTe becomes metal and superconductor under high pressure but before that it undergoes structural phase transition from ZnS phase to NaCl phase. The equilibrium lattice constant, bulk modulus and the phase transition pressure at which the compounds undergo structural phase transition from ZnS to NaCl are predicted from the total energy calculations. The density of states at the Fermi level (N(EF)) gets enhanced after metallization, which leads to the superconductivity in CdTe. In our calculation, the metallization pressure (PM = 1.935 Mbar) and the corresponding reduced volume ((V/V0)M = 0.458) are estimated. Metallization occurs via direct closing of band gap at ? point. (author)

54

Band alignment of semiconductors from density-functional theory and many-body perturbation theory

The band lineup, or alignment, of semiconductors is investigated via first-principles calculations based on density functional theory (DFT) and many-body perturbation theory (MBPT). Twenty-one semiconductors including C, Si, and Ge in the diamond structure, BN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe in the zinc-blende structure, and GaN and ZnO in the wurtzite structure are considered in view of their fundamental and technological importance. Band alignments are determined using the valence and conduction band offsets from heterointerface calculations, the ionization potential (IP) and electron affinity (EA) from surface calculations, and the valence band maximum and conduction band minimum relative to the branch point energy, or charge neutrality level, from bulk calculations. The performance of various approximations to DFT and MBPT, namely the Perdew-Burke-Ernzerhof (PBE) semilocal functional, the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, and the GW approximation with and without vertex corrections in the screened Coulomb interaction, is assessed using the GW?1 approximation as a reference, where first-order vertex corrections are included in the self-energy. The experimental IPs, EAs, and band offsets are well reproduced by GW?1 for most of the semiconductor surfaces and heterointerfaces considered in this study. The PBE and HSE functionals show sizable errors in the IPs and EAs, in particular for group II-VI semiconductors with wide band gaps, but are much better in the prediction of relative band positions or band offsets due to error cancellation. The performance of the GW approximation is almost on par with GW?1 as far as relative band positions are concerned. The band alignments based on average interfacial band offsets for all pairs of 17 semiconductors and branch point energies agree with explicitly calculated interfacial band offsets with small mean absolute errors of both ˜0.1eV, indicating a good overall transitivity of the band offsets. The alignment based on IPs from selected nonpolar surfaces performs comparably well in the prediction of band offsets at most of the considered interfaces. The maximum errors are, however, as large as 0.3, 0.4, and 0.7 eV for the alignments based on the average band offsets, branch point energies, and IPs, respectively. This margin of error should be taken into account when performing materials screening using these alignments.

Hinuma, Yoyo; Grüneis, Andreas; Kresse, Georg; Oba, Fumiyasu

2014-10-01

55

International Nuclear Information System (INIS)

We have investigated the structure of the collective bands in 77,79Sr within our deformed configuration mixing (DCM) shell model based on Hartree-Fock states. In our model, the single particle orbits 1p3/2, 0f5/2, 1p1/2 and 0g9/2 are taken as the configuration space with 56Ni as the core. A modified Kuo interaction for this basis space is used in our calculation. The different levels are classified into collective bands on the basis of the B(E2) values among them. The calculated collective bands and the B(E2) values are compared with available experimental data. Our calculation is able to reproduce the identical bands observed in 77Sr, 78Sr and 78Rb. (author)

56

Band structure mapping of bilayer graphene via quasiparticle scattering

Directory of Open Access Journals (Sweden)

Full Text Available A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as ?0 = 3.1 eV, ?1 = 0.39 eV, and ?4 = 0.22 eV.

Matthew Yankowitz

2014-09-01

57

Liquid Surface Wave Band Structure Instabilities

We study interfacial instabilities between two spatially periodically sheared ideal fluids. Bloch wavefunction decompositions of the surface deformation and fluid velocities result in a nonhermitian secular matrix with an associated band structure that yields both linear oscillating and nonoscillating instabilities, enhanced near Bragg planes corresponding to the periodicity determined by converging or diverging surface flows. The instabilities persist even when the dynamica...

Chou, Tom

1997-01-01

58

Band structure and electronic properties of transition metal hydrides

International Nuclear Information System (INIS)

A calculation of band structures, densities of states, and Fermi surfaces for 3d-element hybrides is made. The results of this calculation are compared with the APW calculation by Switendick and the experimental measured values of electronic specific heat, magnetic susceptibility, and optical absorptivity. By using the Gaspari-Gyorffy theory values of the electron-phonon coupling constant and Tsub(c) are calculated. High critical temperatures of superconductivity are predicted for chromium and nickel dihydrides. (author)

59

X-Band Structure Development at KEK

X-band accelerator structure development at KEK has been driven targeting the linear colliders in worldwide collaborations. It is based on the technologies developed with high-precision machining, precise assembly and bonding method to preserve the precision. With maximally utilizing the merits of such technologies, the long-range wakefield was suppressed in parallel to realize the high gradient. The latter needs more study and development to actually realize the stable operation at a gradient of 100 MV/m or higher in the view point of the present paper. The worldwide collaboration studies are extensively on-going and the understanding of the vacuum breakdown has been advancing. By describing the development at KEK toward the X-band wakefield suppressed high-gradient accelerator structure, this paper shows how such structures have been evolved and may serve to show a room for the future studies.

Higo, Toshiyasu

2015-10-01

60

One-dimensional electromagnetic band gap structures formed by discharge plasmas in a waveguide

International Nuclear Information System (INIS)

We demonstrate the ability to develop one-dimensional electromagnetic band gap structure in X-band waveguide solely by using the positive columns of glow discharges in neon at the middle pressure. Plasma inhomogeneities are distributed uniformly along a typical X-band waveguide with cross section of 23?×?10?mm2. It is shown that electron densities larger than 1014?cm?3 are needed in order to create an effective one-dimensional electromagnetic band gap structure. Some applications for using the one-dimensional electromagnetic band gap structure in waveguide as a control of microwave (broadband filter and device for variation of pulse duration) are demonstrated.

61

Electronic band structures for zinc-blende and wurtzite CdS

International Nuclear Information System (INIS)

The electronic band structures for zinc-blende and wurtzite CdS are calculated within the local-density approximation with the use of first-principles pseudopotentials. Incorporating the d state into the valence band improves substantially the main-valence-band width, and yields valence-band features in good agreement with experiment. The maximum effect of the d band occurs at GAMMA15 for zinc-blende CdS and at GAMMA1,GAMMA6 for wurtzite CdS. We find that the local-density approximation does not predict accurately the position of localized Cd 4d state

62

The effect of band structure on sputtering

International Nuclear Information System (INIS)

Using one-color two step ionisation schemes, we have measured using resonant ionisation spectroscopy (RIS) the number of Ni atoms ejected in the ground state (a3F4) and an electronically excited state (a3D3) state from both Ni3Al and pure Ni surfaces. The ratio of the RIS signal from the a3D3 state to that from the a3F4 state is 1.07±0.03 for Ni3Al and 1.05±0.08 for pure Ni. To within experimental error, we saw no difference between pure Ni and the alloy in the relative sputter yield of Ni atoms in the excited a3D3 state compared to ground a3F4 state. There is a difference in the electronic band structure between pure Ni and Ni3Al. The above result indicates the different band structure does not influence the relative sputter yields of excited states

63

Photo field emission spectroscopy of the tantalum band structure

International Nuclear Information System (INIS)

Photo field emission (PFE) currents of clean and barium covered tantalum tips have been measured with single lines of the mercury arc spectrum and phase-sensitive detection. Field strength and work function were determined from Fowler-Nordheim plots of the FE currents. Shoulders in the PFE current-voltage characteristics could be correlated to transitions in the band structure of tantalum according to a recently proposed two-step PFE model. A comparison with the relativistic calculations of Mattheiss and the nonrelativistic bands of Petroff and Viswanathan shows that Mattheiss' bands are more appropriate. Beside direct transitions several nondirect transitions from the different features composing the upper two density of states maxima below the Fermi edge of tantalum have been found. (Auth.)

64

Segmental structure in banded mongoose calls

Abstract In complex animal vocalizations, such as bird or whale song, a great variety of songs can be produced via rearrangements of a smaller set of 'syllables', known as 'phonological syntax' or 'phonocoding' However, food or alarm calls, which function as referential signals, were previously thought to lack such combinatorial structure. A new study of calls in the banded mongoose Mungos mungo provides the first evidence of phonocoding at the level of single calls. The fi...

Fitch W

2012-01-01

65

Band-structure parameters by genetic algorithm

International Nuclear Information System (INIS)

A genetic algorithm has been used to solve a complex multidimensional parameter-fitting problem. We will focus on the parameters of an empirical tight-binding Hamiltonian. The method is used to approximate the electronic energy band structure if energy values are known for a few wave vectors of high symmetry. Compared to the usual manual procedure this method is more accurate and automatic. This approach, based on the extended Hueckel theory (EHT), has provided a list of EHT parameters for IV-IV and III-V semiconductors with zinc-blende structure and helped us to find a symmetry in the EHT. copyright 1996 The American Physical Society

66

Band-structure parameters by genetic algorithm

A genetic algorithm has been used to solve a complex multidimensional parameter-fitting problem. We will focus on the parameters of an empirical tight-binding Hamiltonian. The method is used to approximate the electronic energy band structure if energy values are known for a few wave vectors of high symmetry. Compared to the usual manual procedure this method is more accurate and automatic. This approach, based on the extended Hückel theory (EHT), has provided a list of EHT parameters for IV-IV and III-V semiconductors with zinc-blende structure and helped us to find a symmetry in the EHT.

Starrost, F.; Bornholdt, S.; Solterbeck, C.; Schattke, W.

1996-05-01

67

4H-SiC band structure investigated by surface photovoltage spectroscopy

International Nuclear Information System (INIS)

The conduction and valence band structure of high-purity 4H-SiC epilayers have been studied by surface photovoltage spectroscopy (SPS). A comparison between defect-free and single-layer stacking fault affected areas is reported. Electronic transitions, determined by SPS, are in good agreement with ab initio calculations. Electronic transitions and changes in band occupation have been observed in stacking fault rich areas below the band gap. Moreover, stacking faults induce the presence of a split-off band below the conduction band and a modification of the electron density of states in the conduction band always at the M point.

68

Electronic band structure of transparent conductor. Nb-doped anatase TiO2

International Nuclear Information System (INIS)

We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature condition band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations. (author)

69

Electronic Structure Calculations of Solids Using Density-Functional Method.

The electronic structures of the II-VI semiconductors in the Wurtzite structure (ZnO, ZnS, ZnSe, CdS, and CdSe) were calculated within the local-density approximation (LDA) using the linear muffin-tin orbitals (LMTO) method. The cation d states were included in the valence band. The calculation shows that for an accurate measurement of the ground state properties of these compounds the inclusion of the cation d states in the valence band is essential. Next, LDA-LMTO method was used to calculate the electronic structure of the CaF_2/Si(lll) interface. The interface states for three different structural models of the Ca-terminated interface are examined. For each model a pair of two-dimensional interface bands is found. The unoccupied interface band around the zone center is a truly localized band while the occupied interface band is buried below the valence-band maximum. The nature of the two -dimensional interface states is also discussed. Finally, the electron band structures of hexagonal and cubic diamond are calculated. Despite similarity between the two crystal structures, significant differences in the electron bands are found. The lowest gap of hexagonal diamond is found to be 4.5eV, which is 1.1eV smaller than the lowest gap of cubic diamond and is from Gamma to K.

Salehpour, Mohammad Reza

70

Relationship between band populations and band structure in the three-band Hubbard model

Energy Technology Data Exchange (ETDEWEB)

We study the two-dimensional three-band Hubbard model by means of a four-pole approximation within the Composite Operator Method framework. The model has been solved by considering as basic composite field a four-component spinor field, which includes the p field, the two Hubbard operators for the d field, and a composite operator describing the p field dressed by the spin excitations of the d field. This solution correctly reproduces many results of numerical simulations. In this manuscript, we investigate the relationship between p-and d- populations and the band structure of the model, by varying the on-site potential, the charge-transfer gap, the doping and the hopping integral between the p orbitals.

Avella, A; Mancini, F; Mancini, F P; Plekhanov, E, E-mail: fpmancini@physics.unisa.it [Dipartimento di fisica ' E.R. Caianiello' , Universita degli Studi di Salerno, 84084 Fisciano (Italy)

2011-01-01

71

Electronic structure of periodic curved surfaces -- topological band structure

Electronic band structure for electrons bound on periodic minimal surfaces is differential-geometrically formulated and numerically calculated. We focus on minimal surfaces because they are not only mathematically elegant (with the surface characterized completely in terms of "navels") but represent the topology of real systems such as zeolites and negative-curvature fullerene. The band structure turns out to be primarily determined by the topology of the surface, i.e., how the wavefunction interferes on a multiply-connected surface, so that the bands are little affected by the way in which we confine the electrons on the surface (thin-slab limit or zero thickness from the outset). Another curiosity is that different minimal surfaces connected by the Bonnet transformation (such as Schwarz's P- and D-surfaces) possess one-to-one correspondence in their band energies at Brillouin zone boundaries.

Aoki, H; Takeda, D; Morise, H

2001-01-01

72

Omnidirectional elastic band gap in finite lamellar structures.

This paper presents a comprehensive theoretical analysis of the occurrence of omnidirectional reflection in one-dimensional phononic crystal structures. We discuss the conditions for a one-dimensional layered structure, made of elastic materials, to exhibit total reflection of acoustic incident waves in a given frequency range, for all incident angles and all polarizations. The property of omnidirectional reflection can be fulfilled with a simple finite superlattice if the substrate from which the incident waves are launched is made of a material with high acoustic velocities (this is very similar to the case of omnidirectional optical mirror where the incident light is generated in vacuum). However, if the substrate is made of a material with low acoustic velocities, we propose two solutions to obtain an omnidirectional band gap, namely, the cladding of a superlattice with a layer of high acoustic velocities, which acts like a barrier for the propagation of phonons, or the association in tandem of two different superlattices in such a way that the superposition of their band structures exhibits an absolute acoustic band gap. We discuss the appropriate choices of the material and geometrical properties to realize such structures. The behavior of the transmission coefficients are discussed in relation with the dispersion curves of the finite structure embedded between two substrates. Both transmission coefficients and densities of states (from which we derive the dispersion curves) are calculated in the framework of a Green's function method. PMID:12513625

Bria, D; Djafari-Rouhani, B

2002-11-01

73

Quasiparticle band structure of Zn-IV-N2 compounds

Electronic energy-band structures of the Zn-IV-N2 compounds, with IV equal to Si, Ge, and Sn calculated in the quasiparticle self-consistent GW approximation and using the full-potential linearized muffin-tin orbital approach, are presented. A comparison is made with local-density approximation results. The bands near the gap are fitted to an effective Kohn-Luttinger-type Hamiltonian appropriate for the orthorhombic symmetry, and conduction-band effective masses are presented. Exciton binding energies and zero-point motion corrections to the gaps are estimated. While ZnSiN2 is found to be an indirect gap semiconductor, ZnGeN2 and ZnSnN2 are direct gap semiconductors. The gaps range from the orange-red to deep UV. The valence-band maximum is split in three levels of different symmetry, even in the absence of spin-orbit coupling, and should show transitions to the conduction band, each for a separate polarization. Spin-orbit effects are found to be surprisingly small, indicating almost exact compensation of the N-2p and Zn-3d contributions.

Punya, Atchara; Lambrecht, Walter R. L.; van Schilfgaarde, Mark

2011-10-01

74

Complex Band Structures and Decay Length in Polyethylene Chains

The complex band structure of an isolated polyethylene chain is calculated within Density Functional Theory (DFT). A plane wave basis and ultrasoft pseudopotentials are used. The results are compared with those obtained via a local basis set. We obtain a gap between the highest occupied molecular orbilar (HOMO) and the antibonding unoccupied molecular orbitals of 9.3 eV and a non-resonant tunneling $\\beta$ parameter of 0.9 per monomer, in reasonable agreement with experiment and with results obtained via local basis. Polyethylene is a negative electron affinity material and the actual gap should be the energy of the HOMO with respect to the vacuum level (in DFT approximation only about 5.14 eV). The Bloch states at imaginary k are mainly free-electron-like parabolic bands which are missing in the local basis. We present also the complex bands of the bulk polyethylene in order to estimate the effects of the chain-chain interactions on the complex band structure. The relevance of these results for the tunnellin...

Picaud, F; Corso, A D; Tosatti, E; Picaud, Fabien; Smogunov, Alexander; Corso, Andrea Dal; Tosatti, Erio

2003-01-01

75

Quasiparticle band structure of bulk hexagonal boron nitride and related systems

International Nuclear Information System (INIS)

The quasiparticle band structure of bulk hexagonal boron nitride is studied within the GW approximation for the self-energy operator. The influence of the interlayer distance on the band structure is investigated both within the local density approximation and the quasiparticle approach, and the importance of an interlayer state in determining the gap is demonstrated. Also, the quasiparticle band structure for an isolated sheet of boron nitride is calculated. We show that the equivalent of the interlayer state in the case of the isolated boron nitride sheet plays the same role as in the bulk case in determing the band gap

76

Band structure in Platinum nuclei (A ? 182)

International Nuclear Information System (INIS)

In this thesis, the author studies the band structure in Platinum nuclei and has divided his work in 5 parts: in the first, the author makes a general presentation of nucleus physics with a high angular momentum and introduces to the deformed nucleus notion -axial, triaxial or mixing of different deformations. The notion of form co-existence will be used to interpret the experimental results. In the second part, the author describes the detection means which have been used to make measurements. An abstract of theoretical notions, usefull for the understanding of fusion-evaporation reaction is presented. The author explains the details, performances and different modes of using of 'Chateau de cristal' and others used spectrometers. In the third part, the author presents all experimental data. He has effected ? coincidence measurements for Pt, Au and Ir nuclei. In the fourth part, for a classical analysis or an interpretation in the frame of cranking model the author presents theoretical models which are adapted at the study of high spin states and band structures

77

Photonic Crystal Narrow Band Filters Using Biperiodic Structures

Directory of Open Access Journals (Sweden)

Full Text Available In this study, we propose a new type of photonic crystal band-pass and band-stop filter based on biperiodic structure. Usually two types of photonic crystal band-pass filters are utilized in optical circuit. In the first one Fabry-Perot cavities are used while in the second one cavity-coupled waveguide are used. We optimize the second structure in this study. Then we show that by little changing in the band-pass structure, a band-stop filter is attained. Using Finite-Difference Time-Domain (FDTD method we achieve output efficiency over 98% for band-pass filter and below 3% for band-stop filter. Also we investigate filter tunability parameters which affecting central frequency of these filters. Further analysis shows that the central frequency of transmission band (stop band can be changed by altering the refractive index of bigger rods in biperiodic structure.

M. Djavid

2008-01-01

78

Segmental structure in banded mongoose calls

Directory of Open Access Journals (Sweden)

Full Text Available Abstract In complex animal vocalizations, such as bird or whale song, a great variety of songs can be produced via rearrangements of a smaller set of 'syllables', known as 'phonological syntax' or 'phonocoding' However, food or alarm calls, which function as referential signals, were previously thought to lack such combinatorial structure. A new study of calls in the banded mongoose Mungos mungo provides the first evidence of phonocoding at the level of single calls. The first portion of the call provides cues to the identity of the caller, and the second part encodes its current activity. This provides the first example known in animals of something akin to the consonants and vowels of human speech. See research article http://www.biomedcentral.com/1741-7007/10/97

Fitch W

2012-12-01

79

Energy band structure of LiNH4SO4 crystals

The genesis of the LiNH4SO4 crystal has been determined, the energy band structure and the total and partial electron densities of states have been calculated within the electron density functional theory. The coordinates of hydrogen atoms in groups have been determined, and the atomic positions and lattice parameters have been refined. It has been found that the band gap of LiNH4SO4 crystals is 5.32 eV.

Rudysh, M. Ya.; Stadnyk, V. Yo.; Brezvin, R. S.; Shchepanskii, P. A.

2015-01-01

80

Band structure and electron level population in plane channeling

International Nuclear Information System (INIS)

The method of band structure calculation of electrons moving in a plane channel is proposed. The energy band structure for the channel (110) in silicon and tungsten is calculated. It is shown that at 20 MeV the bands degenerate into the levels. The band population is investigated depending on electron energy and monocrystal angle of incidence. Population inversion is the greatest at angles of incidence not exceeding the Bragg angle

81

Electron density and carriers of the diffuse interstellar bands

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction to 13 stars. A linear relation was found, that allows the determination of the electron density from the Mg I and Mg II column densities in interstellar medium. The comparison of normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with varying electron density. Therefore the DIBs carriers (1) can be observed only in one ionisation stage, or (2) the DIBs are arising in cloud regions (cores or cloud coronas) for which we can not determine the electron density.

Gnacinski, P; Galazutdinov, G A

2007-01-01

82

Creation of partial band gaps in anisotropic photonic-band-gap structures

The photonic-band-gap (PBG) structure composed of an anisotropic-dielectric sphere in uniform dielectric medium is studied by solving Maxwell’s equations using the plane-wave expansion method. In particular, for a uniaxial material with large principal refractive indices and sufficient anisotropy between them, the photonic band structures possess a full band gap in the whole Brillouin zone for a diamond lattice. Furthermore, in the 1/3 partial Brillouin zone where the Bloch wave vector has ...

Li, Zy; Gu, By; Wang, J.

1998-01-01

83

Energy Technology Data Exchange (ETDEWEB)

Uranium ferromagnet UIr is well-known to a pressure-induced superconductor without an inversion symmetry. In order to clarify the U 5f states of ferromagnetically-ordered UIr below T{sub c} {approx} 46 K and at ambient pressure, we observed 5f-sensitive soft X-ray photoemission spectra (SXPES), and calculated the band structure by a relativistic LAPW method in a local-spin density approximation. The temperature-dependent angle-integrated SXPES near the Fermi energy show a definite energy-shift of the band structures below and above T{sub c}. The exchange splitting of the 5f bands seems to be consistent with an itinerant band picture like Stoner model. Furthermore the angle-resolved SXPES are compared with the theoretical band structure to investigate the relationship between the band shift and the magnetism.

Yamagami, H [Department of Physics, Kyoto Sangyo University, Kyoto 603-8555 (Japan); Ohkochi, T; Fujimori, S-i; Toshimitsu, T; Yasui, A; Okane, T; Saitoh, Y [Synchrotron Radiation Research Center SPring-8, JAEA, Hyogo 679-5148 (Japan); Fujimori, A [Department of Physics, The University of Tokyo, Tokyo (Japan); Haga, Y; Yamamoto, E; Ikeda, S [Advanced Science Research Center, JAEA, Ibaraki 319-1195 (Japan); Onuki, Y, E-mail: yamagami@cc.kyoto-su.ac.j [Graduate School of Science, Osaka University, Osaka 560-0043 (Japan)

2010-01-15

84

Band Structures of the Nucleus 129Cs

International Nuclear Information System (INIS)

High-spin states in 129Cs are populated via the 122Sn (11B, 4n) reaction at beam energies of 55 and 60 MeV. Two additional bands are placed in the level scheme and the previously known bands are extended to higher spins. The results are compared to the cranked shell model calculations and to the systematics of the adjacent Cs isotpoes. One of the new bands is interpreted as the ?-vibrational band built on the ?h11/2 orbital. The possible configuration for another new band is discussed. Upbend caused by (vh11/2)2 alignment is observed both in the favored and unfavored sequences of the ?h11/2 configuration. The band based on the ?g7/2 configuration at low spins forks around spin 17/2, and the two different S-bands are attributed to (vh11/2) and (?h11/2)2 rotational alignments, respectively

85

Band structure calculations of Pt and Pt3Ti

International Nuclear Information System (INIS)

Linearized augmented plane wave (LAPW) calculations of the Pt and Pt3Ti bulk electron structures are presented. We emphasis on hybridization, charge transfer effects and the impact on chemisorption properties densities of states (DOS) and novel band-structures are also provided. The delocalized character of free electron states documents itself by sp-orbitals largely unaffected by alloy formation i.e. the Pt and Ti projected sp-densities are similar and also akin to the distribution of sp-states for pure platinum. The strong Pt-Ti interaction comes from d and f orbitals with a significantly lowered Pt d center-of-mass for the alloy and an intense Ti d structure above the Fermi level. The f-projected density of states gives two narrow peaks: a Ti peak below the Fermi level and an unoccupied Pt peak near Ef. Together the d and f interactions result in a much lowered DOS(Ef). Our results are of direct relevance for adsorption systems modelling titania supported platinum, a well known catalyst for CO hydrogenation. The ordered alloy, Pt3Ti, forms in the industrial catalyst under reducing conditions at elevated temperatures, a phenomenon related to the strong metal support interaction (SMSI) effect in catalysis, and much work has addressed special adsorption sites for enhanced methanation rates. (orig.)

86

Flux density monitoring of radio stars observable by HIPPARCOS at S-band and X-band

Single-dish simultaneous S-band (13 cm) and X-band (3.6 cm) flux density measuremets of radio stars are reported. The aim of the observational program is to monitor the variability and flux density of radio stars with an optical counterpart observable by the future astrometric satellite Hipparcos. From a list of 69 selected radio stars for astrometric purposes, 49 of them have been observed during 1982 with the 64 m antenna of the Madrid DSCC. The number of radio stars that have been detected is 15.

Estalella, R.; Paredes, J. M.; Rius, A.

1983-08-01

87

Electron density and carriers of the diffuse interstellar bands

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction to 13 stars. A linear relation was found, that allows the determination of the electron density from the Mg I and Mg II column densities in diffuse clouds. The comparison of normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range ne=0.01-2.5 cm^-3. There...

Gnacinski, P.; Sikorski, J. K.; Galazutdinov, G. A.

2007-01-01

88

Band edge singularities and density of states in YTaO4 and YNbO4

International Nuclear Information System (INIS)

We study the structural and electronic properties of YTaO4 and YNbO4 by means of accurate first-principle total energy calculations. The calculations are based on density functional theory (DFT). The total energy, electronic band structure, and density of states are calculated via the full potential linear-augmented plane wave approach, as implemented in the WIEN2K code, within the framework of DFT. The results show that the valence bands of tantalate and niobate systems are from O 2p states. Conduction bands are divided into two parts. The lower conduction band is mainly composed of Ta 5d or Nb 4d states and the upper conduction bands involve contribution mainly from Y 4d states of YTaO4 or YNbO4. The efficient band gaps in yttrium tantalate and niobate are determined about 4.8 and 4.1 eV, respectively. The agreement between the calculations and the experimental data is excellent. The efficient band gap and a simple model illustrating excitation and emission process in considered host lattices are discussed. (authors)

89

Electronic structure and electron momentum density in TiSi

International Nuclear Information System (INIS)

We report the electron momentum density in titanium monosilicide using 241Am Compton spectrometer. Experimental Compton profile has been compared with the theoretical profiles computed using linear combination of atomic orbitals (LCAO). The energy bands, density of states and Fermi surface structures of TiSi are reported using the LCAO and the full potential linearized augmented plane wave methods. Theoretical anisotropies in directional Compton profiles are interpreted in terms of energy bands. To confirm the conducting behavior, we also report the real space analysis of experimental Compton profile of TiSi

90

Elucidating the stop bands of structurally colored systems through recursion

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...

Amir, Ariel

2012-01-01

91

Band structure studies of actinide systems

International Nuclear Information System (INIS)

The nature of the f-orbitals in an actinide system plays a crucial role in determining the electronic properties. It has long been realized that when the actinide separation is small enough for the f-orbitals to interact directly, the system will exhibit itinerant electron properties: an absence of local moment due to the f-orbitals and sometimes even superconductivity. However, a number of systems with the larger actinide separation that should imply local moment behavior also exhibit intinerant properties. Such systems (URh3, UIr3, UGe3, UC) were examined to learn something about the other f-interactions. A preliminary observation made is that there is apparently a very large and ansiotropic mass enhancement in these systems. There is very good reason to believe that this is not solely due to large electron--electron correlations but to a large electron--phonon interaction as well. These features of the ''non-magnetic'', large actinide separation systems are discussed in light of our results to date. Finally, the results of some recent molecular calculations on actinide hexafluorides are used to illustrate the shielding effects on the intra-atomic Coulomb term U/sub f-f/ which would appear in any attempt to study the formation of local moments. As one becomes interested in materials for which a band structure is no longer an adequate model, this screened U/sub ff/ is the significant parameter and efforts must be made to evaluate it in solid state systems

92

Band structures of TiO2 doped with N, C and B.

This study on the band structures and charge densities of nitrogen (N)-, carbon (C)- and boron (B)-doped titanium dioxide (TiO(2)) 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. PMID:16532532

Xu, Tian-Hua; Song, Chen-Lu; Liu, Yong; Han, Gao-Rong

2006-04-01

93

Band structures of TiO2 doped with N, C and B*

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. PMID:16532532

Xu, Tian-Hua; Song, Chen-Lu; Liu, Yong; Han, Gao-Rong

2006-01-01

94

Electronic band structure study of colossal magnetoresistance in Tl 2Mn 2O 7

The electronic structure of Tl 2Mn 2O 7 was examined by performing tight binding band calculations. The overlap between the Mn t 2g- and Tl 6 s-block bands results in a partial filling of the Tl 6 s-block bands. The associated Fermi surface consists of 12 cigar-shape electron pockets with each electron pocket about {1}/{1000} of the first Brillouin zone in size. The Tl 6 s-block bands have orbital contributions from the Mn atoms, and the carrier density is very low. These are important for the occurrence of a colossal magnetoresistance in Tl 2Mn 2O 7.

Seo, D.-K.; Whangbo, M.-H.; Subramanian, M. A.

1997-02-01

95

Photonic band structure of highly deformable, self-assembling systems

We calculate the photonic band structure at normal incidence of highly deformable, self-assembling systems - cholesteric elastomers subjected to external stress. Cholesterics display brilliant reflection and lasing owing to gaps in their photonic band structure. The band structure of cholesteric elastomers varies sensitively with strain, showing new gaps opening up and shifting in frequency. A novel prediction of a total band gap is made, and is expected to occur in the vicinity of the previously observed de Vries bandgap, which is only for one polarisation.

Bermel, P A; Bermel, Peter A.; Warner, Mark

2001-01-01

96

Band gap and effective mass of multilayer BN/graphene/BN: van der Waals density functional approach

Using the van der Waals density functional theory method (DFT-D2), we have investigated thickness dependent energy band gaps and effective masses of multilayer BN/graphene/BN structures by changing the stacking order. The band gap is substantially dependent on the stacking order. For instance, the calculated band gap in ABA stacking is about 150 meV, whereas it becomes 31 meV in ABC stacking. No significant thickness dependent band gap is observed in both ABA and ABC stackings although the band gap is gradually increasing with the BN thickness in ABA stacking. In addition, the effective mass is found to be strongly dependent on the stacking order. The effective mass in ABA stacking is much larger than that found in ABC stacking. On the other hand, the effective mass along K-M direction is smaller than that along K-? direction in ABA stacking. However, it is independent on the band direction in ABC stacking. We have found that the inclusion of van der Waals interaction alters thickness dependent band gap and effective mass of BN/graphene/BN multilayer systems compared with those found with standard density functional theory.

Hashmi, Arqum; Hong, Jisang

2014-05-01

97

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

DEFF Research Database (Denmark)

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 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 the band gap closes and the systems become “metallic”. These effects are related to the existence of the built-in electric fields that strongly influence valence- and conduction-band profiles and thus determine effective band gap and emission energies of the superlattices. Varying the widths of the quantum wells and barriers one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.

Gorczyca, I.; Suski, T.

2012-01-01

98

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

Energy Technology Data Exchange (ETDEWEB)

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.

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

99

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

International Nuclear Information System (INIS)

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

100

Mushroomlike photonic band gap (PBG) structures exhibit two band gap characteristics: surface-wave suppression and in-phase reflectivity. The fundamental electromagnetic properties and the relationship between the surface-wave suppression band gap and the plane-wave reflection phase band are investigated and clarified by a finite-element full-wave analysis. The results of the plane-wave bistatic reflection experiments on mushroomlike PBG plates in an anechoic chamber are in good agreement with those of numerical simulation, confirming the phenomenon of dual in-phase reflection, i.e., dual-resonant behavior, for a transverse magnetic polarization plane wave at oblique incidence on a mushroomlike PBG surface. A modified local resonance cavity cell model of a PBG structure is presented to provide insight into the physical mechanism of dual-resonant behavior.

Li, Long; Chen, Qiang; Yuan, Qiawei; Liang, Changhong; Sawaya, Kunio

2008-01-01

101

Band structure and optical properties of diglycine nitrate crystal

Energy Technology Data Exchange (ETDEWEB)

Experimental and theoretical investigations of the electron energy characteristics and optical spectra for diglycine nitrate crystal (DGN) (NH{sub 2}CH{sub 2}COOH){sub 2}.HNO{sub 3}, in the paraelectric phase (T=295K) are presented. Spectral dispersion of light reflection R(E) have been measured in the range of 3-22eV and the optical functions n(E) and k(E) have been calculated using Kramers-Kronig relations. First principal calculations of the electron energy characteristic and optical spectra of DGN crystal have been performed in the frame of density functional theory using CASTEP code (CAmbridge Serial Total Energy Package). Optical transitions forming the low-energy edge of fundamental absorption are associated with the nitrate groups NO{sub 3}. Peculiarities of the band structure and DOS projected onto glycine and NO{sub 3} groups confirm the molecular character of DGN crystal.

Andriyevsky, Bohdan [Faculty of Electronics and Informatics, Technical University of Koszalin, 2 Sniadeckich Str., 75-453 Koszalin (Poland)]. E-mail: bandri@tu.koszalin.pl; Ciepluch-Trojanek, Wioleta [Faculty of Electronics and Informatics, Technical University of Koszalin, 2 Sniadeckich Str., 75-453 Koszalin (Poland); Romanyuk, Mykola [Ivan Franko National University of L' viv, 8 Kyrylo and Mefodii Str., 79005 L' viv (Ukraine); Patryn, Aleksy [Faculty of Electronics and Informatics, Technical University of Koszalin, 2 Sniadeckich Str., 75-453 Koszalin (Poland); Jaskolski, Marcin [Faculty of Electronics and Informatics, Technical University of Koszalin, 2 Sniadeckich Str., 75-453 Koszalin (Poland)

2005-07-15

102

Maturation of Trypanosoma brucei mitochondrial mRNA involves massive posttranscriptional insertion and deletion of uridine residues. This RNA editing utilizes an enzymatic complex with seven major proteins, band I through band VII. We here use RNA interference (RNAi) to examine the band II and band V proteins. Band II is found essential for viability; it is needed to maintain the normal structure of the editing complex and to retain the band V ligase protein. Previously, band III was found es...

O Hearn, Sean F.; Huang, Catherine E.; Hemann, Mike; Zhelonkina, Alevtina; Sollner-webb, Barbara

2003-01-01

103

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

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 on...

Olguin, D.; Baquero, R.

1996-01-01

104

Electronic band structure of lithium, sodium and potassium fluorides

International Nuclear Information System (INIS)

A mixed tight-binding, pseudopotential method is proposed to calculate the energy band structure of large-gap crystals and is tested here on LiF, NaF and KF. Three-centre terms are included in the determination of the valence bands by the tight-binding method and for the conduction bands we use a pseudopotential model proposed by Bassani and Giuliano, modified for the positive ions. By taking into account the polarization corrections, transitions calculated from the energy band structures are compared with experimental data and the agreement is generally good

105

Formation of Deformation Band Structures Normal to the Shear Plane

Recent fieldwork has shown that many of the geometric relationships previously observed in cataclastic deformation bands are based on a two dimensional view of a three dimensional structure. When the bands are viewed in the shear direction lenses and inosculating bands are seen, as first noted by Aydin [1978] and later by many others. When viewed normal to the shear direction "ladder structures" (linked mode II echelon stepovers or duplexes) and conjugate structures are seen [Davis, 1999]. Davis [1999] grouped these two geometries as different classes of deformation bands and many have interpreted the duplexes as strain localization in Riedel shear zones. Here we demonstrate an alternative explanation. When viewed normal to the shear direction, deformation bands display either mutually crosscutting ("conjugate") sets or contractional stepovers ("duplexes") between interacting mode-II bands. Our field observations reveal that the principal difference between the two is the relative distance separating the parallel echelon bands. Bands that are sufficiently close interact mechanically to promote linking bands within the stepovers. These linking bands are younger than the bounding echelon bands. Because the orientations of bands within the stepovers are the same as that of "conjugate" bands that are more widely spaced, we infer that the magnitude of stress within the stepovers was increased somewhat over background values, but that stress rotations within the stepovers were negligible. To accomplish this, the echelon deformation bands must be strong relative to the host rock and accommodate only small offsets, leading to only minor perturbations of the local stress state in their vicinity. Ladder structure, "radiator rock" [Davis, 1999], and linked echelon stepovers demonstrate a progression from distributed to more localized strain of the sandstone, with the scale dependent on the size and offset magnitude of individual bands. Occurrence of linked echelon stepovers along both conjugate directions in spaced arrays argues against Riedel shearing as a mechanism for localizing this class of deformation bands. Aydin, A., Small Faults Formed as Deformation Bands in Sandstone, Pure and Applied Geophysics, 116, 913-930, 1978 Davis, G., Structural Geology of the Colorado Plateau Region of Southern Utah: With Special Emphasis on Deformation Bands, GSA Special Paper 342, 157 pp., 1999

Balasko, C. M.; Schultz, R. A.

2001-12-01

106

Multi-band, highly absorbing, microwave metamaterial structures

A further example of multi-band absorption using ultra-thin, polarization-insensitive, wide-angled metamaterial absorbers that operate in multi-frequency bands within the microwave regime is presented in this work. The basic structure geometry is utilised to create multi-band highly absorbing structures by incorporating the scalability property of the metamaterials. Simulation results verify the structure's ability for high absorption. The multi-band absorbers are promising candidates as absorbing elements in scientific and technical applications because of its multi-band absorption, polarization insensitivity, and wide-angle response. Finally, the current distributions for those structures are presented to gain a better insight into the physics behind the multiple absorption mechanism.

Kollatou, T. M.; Dimitriadis, A. I.; Assimonis, S. D.; Kantartzis, N. V.; Antonopoulos, C. S.

2014-05-01

107

Band structure of metal diboride AlB2 under high pressure

International Nuclear Information System (INIS)

The band structure, density of states (DOS), electronic charge distribution and superconductivity of Aluminium diboride (AlB2) as a function of pressure are investigated. The normal pressure, band structure, DOS and superconducting transition temperature of AlB2 agree well with the previous calculations. The high pressure band structure exhibits significant deviations from the normal pressure band structure. It is found that, the charge transfer from s state to p and d states will cause superconductivity in AlB2. According to the present calculation, at normal pressure, the superconducting transition of AlB2 occurs at 13.57 K which is comparable with the previous theoretical observation of 9K. On further increase of pressure, Tc increases considerably and its maximum value may exceed 32K (at .683 Mbar). The high pressure Tc values are reported for the first time and this metal diboride (AlB2) is identified as pressure induced superconductors. (author)

108

The band energy structure of RbKSO4 crystals

Directory of Open Access Journals (Sweden)

Full Text Available The energy band structure of mechanically free and compressed RbKSO4 single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0], the bottom of the conduction band lies at the ? point, and the minimum direct band gap Eg is equal to 5.80 eV. The bottom of the conduction band is predominantly formed by the K s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients of the energy position of the valence and conduction band states and the band gap Eg are determined.

O.V.Bovgyra

2007-01-01

109

Calculation of semiconductor band gaps with the M06-L density functional

The performance of the M06-L density functional has been tested for band gaps in seven semiconductors plus diamond and MgO. Comparison with the local spin density approximation (LSDA), Becke-Lee-Yang-Parr (BLYP), Perdew-Burke-Eernzerhof (PBE), Tao-Perdew-Staroverov-Scuseria (TPSS), and Heyd-Scuseria-Ernzerhof (HSE) functionals shows that M06-L has improved performance for calculating band gaps as compared to other local functionals, but it is less accurate than the screened hybrid HSE functional for band gaps.

Zhao, Yan; Truhlar, Donald G.

2009-02-01

110

Design of a Miniaturized Dual Wide Band Frequency Selective Structure

This paper deals with the frequency selective property of a structure comprising of a two dimensional array of patches. This frequency selective surface (FSS) acts like a dual band reject filter. The proposed design has been investigated theoretically using Ansoft Designer® software in which the reflection and transmission band have been predicted by the method known as Method of Moment which is most complicated but its accuracy is best. Efforts have been given to achieve dual high band reje...

Shwetanki Singh; Partha Pratim Sarkar; Sarkar, D.; Biswas, S.

2013-01-01

111

Photonic band structure for a superconductor-dielectric superlattice

International Nuclear Information System (INIS)

The photonic band structure in the transversal electric mode for a one-dimensional superconductor-dielectric superlattice is theoretically calculated. By using the Abeles theory for a stratified medium, we first calculate the transmittance spectrum from which all the possible bands can be directly seen. Then we calculate the real photonic band structure based on the transcendental equation derived from the transfer matrix method and Bloch theorem. The band structure is shown to be strongly consistent with the transmittance spectrum. We finally study the three lowest band gaps as a function of penetration of superconductor, permittivity of dielectric, and angle of incidence, respectively. The optical properties in a superconductor-dielectric superlattice thus are well disclosed

112

Shell model description of band structure in 48Cr

International Nuclear Information System (INIS)

The band structure for normal and abnormal parity bands in 48Cr are described using the m-scheme shell model. In addition to full fp-shell, two particles in the 1d3/2 orbital are allowed in order to describe intruder states. The interaction includes fp-, sd- and mixed matrix elements

113

Band structures of bilayer graphene superlattices

We formulate a low energy effective Hamiltonian to study superlattices in bilayer graphene (BLG) using a minimal model which supports quadratic band touching points. We show that a one dimensional (1D) periodic modulation of the chemical potential or the electric field perpendicular to the layers leads to the generation of zero energy anisotropic massless Dirac fermions and finite energy Dirac points with tunable velocities. The electric field superlattice maps onto a couple...

Killi, Matthew; Wu, Si; Paramekanti, Arun

2011-01-01

114

The electronic contribution to the equation of state of copper and nickel is calculated from first principles on the basis of density-functional theory. Two methods are used. The first method is the full quantum statistical model, obtained from an extension of the Thomas-Fermi model; it neglects band-structure effects and yields almost the same results for the two metals. The second method is the finite-temperature linear muffin-tin orbitals band-structure method, based on the local-density approximation. This method yields big differences between the behaviors of the two metals at low temperatures in agreement with experiment. At high temperatures the band-structure effects gradually disappear, and the quantum statistical model results are approached.

Levy, A.; Barak, G.; Ashkenazi, J.

1987-06-01

115

Density changes between sheared zones and their surrounding amorphous matrix as a result of plastic deformation in a cold-rolled metallic glass (melt-spun Al88Y7Fe5) 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. PMID:24713360

Rösner, Harald; Peterlechner, Martin; Kübel, Christian; Schmidt, Vitalij; Wilde, Gerhard

2014-07-01

116

Design of a Miniaturized Dual Wide Band Frequency Selective Structure

Directory of Open Access Journals (Sweden)

Full Text Available This paper deals with the frequency selective property of a structure comprising of a two dimensional array of patches. This frequency selective surface (FSS acts like a dual band reject filter. The proposed design has been investigated theoretically using Ansoft Designer® software in which the reflection and transmission band have been predicted by the method known as Method of Moment which is most complicated but its accuracy is best. Efforts have been given to achieve dual high band reject filtering with high band ratio (approx 3.18.

Shwetanki Singh

2013-01-01

117

First-Principles Study of the Band Gap Structure of Oxygen-Passivated Silicon Nanonets

Directory of Open Access Journals (Sweden)

Full Text Available Abstract A net-like nanostructure of silicon named silicon nanonet was designed and oxygen atoms were used to passivate the dangling bonds. First-principles calculation based on density functional theory with the generalized gradient approximation (GGA were carried out to investigate the energy band gap structure of this special structure. The calculation results show that the indirect–direct band gap transition occurs when the nanonets are properly designed. This band gap transition is dominated by the passivation bonds, porosities as well as pore array distributions. It is also proved that Si–O–Si is an effective passivation bond which can change the band gap structure of the nanonets. These results provide another way to achieve a practical silicon-based light source.

Lin Linhan

2009-01-01

118

Investigation of the XPS valence band structure from Sn chalcogenides

International Nuclear Information System (INIS)

XPS valence band spectra reflect the electronic density of states weighted with cross-sections. The spectra will give additional information about the symmetry of the electron wave function, if they are measured in dependence on the electron emission angles. Valence band spectra of SnS, SnSe, SnTe, and SnS2 are measured partly in dependence on the polar angle THETA. These measurements allow an identification of the symmetry of valence orbitals from intensity variations of the valence band maxima. (author)

119

Coupled Line Band Pass Filter with Defected Ground Structure for Wide Band Application

In this paper a novel wideband microstrip band pass filter is proposed. The band pass filter is designed with coupling between two L-shaped microstriplines and is terminated with a high impedance line. The three circle shapes are etched out at the ground plane and is called defected ground structure (DGS), which provides better return loss as well as it reduces harmonics. Simulated and measured results both are in true agreement with each other. Results show that the defected m...

Abhiruchi Nagpal,; Singhal, Dr P. K.

2014-01-01

120

N-type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridizati...

Mudd, Jj; Lee, T-l; Munoz-sanjose, V.; Zuniga-perez, J.; Payne, Dj; Egdell, Rg; Mcconville, Cf

2014-01-01

121

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

Energy Technology Data Exchange (ETDEWEB)

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 ?–? 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.

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

122

Structure of nearly degenerate dipole bands in 108Ag

The high spin negative parity states of 108Ag have been investigated with the 11B + 100Mo reaction at 39 MeV beam energy using the INGA facility at TIFR, Mumbai. From the ?-? 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 108Ag 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.

Sethi, J.; Palit, R.; Saha, S.; Trivedi, T.; Bhat, G. H.; Sheikh, J. A.; Datta, P.; Carroll, J. J.; Chattopadhyay, S.; Donthi, R.; Garg, U.; Jadhav, S.; Jain, H. C.; Karamian, S.; Kumar, S.; Litz, M. S.; Mehta, D.; Naidu, B. S.; Naik, Z.; Sihotra, S.; Walker, P. M.

2013-08-01

123

Defect band-gap structures for triggering single-photon emission

A 3D analysis of the spontaneous decay of a single dipole embedded in a planar multilayer structure is given, with special emphasis on Kerr-tunable photonic band-gap materials for single-photon emission on demand. It is shown that the change in the density of states near a defect resonance is much more pronounced than that one near the band edges. In particular, operation near the band edge as suggested from a 1D analysis is little suited for controlling the photon emission.

Dung, H T; Welsch, D G; Dung, Ho Trung; Kn\\"{o}ll, Ludwig; Welsch, Dirk-Gunnar

2003-01-01

124

First-principles study of elastic, band structure and optical properties of uranium

International Nuclear Information System (INIS)

We report first principle calculation using plane-wave pseudopotential technique in density-functional theory to investigate the crystal structure, elastic constants, bulk modulus, band structure and optical constants of uranium. Calculated structural properties, bulk modulus and elastic constants compare very favorably with experimental data and other first principle calculations. The calculated optical constants of uranium are compared with experimental data and the contrast is discussed. (authors)

125

Band structure of A2B6 compounds

International Nuclear Information System (INIS)

The band structure of the ZnS, ZnSe, ZnTe, CdS, CdSe and CdTe compounds of the sphalerite modification with provision for the s-, p-, d- states is calculated by the modified method of orthogonalized plane waves (MOPW). The calculation is carried out to find out the nature of interaction between the d electrons of a metal and s, p electrons of a nonmetal. The effect of the Zn and Cd d-states on the energy characteristics of the whole band spectrum of halcogenides of these metals is analyzed in detail. All the compounds under investigation, except CdTe, have the valent d band of the metal higher than the s band of the nonmetal. The first G sub(1c)-d band energy transition corresponds to 13.6-14.0 eV. The energy gaps between the 5s points of the Te band and the lower band of the conductivity band are following Gsub(1c)-Xsub(1v)-11.8 eV, Gsub(1c)-Lsub(1v)-12.1eV, 13.0 eV. The rest experimentally observed CdTe transitions in the > 10 eV range are interpreted as transitions between the Te 5s band and the lower zone of the conductivity band

126

Quantum properties of optical field in photonic band gap structures

A theoretical analysis of the quantum behaviour of radiation field's propagation in photonic band gaps structures is performed. In these initial calculations we consider linear inhomogeneous and nondispersive media. (C) 2001 Optical Society of America.

Severini, S.; Sibilia, C.; Bertolotti, M.; Scalora, M.; Bowden, C.

2001-01-01

127

Band structure calculations for Ba$_{6}$Ge$_{25}$ and Ba$_{4}$Na$_{2}$Ge$_{25}$ clathrates

Electronic band structures for Ba$_{6}$Ge$_{25}$ and Ba$_{4}$Na$_{2}$Ge$_{25}$ clathrates are calculated using linear muffin-tin orbital method within the local density approximation. It is found that barium states strongly contribute to the density of states at the Fermi level and thus can influence the transport properties of the compounds. A sharp peak of the density of states is found just at the Fermi level. It is also shown that the shifting of barium atoms toward experimentally deduced split positions in Ba$_{6}$Ge$_{25}$ produces a splitting of this peak which may therefore be interpreted as a band Jahn-Teller effect. If the locking of the barium atoms at the observed structural phase transition is assumed, this reduction of the density of states at the Fermi level can qualitatively account for the experimentally observed decrease of the magnetic susceptibility and electrical resistivity at the phase transition.

Zerec, I; Thalmeier, P; Grin, Y; Zerec, Ivica; Yaresko, Alexander; Thalmeier, Peter; Grin, Yuri

2002-01-01

128

Band structure characteristics of T-square fractal phononic crystals

International Nuclear Information System (INIS)

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

129

Possibility of experimental determination of band structure parameters in many-valley semiconductors

International Nuclear Information System (INIS)

A possibility to determine some parameters (density-of-state effective mass of different valleys, energy difference between energy valleys, impurity ionization energy levels split from different valleys) of the band structure in many-valley semiconductors based on experimental curve of overall concentration of charge carriers versus temperature is shown

130

Calculation of the energy band structures in semiconductors by RAPW method

International Nuclear Information System (INIS)

To calculate the energy band structures in semiconductors using the relativistic augmented plane wave method, atomic potential and charge density are needed, which are calculated by self-consistent method. Wave function for one electron is determined by solving the Dirac equation with the Hartree-Fock equation based on the slater's exchange potential. The results of calculation for Cu+1 are given. (Author)

131

Measuring large-scale structure with quasars in narrow-band filter surveys

We show that a large-area imaging survey using narrow-band filters could detect quasars in sufficiently high number densities, and with more than sufficient accuracy in their photometric redshifts, to turn them into suitable tracers of large-scale structure. If a narrow-band optical survey can detect objects as faint as i=23, it could reach volumetric number densities as high as 10^{-4} h^3 Mpc^{-3} (comoving) at z~1.5 . Such a catalog would lead to precision measurements of...

Abramo, L. Raul; Strauss, Michael A.; Lima, Marcos; Herna?ndez-monteagudo, Carlos; Lazkoz, Ruth; Moles, Mariano; Oliveira, Cla?udia M.; Sendra, Irene; Sodre? Jr, Laerte

2011-01-01

132

Active Narrow-Band Vibration Isolation of Large Engineering Structures

We present a narrow-band tracking control method using a variant of the Least Mean Squares (LMS) algorithm to isolate slowly changing periodic disturbances from engineering structures. The advantage of the algorithm is that it has a simple architecture and is relatively easy to implement while it can isolate disturbances on the order of 40-50 dB over decades of frequency band. We also present the results of an experiment conducted on a flexible truss structure. The average disturbance rejection achieved is over 40 dB over the frequency band of 5 Hz to 50 Hz.

Rahman, Zahidul; Spanos, John

1994-01-01

133

Banded Electron Structure Formation in the Inner Magnetosphere

Banded electron structures in energy-time spectrograms have been observed in the inner magnetosphere concurrent with a sudden relaxation of geomagnetic activity. In this study, the formation of these banded structures is considered with a global, bounce-averaged model of electron transport, and it is concluded that this structure is a natural occurrence when plasma sheet electrons are captured on closed drift paths near the Earth. These bands do not appear unless there is capture of plasma sheet electrons; convection along open drift paths making open pass around the Earth do not have time to develop this feature. The separation of high-energy bands from the injection population due to the preferential advection of the gradient-curvature drift creates spikes in the energy distribution, which overlap to form a series of bands in the energy spectrograms. The lowest band is the bulk of the injected population in the sub-key energy range. Using the Kp history for an observed banded structure event, a cloud of plasma sheet electrons is captured and the development of their distribution function is examined and discussed.

Liemohn, M. W.; Khazanov, G. V.

1997-01-01

134

We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), Delta, and two inequivalent sub-band ladders, Delta(2) and Delta(4), form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between Delta(2) and Delta(4) and their shift with respect to the bulk value directly from the 2p(3/2)-->Delta transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110). PMID:20302337

Chen, Feng; Ramayya, Edwin B; Euaruksakul, Chanan; Himpsel, Franz J; Celler, George K; Ding, Bingjun; Knezevic, Irena; Lagally, Max G

2010-04-27

135

Electrical properties and band structures of Pb1-x Snx Te alloys

International Nuclear Information System (INIS)

Both p type alloys Pb0.72Sn0.28Te and Pb0.53Sn0.47Te have been studied in the present work. The main obtained results are the following: the materials have a two-valence band structure, the first band following non-parabolic Cohen's dispersion law; at low temperatures, carriers are scattered by ionized impurities; the Coulomb potentials being screened almost completely, impurities act like neutral centers. At room temperature, scattering by acoustic modes can explain lattice mobility behavior; reversing of the thermo-power, for samples with carrier densities of about 1020 cm-3, is possibly due to inter-band scattering between both valence bands; a very simple picture of the band parameters variations as a function of alloy fraction is suggested. (author)

136

Enery band structure and optical properties of wurtzite-structure silicon carbide crystals

Energy Technology Data Exchange (ETDEWEB)

The electroreflection spectra of hexagonal polytypes of SiC, 4H and 6H are measured in the range 1.0 to 5.6 eV. The energies of direct optical transitions are determined using a multiple oscillator model. The electronic band structures of three hexagonal polytypes of SiC (2H, 4H, and 6H) are calculated by the first-principles self-consistent linear muffin-tin orbital (LMTO ASA) method. One-electron energies and densities of states are obtained in the range +-15 eV around the top of valence band. The results calculated are compared with experimental data measured in this work and those available in the literature. (orig.).

Gavrilenko, V.I.; Postnikov, A.V. (Max-Planck-Inst. fuer Festkoerperforschung, Stuttgart (Germany, F.R.)); Klyui, N.I.; Litovchenko, V.G. (Inst. of Semiconductors, Academy of Sciences of the Ukrainian SSR, Kiev (Ukrainian SSR))

1990-12-01

137

Photonic band gap of superconductor-medium structure: Two-dimensional triangular lattice

International Nuclear Information System (INIS)

Highlights: • Plane wave expansion is generalized to superconductor-medium periodic structure. • A wider band gap appears than that in conventional photonic crystals. • Part of original energy levels are rearranged upon consideration of the superconductivity. • Band gap width decreases monotonically with penetration length, but not with the filling factor. • Band gaps can be partially shut down or opened by adjusting filling factor. - Abstract: Based on London theory a general form of wave equation is formulated for both dielectric medium and superconductor. Using the wave equation and applying plane wave expansion, we have numerically calculated the band structures and density of states of a photonic crystal, whose intersection is constructed by a two-dimensional triangular lattice of superconductor padding in dielectric medium. Results indicate a wider band gap in the superconductor-medium photonic crystal than that in conventional photonic crystals. And part of original energy levels are found to be rearranged upon consideration of the superconductivity. The dependence of band gap on penetration length and filling factor is also discussed. Band gap width decreases monotonically with the penetration length, but not with the filling factor. Band gaps can be partially shut down or opened by adjusting filling factor

138

Energy band structures of MgB 2 and related compounds

Energy band structures are calculated for the new superconductor MgB 2 and related compounds by using the local density approximation and an FLAPW method. It is found that the strong three dimensional (3D) network in low-lying ? bands brings about two dimensional ? holes in MgB 2, which should be responsible for the superconductivity. The de Haas-van Alphen frequencies and the cyclotron masses are obtained for the Fermi surfaces. The ? hole is not found in LiBC due to the less 3D network. MgB 2C 2 is found a semiconductor, but the top of the valence bands are similar to the ? bands in MgB 2. The total energy calculations are also performed for Mg 1- xAl xB 2 to investigate the structural phase instability, experimentally observed around x=10%.

Harima, Hisatomo

2002-10-01

139

We report a detailed theoretical study of the structural and vibrational properties of solid nitromethane using first principles density functional calculations. The ground state properties were calculated using a plane wave pseudopotential code with either the local density approximation, the generalized gradient approximation, or with a correction to include van der Waals interactions. Our calculated equilibrium lattice parameters and volume using a dispersion correction are found to be in reasonable agreement with the experimental results. Also, our calculations reproduce the experimental trends in the structural properties at high pressure. We found a discontinuity in the bond length, bond angles, and also a weakening of hydrogen bond strength in the pressure range from 10 to 12 GPa, picturing the structural transition from phase I to phase II. Moreover, we predict the elastic constants of solid nitromethane and find that the corresponding bulk modulus is in good agreement with experiments. The calculated elastic constants show an order of C11> C22 > C33, indicating that the material is more compressible along the c-axis. We also calculated the zone center vibrational frequencies and discuss the internal and external modes of this material under pressure. From this, we found the softening of lattice modes around 8-11 GPa. We have also attempted the quasiparticle band structure of solid nitromethane with the G0W0 approximation and found that nitromethane is an indirect band gap insulator with a value of the band gap of about 7.8 eV with G0W0 approximation. Finally, the optical properties of this material, namely the absorptive and dispersive part of the dielectric function, and the refractive index and absorption spectra are calculated and the contribution of different transition peaks of the absorption spectra are analyzed. The static dielectric constant and refractive indices along the three inequivalent crystallographic directions indicate that this material has a considerable optical anisotropy. PMID:23676062

Appalakondaiah, S; Vaitheeswaran, G; Lebègue, S

2013-05-14

140

Band structure of surface barrier states and resonances

International Nuclear Information System (INIS)

Full text: G. Binnig and H. Rohrer, Nobel Prize Winners for the invention of the Scanning Tunneling Microscope, write in the opening sentence of one of their papers, co-authored with others : 'One of the fundamental problems in surface physics is obtaining knowledge of the electron-metal-surface interaction potential.' Although it is known that the surface barrier has an 'image' asymptotic form and saturates or weakens closer to the crystal surface, the position of the image tail, momentum dependence of the barrier height and saturation closer to the surface have not been agreed upon by different workers and techniques to this day. Ab initio calculations using the density functional approximation produce locations for the position of the image tail which differ by ?50% depending on whether the exiting or incoming electron is considered part of the crystal or a classical charge interacting with the electron gas. Very low energy electron diffraction (VLEED), k-resolved inverse photoemission spectroscopy (KRIPES) and 2-photon photoemission spectroscopy (2PPE) are sensitive to the barrier but analyses to date have not yielded consistent conclusions. In this work we have used our plane-wave scattering method to calculate the barrier energy band structure for Cu (001) over the whole SBZ to compare with experimental results from KRIPES and 2PPE data as well as the calculation of Smith et al. This calculation used a parameterized nearly-free-electron function to represent tly-free-electron function to represent the substrate scattering and could only produce states not resonances which occur outside of bulk band gaps and above the barrier height. As well, no inelastic scattering could be included. We show that inelastic scattering, surface restructuring and an extended data-base must be included for definitive conclusions about details of the barrier. Also, our calculation shows above-barrier resonances are strong and should be measured by experimentalists to extract the momentum dependent saturation and height of the barrier

141

Band structure of fcc-C60 solid state crystal study

Directory of Open Access Journals (Sweden)

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 ? and ? bonds were determined in the band structure. Our results are in good agreement with experiment and pseudopotential calculations.

S Javanbakht

2009-09-01

142

Dispersion Characteristics of a Cylindrical Electromagnetic Band Gap Structure

In this letter, a new analytical expression for the dispersion equation of radially periodic structures is derived. The periodic structure is considered as a set of parallel cylindrical frequency selective surfaces, and the dispersion equation is calculated by using a transmission line model. Using this result, the dispersion proprieties of cylindrical electromagnetic bandgap structures composed of continuous or discontinuous metallic wires are presented. It is shown that the band structures ...

Boutayeb, Halim; Mahdjoubi, Kouroch

2006-01-01

143

Density functional theory and electronic structure

International Nuclear Information System (INIS)

The paper presents an overview of the density functional theory and provides a brief appraisal of the latest developments of the theory. Particular emphasis is placed on the understanding of the electronic structure of materials via the density functional theory. 37 refs, 1 tab

144

Calculation of electronic structure and density of state for BaTiO3

Directory of Open Access Journals (Sweden)

Full Text Available The electronic structure, density of state (DOS and electronic density of state inparaelectric cubic crystal Ba TiO3 are studied using full potential-linearized augmented plane wave (FP-LAPW method in the framework of the density functional theory (DFT with the generalized gradient approximation (GGA by the WIEN2K package. The results show a direct band gap of 1.8 eV at the point in the Brillouin zone. The calculated band structure and density of state of BaTiO3 are in good agreement with theoretical and experimental results.

H. Salehi

2002-12-01

145

Structure of various K bands in 178Hf

International Nuclear Information System (INIS)

Many bands of states are now known experimentally in the nucleus 178Hf. These bands seem to have K as a good quantum number and some of the band-heads are isomers. The structure of this nucleus has been studied by doing Hartree-Fock calculation using a ''modified surface delta interaction'' with one active ma or shell for each of protons and neutrons and with 132Sn as the core. There is axial symmetry in the calculation, but no time-reversal symmetry for the K/O bands. Solutions obtained are K = 0+, 8- and 24- with prolate deformation and a K = 0+ intrinsic state with oblate deformation. Considering the prolate Hartree-Fock solutions and particle-hole excitations across their Fermi surfaces, it has been possible to obtain the bands seen experimentally. The isomerism of the bands can be explained by the microscopic calculation. Two K = 16+ bands, one band with two unpaired protons and two unpaired neutrons, and the other with four unpaired neutrons are obtained. (auth.)

146

Even- and odd-parity band structures in 125Xe

International Nuclear Information System (INIS)

The level structure of 125Xe has been studied with the (3He,3n?) reaction on an enriched 125Te target using in-beam ?-ray spectroscopic methods. In addition to the negative-parity level structure, an extensive previously unknown positive-parity band structure based on different low-j states was observed. The odd-parity hsub(11/2) level system is well reproduced with the triaxial rotor model. However, the positive-parity band structure is qualitatively interpreted with a particle-plus-vibrating-core model. The observed gsub(7/2) band head at 295.9 keV is an isomeric state with a half-life of 140 +- 30 ns. (author)

147

Band structures and localization properties of aperiodic layered phononic crystals

International Nuclear Information System (INIS)

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.

148

Band structures and localization properties of aperiodic layered phononic crystals

Energy Technology Data Exchange (ETDEWEB)

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.

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

149

Solving complex band structure problems with the FEAST eigenvalue algorithm

With straightforward extension, the FEAST eigenvalue algorithm [Polizzi, Phys. Rev. B 79, 115112 (2009)] is capable of solving the generalized eigenvalue problems representing traveling-wave problems—as exemplified by the complex band-structure problem—even though the matrices involved are complex, non-Hermitian, and singular, and hence outside the originally stated range of applicability of the algorithm. The obtained eigenvalues/eigenvectors, however, contain spurious solutions which must be detected and removed. The efficiency and parallel structure of the original algorithm are unaltered. The complex band structures of Si layers of varying thicknesses and InAs nanowires of varying radii are computed as test problems.

Laux, S. E.

2012-08-01

150

Multi bunch dynamics in detuned x-band structures

International Nuclear Information System (INIS)

The multi bunch dynamics of a 2 x 250 GeV version of the JLC is studied. The rf-properties of detuned x-band tubes are calculated with the Computer codes URMEL and MAFIA. The dispersion curve found with these codes is compared with an equivalent circuit model. The calculation of the dipole mode loss parameters is investigated in detail. Tracking calculations are used to investigate the misalignment tolerances for detuned x-band structures. Also tilted x-band tubes are considered. (author)

151

Rotational bands and noncollective structures in 85Zr

International Nuclear Information System (INIS)

The structure of the transitional nucleus 85Zr has been investigated through spectroscopy and lifetime measurements. States up to spins of (45/2)(?/2?) and (39/2)(?/2?) in the positive and negative parity bands, respectively, have been established through ?-? coincidence measurements. The results indicate that there is a competition between collective and noncollective modes of excitation. Two pronounced band crossings have been observed in the positive parity sequence, while none is evident in the negative parity states. The pattern of shape evolution in the positive and negative parity bands is markedly different. The results of the experiments qualitatively corroborate the predictions of total Routhian surface calculations

152

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ?(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH4SO4, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5me. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO4) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.

Andriyevsky, B.; Ciepluch-Trojanek, W.; Stadnyk, V.; Tuzyak, M.; Romanyuk, M.; Kurlyak, V.

2007-10-01

153

Microscopic superconducting parameters of Nb3Al: Importance of the band density of states

International Nuclear Information System (INIS)

Critical-field measurements are carried out in low fields on a series of A15 Nb-Al as-deposited films where structural disorder, as characterized by the residual resistivity rho(T/sub c/), is introduced by varying the composition in the A15 phase. Analysis of the critical-field slopes near T/sub c/ gives relatively low electronic densities of states at Fermi level N/sup b/(0) in agreement with heat-capacity results. The N/sup b/(0)'s remain nearly constant over the resistivity range varied. Lacking data in the low resistivity range, our results neither support nor disagree with the resistive lifetime-broadening model of the density of states. Nonetheless the data unambiguously demonstrate that the conventional model, in which a peak in the band density of states at the Fermi level plays the essential role, is not sufficient to explain the observed systematics in high-T/sub c/ Nb3Al. More specifically, changes in 2>/2> with disorder are found to be responsible for the observed changes in T/sub c/, where the increase of 2> with disorder has been shown earlier in the tunneling studies. Further consideration of similar data for the other superconductors V3Si, Nb3Sn, and Nb3Ge suggests that important variations in 2>/2> may be a general feature of the high-T/sub c/ A15 superconductors. 2>/2> increases as one progresses from V33Si through Nb3Sn and Nb3Al to Nb3Ge in correlation with the relative instability of the A15 phase in these materials

154

Superfluid density and specific heat within a self-consistent scheme for a two-band superconductor

The two gaps in a two-band clean s -wave superconductor are evaluated self-consistently within the quasiclassical Eilenberger weak-coupling formalism with two in-band and one interband pairing potentials. Superfluid density, free energy, and specific heat are given in the form amenable for fitting the experimental data. Well-known two-band MgB2 and V3Si superconductors are used to test the developed approach. The pairing potentials obtained from the fit of the superfluid density data in MgB2 crystal were used to calculate temperature-dependent specific heat C(T) . The calculated C(T) compares well with the experimental data. Advantages and validity of this, which we call the “ ? model,” are discussed and compared with the commonly used empirical (and not self-consistent) “ ? model.” Correlation between the sign of the interband coupling and the signs of the two order parameters is discussed. Suppression of the critical temperature by the interband scattering is evaluated and shown to be severe for the interband repulsion as compared to the attraction. The data on a strong Tc suppression in MgB2 crystals by impurities suggest that the order parameters on two effective bands of this material may have opposite signs, i.e., may have the s± structure similar to proposals for iron-based pnictide superconductors.

Kogan, V. G.; Martin, C.; Prozorov, R.

2009-07-01

155

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

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 included the non common either anion or cation (CdTe/ZnSe), common cation (CdTe/CdSe), and common anion (CdTe/ZnTe) cases. We have aligned the top of the the valence band at the whole interface dominion as the boundary condition. The overall conclusion is that the interface is a very rich space where changes in the band structure with respect to the bulk do occur. This is true not only at interfaces with no common atoms but also at the ones with either common cation or anion atoms irrespective to the fact that the common atomic l...

Olguin, D

1996-01-01

156

International Nuclear Information System (INIS)

Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calculations were used in order to describe the fine structure of the valence band in the Sn/Ni(111) system. The characteristic contributions of each metal in the valence band photoemission spectra obtained with a photon energy of 80 eV and their changes upon the formation of the (?3 x ?3)R30o Sn/Ni(111) surface alloy were also born out in the calculated density-of-states curves in fair agreement with the experiments. The Sn-Ni interaction leads to a considerable broadening of the valence band width at the bimetallic surfaces

157

Measured energy-momentum densities of the valence band of aluminium

International Nuclear Information System (INIS)

The energy-resolved momentum densities of the valence band of a thin polycrystalline aluminium film have been measured using electron momentum spectroscopy (EMS). The spectrometer used for these measurements has estimated energy and momentum resolutions of 0.9 eV and 0.10 atomic units respectively. The valence band of aluminium was clearly resolved, resembling very closely that of a free-election parabola. The measurement has been compared to linear muffin-tin orbital (LMTO) calculations for spherically averaged crystalline aluminium. Comparison has also been made between the experiment and Monte-Carlo simulations which take into account additional elastic and inelastic scattering events not considered in the LMTO calculations. The final agreement obtained between the measurement and theory for the dispersion and relative intensities of the aluminium valence band is excellent when lifetime broadening of the band is allowed for. 21 refs., 11 figs

158

Tuning the electronic band structure of PCBM by electron irradiation

Directory of Open Access Journals (Sweden)

Full Text Available Abstract Tuning the electronic band structures such as band-edge position and bandgap of organic semiconductors is crucial to maximize the performance of organic photovoltaic devices. We present a simple yet effective electron irradiation approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM that is the most widely used organic acceptor material. We have found that the lowest unoccupied molecular orbital (LUMO level of PCBM up-shifts toward the vacuum energy level, while the highest occupied molecular orbital (HOMO level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the molecular structural change of PCBM by electron irradiation.

Yoo Seung

2011-01-01

159

Band structure calculations for Ba$_{6}$Ge$_{25}$ and Ba$_{4}$Na$_{2}$Ge$_{25}$ clathrates

Electronic band structures for Ba$_{6}$Ge$_{25}$ and Ba$_{4}$Na$_{2}$Ge$_{25}$ clathrates are calculated using linear muffin-tin orbital method within the local density approximation. It is found that barium states strongly contribute to the density of states at the Fermi level and thus can influence the transport properties of the compounds. A sharp peak of the density of states is found just at the Fermi level. It is also shown that the shifting of barium atoms toward expe...

Zerec, Ivica; Yaresko, Alexander; Thalmeier, Peter; Grin, Yuri

2002-01-01

160

Coupled Line Band Pass Filter with Defected Ground Structure for Wide Band Application

Directory of Open Access Journals (Sweden)

Full Text Available In this paper a novel wideband microstrip band pass filter is proposed. The band pass filter is designed with coupling between two L-shaped microstriplines and is terminated with a high impedance line. The three circle shapes are etched out at the ground plane and is called defected ground structure (DGS, which provides better return loss as well as it reduces harmonics. Simulated and measured results both are in true agreement with each other. Results show that the defected microstrip filter has a good performance, including a wide pass band of 3.0 GHz to 5.6 GHz at 3dB cut off frequencies with bandwidth of 2.6 GHz, and a small insertion loss. The return loss is found to be higher than 15 dB.

Abhiruchi Nagpal,

2014-07-01

161

Band structure model of magnetic coupling in semiconductors

We present a unified band structure model to explain magnetic ordering in Mn-doped semiconductors. This model is based on the $p$-$d$ and $d$-$d$ level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such as CdTe, GaAs, ZnO, and GaN. This model, therefore, provides a simple guideline for future band structure engineering of magnetic semiconductors.

Dalpian, G M; Gong, X G; Silva, A J R; Fazzio, A; Dalpian, Gustavo M.; Wei, Su-Huai; Silva, Ant\\^onio J. R. da

2005-01-01

162

Electronic band structure and carrier effective mass in calcium aluminates

First-principles electronic band structure investigations of five compounds of the CaO-Al2O3 family, 3CaO.Al2O3, 12CaO.7Al2O3, CaO.Al2O3, CaO.2Al2O3 and CaO.6Al2O3, as well as CaO and alpha-, theta- and kappa-Al2O3 are performed. We find that the conduction band in the complex oxides is formed from the oxygen antibonding p-states and, although the band gap in Al2O3 is almost twice larger than in CaO, the s-states of both cations. Such a hybrid nature of the conduction band l...

Medvedeva, J. E.; Teasley, E. N.; Hoffman, M. D.

2007-01-01

163

Let $X_1,...,X_n$ be a random sample from some unknown probability density $f$ defined on a compact homogeneous manifold $\\mathbf M$ of dimension $d \\ge 1$. Consider a 'needlet frame' $\\{\\phi_{j \\eta}\\}$ describing a localised projection onto the space of eigenfunctions of the Laplace operator on $\\mathbf M$ with corresponding eigenvalues less than $2^{2j}$, as constructed in \\cite{GP10}. We prove non-asymptotic concentration inequalities for the uniform deviations of the linear needlet density estimator $f_n(j)$ obtained from an empirical estimate of the needlet projection $\\sum_\\eta \\phi_{j \\eta} \\int f \\phi_{j \\eta}$ of $f$. We apply these results to construct risk-adaptive estimators and nonasymptotic confidence bands for the unknown density $f$. The confidence bands are adaptive over classes of differentiable and H\\"{older}-continuous functions on $\\mathbf M$ that attain their H\\"{o}lder exponents.

Kerkyacharian, Gerard; Picard, Dominique

2011-01-01

164

The energy band structure of AxFe2Se2 (A = K, Rb) superconductors

International Nuclear Information System (INIS)

We study the band structure of antiferromagnetic AxFe2Se2 (A = K, Rb) superconductors by using first-principles electronic structure calculations which is density functional theory. In the vicinity of iron-vacancy, we identify the valence electrons of AxFe2Se2 will be filled up to the Fermi level and no semiconducting gap is observed. Hence, the AxFe2Se2 is a metallic instead of semiconducting which leads to superconductivity in the orbital-selective Mott phase. Similarly, there is non-vanishing density of states at the Fermi level

165

Novel structural flexibility identification in narrow frequency bands

International Nuclear Information System (INIS)

A ‘Sub-PolyMAX’ method is proposed in this paper not only for estimating modal parameters, but also for identifying structural flexibility by processing the impact test data in narrow frequency bands. The traditional PolyMAX method obtains denominator polynomial coefficients by minimizing the least square (LS) errors of frequency response function (FRF) estimates over the whole frequency range, but FRF peaks in different structural modes may have different levels of magnitude, which leads to the modal parameters identified for the modes with small FRF peaks being inaccurate. In contrast, the proposed Sub-PolyMAX method implements the LS solver in each subspace of the whole frequency range separately; thus the results identified from a narrow frequency band are not affected by FRF data in other frequency bands. In performing structural identification in narrow frequency bands, not in the whole frequency space, the proposed method has the following merits: (1) it produces accurate modal parameters, even for the modes with very small FRF peaks; (2) it significantly reduces computation cost by reducing the number of frequency lines and the model order in each LS implementation; (3) it accurately identifies structural flexibility from impact test data, from which structural deflection under any static load can be predicted. Numerical and laboratory examples are investigated to verify the effectiveness of the proposed method. (paper))

166

BAND STRUCTURE OF NON-STEIOCHIOMETRIC LARGE-SIZED NANOCRYSTALLITES

Directory of Open Access Journals (Sweden)

Full Text Available A band structure of large-sized (from 20 to 35nm non-steichiometric nanocrystallites (NC of the Si2-xCx (1.04 < x < 1.10 has been investigated using different band energy approaches and a modified Car-Parinello molecular dynamics structure optimization of the NC interfaces. The non-steichiometric excess of carbon favors the appearance of a thin prevailingly carbon-contained layer (with thickness of about 1 nm covering the crystallites. As a consequence, one can observe a substantial structure reconstruction of boundary SiC crystalline layers. The numerical modeling has shown that these NC can be considered as SiC reconstructed crystalline films with thickness of about 2 nm covering the SiC crystallites. The observed data are considered within the different one-electron band structure methods. It was shown that the nano-sized carbon sheet plays a key role in a modified band structure. Independent manifestation of the important role played by the reconstructed confined layers is due to the experimentally discovered excitonic-like resonances. Low-temperature absorption measurements confirm the existence of sharp-like absorption resonances originating from the reconstructed layers.

I.V.Kityk

2004-01-01

167

Trends in the band structures of the group-I and -II oxides.

Measured and calculated band structures for the six lightest group-I and -II oxides are reported. Band structures have been measured using electron momentum spectroscopy, a technique that maps the ground-state occupied orbitals resolved both in energy and momentum. Measurements are compared with first-principles calculations carried out within the linear combination of atomic orbitals approximation using both Hartree-Fock (HF) and density functional (DFT) methods. Three DFT functionals are used representative of the local density approximation, the generalized gradient approximation, and a hybrid method incorporating exact exchange. The calculated O 2p bandwidths and O 2p-2s band gaps generally scale linearly with the inverse of the oxygen-oxygen separation squared, but consistently show an anomaly at Li(2)O. These trends, including the anomaly, are also observed in the experimental data. HF calculations consistently overestimate the oxygen 2p-2s band gap by almost a factor of two. Measured band gaps lie within the range of the three DFT functionals employed, with evidence that the description of exchange becomes more important as the cation size increases. Both HF and DFT calculations overestimate the oxygen valence bandwidths, with DFT giving more accurate predictions. Both observed and calculated bandwidths converge as the cation size increases, indicating that exchange-correlation effects become less important as the metallic ion becomes larger. PMID:15268107

Mikajlo, Elisabeth A; Dorsett, Helen E; Ford, Michael J

2004-06-01

168

Transmission coefficient, eigen states and tunneling current density of a potentially symmetric quantum double barrier structure has been numerically computed using transfer matrix technique for qualitative analysis of resonant tunneling probability when realistic band structure of higher band gap material is taken into account. GaAs/AlxGa1-xAs material composition is taken as an example for calculation, and thickness of the barrier and well regions are varied along with material compositions...

Arpan Deyasi and Gourab Kumar Ghosh

2012-01-01

169

Band-gap corrected density functional theory calculations for InAs/GaSb type II superlattices

We performed pseudopotential based density functional theory (DFT) calculations for GaSb/InAs type II superlattices (T2SLs), with bandgap errors from the local density approximation mitigated by applying an empirical method to correct the bulk bandgaps. Specifically, this work (1) compared the calculated bandgaps with experimental data and non-self-consistent atomistic methods; (2) calculated the T2SL band structures with varying structural parameters; (3) investigated the interfacial effects associated with the no-common-atom heterostructure; and (4) studied the strain effect due to lattice mismatch between the two components. This work demonstrates the feasibility of applying the DFT method to more exotic heterostructures and defect problems related to this material system.

Wang, Jianwei; Zhang, Yong

2014-12-01

170

The band structure of MgB$_{2}$ with different lattice constants

We report a detailed study of the electronic structure of the MgB$_{2}$ with different lattice constants by using the full-potential linearized augmented plane wave(FPLAPW) method. It is found that the lattice parameters have great effect on the $\\sigma$ band of Boron. Our results indicate that increasing lattice constant along the {\\it c} axis will increase the density of states(DOS) at the Fermi level, making the $\\sigma$ band upward shift, and increasing hole number in th...

Wan, Xiangang; Dong, Jinming; Weng, Hongming; Xing, D. Y.

2001-01-01

171

New band structures and an unpaired crossing in 78Kr

International Nuclear Information System (INIS)

High-spin states in 78Kr were studied using the 58Ni(23Na,3p) reaction at 70 MeV and the 58Ni(28Si,?4p) reaction at 130 MeV. Prompt ?-? coincidences were measured using the Pitt-FSU detector array and the GAMMASPHERE-MICROBALL array. Results from these experiments have led to 26 new excitation levels, some of which have been grouped into 3 new bands. Spins were assigned based on directional correlations of oriented nuclei. Two of the new negative-parity bands appear to form a signature-partner pair based on a two-quasineutron structure, in contrast to the previously known two-quasiproton negative-parity bands. A forking has been observed at the 24+ state in the yrast band, which calculations suggest may result from an unpaired crossing. The available evidence suggests oblate shapes in the yrast band coexist with prolate shapes in the negative-parity bands. copyright 1999 The American Physical Society

172

Band-structure loops and multistability in cavity QED

International Nuclear Information System (INIS)

We calculate the band structure of ultracold atoms located inside a laser-driven optical cavity. For parameters where the atom-cavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for Bose-Einstein condensates in ordinary (noncavity) optical lattices. However, in our case the nonlinearity derives from the cavity back-action rather than from direct interatomic interactions. We find both bi- and tristable regimes associated with the lowest band, and show that the multistability we observe can be analyzed in terms of swallowtail catastrophes. Dynamic and energetic stability of the mean-field solutions is also discussed, and we show that the bistable solutions have, as expected, one unstable and two stable branches. The presence of loops in the atomic band structure has important implications for proposals concerning Bloch oscillations of atoms inside optical cavities [Peden et al., Phys. Rev. A 80, 043803 (2009); Prasanna Venkatesh et al., Phys. Rev. A 80, 063834 (2009)].

173

International Nuclear Information System (INIS)

Photoemission, LEED, and AES measurements were made on the mechanically polished (111) surface of a type IIa diamond. No emission from filled states in the fundamental gap was found over the photon energy range 13.3 eV< or =h?< or =200 eV. This result, coupled with the sharp 1 x 1 LEED patterns which were obtained and the relative cleanliness (of elements which can be detected by AES) of the diamond (< or approx. =1 at.% oxygen, <0.5 at.% Si) suggests hydrogen termination of the lattice. Photoelectric yield measurements demonstrate the photoelectric threshold to be at band gap energy radiation. Investigation of the photoemission electron distribution curves (EDC's) shows that, while the electron affinity at the surface is always positive, band bending is sufficient to result in an effective negative electron affinity under certain conditions. A variable surface dipole on the atomic scale, possibly due to the adsorption--desorption of a background gas, is reported. A study of the relative cross section of the upper (p-like) versus the lower (s-like) portion of the diamond valence band indicates comparable cross sections at a photon energy h?=160 eV

174

ELECTRONIC BAND STRUCTURE AND PHOTOEMISSION : A REVIEW AND PROJECTION

A brief review of electronic-structure calculations in solids, as a means of interpreting photoemission spectra, is presented. The calculations are, in general, of three types : (i) ordinary one-electron-like band structures, which apply to bulk solids and are the basis of all other calculations ; (ii) surface modified calculations, which take into account, self-consistently if at all possible, the presence of a vacuum-solid interface and of the electronic modifications caused thereby ; (iii)...

Falicov, L.

1987-01-01

175

GaN m -plane: Atomic structure, surface bands, and optical response

Density-functional-theory calculations are combined with many-body perturbation theory in order to elucidate the geometry, electronic, and optical properties of the w z -GaN (1 1 ¯00 ) surface, i.e., the so-called m -plane. The optical absorption and reflection anisotropy related to electronic transitions between surface states are identified by comparison with measured data covering transition energies from 2.4 up to 5.4 eV. Our results show a surface relaxation mechanism consistent with the electron counting rule that causes a moderate buckling of the GaN surface dimers and gives rise to two distinct surface states: The doubly occupied N dangling bonds form a surface band that is resonant with the GaN valence-band edge at the center of the Brillouin zone, whereas the empty Ga dangling bonds occur within the GaN band gap closely following the dispersion of the conduction-band edge. These two states contribute strongly to the formation of surface excitons that redshift the optical absorption with respect to the bulk optical response. The surface optical absorption i.e., the excitonic onset below the bulk band gap followed by a broad absorption band at higher energies related to the dispersion of the surface band structure, is calculated in agreement with the experimental data.

Landmann, M.; Rauls, E.; Schmidt, W. Â. G.; Neumann, M. Â. D.; Speiser, E.; Esser, N.

2015-01-01

176

Band structure and optical properties of amber studied by first principles

International Nuclear Information System (INIS)

The band structure and density of states of amber is studied by the first principles calculation based on density of functional theory. The complex structure of amber has 214 atoms and the band gap is 5.0 eV. The covalent bond is combined C/O atoms with H atoms. The O 2p orbital is the biggest effect near the Fermi level. The optical properties' results show that the reflectivity is low, and the refractive index is 1.65 in visible light range. The highest absorption coefficient peak is at 172 nm and another higher peak is at 136 nm. These convince that the amber would have a pretty sheen and that amber is a good and suitable crystal for jewelry and ornaments

177

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

International Nuclear Information System (INIS)

The electronic structures of Ag-doped rutile and anatase TiO2 are studied by first-principles band calculations based on density functional theory with the full-potential linearized-augmented-plane-wave method. New occupied bands are found between the band gaps of both Ag-doped rutile and anatase TiO2. The formation of these new bands can 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. (condensed matter: electronicstructure, electrical, magnetic, and opticalproperties)

178

Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors

International Nuclear Information System (INIS)

A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k?) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg1?xCdxTe, and In1?xGaxAsyP1?y lattice matched to InP, as example of III–V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors

179

Photonic crystal digital alloys and their band structure properties.

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. PMID:21996867

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

2011-09-26

180

Electronic band structure of magnetic bilayer graphene superlattices

Energy Technology Data Exchange (ETDEWEB)

Electronic band structure of the bilayer graphene superlattices with ?-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

Pham, C. Huy; Nguyen, T. Thuong [Theoretical and Computational Physics Department, Institute of Physics, VAST, 10 Dao Tan, Ba Dinh Distr., Hanoi 10000 (Viet Nam); SISSA/International School for Advanced Study, Via Bonomea 265, I-34136 Trieste (Italy); Nguyen, V. Lien, E-mail: nvlien@iop.vast.ac.vn [Theoretical and Computational Physics Department, Institute of Physics, VAST, 10 Dao Tan, Ba Dinh Distr., Hanoi 10000 (Viet Nam); Institute for Bio-Medical Physics, 109A Pasteur, 1st Distr., Hochiminh City (Viet Nam)

2014-09-28

181

Multi-band and broadband acoustic metamaterial with resonant structures

International Nuclear Information System (INIS)

We design an acoustic metamaterial (AM) with multi-band of negative modulus composed of different sized split hollow spheres (SHSs). From acoustic transmitted experiment, the AM exhibits simultaneously negative modulus at frequencies 914, 1298 and 1514 Hz. Based on the multi-band designed concept, broadband AM is fabricated by arraying gradually sized SHS. The transmission results indicate that this medium can achieve negative modulus at the frequency range from 900 to 1500 Hz. This kind of broadband AM is very convenient to couple with other structures to gain the double-negative AM.

182

Waveguiding in surface plasmon polariton band gap structures

DEFF Research Database (Denmark)

Using near-held optical microscopy, we investigate propagation and scattering of surface plasmon polaritons (SPP's) excited in the wavelength range of 780-820 nm at nanostructured gold-film surfaces with areas of 200-nm-wide scatterers arranged in a 400-nm-period triangular lattice containing line defects. We observe the SPP reflection by such an area and SPP guiding along line defects at 782 nm, as well as significant deterioration of these effects is 815 nm, thereby directly demonstrating the SPP band gap effect and showing first examples of SPP channel waveguides in surface band gap structures.

Bozhevolnyi, S.I.; Østergaard, John Erland

2001-01-01

183

Electronic band structure of magnetic bilayer graphene superlattices

International Nuclear Information System (INIS)

Electronic band structure of the bilayer graphene superlattices with ?-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

184

Electronic band structure of magnetic bilayer graphene superlattices

Electronic band structure of the bilayer graphene superlattices with ?-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

Pham, C. Huy; Nguyen, T. Thuong; Nguyen, V. Lien

2014-09-01

185

Electronic band structure of TiFese2 in ferromagnetic phase

International Nuclear Information System (INIS)

Electronic band structure of crystalline TiFeSe2 has been calculated using full-potential method of Linear Augmented Plane Wave (LAPW) in density-functional approach with exchange-correlation potential taken in Generalized Gradient Approximation (GGA). The chemical bond in TiFeSe2 is shown to be metallic because energies of 3d-electrons localized at iron atoms are close to Fermi energy level

186

Extended Hückel theory for band structure, chemistry, and transport. I. Carbon nanotubes

We describe a semiempirical atomic basis extended Hückel theoretical (EHT) technique that can be used to calculate bulk band structure, surface density of states, electronic transmission, and interfacial chemistry of various materials within the same computational platform. We apply this method to study multiple technologically important systems, starting with carbon nanotubes and their interfaces and silicon-based heterostructures in our follow-up paper [D. Kienle et al., J. Appl. Phys. 100...

Kienle, D.; Cerda?, J. I.; Ghosh, A. W.

2006-01-01

187

Intrinsic spin-Hall accumulation in honeycomb lattices: Band structure effects

Local spin and charge densities on a two-dimensional honeycomb lattice are calculated by the Landauer-Keldysh formalism (LKF). Through the empirical tight-binding method, we show how the realistic band structure can be brought into the LKF. Taking the Bi(111) surface, on which strong surface states and Rashba spin-orbit coupling are present [Phys. Rev. Lett. 93, 046403 (2004)], as a numeric example, we show typical intrinsic spin-Hall accumulation (ISHA) patterns thereon. Th...

Liu, Ming-hao; Bihlmayer, Gustav; Blu?gel, Stefan; Chang, Ching-ray

2007-01-01

188

The electronic structure of NaIrO$_3$, Mott insulator or band insulator?

Motivated by the unveiled complexity of nonmagnetic insulating behavior in pentavalent post-perovskite NaIrO$_3$, we have studied its electronic structure and phase diagram in the plane of Coulomb repulsive interaction and spin-orbit coupling (SOC) by using the newly developed local density approximation plus Gutzwiller method. Our theoretical study proposes the metal-insulator transition can be generated by two different physical pictures: renormalized band insulator or Mot...

Du, Liang; Sheng, Xianlei; Weng, Hongming; Dai, Xi

2012-01-01

189

Effect of metal vacancies on the electronic band structure of hexagonal Nb, Zr and Y diborides

Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$ (M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The metal vacancies change the density of states near the Fermi level and this effect is quite different for III-V group transition metal diborides. Contradictory data on superconductivity in diborides may be supposed to be connected with nonstoichiometry of samples. Vacancy formation energies are estimated and analyzed.

Shein, I R; Medvedeva, N I; Ivanovskii, A L

2002-01-01

190

Structurally tunable resonant absorption bands in ultrathin broadband plasmonic absorbers.

Light absorption is a fundamental optical process playing significantly important role in wide variety of applications ranging from photovoltaics to photothermal therapy. Semiconductors have well-defined absorption bands with low-energy edge dictated by the band gap energy, therefore it is rather challenging to tune the absorption bandwidth of semiconductors. However, resonant absorbers based on plasmonic nanostructures and optical metamaterials emerged as alternative light absorbers due to spectrally selective absorption bands resulting from optical resonances. Recently, a broadband plasmonic absorber design was introduced by Aydin et al. with a reasonably high broadband absorption. Based on that design, here, structurally tunable, broadband absorbers with improved performance are demonstrated. This broadband absorber has a total thickness of 190 nm with 80% average measured absorption (90% simulated absorption) over the entire visible spectrum (400 - 700 nm). Moreover, the effect of the metal and the oxide thicknesses on the absorption spectra are investigated and results indicate that the shorter and the longer band-edge of broadband absorption can be structurally tuned with the metal and the oxide thicknesses, as well as with the resonator size. Detailed numerical simulations shed light on the type of optical resonances that contribute to the broadband absorption response and provide a design guideline for realizing plasmonic absorbers with structurally tunable bandwidths. PMID:25321029

Butun, Serkan; Aydin, Koray

2014-08-11

191

Tight binding modelling of energy band structure in nitride heterostructures.

We studied the electronic structure of group III-V nitride ternary/binary heterostructures by using a semi-empirical sp(3)s* tight binding theory, parametrized to provide accurate description of both valence and conductions bands. It is shown that the sp(3)s* basis, along with the second nearest neighbor (2NN) interactions, spin-orbit splitting of cation and anion atoms, and nonlinear composition variations of atomic energy levels and bond length of ternary, is sufficient to describe the electronic structure of III-V ternary/binary nitride heterostructures. Comparison with experiment shows that tight binding theory provides good description of band structure of III-V nitride semiconductors. The effect of interface strain on valence band offsets in the conventional Al(1-x)Ga(x)N/GaN and In(1-x) Ga(x) N/GaN and dilute GaAs(1-x)Nx/GaAs nitride heterostructures is found to be linear function of composition for the entire composition range (0 < or = x < or = 1) because of smaller valence band deformations. PMID:18464368

Akinci, Ozden; Gürel, H Hakan; Unlü, Hilmi

2008-02-01

192

Chaos and structure of level densities

Energy Technology Data Exchange (ETDEWEB)

The energy region of the first few MeV above the ground state shows interesting features of the nucleus. Beyond an ordered energy region just above the ground-state the dynamics changes, and chaotic features are observed in the neutron resonance region. The statistical properties of energies and wave-functions are common to all chaotic nuclei. However, if instead a global property, like the local level-density function is studied, strong structure effects emerge. In this contribution we discuss these two different facets of warm nuclei. In section 2 the onset of chaos with increasing excitation energy is discussed, with both experimental observations and proposed theoretical mechanisms as starting points. The structure of level densities in the same excitation energy region based on the two different starting points, is treated in section 3, where we give a short presentation of a newly developed combinatorial level-density modell. Some results from the model are presented and discussed. Two coexisting facets of warm nuclei, quantum chaos and structure of the level density, are considered. A newly developed combinatorial level-density model is presented, and the role of collective enhancements discussed. An example of extreme parity enhancement is shown.

Moller, Peter [Los Alamos National Laboratory; Aberg, Sven [LUND SWEDEN; Uhrenholt, Henrik [LUND SWEDEN; Ickhikawa, Takatoshi [RIKEN

2008-01-01

193

Conduction-band-edge charge densities in elemental and compound semiconductors

International Nuclear Information System (INIS)

The empirical pseudopotential method is used to compute electronic charge densities at the ?, L, and X k points of the conduction-band edge in selected group-IV elemental (Si, Ge, ?-Sn) and III-V compound (GaP, GaAs, GaSb, InP, InAs, InSb) semiconductors. We find that these charge distributions are essentially independent of the system under study and are strongly dependent upon the symmetry of the wave function of a particular state. As a direct consequence of this analysis, we find that the ?1/sup c/ conduction-band state for the heteropolar semiconductors contains an abundance of charge density associated with the anion as opposed to the cation in support of a previous observation by Baldereschi and Maschke, and that the symmetry of the wave function of the conduction-band edge at X/sup c/ for GaSb is actually X3/sup c/, and not X1/sup c/ as has been previously assigned in the literature. We therefore predict an increase in both the direct and indirect band gaps of the Nowotny-Juza alloy ?-LiZnSb upon applying the interstitial insertion rule of Wood, Zunger, and de Groot to GaSb

194

International Nuclear Information System (INIS)

We present a systematic study of a recently proposed deconvolution of valence-band spectra of alloys into partial densities of states by x-ray photoelectron diffraction. The deconvolution is performed along the [111], [112], [113], and [114] directions of a AuCu3 (001) crystal. As expected, there are only small variations in the partial densities of states as a function of direction, except for the [111] direction. The mathematical assumptions and experimental limitations of the method are discussed in detail

195

Three-dimensional photonic band gaps in woven structures

International Nuclear Information System (INIS)

In this paper, we studied the photonic properties of dielectric fibres woven into three-dimensional (3D) structures. Such fibres can be fabricated on the micrometre scale, and hence the gaps are in the far-infrared to the infrared regime. The vector-wave transfer matrix method is applied to evaluate the photonic band structures. We have also employed the constant-frequency dispersion surface scheme to investigate the development of a full band gap. Such a 3D absolute gap is observed in a rectangular lattice, but at a fairly large dielectric constant for the fibres. Ways to improve on this have been suggested. Our study indicates that woven structures are promising materials for realizing the 3D photonic insulator in the infrared regime. (author)

196

Three-dimensional photonic band gaps in woven structures

In this paper, we studied the photonic properties of dielectric fibres woven into three-dimensional (3D) structures. Such fibres can be fabricated on the micrometre scale, and hence the gaps are in the far-infrared to the infrared regime. The vector-wave transfer matrix method is applied to evaluate the photonic band structures. We have also employed the constant-frequency dispersion surface scheme to investigate the development of a full band gap. Such a 3D absolute gap is observed in a rectangular lattice, but at a fairly large dielectric constant for the fibres. Ways to improve on this have been suggested. Our study indicates that woven structures are promising materials for realizing the 3D photonic insulator in the infrared regime. (author)

Tsai Ya Chih; Pendry, J B

1998-01-01

197

Doping dependent quasiparticle band structure in cuprate superconductors

We present an exact diagonalization study of the single particle spectral function in the so-called t-t'-t''-J model in 2D. As a key result, we find that unlike the `pure' t-J model, hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasiparticle states near (pi,0) are deep below the top of the band at (pi/2,pi/2), hole doping shifts these states up to E_F, resulting in extended flat band regions close to E_F and around (pi,0). This strong doping-induced deformation can be directly compared to angle resolved photoemission results on Sr_2 Cu Cl_2 O_2, underdoped Bi2212 and optimally doped Bi2212. We propose the interplay of long range hopping and decreasing spin correlations as the mechanism of this deformation.

Eder, R; Sawatzky, G A

1996-01-01

198

QUANTITATIVE ANALYSIS OF BANDED STRUCTURES IN DUAL-PHASE STEELS

Directory of Open Access Journals (Sweden)

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.

Benoit Krebs

2011-05-01

199

Mid-frequency Band Dynamics of Large Space Structures

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.

Coppolino, Robert N.; Adams, Douglas S.

2004-01-01

200

Joint density of states of wide-band-gap materials by electron energy loss spectroscopy

International Nuclear Information System (INIS)

Kramers-Kronig analysis for parallel electron energy loss spectroscopy (PEELS) data is developed as a software package. When used with a JEOL 4000EX high-resolution transmission electron microscope (HRTEM) operating at 100 keV this allows us to obtain the dielectric function of relatively wide band gap materials with an energy resolution of approx 1.4 eV. The imaginary part of the dielectric function allows the magnitude of the band gap to be determined as well as the joint-density-of-states function. Routines for obtaining three variations of the joint-density of states function, which may be used to predict the optical and dielectric response for angle-resolved or angle-integration scattering geometries are also described. Applications are presented for diamond, aluminum nitride (AlN), quartz (SiO2) and sapphire (Al2O3). The results are compared with values of the band gap and density of states results for these materials obtained with other techniques. (authors)

201

We have developed an 8-band Effective Mass Approximation model that describes the zero field spin splitting in the band structure of zincblende heterostructures due to bulk inversion asymmetry (BIA). We have verified that our finite difference Hamiltonian transforms in almost all situations according to the true $D_{2d}$ or $C_{2v}$ symmetry of [001] heterostructures. This makes it a computationally efficient tool for the accurate description of the band structure of heteros...

Cartoixa, X.; Ting, D. Z. -y; Mcgill, T. C.

2002-01-01

202

Development of X-band accelerating structures for high gradients

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.

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

2012-07-01

203

Engineering Design of a Multipurpose X-band Accelerating Structure

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...

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

2010-01-01

204

The quiet Sun's magnetic flux estimated from CaIIH bright inter-granular G-band structures

We determine the number density and area contribution of small-scale inter-granular calcium-II bright G-band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations. In a 149" x 117" G-band image of the disk center at the activity minimum, 7593 small inter-granular structures ['IGS']were segmented with the `multiple-level tracking' pattern recognition algorithm ['MLT_4']. The scatter-plot of the continuum versus the G-band brightness sho...

Bovelet, Burkart; Wiehr, Eberhard

2012-01-01

205

Charge-Density-Wave Phase of 1T-TiSe2: The Influence of Conduction Band Population

The charge-density-wave phase of TiSe2 was studied by angle-resolved photoelectron spectroscopy and resistivity measurements investigating the influence of the band gap size and of a varying population of the conduction band. A gradual suppression of the charge-density-wave-induced electronic superstructure is observed for a variation of the band gap in the ternary compounds TiCxSe2-x with C=(S,Te) as well as for an occupation of only the conduction band by H2O adsorption-induced band bending. These observations point to an optimum band gap and support an excitonic driving force for the charge-density wave.

May, Matthias M.; Brabetz, Christine; Janowitz, Christoph; Manzke, Recardo

2011-10-01

206

Electronic structure of BaCu$_2$As$_2$ and SrCu$_2$As$_2$: sp-band metals

The electronic structures of ThCr$_2$Si$_2$ structure BaCu$_2$As$_2$ and SrCu$_2$As$_2$ are investigated using density functional calculations. The Cu $d$ orbitals are located at 3 eV and higher binding energy, and are therefore chemically inert with little contribution near the Fermi energy. These materials are moderate density of states, sp-band metals with large Fermi surfaces and low anisotropy.

Singh, D. J.

2009-01-01

207

Effective band structure of correlated materials: the case of VO2

International Nuclear Information System (INIS)

Vanadium dioxide VO2 and its metal-insulator transition at T = 340 K continue to receive considerable interest. The question whether the physics of the insulating low-temperature phase is dominated by the Mott or the Peierls scenario, i.e. by correlation or band effects, is still under debate. A recent cluster dynamical mean field theory calculation (Biermann et al 2005 Phys. Rev. Lett. 94 026404) suggests a combination of both effects, characterizing the transition as of a correlation-assisted Peierls type. In this paper we present a detailed analysis of the excitation spectrum of the insulating M1 phase of VO2, based on this calculation. We implement a scheme to analytically continue self-energies from Matsubara to real frequencies, and study the physics of the strong interactions, as well as the corresponding changes with respect to the density functional theory band structure within the local density approximation (LDA). We find that in the M1 phase lifetime effects are rather negligible, indeed allowing for an effective band structure description. A frequency-independent but orbital-dependent potential, constructed as an approximation to the full cluster dynamical mean field self-energy, turns out to satisfactorily reproduce the fully interacting one-particle spectrum, acting as a scissors operator which pushes the a1g bonding and eg? bands apart and, thus, opens the gapgap

208

Electronic band structure of the superconductor Sr2RuO4

International Nuclear Information System (INIS)

A local-density electronic-band-structure calculation was performed for a recently discovered non-copper-layered perovskite superconductor, Sr2RuO4. It was found that the electronic structure near the Fermi energy is essentially described by antibonding bands of the Ru d var-epsilon and O p? states. Although two holes in the bands are predominantly situated in a d var-epsilon(xy)-p? state in the ab plane, the hole occupations in the other d var-epsilon-p? states vertical to the plane are not negligibly small, possibly in conjunction with the smallness of tetragonal distortion of the RuO6 octahedron. Associated with the antibonding d var-epsilon-p? bands, the density of states at the Fermi energy is relatively high (4.36 states/eVcell) but not enough to account for the observed specific-heat constant ?exp and temperature-independent magnetic susceptibility ?exp. We found a large Stoner factor, which may explain most of the mass enhancement involved in ?exp. Certain similarities and dissimilarities in the electronic properties to the cuprate superconductors are discussed

209

Electronic band structure of alfa-(Per)2M(mnt)2 compounds

The band structure of alfa-(Per)2M(mnt)2, leads to a description of these materials as nearly perfectly one dimensional conductors. The conduction is mainly along the stacking direction of the partially oxidized perylene molecules, (Per)1/2+, with virtually no interchain bandwidth. However, recent high magnetic field experiments suggest orbital coupling of the magnetic field to the electronic structure, indicating a finite interchain bandwidth. The details of the band structure and the possible variances, at low energies, from a perfect one dimensional system are examined. In particular, multiple quasi-one dimensional Fermi surface sheets, which would become important at low temperatures, may lead to an explanation for the experimental finding of a magnetic field induced high field charge density wave ground state. Experimental tests to observe the effects of the finite interchain bandwidth are proposed.

Canadell, E; Brooks, J; Canadell, Enric; Almeida, Manuel; Brooks, James

2004-01-01

210

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

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 Ge X 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 band (CB) edge, that valence band (VB) edge degeneracy is partially lifted and that the electron mass is unaltered 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.

Song, Jianjun; Zhang, Heming; Hu, Huiyong; Fu, Qiang

2009-04-01

211

Band gap and electronic structure of MgSiN2

International Nuclear Information System (INIS)

Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN2 is similar to that of wurtzite AlN. A band gap of 5.7?eV was calculated for both MgSiN2 (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN2 is 6.2?eV. MgSiN2 has an additional direct gap of 6.3?eV at the ? point.

212

Band gap and electronic structure of MgSiN{sub 2}

Energy Technology Data Exchange (ETDEWEB)

Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN{sub 2} is similar to that of wurtzite AlN. A band gap of 5.7?eV was calculated for both MgSiN{sub 2} (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN{sub 2} is 6.2?eV. MgSiN{sub 2} has an additional direct gap of 6.3?eV at the ? point.

Quirk, J. B., E-mail: james.quirk09@imperial.ac.uk; Råsander, M.; McGilvery, C. M.; Moram, M. A. [Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ (United Kingdom); Palgrave, R. [Department of Chemistry, University College London, Gordon Street WC1H 0AJ (United Kingdom)

2014-09-15

213

Spin-dependent band structure of the ferromagnetic semimetal EuB6

The spin polarization of EuB6 crystals has been measured using Andreev reflection spectroscopy. The conductance spectra of the EuB6/Pb junctions are well-described by the spin-polarized BTK model, which yields a spin polarization of about 56%. The results demonstrate that ferromagnetic EuB6 is not half-metallic. Further analyses of the Hall effect and magnetoresistivity indicate a semi-metallic band structure with complete spin polarization for the hole band only. The values and the spread of the measured spin polarization are quantitatively consistent with Fermi surface determined by quantum oscillation measurements^1 and carrier densities obtained from standard two-band model fits to the low temperature magnetoresistivity and Hall resistivity. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. ^1R. Goodrich et al., PRB 58, 14896 (1998); M. Aronson et al., PRB 59, 4720 (1999).

Xiong, Peng; Zhang, X.; von Molnar, S.; Fisk, Z.

2008-03-01

214

A Brief Introduction to Band Structure in Three Dimensions

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 potenti...

Iannucci, Peter

2011-01-01

215

Structure of superdeformed bands in the A ? 150 mass region

International Nuclear Information System (INIS)

The structure of superdeformed rotational bands recently discovered around 152Dy is discussed within the deformed shell model based on an average Woods-Saxon potential with a monopole pairing force. A comparison with available experimental data is provided and detailed predictions for yet unobserved cases are given. Pronounced variations in the observed rotational pattern are attributed to the angular momentum alignment of the high-N intruder (quasi)particles. (orig.)

216

Nonlocal pseudopotentials in complex band-structure and photoemission calculations

International Nuclear Information System (INIS)

Based on a recently proposed localization procedure, a nonlocal pseudopotential scheme is derived to calculate potential coefficients Vrvec Grvec G'(rvec k) which can be decomposed into terms, each being quadratic in rvec k and multiplied by a function of (rvec G-rvec G'), thus making them applicable to some important cases where local potential coefficients are required. Electronic structure calculations for semiconductors are in agreement with well-known semiempirical local pseudopotential band structures, as shown for GaAs. Nevertheless, the potential may significantly deviate from the semiempirical results. In order to test wave functions and transition probabilities, we prove the success of the procedure in a more troublesome case, i.e., applying it to a transition-metal compound as the experimentally well-investigated layered crystal TiSe2, which up to now was not treated with a pseudopotential. Photoemission spectra within the one-step model are presented using Pendry's method of complex band-structure calculation. The latter formalism had to be slightly generalized for the quasilocal properties of the potential. The agreement of the spectra with experimental data shows this method to be a reliable and practical tool to use nonlocal pseudopotentials for conduction-band wave functions of electron spectroscopies. copyright 1997 The American Physical Society

217

Band structure, optical properties and infrared spectrum of glycine sodium nitrate crystal

Glycine-sodium nitrate, GSN, crystals were grown from a stoichiometric solution by slow cooling technique and were characterized by optical absorption and FTIR spectroscopy. The data collected by FTIR were compared with the vibrational spectrum theoretically obtained by using DMol code in the local density approximation LDA. Moreover, the crystal band structure, the density of states, and the optical absorption data were calculated by using the CASTEP code within the framework of LDA and the generalized gradient approximation GGA. The calculations are in good agreement with the structure and properties of GSN; e.g., the optical transparency in visible region, the low density, the insulate character, and the bipolar form of glycine molecule.

Hernández-Paredes, J.; Glossman-Mitnik, D.; Esparza-Ponce, H. E.; Alvarez-Ramos, M. E.; Duarte-Moller, A.

2008-03-01

218

Nuclear energy density optimization: Shell structure

Background: Nuclear density functional theory is the only microscopical theory that can be applied throughout the entire nuclear landscape. Its key ingredient is the energy density functional. Purpose: In this work, we propose a new parametrization unedf2 of the Skyrme energy density functional. Methods: The functional optimization is carried out using the pounders optimization algorithm within the framework of the Skyrme Hartree-Fock-Bogoliubov theory. Compared to the previous parametrization unedf1, restrictions on the tensor term of the energy density have been lifted, yielding a very general form of the energy density functional up to second order in derivatives of the one-body density matrix. In order to impose constraints on all the parameters of the functional, selected data on single-particle splittings in spherical doubly-magic nuclei have been included into the experimental dataset. Results: The agreement with both bulk and spectroscopic nuclear properties achieved by the resulting unedf2 parametrization is comparable with unedf1. While there is a small improvement on single-particle spectra and binding energies of closed shell nuclei, the reproduction of fission barriers and fission isomer excitation energies has degraded. As compared to previous unedf parametrizations, the parameter confidence interval for unedf2 is narrower. In particular, our results overlap well with those obtained in previous systematic studies of the spin-orbit and tensor terms. Conclusions: unedf2 can be viewed as an all-around Skyrme EDF that performs reasonably well for both global nuclear properties and shell structure. However, after adding new data aiming to better constrain the nuclear functional, its quality has improved only marginally. These results suggest that the standard Skyrme energy density has reached its limits, and significant changes to the form of the functional are needed.

Kortelainen, M.; McDonnell, J.; Nazarewicz, W.; Olsen, E.; Reinhard, P.-G.; Sarich, J.; Schunck, N.; Wild, S. M.; Davesne, D.; Erler, J.; Pastore, A.

2014-05-01

219

Tuning of X-band traveling-wave accelerating structures

International Nuclear Information System (INIS)

This paper derives and explains the full procedure used to tune an X-band traveling-wave accelerating structure from the “bead-pull” measurement data. The local reflection of each cell caused by frequency detuning is calculated from the measured field profile deduced from the reflection from a dielectric perturbation (so-called “bead”) and is corrected by monitoring the input reflection coefficient. The structure output matching is made by tuning the last two cells to cancel the reflected wave. This method also gives the relationship of the reflection and frequency detuning, making it possible to evaluate the dimensional tolerances achieved in manufacture

220

The crystal and electronic band structure of the diamond-like semiconductor Ag2ZnSiS4

International Nuclear Information System (INIS)

Highlights: ? The structure of Ag2ZnSiS4 is solved and refined in the space group Pn using single crystal X-ray diffraction. ? Electronic band structure calculations show that Ag2ZnSiS4 is a direct band gap semiconductor with a calculated band gap of 1.88 eV. ? The optical band gap of Ag2ZnSiS4 was experimentally determined as 3.28 eV. - Abstract: Single crystals of the new diamond-like semiconductor Ag2ZnSiS4 have been synthesized using high-temperature, solid state synthesis at 800 °C. The compound crystallizes in the monoclinic, noncentrosymmetric space group Pn with a = 6.4052(1) ?, b = 6.5484(1) ?, c = 7.9340(1) ?, ? = 90.455(1)° and R1 (for all data) = 2.42%. The electronic band structure and density of states were calculated using density functional theory (DFT) and the full potential linearized augmented plane wave (LAPW) method within the Wien2k program suite. The calculated band structure suggests that Ag2ZnSiS4 is a direct band gap semiconductor with a calculated band gap of 1.88 eV at the ?-point. The calculated density of states of Ag2ZnSiS4 is compared with that of AgGaS2. The band gap of Ag2ZnSiS4 was also determined experimentally as 3.28 eV via optical diffuse reflectance spectroscopy.

221

International Nuclear Information System (INIS)

Photoelectron energy distribution curves from solid nitrogen have been measured for excitation energies ranging from threshold (14.2 eV) to 40 eV using Synchrotron Radiation. The partial cross sections for the emission from the 3sigmasub(g), 1?sub(u) and 2sigmasub(u) derived valence bands show pronounced maxima 3.4 eV, 2.9 eV and 3.0 eV above the vacuum level respectively which we interpret as being due to a high density of conduction band final states. These states are closely related to the ?sub(g)* negative-ion shape resonance for molecular nitrogen. (orig.)

222

Spin current in junctions composed of multi-band superconductors with a spin-density wave

We calculate a nondissipative spin current and show that it can flow with or without a charge current. We consider a two-band model which can be applied to the description of Fe-based pnictides in coexistence regime of superconductivity and spin-density wave. Using quasiclassical Green?s functions approach and tunneling Hamiltonian method we show that there exists a possibility to switch off the Josephson current while leaving the spin current finite. Moreover, it is possible to have the critical Josephson current and the critical spin current being proportional to each other, thus giving a possibility to measure the spin current via the Josephson current. The underlying mechanism is the interfering hopping of electron–hole pairs between different bands of the superconductors composing the junction. This is an intrinsic property of the system and provides a unique and natural way to utilize junctions made solely of pnictides in promising applications in spintronics devices.

Moor, Andreas; Volkov, Anatoly F.; Efetov, Konstantin B.

2015-02-01

223

Several ultrathin films of oriented alkane chains were studied by ultraviolet photoelectron spectroscopy using UV photons in the energy range of 20-200 eV. From the experimental data the full valence band structure has been determined for self-assembled films of long-chain n-alkanethiols, Langmuir-Blodgett films of Cd-arachidate and thin films of hexatriacontane. Significant deviations from band structures obtained by ab initio calculations of Karpfen are found at the boundary of the one-dimensional Brillouin zone. Furthermore it is demonstrated that a mapping of the one-dimensional band structure can be used for precisely determining tilt angles of alkane chains.

Zubrägel, Ch.; Schneider, F.; Neumann, M.; Hähner, G.; Wöll, Ch.; Grunze, M.

1994-03-01

224

Band structure mapping and calculations of CuInS2(001)

International Nuclear Information System (INIS)

The ternary compound semiconductor CuInS2 is used as an absorber material for thin film solar cells. A better understanding of the detailed electronic structure might lead to an improvement of the junction properties with respect to the still limited photo voltage of the present devices. We report on band structure mapping via ARUPS on thin epitaxial layers of CuInS2(001) prepared on sulfur passivated GaAs(100). To have a better control on the deposition process we introduced a MOMBE type deposition with an organic sulfur precursor. Samples were prepared and precharacterized in a dedicated UHV deposition and analysis system. ARUPS measurements were conducted at the beamline TGM7 at BESSY II. We discuss the experimentally determined band structure in comparison to recent augmented spherical wave (ASW) calculations as based on density functional theory and the local density approximation (LDA). Beside earlier calculations presented by Zunger for some high symmetry directions along ? perpendicular to our calculations include for ? parallel the experimental parameters of an ARUPS experiment thus allowing a direct comparison with our experimentally derived data rather than an idealistic run on the boundary of the Brillouin-zone. Additionally we derive the effective mass from the valence band curvature

225

Cell and band structures in cold rolled polycrystalline copper

DEFF Research Database (Denmark)

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 dislocation walls (DDWs) and cells develop during the initial stages of cold rolling. Grains having a high density of DDWs are described as high wall density (HWD) structures, and grains having a low density of DDWs are described as low wall density (LWD) structures. These structures are characterised by cell size, misorientation across the cell walls, and the crystallographic orientation of the grains in which they appear. The DDWs in the HWD structures have special characteristics, extending along several cells and having a misorientation across them greater than that across ordinary cell boundaries at the same strain. The DDWs appear to have a macroscopically determined orientation. Analysis of their crystallographic orientation shows that they are not, as frequently reported in the literature, parallel to {111} planes. It is suggested that the DDWs separate regions of the crystal having different operating slip systems. Two generations of microbands are found to develop with increasing deformation. The first generation microbands are related to a continuous development of the structure according to the principle of grain subdivision, whereas the second generation microbands relate to localised shear on {111}. Finally, the evolution of the deformation microstructure in copper is compared with that observed in other face centred cubic metals, especially aluminium.

Ananthan, V.S.; Leffers, Torben

1991-01-01

226

Crystal structure, electrical properties and electronic band structure of tantalum ditelluride

Motivated by the unexpectedly strong influence of the Te atoms on the structural and bonding properties of the transition metal tellurides, we have performed a detailed study of TaTe sub 2. Experimentally, this comprises a crystal structure determination as well as electrical resistivity measurements. The former analysis leads to an accurate update of the structural data reported in the 1960s, while the latter provides evidence for the mainly electronic character of scattering processes leading to the electrical conductivity. In addition, the electronic properties of TaTe sub 2 have been calculated using the TB-LMTO method. The partial density of states reflects the close connection of the Ta zigzag chains and the Te-Te network. This finding explains the charge transfer in the system in a rather simple way. The orthogonal-orbital character of the bands proved the existence of pi-bonds. The Fermi-surface study supports the interpretation of the experimental resistivity measurements. (author)

Vernes, A; Bensch, W; Heid, W; Naether, C

1998-01-01

227

Robust one-dimensional metallic band structure of silicide nanowires.

Angle-resolved photoemission (ARP) is employed to investigate the electronic structure of an extremely anisotropic form of nanocrystals--GdSi(2-x) nanowires on Si(100). Using a stepped Si(100) surface, a well-ordered and uniformly oriented array of nanowires is formed along the step edges as confirmed by diffraction and microscopy. The ARP measurement discloses two distinct electronic bands near the Fermi level, which disperse one dimensionally along the nanowires. These bands are metallic with the electron filling of 1/4 and 2/5, respectively, and with the effective mass close to that of a free electron along the wires. The metallicity is robust down to 20 K, in contrast to metallic surface atomic chain systems, paving a way to further studies on one-dimensional physics of metallic nanowires. PMID:16384071

Yeom, H W; Kim, Y K; Lee, E Y; Ryang, K-D; Kang, P G

2005-11-11

228

Dual-band metamaterial with a windmill-like structure

A broadband negative refractive index metamaterial based on a windmill-like structure is proposed, and investigated numerically and experimentally at the microwave frequency range. From the numerical and experimental results, effect media parameters are retrieved, which clearly show that two broad frequency bands exist in which the permittivity and permeability are negative. The two negative bands are from 9.1 GHz to 10.5 GHz and from 12.05 GHz to 14.65 GHz respectively, and the negative bandwidth is 4 GHz. Due to the good bandwidth performance, the metallic cell with double negative property obtained in this paper is suitable for use in the design of multiband or broadband microwave devices.

Xiong, Han; Hong, Jing-Song; Jin, Da-Lin

2013-01-01

229

Dual-band metamaterial with a windmill-like structure

International Nuclear Information System (INIS)

A broadband negative refractive index metamaterial based on a windmill-like structure is proposed, and investigated numerically and experimentally at the microwave frequency range. From the numerical and experimental results, effect media parameters are retrieved, which clearly show that two broad frequency bands exist in which the permittivity and permeability are negative. The two negative bands are from 9.1 GHz to 10.5 GHz and from 12.05 GHz to 14.65 GHz respectively, and the negative bandwidth is 4 GHz. Due to the good bandwidth performance, the metallic cell with double negative property obtained in this paper is suitable for use in the design of multiband or broadband microwave devices. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)

230

Fabrication of x-band accelerating structures at Fermilab

Energy Technology Data Exchange (ETDEWEB)

The RF Technology Development group at Fermilab is working together with the NLC and GLC groups at SLAC and KEK on developing technology for room temperature X-band accelerating structures for a future linear collider. We built six 60-cm long, high phase advance, detuned structures (HDS or FXB series). These structures have 150 degrees phase advance per cell, and are intended for high gradient tests. The structures were brazed in a vacuum furnace with a partial pressure of argon, rather than in a hydrogen atmosphere. We have also begun to build 60-cm long, damped and detuned structures (HDDS or FXC/FXD series). We have built 5 FXC and 1 FXD structures. Our goal was to build six structures for the 8-pack test at SLAC by the end of March 2004, as part of the GLC/NLC effort to demonstrate the readiness of room temperature RF technology for a linear collider. This paper describes the RF structure factory infrastructure (clean rooms, vacuum furnaces, vacuum equipment, RF equipment etc.), and the fabrication techniques utilized (the machining of copper cells/couplers, quality control, etching, vacuum brazing, cleanliness requirements etc.) for the production of FXB and FXC/FXD structures.

Tug T Arkan et al.

2004-07-20

231

Energy Technology Data Exchange (ETDEWEB)

The effects of impurities in room-temperature monoclinic WO3 were studied by using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO{sub 3}, where both substitutional and interstitial cases were considered. We have also considered transition-metal atom impurities and some codoping approaches in WO{sub 3}. We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results therefore provide guidance for making WO3 a suitable candidate for photoelectrodes for hydrogen generation by water splitting.

Huda, M. N.; Yan, Y.; Moon, C. Y.; Wei, S. H.; Al-Jassim, M. M.

2008-01-01

232

Density-functional theory study of the effects of atomic doping on the band edges of monoclinic WO3

The effects of impurities in room temperature monoclinic WO3 were studied using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO3, where both substitutional and interstitial dopings were considered. We have also considered doping with transition-metal atoms and some co-doping approaches in WO3. We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results, therefore, provide guidance for making WO3 a suitable candidate for photo-electrodes for hydrogen generation by water splitting.

Huda, Muhammad N.; Yan, Yanfa; Wei, Su-Huai; Al-Jassim, Mowafak M.

2008-03-01

233

The effects of impurities in room-temperature monoclinic WO3 were studied by using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO3 , where both substitutional and interstitial cases were considered. We have also considered transition-metal atom impurities and some codoping approaches in WO3 . We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results therefore provide guidance for making WO3 a suitable candidate for photoelectrodes for hydrogen generation by water splitting.

Huda, Muhammad N.; Yan, Yanfa; Moon, Chang-Yoon; Wei, Su-Huai; Al-Jassim, Mowafak M.

2008-05-01

234

Air Density Measurements in a Mach 10 Wake Using Iodine Cordes Bands

An exploratory study designed to examine the viability of making air density measurements in a Mach 10 flow using laser-induced fluorescence of the iodine Cordes bands is presented. Experiments are performed in the NASA Langley Research Center 31 in. Mach 10 air wind tunnel in the hypersonic near wake of a multipurpose crew vehicle model. To introduce iodine into the wake, a 0.5% iodine/nitrogen mixture is seeded using a pressure tap at the rear of the model. Air density was measured at 56 points along a 7 mm line and three stagnation pressures of 6.21, 8.62, and 10.0 MPa (900, 1250, and 1450 psi). Average results over time and space show rho(sub wake)/rho(sub freestream) of 0.145 plus or minus 0.010, independent of freestream air density. Average off-body results over time and space agree to better than 7.5% with computed densities from onbody pressure measurements. Densities measured during a single 60 s run at 10.0 MPa are time-dependent and steadily decrease by 15%. This decrease is attributed to model forebody heating by the flow.

Balla, Robert J.; Everhart, Joel L.

2012-01-01

235

Band structure quantization in nanometer sized ZnO clusters

Nanometer sized ZnO clusters are produced in the gas phase and subsequently deposited on clean Au(111) surfaces under ultra-high vacuum conditions. The zinc blende atomic structure of the approximately spherical ZnO clusters is resolved by high resolution scanning transmission electron microscopy. The large band gap and weak n-type conductivity of individual clusters are determined by scanning tunnelling microscopy and spectroscopy at cryogenic temperatures. The conduction band is found to exhibit clear quantization into discrete energy levels, which can be related to finite-size effects reflecting the zero-dimensional confinement. Our findings illustrate that gas phase cluster production may provide unique possibilities for the controlled fabrication of high purity quantum dots and heterostructures that can be size selected prior to deposition on the desired substrate under controlled ultra-high vacuum conditions.Nanometer sized ZnO clusters are produced in the gas phase and subsequently deposited on clean Au(111) surfaces under ultra-high vacuum conditions. The zinc blende atomic structure of the approximately spherical ZnO clusters is resolved by high resolution scanning transmission electron microscopy. The large band gap and weak n-type conductivity of individual clusters are determined by scanning tunnelling microscopy and spectroscopy at cryogenic temperatures. The conduction band is found to exhibit clear quantization into discrete energy levels, which can be related to finite-size effects reflecting the zero-dimensional confinement. Our findings illustrate that gas phase cluster production may provide unique possibilities for the controlled fabrication of high purity quantum dots and heterostructures that can be size selected prior to deposition on the desired substrate under controlled ultra-high vacuum conditions. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr33989k

Schouteden, Koen; Zeng, Yu-Jia; Lauwaet, Koen; Romero, Christian P.; Goris, Bart; Bals, Sara; van Tendeloo, Gustaaf; Lievens, Peter; Van Haesendonck, Chris

2013-04-01

236

Crossing points in the electronic band structure of vanadium oxide

Directory of Open Access Journals (Sweden)

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.

Keshav N. Shrivastava

2010-03-01

237

International Nuclear Information System (INIS)

Background and purpose: We previously determined that the density of a rapidly migrating DNA end-binding complex (termed 'band-A') predicts radiosensitivity of human normal and tumor cells. The goal of this study was first to identify the protein components of band-A and to determine if the protein levels of band-A components would correlate with band-A density and radiosensitivity. Patients and methods: DNA end-binding protein complex (DNA-EBC) protein components were identified by adding antibodies specific for a variety of DNA repair-associated proteins to the DNA-EBC reaction and then noting which antibodies super-shifted various DNA-EBC bands. Band-A levels were correlated with SF2 for a panel of primary human fibroblasts heterozygous for sequence-proven mutations in BRCA1 or BRCA2. The nuclear protein levels of band-A components were determined in each BRCA1 heterozygote by western hybridization. Results: DNA-EBC analysis of human nuclear proteins revealed 10 identifiable bands. The density of the most rapidly migrating DNA-EBC band correlated closely with both BRCA-mutation status and radiosensitivity (r2=0.85). This band was absent in cells with homozygous mutations in their ataxia-telangiectasia-mutated protein (ATM) genes. This band was also completely supershifted by the addition of antibodies to ATM, Ku70, DNA ligase III, Rpa32, Rpa14, DNA ligase IV, XRCC4, WRN, BLM, RAD51 and p53. However, the intranuclear concentrations of these proteins did near concentrations of these proteins did not correlate with either the SF2 or DNA-EBC density. Neither BRCA1 or BRCA2 could be detected in band-A. Conclusions: DNA-EBC analysis of human nuclear extracts resulted in 10 bands, at least six of which contained ATM. The density of one of the DNA-EBCs predicted the radiosensitization caused by BRCA haploinsufficiency, and this band contains Ku70, ATM, DNA ligase III, Rpa32, Rpa14, DNA ligase IV, XRCC4, WRN, BLM, RAD51 and p53 but not BRCA 1 or 2. The density of band-A was independent of the nuclear concentration of any of its known component

238

Effect of Electric Field on the Band Structure of Graphene/BN and BN/BN Bilayers

Effect of electric field on the band structures of graphene/BN and BN/BN bilayers is investigated within the framework of density functional theory. A relatively large degree of modulation is predicted for the graphene/BN bilayer. The calculated band gap of the graphene/BN bilayer increases with applied electric field, which is not the case of BN/BN bilayer; its band gap decreases with the applied field. Both features in the bilayers considered appear to be related to the na...

Balu, Radhakrishnan; Zhong, Xiaoliang; Pandey, Ravindra; Karna, Shashi P.

2011-01-01

239

Energy Technology Data Exchange (ETDEWEB)

We have investigated trap density of states (trap DOS) in n-channel organic field-effect transistors based on N,N?’-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI). A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical V{sub G} above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge.

Cho, Joung-min, E-mail: cho.j.ad@m.titech.ac.jp; Akiyama, Yuto; Kakinuma, Tomoyuki [Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152-8552 (Japan); Mori, Takehiko [Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152-8552 (Japan); ACT-C, JST, Honcho, Kawaguchi, Saitama 332-0012 (Japan)

2013-10-15

240

We have investigated trap density of states (trap DOS) in n-channel organic field-effect transistors based on N,N '-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI). A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical VG above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge.

Cho, Joung-min; Akiyama, Yuto; Kakinuma, Tomoyuki; Mori, Takehiko

2013-10-01

241

International Nuclear Information System (INIS)

We have investigated trap density of states (trap DOS) in n-channel organic field-effect transistors based on N,N?’-bis(cyclohexyl)naphthalene diimide (Cy-NDI) and dimethyldicyanoquinonediimine (DMDCNQI). A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical VG above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge

242

Directory of Open Access Journals (Sweden)

Full Text Available We have investigated trap density of states (trap DOS in n-channel organic field-effect transistors based on N,N?’-bis(cyclohexylnaphthalene diimide (Cy-NDI and dimethyldicyanoquinonediimine (DMDCNQI. A new method is proposed to extract trap DOS from the Arrhenius plot of the temperature-dependent transconductance. Double exponential trap DOS are observed, in which Cy-NDI has considerable deep states, by contrast, DMDCNQI has substantial tail states. In addition, numerical simulation of the transistor characteristics has been conducted by assuming an exponential trap distribution and the interface approximation. Temperature dependence of transfer characteristics are well reproduced only using several parameters, and the trap DOS obtained from the simulated characteristics are in good agreement with the assumed trap DOS, indicating that our analysis is self-consistent. Although the experimentally obtained Meyer-Neldel temperature is related to the trap distribution width, the simulation satisfies the Meyer-Neldel rule only very phenomenologically. The simulation also reveals that the subthreshold swing is not always a good indicator of the total trap amount, because it also largely depends on the trap distribution width. Finally, band transport is explored from the simulation having a small number of traps. A crossing point of the transfer curves and negative activation energy above a certain gate voltage are observed in the simulated characteristics, where the critical VG above which band transport is realized is determined by the sum of the trapped and free charge states below the conduction band edge.

Joung-min Cho

2013-10-01

243

Band structure approach to the resonant x-ray scattering

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 the phase of the scattering, is a direct measure of this p-d hybridization.and obtain quantitative agreement with experiment. A simple physical picture involving a product of dipole and quadrupolar transition matrix elements explains this behaviour and shows that it should be generally true for cases where the resonating atom is not at an inversion center. This has strong implications for the description of the resonance behavior of x-ray scattering in materials where the resonant atom is not at an inversion center such as ...

Elfimov, I S; Anisimov, V I; Sawatzky, G A

2002-01-01

244

Birefringence and band structure of CdP2 crystals

International Nuclear Information System (INIS)

The spatial dispersion in CdP2 crystals was investigated. The dispersion is positive (nk||c>nk||y) at ?>?0 and negative (nk||ck||y) at ?0. CdP2 crystals are isotropic for wavelength ?o=896 nm. Indirect transitions in excitonic region Egx are nonpolarized due to one pair of bands. Minimal direct energy intervals correspond to transitions ?1??1 for E||c and ?2??1 for E?c. The temperature coefficient of energy gap sifting in the case of temperature changing between 2 and 4.2 K equals to 10.6 meV/K and 3.2 mev/K for ?1??1 and ?2??1 band gap correspondingly. Reflectivity spectra were measured for energy interval 1.5–10 eV and optical functions (n, k, ?1, ?2,d2?1/dE2 and d2?2/dE2) were calculated by using Kramers–Kronig analyses. All features were interpreted as optical transitions on the basis of both theoretical calculations of band structure

245

International Nuclear Information System (INIS)

Direct carrier multiplication (DCM) occurs when a highly excited electron-hole pair decays by transferring its excess energy to the electrons rather than to the lattice, possibly exciting additional electron-hole pairs. Atomistic electronic structure calculations have shown that DCM can be induced by electron-hole Coulomb interactions, in an impact-ionization-like process whose rate is proportional to the density of biexciton states ?XX. Here we introduce a DCM 'figure of merit' R2(E) which is proportional to the ratio between the biexciton density of states ?XX and the single-exciton density of states ?x, restricted to single-exciton and biexciton states that are coupled by Coulomb interactions. Using R2(E), we consider GaAs, InAs, InP, GaSb, InSb, CdSe, Ge, Si, and PbSe nanocrystals of different sizes. Although DCM can be affected by both quantum-confinement effects (reflecting the underly electronic structure of the confined dot-interior states) and surface effects, here we are interested to isolate the former. To this end the nanocrystal energy levels are obtained from the corresponding bulk band structure via the truncated crystal approximation. We find that PbSe, Si, GaAs, CdSe, and InP nanocrystals have larger DCM figure of merit than the other nanocrystals. Our calculations suggest that high DCM efficiency requires high degeneracy of the corresponding bulk band-edge states. Interestingly, by consideringe states. Interestingly, by considering band structure effects we find that as the dot size increases the DCM critical energy E0 (the energy at which R2(E) becomes (ge)1) is reduced, suggesting improved DCM. However, whether the normalized E0/(varepsilon)g increases or decreases as the dot size increases depends on dot material

246

A study of higher-band dipole wakefields in X-band accelerating structures for the G/NLC

The X-band linacs for the GLC/NLC (Global Linear Collider/Next Linear Collider) have evolved from the DDS (Damped Detuned Structure) series [1,2]. The present series of accelerating structures are each 60 cm in length and incorporate both damping and detuning of the dipole modes which comprise the wakefield. In order to adequately damp the wakefield the dipole frequencies of adjacent structures are interleaved. The properties of the first dipole band have been extensively studied. However, limited analysis has been done on the higher order dipole bands. Here, we calculate the contribution of the higher order bands of the interleaved structures to the wakefield using a mode matching computer code [3]. Beam dynamics issues are also studied by tracking the beam through the complete linac using the particle beam tracking code LIAR [4].

Jones, R M

2004-01-01

247

Correlation between surface chemistry, density and band gap in nanocrystalline WO3 thin films

Energy Technology Data Exchange (ETDEWEB)

Nanocrystalline WO3 thin films were produced by sputter-deposition by varying the ratio of argon to oxygen in the reactive gas mixture during deposition. The surface chemistry, physical characteristics, and optical properties of nanocrystalline WO3 films were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray reflectivity (XRR), and spectrophotometric measurements. The effect of ultra-microstructure was significant on the optical properties of WO3 films. The XPS analyses indicate the formation of stoichiometric WO3 with tungsten existing in fully oxidized valence state (W6+). However, WO3 films grown at high oxygen concentration (>60%) in the sputtering gas mixture were over stoichiometric with excess oxygen. XRR simulations, which are based on isotropic WO3 film - SiO2 interface - Si substrate model, indicate that the density of WO3 films is sensitive to the oxygen content in the sputtering gas. The spectral transmission of the films increased with the increasing oxygen. The band gap of these films increases from 2.78 eV to 3.25 eV with increasing oxygen. A direct correlation between the film-density and band gap in nanocrystalline WO3 films is established based on the observed results.

Vemuri, Venkata Rama Ses; Engelhard, Mark H.; Ramana, C.V.

2012-03-01

248

Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy

Energy Technology Data Exchange (ETDEWEB)

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.

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

2008-12-10

249

Tunneling and the band structure of chaotic systems

International Nuclear Information System (INIS)

The dispersion laws of chaotic periodic systems are computed using the semiclassical periodic orbit theory to approximate the trace of the powers of the evolution operator. Aside from the usual real trajectories, complex orbits are also included. These turn out to be fundamental for a proper description of the band structure since they incorporate conduction processes through tunneling mechanisms. The results obtained, illustrated with the kicked-Harper model, are in excellent agreement with numerical simulations, even in the extreme quantum regime. (authors). 14 refs., 1 fig

250

Electronic band structure of calcium selenide under pressure

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.

Louail, L.; Haddadi, K.; Maouche, D.; Ali Sahraoui, F.; Hachemi, A.

2008-09-01

251

Band structure of odd-mass lanthanum nuclei

International Nuclear Information System (INIS)

Negative parity energy states in 121–131La have been studied using Projected Shell Model (PSM). Some nuclear structure properties like yrast spectra, back-bending in moment of inertia, reduced transition probabilities and band diagrams have been described. The experimental feature of the co-existence of prolate–oblate shapes in 125–131La isotopes has been satisfactorily explained by PSM results. Comparison of the theoretical data with their experimental counterparts has also been made. From the calculations, it is found that the yrast states arise because of multi-quasiparticle states. (author)

252

Electronic band structure of a type-II ‘W’ quantum well calculated by an eight-band k · p model

International Nuclear Information System (INIS)

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. (general)

253

Directory of Open Access Journals (Sweden)

Full Text Available The electronic structure, energy band structure, total density of states (DOS and electronic density of perovskite SrTiO_{3} in the cubic phase are calculated by the using full potential-linearized augmented plane wave (FP-LAPW method in the framework density functional theory (DFT with the generalized gradient approximation (GGA by WIEN2k package. The calculated band structure shows a direct band gap of 2.5 eV at the ? point in the Brillouin zone.The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, I have conclude that the bonding between Sr and TiO_{2} is mainly ionic and that the TiO_{2} entities bond covalently.The calculated band structure and density of state of SrTiO_{3} are in good agreement with theoretical and experimental results.

H. Salehi

2011-09-01

254

Density banding in skeletons of reef-building corals is a valuable source of proxy environmental data. However, skeletal growth strategy has a significant impact on the apparent timing of density-band formation. Some corals employ a strategy where the tissue occupies previously formed skeleton during as the new band forms, which leads to differences between the actual and apparent band timing. To investigate this effect, we collected cores from female and male colonies of Siderastrea siderea and report tissue thicknesses and density-related growth parameters over a 17-yr interval. Correlating these results with monthly sea surface temperature (SST) shows that maximum skeletal density in the female coincides with low winter SSTs, whereas in the male, it coincides with high summer SSTs. Furthermore, maximum skeletal densities in the female coincide with peak Sr/Ca values, whereas in the male, they coincide with low Sr/Ca values. Both results indicate a 6-month difference in the apparent timing of density-band formation between genders. Examination of skeletal extension rates also show that the male has thicker tissue and extends faster, whereas the female has thinner tissue and a denser skeleton—but both calcify at the same rate. The correlation between extension and calcification, combined with the fact that density banding arises from thickening of the skeleton throughout the depth reached by the tissue layer, implies that S. siderea has the same growth strategy as massive Porites, investing its calcification resources into linear extension. In addition, differences in tissue thicknesses suggest that females offset the greater energy requirements of gamete production by generating less tissue, resulting in differences in the apparent timing of density-band formation. Such gender-related offsets may be common in other corals and require that environmental reconstructions be made from sexed colonies and that, in fossil corals where sex cannot be determined, reconstructions must be duplicated in different colonies.

Carricart-Ganivet, J. P.; Vásquez-Bedoya, L. F.; Cabanillas-Terán, N.; Blanchon, P.

2013-09-01

255

Electronic band structure of the CeNi4Ga compound

New results of the electronic properties of the CeNi4Ga compound are reported. The magnetic susceptibility, X-ray photoemission spectroscopy and theoretical calculations suggest that this compound exhibits features typical of paramagnetic state and mixed-valence behavior. The electronic structure of the ternary CeNi4Ga compound, crystallizing in the hexagonal CaCu5 structure is studied by ab-initio calculations. Below the Fermi energy the total density of states contains mainly the d-states of Ni atoms. The narrow peak of the f-states of Ce atoms is found above the Fermi level. The density of states is N(EF) = 7.33 states/(eV f.u.) at the Fermi level. The electronic specific heat coefficient estimated from the band calculations is about 28.7 mJ/mol K2.

Kowalczyk, A.; Pugaczowa-Michalska, M.; Toliski, T.

2005-02-01

256

Electronic band structure of silver-deficient hexagonal AgB$_2$

Structural, cohesive properties as well as energy band structure of metastable hexagonal AgB$_2$ and silver-deficient borides Ag$_{0.875}$B$_2$ and Ag$_{0.750}$B$_2$ were investigated by means of the projected augmented wave method in the framework of the density functional theory (VASP package). We found that the density of states at the Fermi level for nonstoichiometric diborides is almost constant within a range of vacancy content up to 25%. The formation energy of metal vacancies in silver diboride is the least among all 4d metal diborides, i.e. for AgB$_2$ is possible to expect the wide homogeneity region.

Shein, I R; Ivanovskii, A L

2004-01-01

257

Energy Technology Data Exchange (ETDEWEB)

The electronic band structures and excited states of III-V semiconductors such as GaP, AlP, AlAs, and AlSb for various polytypes are determined employing the screened-exchange density functional calculations implemented in the full-potential linearized augmented plane-wave methods. We demonstrate that GaP and AlSb in the wurtzite (WZ) structure have direct gap while III-V semiconductors in the zinc blende, 4H, and 6H structures considered in this study exhibit an indirect gap. Furthermore, we find that inclusion of Al atoms less than 17% and 83% in the hexagonal Al{sub x}Ga{sub 1?x}P and Al{sub x}Ga{sub 1?x}As alloys, respectively, leads to a direct transition with a gap energy of ?2.3 eV. The feasibility of III-V semiconductors with a direct gap in WZ structure offers a possible crystal structure engineering to tune the optical properties of semiconductor materials.

Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori [Department of Physics Engineering, Mie University, 1577 Kurima-Machiya, Tsu 514-8507 (Japan); Freeman, Arthur J. [Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 (United States)

2014-03-31

258

The electronic band structures and excited states of III-V semiconductors such as GaP, AlP, AlAs, and AlSb for various polytypes are determined employing the screened-exchange density functional calculations implemented in the full-potential linearized augmented plane-wave methods. We demonstrate that GaP and AlSb in the wurtzite (WZ) structure have direct gap while III-V semiconductors in the zinc blende, 4H, and 6H structures considered in this study exhibit an indirect gap. Furthermore, we find that inclusion of Al atoms less than 17% and 83% in the hexagonal AlxGa1-xP and AlxGa1-xAs alloys, respectively, leads to a direct transition with a gap energy of ˜2.3 eV. The feasibility of III-V semiconductors with a direct gap in WZ structure offers a possible crystal structure engineering to tune the optical properties of semiconductor materials.

Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori; Freeman, Arthur J.

2014-03-01

259

Quasiparticle semiconductor band structures including spin-orbit interactions.

We present first-principles calculations of the quasiparticle band structure of the group IV materials Si and Ge and the group III-V compound semiconductors AlP, AlAs, AlSb, InP, InAs, InSb, GaP, GaAs and GaSb. Calculations are performed using the plane wave pseudopotential method and the 'one-shot' GW method, i.e. G(0)W(0). Quasiparticle band structures, augmented with the effects of spin-orbit, are obtained via a Wannier interpolation of the obtained quasiparticle energies and calculated spin-orbit matrix. Our calculations explicitly treat the shallow semicore states of In and Ga, which are known to be important in the description of the electronic properties, as valence states in the quasiparticle calculation. Our calculated quasiparticle energies, combining both the ab initio evaluation of the electron self-energy and the vector part of the pseudopotential representing the spin-orbit effects, are in generally very good agreement with experimental values. These calculations illustrate the predictive power of the methodology as applied to group IV and III-V semiconductors. PMID:23396813

Malone, Brad D; Cohen, Marvin L

2013-03-13

260

The band structure and electronic transport properties of pristine ZrN nanotube, oxygen, fluorine and niobium substituted ZrN are successfully optimized using density functional theory. The transport properties of ZrN nanotube are studied in terms of band structure, density of states, electron density and transmission spectrum of ZrN nanotube. The band structure reveals that the nanostructures show metallic nature due to orbital overlapping of zirconium and nitrogen atoms. The density of states gives the information of localization of charges in energy intervals. The major contribution in density of states arises from p and d orbitals of zirconium and nitrogen atoms. The electron density is observed more in nitrogen sites for pristine and impurity substituted ZrN nanotube. The electrons near the Fermi level contributes more to the transmission, the impact in the transmission is seen due to substitution impurity and position of the defect in the ZrN nanotube. The results of the present work focus light to tailor ZrN nanotube with enhanced electronic properties in nanoelectronics applications. PMID:25459628

Chandiramouli, R; Nagarajan, V

2015-02-01

261

High gradient experiment for X-band travelling wave structure

International Nuclear Information System (INIS)

High gradient performance of a travelling wave X-band accelerating structure 20 cm long was examined. Various observables were measured such as RF pulse waveform at various positions, dark current versus accelerating field level, energy spectra of emitted electrons downstream and so on. The field level of 80 MV/m at the input coupler cell was obtained after 500 hours' conditioning at a typical repetition rate of 10 pps. This field level is equivalent to 70 MV/m accelerating field in 20 cm long structure. Peak dark current decreased down to 1 ?A level at accelerating field of 50 MV/m at the input coupler cell. (Author) 5 refs., 7 figs., tab

262

Band structure and electron-electron interaction in samarium monosulphide

International Nuclear Information System (INIS)

The method of augmented plane wave (APW) is used to obtain the band structure of the SmS compound in the semiconductor and metal phases. The noncentral part of the Coulomb electron-electron interaction is taken into account in the first order perturbation theory. In this case the radial part of the wave APW-function is taken as a zero approximation function. A multiplet structure of the excited configuration f5d, which provides a good description of the X-ray photoelectron spectrum and optical spectrum epsilon2(?), is obtained. The configuration fd is calculated for the interpretation of the optical absorption spectrum of the samarium monosulfide metal phase. (author)

263

Energy Technology Data Exchange (ETDEWEB)

We present a divergence-free method to determine the characteristics of band structures and projected band structures of transverse acoustic phonons in Fibonacci superlattices. A set of bandedge equations is formulated to solve the band structures for the phonon instead of using the traditional dispersion relation. Numerical calculations show band structures calculated by the present method for the Fibonacci superlattice without numerical instability, which may occur in traditional methods. Based on the present formalism, the band structure for the acoustic phonons has been characterized by closure points and the projected bandgaps of the forbidden bands. The projected bandgaps are determined by the projected band structure, which is characterized by the cross points of the projected bandedges. We observed that the band structure and projected band structure and their characteristics were quite different for different generation orders and the basic layers for the Fibonacci superlattice. In this study, concise rules to determine these characteristics of the band structure and the projected band structure, including the number and the location of closure points of forbidden bands and those of projected bandgaps, in Fibonacci superlattices with arbitrary generation order and basic layers are proposed.

Hsueh, W J; Chen, R F; Tang, K Y [Department of Engineering Science, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 10660, Taiwan (China)], E-mail: hsuehwj@ntu.edu.tw

2008-11-05

264

International Nuclear Information System (INIS)

We present a divergence-free method to determine the characteristics of band structures and projected band structures of transverse acoustic phonons in Fibonacci superlattices. A set of bandedge equations is formulated to solve the band structures for the phonon instead of using the traditional dispersion relation. Numerical calculations show band structures calculated by the present method for the Fibonacci superlattice without numerical instability, which may occur in traditional methods. Based on the present formalism, the band structure for the acoustic phonons has been characterized by closure points and the projected bandgaps of the forbidden bands. The projected bandgaps are determined by the projected band structure, which is characterized by the cross points of the projected bandedges. We observed that the band structure and projected band structure and their characteristics were quite different for different generation orders and the basic layers for the Fibonacci superlattice. In this study, concise rules to determine these characteristics of the band structure and the projected band structure, including the number and the location of closure points of forbidden bands and those of projected bandgaps, in Fibonacci superlattices with arbitrary generation order and basic layers are proposed.

265

Electronic Structures of Carbon-Doped Hexagonal Boron Nitride Sheet: A Density-Functional Study

We study electronic structures of carbon-doped hexagonal boron nitride sheets using a first-principles electronic structure calculation within the density-functional theory. We calculate the variation of the energy bands and the C-induced states under the biaxially applied tensile strain. We also show the work functions and the ionization energies of C-induced states in unstrained and 5% tensile strained h-BN sheets, and discuss the effects of tensile strain on their electronic structures.

Fujimoto, Yoshitaka; Koretsune, Takashi; Saito, Susumu

266

Corals are a key reservoir of biodiversity in coastal, shallow water tropical marine environments, and density banding in their aragonite skeletons is used as a sensitive record of paleoclimate. Therefore, the cellular response of corals to environmental change and its expression in skeletal structure is of significant importance. Chromatophores, pigment-bearing cells within the ectoderm of hermatypic corals, serve to both enhance the photosynthetic activity of zooxanthellae symbionts, as well as protect the coral animal from harmful UV radiation. Yet connections have not previously been drawn between chromatophore tissue density and the development of skeletal density bands. A histological analysis of the coral Montastrea faveolata has therefore been conducted across a bathymetric gradient of 1-20 m on the southern Caribbean island of Curaçao. A combination of field and laboratory photography, serial block face imaging (SBFI), two-photon laser scanning microscopy (TPLSM), and 3D image analysis has been applied to test whether M. faveolata adapts to increasing water depth and decreasing photosynthetically active radiation by shifting toward a more heterotrophic lifestyle (decreasing zooxanthellae tissue density, increasing mucocyte tissue density, and decreasing chromatophores density). This study is among the first to collect and evaluate histological data in the spatial context of an entire unprocessed coral polyp. TPLSM was used to optically thin section unprocessed tissue biopsies with quantitative image analysis to yield a nanometer-scale three-dimensional map of the quantity and distribution of the symbionts (zooxanthellae) and a host fluorescent pigments (chromatophores), which is thought to have photoprotective properties, within the context of an entire coral polyp. Preliminary results have offered new insight regarding the three-dimensional distribution and abundance of chromatophores and have identified: (1) M. faveolata tissue collected from 8M SWD do not contain the abundant chromatophores present in M. faveolata collected from 20M SWD; and (2) a distinct difference in size and distribution of chromatophores between M. faveolata collected from 8-20M SWD. These results suggest that chromatophore cells may have an important photoenhancing function (reflection of light to help facilitate the collection of usable light that reaches the symbiotic algae for effective photosynthesis) rather than a photoinhibitive function (absorbing or refract light that may be harmful to zooxanthellae) which has been previously hypothesized.

Ardisana, R. N.; Miller, C. A.; Sivaguru, M.; Fouke, B. W.

2013-12-01

267

International Nuclear Information System (INIS)

Opportunities of the method of cathode absorption when studying band structure and properties of nonequilibrium charge carriers in solid are considered. Experimental temperature dependences of effective energy-level densities in the conduction and valence bands of CdCr2Se4 magnetic semiconductor are obtained. Electron and hole relaxation in n- and p-type samples is studied by the optical method

268

Band Structure and Transport Properties of $CrO_{2}$

Local Spin Density Approximation (LSDA) is used to calculate the energy bands of both the ferromagnetic and paramagnetic phases of metallic CrO_2. The Fermi level lies in a peak in the paramagnetic density of states, and the ferromagnetic phase is more stable. As first predicted by Schwarz, the magnetic moment is 2 \\mu_B per Cr atom, with the Fermi level for minority spins lying in an insulating gap between oxygen p and chromium d states ("half-metallic" behavior.) The A_1g Raman frequency is predicted to be 587 cm^{-1}. Drude plasma frequencies are of order 2eV, as seen experimentally by Chase. The measured resistivity is used to find the electron mean-free path l, which is only a few angstroms at 600K, but nevertheless, resistivity continues to rise as temperature increases. This puts CrO_2 into the category of "bad metals" in common with the high T_c superconductors, the high T metallic phase of VO_2, and the ferromagnet SrRuO_3. In common with both SrRuO_3 and Sr_2RuO_4, the measured specific heat \\gamma ...

Lewis, S P; Sasaki, T; Lewis, Steven P.; Allen, Phillip B.; Sasaki, Taizo

1996-01-01

269

International Nuclear Information System (INIS)

The electronic structure of the high-Tc copper oxides is calculated by means of an extended two-dimensional three-band Hubbard model in the unrestricted Hartree-Fock approximation. The influence of the coupling parameters on the obtained bands, as well as their doping dependence are investigated especially at the Fermi surface. Results are discussed in the light of recent experimental data for the cuprate Fermi surfaces. A comparative analysis of these conflicting data on the basis of our results sheds some light on the interpretation of the measured band structures. The direct oxygen-oxygen hopping interaction is found to be essential in fitting experimental results, suggesting that, in the doped regime, the oxygen band plays a key role at least in the near-EF region. Antiferromagnetic correlations among copper atoms turn out as well to be crucial. The results agree remarkably well with previous local-density calculations and with spectroscopic measurements

270

The electronic structure of the high-Tc copper oxides is calculated by means of an extended two-dimensional three-band Hubbard model in the unrestricted Hartree-Fock approximation. The influence of the coupling parameters on the obtained bands, as well as their doping dependence are investigated especially at the Fermi surface. Results are discussed in the light of recent experimental data for the cuprate Fermi surfaces. A comparative analysis of these conflicting data on the basis of our results sheds some light on the interpretation of the measured band structures. The direct oxygen-oxygen hopping interaction is found to be essential in fitting experimental results, suggesting that, in the doped regime, the oxygen band plays a key role at least in the near-EF region. Antiferromagnetic correlations among copper atoms turn out as well to be crucial. The results agree remarkably well with previous local-density calculations and with spectroscopic measurements.

López Sancho, M. P.; Rubio, J.; Refolio, M. C.; López Sancho, J. M.

1995-09-01

271

Band structure and UV optical spectra of TGS crystals in the range of 4-10eV

International Nuclear Information System (INIS)

Theoretical and experimental studies of the band energy structure and optical spectra for triglycine sulphate crystal (TGS) (NH2CH2COOH)3.H2SO4, in the ferroelectric phase have been performed for the first time. First principal DFT calculations of the band structure, density of states and dielectric functions spectra ?'(E) and ?''(E) of TGS crystal have been done using the computer package Cambridge Serial Total Energy Package (CASTEP) code. Experimental spectral dispersions of the complex reflection ratio ?(E) have been measured using the synchrotron radiation at BESSY synchrotron source in the spectral range of 4-10eV and the pseudo-dielectric functions ='>+i''> were evaluated. Experimental data and theoretically calculated dielectric functions have demonstrated a good agreement. The band energy dispersion of valence and conducting bands have been analyzed and were used to identify the dielectric functions peculiarities

272

Energy Technology Data Exchange (ETDEWEB)

The Hopfield parameter eta of the electron-phonon coupling is calculated for twelve transition metals (Rh, Pd, Ag, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au) in the rigid muffin-tin approximation. Using relativistic APW band structure data and a generalized Gaspari-Gyorffy formula the influence of relativistic effects is completely taken into account. It is found that eta may be calculated in remarkably good approximation using spin-weighted averages of phase shifts and density matrix coefficients. The theoretical results for the electron-phonon parameter lambda and the electronic specific heat are compared with the experimental data.

John, W.; Nemoshkalenko, V.V.; Antonov, V.N.; Antonov, V.N.

1984-01-01

273

Energy Technology Data Exchange (ETDEWEB)

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.

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

274

Fine structure of the valence band top of 3C BN crystal with nanopore

International Nuclear Information System (INIS)

The electron energy structure of the 3C BN boron nitride with pores (the radius r ? 0.3 nm) statistically distributed on the crystal is calculated by the method of the local coherent potential in approximation of multiple scattering. The crystalline potential is determined in the muffin-tin (MT) approximation. The contributions of the neighboring atoms to the electron density and the Coulomb potential of fifteen coordination spheres are accounted for. Comparison of the valence band tops of the crystalline (stoichiometric) and porous boron nitride with the X-ray photoelectron spectrum (XPS) BN and nitrogen emission spectra (SXES) is carried out. The nature of the short-wave floating in the XPS and also in the nitrogen K?-emission bands XES and SXES in the binary nitride is discussed

275

Band structure and optical parameters of the SnO2(110) surface

International Nuclear Information System (INIS)

With a first-principles-density-functional method, combined with two different pseudopotentials, ideal oxidized and reduced surfaces of tin oxide are studied. The band structures of bulk and the surface systems are calculated and compared. The nature of the surface Sn2+ ions, their outward relaxation, associated 'dangling bonds' and band gap states are considered. Also ultraviolet optical constants are determined by using the electric dipole approximation with a scissor correction, and noted to agree with experiments. The presence of the surface, and more significantly, its removed bridging oxygen atoms, becomes apparent in a formation of a new absorption feature. This is predicted to cause about 0.7 eV decrease of the absorption edge

276

Compton profiles and band structure calculations of CdS and CdTe

International Nuclear Information System (INIS)

In this paper we present the isotropic Compton profiles of zinc-blende CdS and CdTe measured at an intermediate resolution of 0.39 a.u. using our 20 Ci 137Cs Compton spectrometer. The electronic band structure calculations for both the zinc-blende structure compounds and also wurtzite CdS have been undertaken using various schemes of ab-initio linear combination of atomic orbitals calculations implemented in CRYSTAL03 code. The band structure and Mulliken's populations are reported using density functional scheme. In case of wurtzite CdS, our theoretical anisotropies in directional Compton profiles are compared with available experimental data. In case of both the zinc-blende compounds, the isotropic experimental profiles are found to be in better agreement with the present Hartree-Fock calculations. A study of the equal-valence-electron-density experimental profiles of zinc-blende CdS and CdTe shows that the CdS is more ionic than CdTe. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

277

Compton profiles and band structure calculations of CdS and CdTe

Energy Technology Data Exchange (ETDEWEB)

In this paper we present the isotropic Compton profiles of zinc-blende CdS and CdTe measured at an intermediate resolution of 0.39 a.u. using our 20 Ci {sup 137}Cs Compton spectrometer. The electronic band structure calculations for both the zinc-blende structure compounds and also wurtzite CdS have been undertaken using various schemes of ab-initio linear combination of atomic orbitals calculations implemented in CRYSTAL03 code. The band structure and Mulliken's populations are reported using density functional scheme. In case of wurtzite CdS, our theoretical anisotropies in directional Compton profiles are compared with available experimental data. In case of both the zinc-blende compounds, the isotropic experimental profiles are found to be in better agreement with the present Hartree-Fock calculations. A study of the equal-valence-electron-density experimental profiles of zinc-blende CdS and CdTe shows that the CdS is more ionic than CdTe. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Heda, N.L.; Mathur, S.; Ahuja, B.L. [Department of Physics, University College of Science, M.L. Sukhadia University, Udaipur, Rajasthan (India); Sharma, B.K. [Department of Physics, University of Rajasthan, Jaipur (India)

2007-03-15

278

Analysis of photonic band-gap structures in stratified medium

DEFF Research Database (Denmark)

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 in solving the Maxwell's equations numerically. It expands the temporal derivatives using the finite differences, while it adopts the Fourier transform (FT) properties to expand the spatial derivatives in Maxwell's equations. In addition, the method makes use of the chain-rule property in calculus together with the transformed space technique in order to make the algorithm flexible in terms of non-uniform spatial sampling. Findings - Through the studies of the wave propagation characteristics on PBG structures in stratified medium, it has been found that the proposed method retains excellent accuracy in the occasions where the spatial distributions contain step of up to five times larger than the original size, while simultaneously the flexibility of non-uniform sampling offers further savings on computational storage. Research limitations/implications - Research has been mainly limited to the simple one-dimensional (1D) periodic and defective cases of PBG structures. Nevertheless, the findings reveal strong implications that flexibility of sampling and memory savings can be realized in multi-dimensional structures. Practical implications - The proposed method can be applied to various practical structures in electromagnetic and microwave applications once the Maxwell's equations are appropriately modeled. Originality/value - The method validates its values and properties through extensive studies on regular and defective 1D PBG structures in stratified medium, and it can be further extended to solving more complicated structures. CPY Emerald Group Publishing Limited.

Tong, Ming-Sze; Yinchao, Chen

2005-01-01

279

Quasiparticle band structure based on a generalized Kohn-Sham scheme

We present a comparative full-potential study of generalized Kohn-Sham (gKS) schemes with explicit focus on their suitability as starting point for the solution of the quasiparticle equation. We compare G0W0 quasiparticle band structures calculated upon local-density approximation (LDA), screened-exchange, HSE03, PBE0, and Hartree-Fock functionals for exchange and correlation (XC) for Si, InN, and ZnO. Furthermore, the HSE03 functional is studied and compared to the generalized gradient approximation (GGA) for 15 nonmetallic materials for its use as a starting point in the calculation of quasiparticle excitation energies. For this case, the effects of self-consistency in the GW self-energy are also analyzed. It is shown that the use of a gKS scheme as a starting point for a perturbative quasiparticle correction can improve upon the deficiencies found for LDA or GGA starting points for compounds with shallow d bands. For these solids, the order of the valence and conduction bands is often inverted using local or semilocal approximations for XC, which makes perturbative G0W0 calculations unreliable. The use of a gKS starting point allows for the calculation of fairly accurate band gaps even in these difficult cases, and generally single-shot G0W0 calculations following calculations using the HSE03 functional are very close to experiment.

Fuchs, F.; Furthmüller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G.

2007-09-01

280

A Brief Introduction to Band Structure in Three Dimensions

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...

Iannucci, Peter

2011-01-01

281

Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering

Semiconductor nanocrystals are tiny light-emitting particles on the nanometer scale. Researchers have studied these particles intensely and have developed them for broad applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. A major feature of semiconductor nanocrystals is the quantum confinement effect, which leads to spatial enclosure of the electronic charge carriers within the nanocrystal. Because of this effect, researchers can use the size and shape of these “artificial atoms” to widely and precisely tune the energy of discrete electronic energy states and optical transitions. As a result, researchers can tune the light emission from these particles throughout the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. These particles also span the transition between small molecules and bulk crystals, instilling novel optical properties such as carrier multiplication, single-particle blinking, and spectral diffusion. In addition, semiconductor nanocrystals provide a versatile building block for developing complex nanostructures such as superlattices and multimodal agents for molecular imaging and targeted therapy. In this Account, we discuss recent advances in the understanding of the atomic structure and optical properties of semiconductor nanocrystals. We also discuss new strategies for band gap and electronic wave function engineering to control the location of charge carriers. New methodologies such as alloying, doping, strain-tuning, and band-edge warping will likely play key roles in the further development of these particles for optoelectronic and biomedical applications. PMID:19827808

SMITH, ANDREW M.; NIE, SHUMING

2010-01-01

282

Band structure of ABC-trilayer graphene superlattice

We investigate the effect of one-dimensional periodic potentials on the low energy band structure of ABC trilayer graphene first by assuming that all the three layers have the same potential. Extra Dirac points having the same electron hole crossing energy as that of the original Dirac point are generated by superlattice potentials with equal well and barrier widths. When the potential height is increased, the numbers of extra Dirac points are increased. The dispersions around the Dirac points are not isotropic. It is noted that the dispersion along the ky direction for kx = 0 oscillates between a non-linear dispersion and a linear dispersion when the potential height is increased. When the well and barrier widths are not identical, the symmetry of the conduction and valence bands is broken. The extra Dirac points are shifted either upward or downward depending on the barrier and well widths from the zero energy, while the position of the central Dirac point oscillates with the superlattice potential height. By considering different potentials for different layers, extra Dirac points are generated not from the original Dirac points but from the valleys formed in the energy spectrum. Two extra Dirac points appear from each pair of touched valleys, so four Dirac points appeared in the spectrum at particular barrier height. By increasing the barrier height of superlattice potential two Dirac points merge into the original Dirac point. This emerging and merging of extra Dirac points is different from the equal potential case.

Uddin, Salah; Chan, K. S.

2014-11-01

283

N-type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridization with the valence-band (VB) states. In order to test theoretical approaches, comparisons to experiment need to be made. Here, synchrotron-radiation photoelectron spectroscopy (SR-PES) measurements are presented, and comparison with three theoretical approaches are made. In particular the position of the Cd 4d state is measured with hard x-ray PES, and the orbital character of the VB is probed by photon energy dependent measurements. It is found that LDA + U using a theoretical U value of 2.34 eV is very successful in predicting the position of the Cd 4d state. The VB photon energy dependence reveals the O 2p photoionization cross section is underestimated at higher photon energies, and that an orbital contribution from Cd 5p is underestimated by all the DFT approaches.

Mudd, J. J.; Lee, Tien-Lin; Muñoz-Sanjosé, V.; Zúñiga-Pérez, J.; Payne, D. J.; Egdell, R. G.; McConville, C. F.

2014-04-01

284

We study the Q/B-band intensity ratio (IQ/B) of zinc tetraphenylporphyrin (ZnTPP) in various solvents from its electronic structure by utilizing the polarizable continuum model and density functional theory (DFT). The selected solvents are benzene, chloroform, dichloromethane, pyridine and methanol. The Q/B-band intensity ratio is simplified by dipole strength of HOMO ? LUMO and HOMO?1 ? LUMO from a two-level system, or fQ/B. The results show the linear correlation between the calculated fQ/B and the experimental IQ/B. This suggests that IQ/B of ZnTPP in solvents can be studied from dipole strength ratio from a two-level system, where the orbitals are determined by DFT. The change of fQ/B is due to the change of frontier molecular orbital shape of ZnTPP which are dominantly constructed by pz-orbital. In the core part of ZnTPP, the change of pz-orbital shape is inversely proportional to the solvent dielectric constant, ?.

Rusydi, Febdian; Gandaryus Saputro, Adhitya; Kasai, Hideaki

2014-08-01

285

A Tri-band-notched UWB Antenna with Low Mutual Coupling between the Band-notched Structures

Directory of Open Access Journals (Sweden)

Full Text Available A compact printed U-shape ultra-wideband (UWB antenna with triple band-notched characteristics is presented. The proposed antenna, with compact size of 24×33 mm2, yields an impedance bandwidth of 2.8-12GHz for VSWR less than 2, except the notched bands. The notched bands are realized by introducing two different types of slots. Two C-shape half-wavelength slots are etched on the radiating patch to obtain two notched bands in 3.3-3.7GHz for WiMAX and 7.25-7.75GHz for downlink of X-band satellite communication systems. In order to minimize the mutual coupling between the band-notched structures, the middle notched band in 5-6GHz for WLAN is achieved by using a U-slot defected ground structure. The parametric study is carried out to understand the mutual coupling. Surface current distributions and equivalent circuit are used to illustrate the notched mechanism. The performance of this antenna both by simulation and by experiment indicates that the proposed antenna is suitable and a good candidate for UWB applications.

C. M. Luo

2013-12-01

286

Importance of static correlation in the band structure of high-temperature superconductors

International Nuclear Information System (INIS)

Recently the authors presented a new band structure for La2-xSrxCuO4 and other high-temperature superconductors in which a second narrow band was seen to cross the primary band at the Fermi level. The existence of this second Fermi level band is in complete disagreement with the commonly accepted LDA band structure. Yet it provided a crucial piece of physics which led to an explanation for superconductivity and other unusual phenomena in these materials. In this work, details as to the nature of the failure of conventional methods in deriving the band structure of the cuprates are presented. In particular, a number of chemical analogues describe the problem of static correlation in the band structure calculations and show how this can be corrected with the predictable outcome of a Fermi level band crossing

287

Band structures of Ge and InAs: A 20 k.p model

The band structure of direct-band-gap semiconductor (InAs) and indirect-band-gap semiconductor (Ge) is described theoretically using a 20×20 k.p model and including far-level contribution (essentially the d levels). By using this model, we obtained a quantitatively correct description of the top of the valence band and the lowest two conduction bands both in terms of energetic positions and band curvatures. In particular, the k.p Hamiltonian parameters are adjusted such that the transverse mass of the germanium conduction band is equal to the experimental value of 0.081.

Radhia, S. Ben; Ridene, S.; Boujdaria, K.; Bouchriha, H.; Fishman, G.

2002-10-01

288

Anomalous decay of an atom in structured band gap reservoirs

We analyze the spontaneous emission of a two-level atom interacting with a special class of structured reservoirs of field modes with band gap edge coinciding with the atomic transition frequency. The exact time evolution of the population of the excited level is evaluated analytically through series of Fox-$H$ functions. Over estimated long time scales, inverse power law relaxations emerge, with powers decreasing continuously to $2$ according to the choice of the special reservoir. No trapping of the population of the excited level emerges. The same results are recovered in presence of $N-1$ atoms, each one in the ground state, described by the Dicke model. The power of the inverse power law decay results to be independent of $N$. A critical number $N_{\\alpha}^{\\left(\\star\\right)}$ is evaluated, such that, for $N \\gg N_{\\alpha}^{\\left(\\star\\right)}$, the inverse power law decay vanishes.

Giraldi, Filippo

2010-01-01

289

Parallelising the LMTO band-structure program on transputer arrays

We describe our experiences at parallelising a large Fortran program on a distributed-memory parallel computer. The program is the first-principles linear muffin tin orbital (LMTO) band-structure program which is used widely to calculate the electronic properties of crystalline solids, multilayers, polymers and complex molecules. The parallelising was done with (1) 3L parallel Fortran running on a quadputer, (2) Meiko's Occam Programming System (OPS) and (3) CS Tools on the Meiko Surface. We found that farming was not practicable because of livelock, the most efficient system was OPS, and that the speed of the program exceeds that of the CRAY when it is run on the larger domains of the Edinburgh Supercomputer. Also large numbers of k points can be easily handled on these domains. We give and discuss the efficiency of speed-up from our parallelisation.

Yeung, W.

1991-07-01

290

International Nuclear Information System (INIS)

We use Hedin's GW framework to compute quasiparticle bands for group-II monoxides in wurtzite (wz) structure. Modern hybrid density functional theory is applied to compute the starting electronic structure for the solution of the quasiparticle equation. We derive band parameters and effective masses for the three uppermost valence bands as well as the lowest conduction band including spin-orbit coupling. Optical transition matrix elements and exciton binding energies are also calculated. In addition to the prediction of parameters for wz-MgO and wz-CdO we discuss the chemical trend. (Abstract Copyright [2009], Wiley Periodicals, Inc.)

291

Band-structure analysis from photoreflectance spectroscopy in (Ga,Mn)As

Energy Technology Data Exchange (ETDEWEB)

Modulation photoreflectance spectroscopy has been applied to study the band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn content. 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 in the (Ga,Mn)As layers with increasing Mn content are interpreted in terms of a disordered valence band, extended within the band gap, formed, in highly Mn-doped (Ga,Mn)As, as a result of merging the Mn-related impurity band with the host GaAs valence band.

Yastrubchak, Oksana; Gluba, Lukasz; Zuk, Jerzy [Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin (Poland); Wosinski, Tadeusz; Andrearczyk, Tomasz; Domagala, Jaroslaw Z. [Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw (Poland); Sadowski, Janusz [Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland and MAX-Lab, Lund University, 22100 Lund (Sweden)

2013-12-04

292

Experimental determination of the band structure of photonic crystals of colloidal silica spheres

International Nuclear Information System (INIS)

A photonic band structure of colloidal crystals of silica spheres is analytically determined by a band model with three fitting parameters: the sphere size, the effective refractive index, and the band-gap. Optical properties of the crystals annealed at various temperatures were characterized by a procedure similar to X-ray diffraction technique, and the width of photonic band-gap measured from the transmission spectra experimentally servers as an additional check on the validation of the model. The photonic band structures defined by the band-gap, the refractive index, and the Brillouin zone are obviously superior to the use of the Bragg's expression involving simple zone folding.

293

Band structure calculations of CuAlO2, CuGaO2, CuInO2 and CuCrO2 by screened exchange

We report density functional theory (DFT) band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2 and CuCrO2. The use of the hybrid functional sX-LDA leads to considerably improved electronic properties compared to standard local density approximation (LDA) and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations and sh...

Gillen, Roland; Robertson, John

2011-01-01

294

Quasiparticle band structures of six II-VI compounds: ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe

International Nuclear Information System (INIS)

Quasiparticle band structures of six II-VI compounds (ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe in the zinc-blende and wurtzite structures) are calculated using the GW approximation. Results for band structures, calculated using the ab initio pseudopotential method within the local-density approximation (LDA), are given along with the energies of the quasiparticle excitations at symmetry points. Comparisons are made with measured values and trends in the GW corrections to the LDA band structures are examined

295

Three dimensional structure of low-density nuclear matter

We numerically explore the pasta structures and properties of low-density nuclear matter without any assumption on the geometry. We observe conventional pasta structures, while a mixture of the pasta structures appears as a metastable state at some transient densities. We also discuss the lattice structure of droplets.

Okamoto, Minoru; Yabana, Kazuhiro; Tatsumi, Toshitaka

2011-01-01

296

Three-dimensional structure of low-density nuclear matter

Energy Technology Data Exchange (ETDEWEB)

We numerically explore the pasta structures and properties of low-density nuclear matter without any assumption on the geometry. We observe conventional pasta structures, while a mixture of the pasta structures appears as a metastable state at some transient densities. We also discuss the lattice structure of droplets.

Okamoto, Minoru, E-mail: okamoto@nucl.ph.tsukuba.ac.jp [Graduate School of Pure and Applied Science, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan); Advanced Science Research Center, Japan Atomic Energy Agency, Shirakata Shirane 2-4, Tokai, Ibaraki 319-1195 (Japan); Maruyama, Toshiki, E-mail: maruyama.toshiki@jaea.go.jp [Advanced Science Research Center, Japan Atomic Energy Agency, Shirakata Shirane 2-4, Tokai, Ibaraki 319-1195 (Japan); Graduate School of Pure and Applied Science, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan); Yabana, Kazuhiro, E-mail: yabana@nucl.ph.tsukuba.ac.jp [Graduate School of Pure and Applied Science, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan); Center of Computational Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan); Tatsumi, Toshitaka, E-mail: tatsumi@ruby.scphys.kyoto-u.ac.jp [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan)

2012-07-09

297

Electronic band structure of LaO1-xFxBiS2: A recently invented family of superconductors

In this paper we present electronic band structure calculations of newly discovered BiS2 layer based LaO0.5F0.5BiS2 superconductor using density functional theory. The force minimization results of atomic positions are in agreement with experiments. From band structure analysis the parent compound LaOBiS2 is found to be an insulator for relaxed atomic positions whereas it exhibits metallic state for experimental coordinates. The substitution of F at O site is found to affect the electronic structure in non-rigid band scenario. The doped compound is found to be metallic having electrons as dominant charge carriers. The major contribution to states at Fermi level in LaFBiS2 comes from Bi-p and La-d orbitals.

Kumar, Jagdish; Ahluwalia, P. K.; Awana, V. P. S.

2013-02-01

298

Ab initio complex band structure of conjugated polymers: Effects of hydrid DFT and GW schemes

The non-resonant tunneling regime for charge transfer across nanojunctions is critically dependent on the so-called \\beta{} parameter, governing the exponential decay of the current as the length of the junction increases. For periodic materials, this parameter can be theoretically evaluated by computing the complex band structure (CBS) -- or evanescent states -- of the material forming the tunneling junction. In this work we present the calculation of the CBS for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the GW approximation. We compare the description of localization and \\beta{} parameters among the adopted methods and with experimental data. We show that local and semilocal density functionals systematically underestimate the \\beta{} parameter, while hybrid-exchange schemes partially correct for this discrepancy, resulting in a much better agreement with GW calculations an...

Ferretti, Andrea; Martin-Samos, Layla; Bussi, Giovanni; Ruini, Alice; Montanari, Barbara; Harrison, Nicholas M

2012-01-01

299

Handbook of the band structure of elemental solids from Z = 1 to Z = 112

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...

Papaconstantopoulos, Dimitris A

2015-01-01

300

The state of bistable defects in crystalline silicon such as iron-boron pairs or the boron-oxygen defect can be changed at room temperature. In this letter, we experimentally demonstrate that the chemical state of a group of defects can be changed to represent a bit of information. The state can then be read without direct contact via the intensity of the emitted band-band photoluminescence signal of the group of defects, via their impact on the carrier lifetime. The theoretical limit of the information density is then computed. The information density is shown to be low for two-dimensional storage but significant for three-dimensional data storage. Finally, we compute the maximum storage capacity as a function of the lower limit of the photoluminescence detector sensitivity.

Rougieux, F. E.; Macdonald, D.

2014-03-01

301

Bi2Te3 band structure reconstruction under hydrostatic compression

International Nuclear Information System (INIS)

The effect of hydrostatic compression on the Shubnikov-de Haas phenomenon in n- and p-Bi2Te3 samples with different degrees of filling of the valence and conductivity bands is investigated. A flow of carriers between nonequivalent extrema in the bands induced by pressure is observed. It is shown that the energy gap between the extrema increases in the conductivity band and decreases in the valence band under pressure. At a pressure P approximately 7 kbar an electron transition consisting of a change of the extremum of the valence band occurs

302

Valence-band density of states and surface electron accumulation in epitaxial SnO2 films

The surface band bending and electronic properties of SnO2(101) films grown on r-sapphire by plasma-assisted molecular beam epitaxy have been studied by Fourier-transform infrared spectroscopy (FTIR), x-ray photoemission spectroscopy (XPS), Hall effect, and electrochemical capacitance-voltage measurements. The XPS results were correlated with density functional theory calculation of the partial density of states in the valence-band and semicore levels. Good agreement was found between theory and experiment with a small offset of the Sn 4d levels. Homogeneous Sb-doped SnO2 films allowed for the calculation of the bulk Fermi level with respect to the conduction-band minimum within the k .p carrier statistics model. The band bending and carrier concentration as a function of depth were obtained from the capacitance-voltage characteristics and model space charge calculations of the Mott-Schottky plots at the surface of Sb-doped SnO2 films. It was quantitatively demonstrated that SnO2 films have downward band bending and surface electron accumulation. The surface band bending, unoccupied donor surface-state density, and width of the accumulation region all decrease with increasing Sb concentration.

Vasheghani Farahani, S. K.; Veal, T. D.; Mudd, J. J.; Scanlon, D. O.; Watson, G. W.; Bierwagen, O.; White, M. E.; Speck, J. S.; McConville, C. F.

2014-10-01

303

Influence of banded structure on the mechanical properties of a high-strength maraging steel

Energy Technology Data Exchange (ETDEWEB)

Chemical inhomogeneity results in the formation of banded structure in high-strength maraging steels. Segregation of titanium and molybdenum was found to be the primary cause of banded structure formation. When the concentrations of these elements increased beyond certain critical levels, bands comprising different grain sizes formed. The inclusions existed preferentially along the interface of the bands. A high-temperature homogenization treatment substantially reduced or eliminated the banded structure. The large grain size resulting from the homogenization treatment was subsequently reduced by a grain refinement treatment. The mechanical properties of the steel substantially improved following homogenization and grain refinement.

Ahmed, M.; Salam, I.; Hashmi, F.H.; Khan, A.Q. [Dr. A.Q. Khan Research Labs., Rawalpindi (Pakistan). Metallurgy Div.

1997-04-01

304

International Nuclear Information System (INIS)

Strong spatial asymmetries in the spectrum and magnitude of low-frequency (?<density fluctuations have recently been measured in the TEXT tokamak. Large amplitude, narrow-band (as low as ??/?approx. =0.1) modes have been observed on the high-field torus inside in addition to the typical broad-band (??/?approx. =1) microturbulence which is present throughout the plasma cross section. These narrow-band fluctuations are only detected at the plasma interior, being localized towards the midplane, and are not seen at the edge

305

Where the doped holes reside in cuprate superconductors has crucial implications for the understanding of the mechanism responsible for their high-temperature superconductivity. It has been generally assumed that doped holes reside in hybridized Cu dx2-y2-O p ? orbitals in the CuO2 planes, based on results of density functional band-structure calculations. Instead, we propose that doped holes in the cuprates reside in O p ? orbitals in the plane, perpendicular to the Cu -O bond, that are raised to the Fermi energy through local orbital relaxation, that is not taken into account in band-structure calculations that place the bands associated with these orbitals well below the Fermi energy. We use a dynamic Hubbard model to incorporate the orbital relaxation degree of freedom and find in exact diagonalization of a small Cu4O4 cluster that holes will go to the O p ? orbitals for relaxation energies comparable to what is expected from atomic properties of oxygen anions. The bandwidth of this band becomes significantly smaller than predicted by band-structure calculations due to the orbital relaxation effect. Within the theory of hole superconductivity the heavy hole carriers in this almost full band will pair and drive the system superconducting through lowering of their quantum kinetic energy.

Hirsch, J. E.

2014-11-01

306

Hyperspectral bands prediction based on inter-band spectral correlation structure

Hyperspectral imaging has been widely studied in many applications; notably in climate changes, vegetation, and desert studies. However, such kind of imaging brings a huge amount of data, which requires transmission, processing, and storage resources for both airborne and spaceborne imaging. Compression of hyperspectral data cubes is an effective solution for these problems. Lossless compression of the hyperspectral data usually results in low compression ratio, which may not meet the available resources; on the other hand, lossy compression may give the desired ratio, but with a significant degradation effect on object identification performance of the hyperspectral data. Moreover, most hyperspectral data compression techniques exploits the similarities in spectral dimensions; which requires bands reordering or regrouping, to make use of the spectral redundancy. In this paper, we analyze the spectral cross correlation between bands for AVIRIS and Hyperion hyperspectral data; spectral cross correlation matrix is calculated, assessing the strength of the spectral matrix, we propose new technique to find highly correlated groups of bands in the hyperspectral data cube based on "inter band correlation square", and finally, we propose a new technique of band regrouping based on correlation values weights for different group of bands as network of correlation.

Ahmed, Ayman M.; Sharkawy, Mohamed El.; Elramly, Salwa H.

2013-02-01

307

Solar spectral fine structure in 18-23 GHz band

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english On 30th June 1989 high sensitivity-spectral resolution observations of solar radio bursts were carried out in the frequency range of 18 - 23 GHz. The burst observed at 17:46 UT was different from the 60 bursts observed so far in the sense that it exhibited a frequency fine structure superimposed on [...] the ongoing burst in its rising phase, i.e. an additional enhancement of the flux density of the order of 10 SFU, observed only in the 21 and 22 GHz frequency channels, lasting for about 4 s. Interaction of an emerging loop with an adjacent loop accelerated particles in that loop from which the broadband burst was emitted due to the gyrosynchrotron emission. The observed fine structure is interpreted as due to thermal gyro-emission at 6th harmonic of the gyrofrequency originated from a hot kernel with short lifetime located at the top of emerging loop. We derived the hot kernel source parameters, th e temperature as 8 ´10(7) K, the magnetic field as 1250 G and the density as 5 ×10(12) cm-3.

J. R., Cecatto; K. R., Subramanian; H. S., Sawant.

1999-09-01

308

Band structure, metallization and superconductivity of GaP and GaN under high pressure

International Nuclear Information System (INIS)

The electronic band structure, metallization, structural phase transition and superconductivity of cubic zinc blende-type GaP and GaN are investigated. The equilibrium lattice constant, bulk modulus and pressure at which the compounds undergo a structural phase transition from ZnS structure to NaCl structure are predicted from the total energy calculations. The density of states at the Fermi level (N(EF)) is enhanced after metallization, which leads to the superconductivity in GaP and GaN. The superconducting transition temperatures (Tc) of GaP and GaN are obtained as a function of pressure for both the ZnS and NaCl structures and GaP and GaN come under the category of pressure-induced superconductors. When pressure is increased Tc increases in both the normal and high-pressure structures. The dependence of Tc on the electron-phonon mass enhancement factor ? shows that GaP and GaN are electron-phonon-mediated superconductors. Also, it is found that GaP and GaN retained in their normal structure under high pressure give appreciably high Tc. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

309

True photonic band-gap mode-control in VCSEL structures

DEFF Research Database (Denmark)

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.

Romstad, F.; Madsen, M.

2003-01-01

310

Vector k small middle dotp approach for photonic band structures

We point out that k small middle dotp treatments of photonic band gap materials based on the usual master equation must employ not only the physical photonic band solutions of that equation, but also unphysical solutions, in order to form a complete set. Nonetheless, it is possible to construct correct k small middle dotp expressions for the group velocity and its dispersion in terms of matrix elements involving only the photonic band solutions. PMID:11089125

Sipe

2000-10-01

311

Nuclear energy density optimization: Shell structure

Nuclear density functional theory is the only microscopical theory that can be applied throughout the entire nuclear landscape. Its key ingredient is the energy density functional. In this work, we propose a new parameterization UNEDF2 of the local Skyrme energy density functional. The functional optimization is carried out using the POUNDerS optimization algorithm within the framework of the Skyrme Hartree-Fock-Bogoliubov theory. Compared to the previous parameterization UNEDF1, restrictions on the tensor term of the energy density have been lifted, yielding the most general form of the Skyrme energy density functional up to second order in derivatives of the one-body local density. In order to impose constraints on all the parameters of the functional, selected data on single-particle splittings in spherical doubly-magic nuclei have been included into the experimental dataset. The agreement with both bulk and spectroscopic nuclear properties achieved by the resulting UNEDF2 parameterization is comparable wi...

Kortelainen, M; Nazarewicz, W; Olsen, E; Reinhard, P -G; Sarich, J; Schunck, N; Wild, S M; Davesne, D; Erler, J; Pastore, A

2013-01-01

312

International Nuclear Information System (INIS)

This paper presents a three-dimensional particle-in-cell (PIC) simulation of a Ka-band relativistic Cherenkov source with a slow wave structure (SWS) consisting of metal photonic band gap (PBG) structures. In the simulation, a perfect match layer boundary is employed to absorb passing band modes supported by the PBG lattice with an artificial metal boundary. The simulated axial field distributions in the cross section and surface of the SWS demonstrate that the device operates in the vicinity of the ? point of a TM01-like mode. The Fourier transformation spectra of the axial fields as functions of time and space show that only a single frequency appears at 36.27 GHz, which is in good agreement with that of the intersection of the dispersion curve with the slow space charge wave generated on the beam. The simulation results demonstrate that the SWS has good mode selectivity

313

High spin structure in a coupled bands model

International Nuclear Information System (INIS)

A coupled bands model proposed earlier is developed. It is a modification of the band crossing models with Coriolis interaction and conserves rotational invariance. It is applied to the phenomenological description of energies and transition rates for crossing rotational bands in even-even nuclei. Information on the strengths of band interactions has been deduced by a fit to experimental energies. The strange clockwise circling dependence of reduced transition probabilities on squared rotational frequencies in the yrast line has been described, and information on interband E2 moment matrix elements has been deduced by a fit to experimental transition rates. (author)

314

Band structure in even-mass nuclei around A = 70

International Nuclear Information System (INIS)

Studies of in-beam gamma rays from heavy-ion induced reactions were used to extensively explore the higher spin states of some of the even-mass nuclei around mass 70. This not only allowed observation of higher members of the ground-state band (up to spins of to 16), but also revealed a surprising variety of other bands. The various bands in such nuclei are reviewed, and some possible theoretical explanations are discussed. Five types of bands are considered. 32 references

315

Coulomb (or strong) correlation effects in the band structure of Eu2O3

International Nuclear Information System (INIS)

Two band-structure calculations of Eu2O3 are performed, one of them in the metallic phase and the other in the semiconducting phase; the difference between the corresponding band Hamiltonians is the occupation potential U(r) that includes the Coulomb correlation effects. On both phases, the 4f occupied bands are mixed with the 2p bands; this fact broadens these 4f bands so that its calculation is possible in this complex crystal lattice T/sub h/7. In the semiconducting band structure, the 4f empty bands are localized in the d-zone energy showing (4f, 5d) hybridization that produces a broadening still greater than for the 4f occupied bands. The semiconducting gap is GAMMA1+ - H1-and its value is 3.7 eV. (author)

316

Directory of Open Access Journals (Sweden)

Full Text Available Transmission coefficient, eigen states and tunneling current density of a potentially symmetric quantum double barrier structure has been numerically computed using transfer matrix technique for qualitative analysis of resonant tunneling probability when realistic band structure of higher band gap material is taken into account. GaAs/AlxGa1-xAs material composition is taken as an example for calculation, and thickness of the barrier and well regions are varied along with material compositions of AlxGa1-xAs to study the effect on electrical parameters; and also to observe the existence of quasi-bound states. Effective mass mismatch at junctions is considered following envelope function approximation, and conduction band discontinuity is taken into account for computational purpose. Under low biasing condition, negative differential regions (NDR can be obtained which speaks in favor of tunneling current.

Arpan Deyasi and Gourab Kumar Ghosh

2012-12-01

317

Novel semiconductor solar cell structures: The quantum dot intermediate band solar cell

International Nuclear Information System (INIS)

The Quantum Dot Intermediate Band Solar Cell (QD-IBSC) has been proposed for studying experimentally the operating principles of a generic class of photovoltaic devices, the intermediate band solar cells (IBSC). The performance of an IBSC is based on the properties of a semiconductor-like material which is characterised by the existence of an intermediate band (IB) located within what would otherwise be its conventional bandgap. The improvement in efficiency of the cell arises from its potential (i) to absorb below bandgap energy photons and thus produce additional photocurrent, and (ii) to inject this enhanced photocurrent without degrading its output photo-voltage. The implementation of the IBSC using quantum dots (QDs) takes advantage of the discrete nature of the carrier density of states in a 0-dimensional nano-structure, an essential property for realising the IB concept. In the QD-IBSC, the IB arises from the confined electron states in an array of quantum dots. This paper reviews the operation of the first prototype QD-IBSCs and discusses some of the lessons learnt from their characterisation

318

Magnetic response of spin-density waves in a quasilinear half-filled Hubbard band

International Nuclear Information System (INIS)

The linear and nonlinear response of spin-density waves (SDW) to a magnetic field is studied in a quasi-one-dimensional half-filled Hubbard band within a mean-field approximation. We find that the linear transverse susceptibility remains nearly temperature independent below the SDW transition, while the longitudinal susceptibility drops exponentially with the temperature. At low fields we prove that the free energy is minimum when the field is perpendicular to the spontaneous magnetization and maximum when it is parallel. Regardless of its magnitude, a transverse field is shown to have no significant effect on the temperature dependence of the spontaneous magnetization and the susceptibility in agreement with data. On the other hand, a large longitudinal field destroys the SDW. In this case the free energy is maximum and a much lower energy is obtained by reorienting the spontaneous magnetization perpendicular to the field. The relevance of the results to the organic conductors (TMTSF)2X (tetramethyltetraselenafulvalene; X = PF6 and AsF6) is discussed

319

The Importance of Static Correlation in the Band Structure of High Temperature Superconductors

Recently we presented a new band structure for La(2-x)Sr(x)CuO(4) and other high temperature superconductors in which a second narrow band was seen to cross the primary band at the Fermi level. The existence of this second Fermi level band is in complete disagreement with the commonly accepted LDA band structure. Yet it provided a crucial piece of physics which led to an explanation for superconductivity and other unusual phenomena in these materials. In this work we present...

Perry, Jason K.

1999-01-01

320

Collective band structures of neutron-rich 112,114,116Pd nuclei

International Nuclear Information System (INIS)

The high spin level structures in very neutron-rich nuclei, 112,114,116Pd have been studied by observing the prompt ?-rays from the spontaneous fission of 252Cf. The ground state bands are expanded up to 16+ and the ?-vibrational bands have been identified. New octupole bands possibly based on the proton configurations are also observed in 114,116Pd. The collective band behavior is discussed

321

Self-consistent treatment of v-groove quantum wire band structure in no parabolic approximation

Directory of Open Access Journals (Sweden)

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.

Crnjanski Jasna V.

2004-01-01

322

The Born effective charge tensors of Barium Titanate have been calculated for each of its 4 phases. Large effective charges of Ti and O, also predicted by shell model calculations and made plausible by a simplified model, reflect the partial covalent character of the chemical bond. A band by band decomposition confirms that orbital hybridization is not restricted to Ti and O atoms but also involves Ba which appears more covalent than generally assumed. Our calculations reveal a strong dependence of the effective charges on the atomic positions contrasting with a relative insensitivity on isotropic volume changes.

Ghosez, P; Lambin, P; Michenaud, J P; Ghosez, Ph.; Lambin, Ph.

1994-01-01

323

Complex band structure of topologically protected edge states

One of the great successes of modern condensed matter physics is the discovery of topological insulators (TIs). A thorough investigation of their properties could bring such materials from fundamental research to potential applications. Here, we report on theoretical investigations of the complex band structure (CBS) of two-dimensional (2D) TIs. We utilize the tight-binding form of the Bernevig, Hughes, and Zhang model as a prototype for a generic 2D TI. Based on this model, we outline the conditions that the CBS must satisfy in order to guarantee the presence of topologically protected edge states. Furthermore, we use the Green's function technique to show how these edge states are localized, highlighting the fact that the decay of the edge-state wave functions into the bulk of a TI is not necessarily monotonic and, in fact, can exhibit an oscillatory behavior that is consistent with the predicted CBS of the bulk TI. These results may have implications for electronic and spin transport across a TI when it is used as a tunnel barrier.

Dang, Xiaoqian; Burton, J. D.; Kalitsov, Alan; Velev, Julian P.; Tsymbal, Evgeny Y.

2014-10-01

324

Global Kinetic Modeling of Banded Electron Structures in the Plasmasphere

Significant fluxes of 10 eV to 30 keV electrons have been detected in the plasmasphere, appearing as banded structures in energy with broad spatial extents and slowly evolving over several days. It is thought that these populations are decaying plasma sheet electrons injected into the corotating region of near-Earth space. This capture can occur when the convective electric field drops rapidly and the Alfven boundary suddenly outward, trapping the inner edge of the plasma sheet along closed drift paths. Our bounce-averaged kinetic model of superthermal electron transport is able to simulate this capture and the subsequent drift, diffusion, and decay of the plasma cloud. Results of this simulation will be shown and discussed, from the initial injection during the elevated convection to the final loss of the particles. It is thought that not only Coulomb collisions but also wave-particle interactions play a significant role in altering the plasma cloud. Quasilinear diffusion is currently being incorporated into the model and the importance of this mechanism will be examined. Also, the high anisotropy of the trapped population could be unstable and generate plasma waves. These and other processes will be investigated to determine the final fate of the cloud and to quantify where, how, and when the energy of the plasma cloud is deposited. Comparisons with CRRES observations of these events are shown to verify the model and explain the data.

Liemohn, M. W.; Khazanov, G. V.

1997-01-01

325

Graphene band structure and its 2D Raman mode

High-precision simulations are used to generate the 2D Raman mode of graphene under a range of screening conditions and laser energies EL. We reproduce the decreasing trend of the 2D mode FWHM vs EL and the nearly linearly increasing dispersion ??2D/?EL seen experimentally in freestanding (unscreened) graphene, and propose relations between these experimentally accessible quantities and the local, two-dimensional gradients |? | of the electronic and TO phonon bands. In light of state-of-the-art electronic structure calculations that acutely treat the long-range e-e interactions of isolated graphene and its experimentally observed 2D Raman mode, our calculations determine a 40% greater slope of the TO phonons about K than given by explicit phonon measurements performed in graphite or GW phonon calculations in graphene. We also deduce the variation of the broadening energy ? [EL] for freestanding graphene and find a nominal value ? ˜140 meV, showing a gradually increasing trend for the range of frequencies available experimentally.

Narula, Rohit; Reich, Stephanie

2014-08-01

326

We present an experimental study of the emission characteristics and photonic band structure of a distributed feedback polymer laser, based on the material poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]. We use measurements of the photonic band dispersion to explain how the substrate microstructure modifies both spontaneous and stimulated emission. The lasing structure exhibits a one-dimensional photonic band gap around 610 nm, with lasing occurring at one of the two associated band edges. The band edge (frequency) selection mechanism is found to be a difference in the level of output coupling of the modes associated with the two band edges. This is a feature of the second-order distributed feedback mechanism we have employed and is clearly evident in the measured photonic band structure.

Turnbull, G. A.; Andrew, P.; Jory, M. J.; Barnes, W. L.; Samuel, I. D. W.

2001-09-01

327

Electronic density of states and the x-ray photoelectron spectra of the valence band of Cu-Pd alloys

International Nuclear Information System (INIS)

We present self-consistent-field Korringa-Kohn-Rostoker coherent-potential-approximation calculations of the electronic density of states of random Cu/sub c/Pd/sub 1-c/ alloys. We find strong hybridization of the palladium d bands with the copper d bands over the entire concentration range. We do not obtain a palladium virtual bound state for the copper-rich alloys and therefore contradict the interpretation generally placed on valence-band x-ray photoelectron spectroscopy (XPS) spectra for Cu-Pd. Nevertheless, our first-principles calculations of the XPS spectra are in excellent agreement with recent measurements, and we discuss why this is so. Furthermore, we compare our density of states at the Fermi energy with specific-heat measurements

328

Electronic structure and band parameters for ZnX (X = O, S, Se, Te)

First-principles density-functional calculations have been performed for zinc monochalcogenides with zinc-blende- and wurtzite-type structures. It is shown that the local-density approximation underestimates the band gap, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field splitting energy. Without spinorbit coupling, the order of the states at the top of VB is found to be normal for all the ZnX phases considered. Upon inclusion of the spinorbit coupling in calculations, ZnO in zinc-blende- and wurtzite-type phases become anomalous. It is shown that the Zn-3d electrons are responsible for the anomalous order. The effective masses of electrons and holes have been calculated and found that holes are much anisotropic and heavier than the electrons in agreement with experimental findings. The typical errors in calculated band gaps and related parameters originate from strong Coulomb correlations, which are found to be highly significant in ZnO. The LDA+U approach is found to corre...

Karazhanov, S Z; Grossner, U; Kjekhus, A; Fjellvag, H; Svensson, B G

2006-01-01

329

Atomic and electronic structures of novel ternary and quaternary narrow band-gap semiconductors

This thesis concerns mainly with atomic and electronic structures of novel ternary and quaternary chalcogenide narrow band-gap semiconductors which are of great interest for infrared devices, photovoltaics, and high temperature thermoelectrics. More specifically, it presents studies of charge ordering and self-assembled nanostructures in a class of quaternary systems and their phase diagram, defect clustering and nanostructure formation in bulk thermoelectrics, atomic and electronic structures of ternary chalcogenides, and nature of defect states in narrow band-gap semiconductors. Studies are carried out using Monte Carlo (MC) and ab initio methods to understand the nanostructuring phenomenon observed in AgPbmSbTem+2 and similar systems. MC simulations in these quaternaries using an ionic model show a distinct phase diagram and a variety of structural orderings depending on the concentration of the monovalent and trivalent atoms as a result of the long-range nature of the Coulomb interaction. Ab initio density functional theory (DFT) based calculations also show that monovalent and trivalent impurities in PbTe, SnTe, and GeTe-based bulk thermoelectric materials like to come close to each other and form clusters or some sort of embedded nanostructures. Interplay of atomic and electronic structures and band gap formation in I-V-VI2 and TI-based III-V-VII2 tertiary chalcogenides (I=Ag, Cu, Au, Na, K; V=As, Sb, Bi; VII=S, Se, Te) are studied using ab initio electronic structure calculations. These calculations have been able to identify low energy ordered structures which are consistent with experiments. Several intriguing physical properties of these materials can be understood in terms of the calculated electronic structure. This thesis suggests how to modify a certain ternary by replacing its constituting elements) such that the electronic structure shows desired features for different applications. Comprehensive studies of the nature of defect-induced electronic states associated with a large class of substitutional impurities and native point defects in PbTe, SnTe, and GeTe are carried out using DFT and supercell models. Calculations are also carried out in PbTe thin films and nanoclusters to study how the defect states change in going from one geometry to another. This thesis also concerns with energetics of the defects, particularly defect formation energy, which may be able to give some information on the doping mechanism and the distribution of the defects in these systems. Based on the calculated electronic structures, one can explain the peculiar properties of PbTe doped with group III (Ga, In, T1) impurities and the observed transport properties of PbTe, SnTe, and GeTe-based thermoelectrics.

Hoang, Khang

330

The full-potential linearized augmented plane wave method with the generalized gradient approximation for the exchange-correlation potential (FLAPW-GGA) is used to predict the electronic and elastic properties of the newly discovered superconducting nanolaminate Ti2InC. The band structure, density of states and Fermi surface features are discussed. The optimized lattice parameters, independent elastic constants, bulk and shear moduli, compressibility are evaluated and discus...

Shein, I. R.; Ivanovskii, A. L.

2008-01-01

331

Systematic design of phononic band-gap materials and structures by topology optimization

DEFF Research Database (Denmark)

Phononic band-gap materials prevent elastic waves in certain frequency ranges from propagating, and they may therefore be used to generate frequency filters, as beam splitters, as sound or vibration protection devices, or as waveguides. In this work we show how topology optimization can be used to design and optimize periodic materials and structures exhibiting phononic band gaps. Firstly, we optimize infinitely periodic band-gap materials by maximizing the relative size of the band gaps. Then, finite structures subjected to periodic loading are optimized in order to either minimize the structural response along boundaries (wave damping) or maximize the response at certain boundary locations (waveguiding).

Sigmund, Ole; Jensen, Jakob SØndergaard

2003-01-01

332

Efficiency of tunable band-gap structures for single-photon emission

The efficiency of recently proposed single-photon emitting sources based on tunable planar band-gap structures is examined. The analysis is based on the study of the total and ``radiative'' decay rates, the expectation value of emitted radiation energy and its collimating cone. It is shown that the scheme operating in the frequency range near the defect resonance of a defect band-gap structure is more efficient than the one operating near the band edge of a perfect band-gap structure.

Dung, H T; Welsch, D G; Dung, Ho Trung; Kn\\"{o}ll, Ludwig; Welsch, Dirk-Gunnar

2004-01-01

333

We investigate an atomic $\\Lambda$-system with one transition coupled to a laser field and a flat continuum of vacuum modes and the other transition coupled to field modes near the edge of a photonic band gap. The system requires simultaneous treatment of Markovian and non-Markovian dissipation processes, but the photonic band gap continuum can not be eliminated within a density matrix treatment. Instead we propose a formalism based on Monte-Carlo wavefunctions, and we present results relevant to an experimental characterization of a structured continuum.

Bay, S D; Mølmer, K; Bay, Soren

1997-01-01

334

Application of Hyperspectral Band Elimiation Technique to PVT Images of Composite Structures

A new approach to NDT of composite structures using Band Elimination of the analyzed image index by Hyperspectral image analysis approach is presented and discussed. The matrix Band Elimination technique allows the monitoring and analysis of a components structure based on Filtering of bands and correlation between sequentially pulsed thermal images and their indices. The technique produces several matrices resulting from frame deviation and pixel redistribution calculations for intelligent c...

Mahmoud Zaki Iskandarani

2012-01-01

335

Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of opt...

Ho, Ching-hwa; Chen, Hsin-hung

2014-01-01

336

Electronic Structure and Valence Band Spectra of Bi4Ti3O12

The x-ray photoelectron valence band spectrum and x-ray emission valence-band spectra (Ti K _beta_5, Ti L_alpha, O K_alpha) of Bi4Ti3O12 are presented (analyzed in the common energy scale) and interpreted on the basis of a band-structure calculation for an idealized I4/mmm structure of this material.

Postnikov, A. V.; Bartkowski, St; Mersch, F.; Neumann, M.; Kurmaev, E. Z.; Cherkashenko, V. M.; Nemnonov, S. N.; Galakhov, V. R.

1995-01-01

337

International Nuclear Information System (INIS)

X-ray spectra of the ZrC-ZrN solid solution valence band are obtained. Transfer properties and their temperature dependence are studied. They serve as basis for calculating main electronic structure parameters in terms of a two-band model of degenerate electrons and undegenerate holes. The energy equality of C and N atoms in interstitial octa-pores and an additive pattern of the band structure variations when carbon is replaced by nitrogen are shown

338

In this paper, the propagation of acoustic waves in the phononic crystals (PC) of 3D with rhombohedral(II) lattice is studied theoretically. The PC are constituted of nickel spheres embedded in epoxy. The calculations of the band structure and density of states are performed with the plane wave expansion method in the irreducible part of the Brillouin zone (BZ). In this study, we analyze the dependence of the band structures inside (the complete band gap width) and outside the complete band gap (negative refraction of acoustic wave) on the lattice angle in the irreducible part of the first BZ. Also the effect of lattice angle has been analyzed on the band structure of the () and (122) planes. Then, the equifrequency surface is calculated for the high symmetry point in the [111] and [100] directions. The results show that the maximum width of AEBG (0.022) in the irreducible part of the BZ of RHL2 is formed for (105?) and no AEBG is found for ? > 150?. Also, the maximum of the first and second AEBG width are 0.1076 and 0.0523 for ? = 133? in the () plane and the maximum of the first and second AEBG width are 0.1446 and 0.0998 for ? = 113? in the (122) plane. In addition, we have found that frequencies in which negative refraction occurs is constant for all lattice angles.

Aryadoust, M.; Salehi, H.

2014-12-01

339

Structural and electronic properties of Cu(In,Ga)S2 (CIGS) alloys as a function of the In/Ga ratio are studied by combining near edge X-ray absorption fine structure and extended X-ray absorption fine structure spectroscopy with density functional theory calculations. While the S absorption edge shifts with the change of the In/Ga ratio, consistently with the corresponding variation of the band gap, the edges of the other elements are largely independent of composition. Our calculations repro...

Sarmiento-pe?rez, Rafael; Botti, Silvana; Schnohr, Claudia S.; Lauermann, Iver; Rubio Secades, A?ngel; Johnson, Benjamin

2014-01-01

340

Energy Technology Data Exchange (ETDEWEB)

The detailed energy-band structure of hexagonal-close-packed technetium, corresponding to the atomic configuration 4d/sup 5/5s/sup 2/ of its seven outermost valence electrons, has been obtained throughout the Brillouin zone using the composite-wave variational version of the augmented-plane-wave (APW) method in conjunction with the X..cap alpha.. (..cap alpha.. = 0.702 99) exchange approximation for obtaining the potentials. From the band-structure data the electronic density of states (DOS) and the angular-momentum--decomposed DOS were calculated by the accurate Gilat-Raubenheimer method. These quantities were used to calculate the electron-phonon coupling constant and the transition temperature (T/sub c/) using the theories of Gaspari and Gyorffy and of McMillan. Also studied were the Fermi surface and the optical properties of Tc via the imaginary part of the interband dielectric constant for bound electrons, the latter being the first of such a study on Tc to date. The superconducting properties and the Fermi surface which could be compared to experiments show, in general, satisfactory agreement.

Chatterjee, P.

1983-04-15

341

International Nuclear Information System (INIS)

The detailed energy-band structure of hexagonal-close-packed technetium, corresponding to the atomic configuration 4d55s2 of its seven outermost valence electrons, has been obtained throughout the Brillouin zone using the composite-wave variational version of the augmented-plane-wave (APW) method in conjunction with the X? (? = 0.702 99) exchange approximation for obtaining the potentials. From the band-structure data the electronic density of states (DOS) and the angular-momentum--decomposed DOS were calculated by the accurate Gilat-Raubenheimer method. These quantities were used to calculate the electron-phonon coupling constant and the transition temperature (T/sub c/) using the theories of Gaspari and Gyorffy and of McMillan. Also studied were the Fermi surface and the optical properties of Tc via the imaginary part of the interband dielectric constant for bound electrons, the latter being the first of such a study on Tc to date. The superconducting properties and the Fermi surface which could be compared to experiments show, in general, satisfactory agreement

342

The spin polarized band structure of strained thin films of gadolinium

International Nuclear Information System (INIS)

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

343

Single Layer MoS2 Band Structure and Transport

Introduction: Ultrathin two dimensional materials have significant potential for application in nano-electronic devices. Graphene is a popular two-dimensional material due to its intrinsic high mobility. However, Graphene does not have any band gap, which makes it hard to be used for field effect transistors (FET)[1]. On the other hand, monolayer Molybdenum Disulfide (MoS2) is a thin two dimensional material with large intrinsic band gap. These characteristics and recent successful fabricatio...

Salmani-jelodar, Mehdi; Tan, Yaohua; Klimeck, Gerhard

2011-01-01

344

Disclination density in atomic structures described in curved spaces

The curvature of a space and the density of disclinations are two related quantities. There is an exact relation in 2-D spaces. We show how an approximate solution can be used in 3-D space. Applications to the ?-W structure and the Laves phase are presented The coordination number in dense random structures is explained in terms of disclination density.

Sadoc, J. F.; Mosseri, R.

1984-01-01

345

Band structure in two-dimensional fiber-air phononic crystals

International Nuclear Information System (INIS)

A two-dimensional phononic crystal (PC) composed of textile fiber and air is initially discussed in this paper, which is different from the previous PCs with rigid inclusions. The plain wave expansion method is used to calculate band structure of different PCs by altering fiber material properties and structure parameters. Numerical results show that the effect of material properties of soft fiber on band structure of phononic crystal can be ignored, while the effect of structural parameters is obvious.

346

We investigate the in-plane oscillations of relaxed-shape graphene due to externally applied tensile edge stress along both the armchair and zigzag directions. We show that the total elastic energy density is enhanced with temperature for the case of applied tensile edge stress along the zigzag direction. Thermoelectromechanical effects are treated via pseudomorphic vector potentials to analyze the influence of these coupled effects on the band structures of bilayer-graphene quantum dots. We report that the level crossing between the ground and first-excited states in the localized edge states can be achieved with accessible values of temperature. In particular, the level-crossing point extends to higher temperatures with decreasing externally applied tensile edge stress along the armchair direction. This kind of level crossing is absent in the states formed at the center of the graphene sheet due to the presence of threefold symmetry.

Prabhakar, Sanjay; Melnik, Roderick; Bonilla, Luis L.; Badu, Shyam

2014-11-01

347

Shell-like structure of valence band orbitals of silicon nanocrystals in silica glass

Energy Technology Data Exchange (ETDEWEB)

Quasi-spherical Si nanocrystals with diameters from 0.8 to 1.6 nm are embedded into amorphous SiO{sub 2} networks and further optimized using density functional methods. After relaxation the nanocrystals exhibit a smooth interface without defects to the glass matrix. Tensile strain is observed within the nanocrystalline core, with a maximum strain for sub-surface Si atoms close to the suboxide interface. We find the highest occupied states of the valence band being located at Si-Si bonds close to the interface forming a shell-like structure around the central core of the nanocrystal. The lowest unoccupied states are centered within the nanocrystal. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Kroll, Peter; Schulte, Hendrik J. [Institut fuer Anorganische Chemie, RWTH Aachen, Landoltweg 1, 52056 Aachen (Germany)

2006-05-15

348

The simulated annealing basin-hopping method incorporating the penalty function was used to predict the lowest-energy structures for ultrathin tungsten nanowires and nanotubes of different sizes. These predicted structures indicate that tungsten one-dimensional structures at this small scale do not possess B.C.C. configuration as in bulk tungsten material. In order to analyze the relationship between multi-shell geometries and electronic transfer, the electronic and structural properties of tungsten wires and tubes including partial density of state and band structures which were determined and analyzed by quantum chemistry calculations. In addition, in order to understand the application feasibility of these nanowires and tubes on nano-devices such as field emitters or chemical catalysts, the electronic stability of these ultrathin tungsten nanowires was also investigated by density functional theory calculations.

Sun, Shih-Jye; Lin, Ken-Huang; Ju, Shin-Pon; Li, Jia-Yun

2014-10-01

349

International Nuclear Information System (INIS)

The simulated annealing basin-hopping method incorporating the penalty function was used to predict the lowest-energy structures for ultrathin tungsten nanowires and nanotubes of different sizes. These predicted structures indicate that tungsten one-dimensional structures at this small scale do not possess B.C.C. configuration as in bulk tungsten material. In order to analyze the relationship between multi-shell geometries and electronic transfer, the electronic and structural properties of tungsten wires and tubes including partial density of state and band structures which were determined and analyzed by quantum chemistry calculations. In addition, in order to understand the application feasibility of these nanowires and tubes on nano-devices such as field emitters or chemical catalysts, the electronic stability of these ultrathin tungsten nanowires was also investigated by density functional theory calculations.

350

Energy Technology Data Exchange (ETDEWEB)

The simulated annealing basin-hopping method incorporating the penalty function was used to predict the lowest-energy structures for ultrathin tungsten nanowires and nanotubes of different sizes. These predicted structures indicate that tungsten one-dimensional structures at this small scale do not possess B.C.C. configuration as in bulk tungsten material. In order to analyze the relationship between multi-shell geometries and electronic transfer, the electronic and structural properties of tungsten wires and tubes including partial density of state and band structures which were determined and analyzed by quantum chemistry calculations. In addition, in order to understand the application feasibility of these nanowires and tubes on nano-devices such as field emitters or chemical catalysts, the electronic stability of these ultrathin tungsten nanowires was also investigated by density functional theory calculations.

Sun, Shih-Jye [Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan (China); Lin, Ken-Huang; Li, Jia-Yun [Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan (China); Ju, Shin-Pon, E-mail: jushin-pon@mail.nsysu.edu.tw [Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan (China); Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan (China)

2014-10-07

351

Elastic band structures of two-dimensional solid phononic crystal with negative Poisson's ratios

International Nuclear Information System (INIS)

In this paper, the elastic band structures of two-dimensional solid phononic crystals (PCs) with both negative and positive Poisson's ratios are investigated based on the finite difference domain method. Systems with different combinations of mass density ratio and shear modulus ratio, filling fractions and lattices are considered. The numerical results show that for the PCs with both large mass density ratio and shear modulus ratio, the first bandgap becomes narrower with its upper edge becoming lower as Poisson's ratio of the scatterers decreases from -0.1 to -0.9. Generally, introducing the material with a negative Poisson's ratio for scatterers will make this bandgap lower and narrower. For the PCs with large mass density ratio and small shear modulus ratio, the first bandgap becomes wider with Poisson's ratio of the scatterers decreasing and that of the host increasing. It is easy to obtain a wide low-frequency bandgap by embedding scatterers with a negative Poisson's ratio into the host with a positive Poisson's ratio. The PCs with large filling fractions are more sensitive to the variations of Poisson's ratios. Use of negative Poisson's ratio provides us a way of tuning bandgaps.

352

Collective motions and band structures in A = 60 to 80, even--even nuclei

International Nuclear Information System (INIS)

Evidence for and the theoretical understanding of the richness of the collective band structures as illustrated by at least seven bands seen in levels of 68Ge, 74Se are reviewed. The experimental data on even-even nuclei in the A = 60 to 80 region have now revealed a wide variety of collective bands with different structures. The even parity yrast cascades alone are seen to involve multiple collective structures. In addition to the ground-state bands, strong evidence is presented for both neutron and proton rotation-aligned bands built on the same orbital, (g9/2)2, in one nucleus. Several other nuclei also show the crossing of RAL bands around the 8+ level in this region. Evidence continues to be strong experimentally and supported theoretically that there is some type of shape transition and shape coexistence occurring now both in the Ge and Se isotopes around N = 40. Negative parity bands with odd and even spins with very collective nature are seen in several nuclei to high spin. These bands seem best understood in the RAL model. Very collective bands with ?I = 1, extending from 2+ to 9+ are seen with no rotation-alignment. The purity of these bands and their persistence to such high spin establish them as an independent collective mode which is best described as a gamma-type vibration band in a deformed nucleus. In addition to all of the above bands, new bands are seen in 76Kr and 74Se. The nature of these bands is not presently known. 56 references

353

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding and impurity Green's function approaches. Density-functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three sp2? dangling bonds adjacent to the vacancy introduce localized states (V?) in the mid-gap region, which split due to the crystal field and a Jahn-Teller distortion, while the pz? states introduce a sharp resonance state (V?) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the ? and the ? states, so that these bands are clearly identifiable in the calculated band structure. As to the magnetic moment of the vacancy, the Hund's rule coupling aligns the spins of the four localized V?1??, V?2? and V?? electrons, resulting in an S = 1 state, with a magnetic moment of 2?B, which is reduced by about 0.3?B due to the anti-ferromagnetic spin polarization of the ? band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of 1.7?B. Using the Lippmann-Schwinger equation, we reproduce the well-known ˜1/r decay of the localized V? wave function with distance, and in addition, find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the V? wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.

Nanda, B. R. K.; Sherafati, M.; Popovi?, Z. S.; Satpathy, S.

2012-08-01

354

We report quasiparticle-energy calculations of the electronic bandstructure as measured by valence-band photoemission for selected II-VI compounds and group-III-nitrides. By applying GW as perturbation to the ground state of the fictitious, non-interacting Kohn-Sham electrons of density-functional theory (DFT) we systematically study the electronic structure of zinc-blende GaN, ZnO, ZnS and CdS. Special emphasis is put on analysing the role played by the cation semicore d-el...

Rinke, Patrick; Qteish, Abdallah; Neugebauer, Joerg; Freysoldt, Christoph; Scheffler, Matthias

2005-01-01

355

Estimating tropical forest structure using LIDAR AND X-BAND INSAR

Tropical forests are considered the most structurally complex of all forests and are experiencing rapid change due to anthropogenic and climatic factors. The high carbon stocks and fluxes make understanding tropical forests highly important to both regional and global studies involving ecosystems and climate. Large and remote areas in the tropics are prime targets for the use of remotely sensed data. Radar and lidar have previously been used to estimate forest structure, with an emphasis on biomass. These two remote sensing methods have the potential to yield much more information about forest structure, specifically through the use of X-band radar and waveform lidar data. We examined forest structure using both field-based and remotely sensed data in the Tapajos National Forest, Para, Brazil. We measured multiple structural parameters for about 70 plots in the field within a 25 x 15 km area that have TanDEM-X single-pass horizontally and vertically polarized radar interferometric data. High resolution airborne lidar were collected over a 22 sq km portion of the same area, within which 33 plots were co-located. Preliminary analyses suggest that X-band interferometric coherence decreases by about a factor of 2 (from 0.95 to 0.45) with increasing field-measured vertical extent (average heights of 7-25 m) and biomass (10-430 Mg/ha) for a vertical wavelength of 39 m, further suggesting, as has been observed at C-band, that interferometric synthetic aperture radar (InSAR) is substantially more sensitive to forest structure/biomass than SAR. Unlike InSAR coherence versus biomass, SAR power at X-band versus biomass shows no trend. Moreover, airborne lidar coherence at the same vertical wavenumbers as InSAR is also shown to decrease as a function of biomass, as well. Although the lidar coherence decrease is about 15% more than the InSAR, implying that lidar penetrates more than InSAR, these preliminary results suggest that X-band InSAR may be useful for structure and biomass estimation over large spatial scales not attainable with airborne lidar. In this study, we employed a set of less commonly used lidar metrics that we consider analogous to field-based measurements, such as the number of canopy maxima, measures of canopy vegetation distribution diversity and evenness (entropy), and estimates of gap fraction. We incorporated these metrics, as well as lidar coherence metrics pulled from discrete Fourier transforms of pseudowaveforms, and hypothetical stand characteristics of best-fit synthetic vegetation profiles into multiple regression analysis of forest biometric properties. Among simple and complex measures of forest structure, ranging from tree density, diameter at breast height, and various canopy geometry parameters, we found strong relationships with lidar canopy vegetation profile parameters. We suggest that the sole use of lidar height is limited in understanding biomass in a forest with little variation across the landscape and that there are many parameters that may be gleaned by lidar data that inform on forest biometric properties.

Palace, M. W.; Treuhaft, R. N.; Keller, M. M.; Sullivan, F.; Roberto dos Santos, J.; Goncalves, F. G.; Shimbo, J.; Neumann, M.; Madsen, S. N.; Hensley, S.

2013-12-01

356

A Half-Metallic Band Structure and Fe2MnZ (Z=Al, Si, P)

Band calculation results show that Heusler compounds Fe2MnZ (Z=Al,Si,P) have a band gap near the Fermi energy for one spin direction. Except for Fe2MnSi, the compounds does not have a half-metallic character because there is a slight density of states at the Fermi energy in the minority-spin state. An analysis of density of states indicates that there exists sufficiently strong d-d hybridization between metal atoms and weak p-d hybridization between Si and metal atoms. Due to this weakness, the replacement of Si by Al or P does not cause considerable change in the DOS structure. However, the weak hybridization defines the electronic character of the system.

Fujii, Shinpei; Ishida, Shoji; Asano, Setsuro

1995-01-01

357

Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to the presence of time-reversal symmetry and the absence of inversion symmetry, making them promising candidates for valleytronics. In contrast, in bilayer TMDCs both symmetries are present and these desirable valley-selective properties are lost. Here, by using density-functional tight-binding electronic structure simulations and revised periodic boundary conditions, we show that bending of bilayer MoS2 sheets breaks band degeneracies and localizes states on separate layers due to bending-induced strain gradients across the sheets. We propose a strategy for employing bending deformations in bilayer TMDCs as a simple yet effective means of dynamically and reversibly tuning their band gaps while simultaneously tuning valley-selective physics.

Koskinen, Pekka; Fampiou, Ioanna; Ramasubramaniam, Ashwin

2014-05-01

358

Topological equivalence of crystal and quasicrystal band structures

A number of recent articles have reported the existence of topologically nontrivial states and associated end states in one-dimensional incommensurate lattice models that would usually only be expected in higher dimensions. Using an explicit construction, we here argue that the end states have precisely the same origin as their counterparts in commensurate models and that incommensurability does not in fact provide a meaningful connection to the topological classification of systems in higher dimensions. In particular, we show that it is possible to smoothly interpolate between states with commensurate and incommensurate modulation parameters without closing the band gap and without states crossing the band gap.

Madsen, Kevin A.; Bergholtz, Emil J.; Brouwer, Piet W.

2013-09-01

359

Structural-configurated magnetic plasmon bands in connected ring chains

Magnetic resonance coupling between connected split ring resonators (SRRs) and magnetic plasmon (MP) excitations in the connected SRR chains were theoretically studied. By changing the connection configuration, two different coupling behaviors were observed, and therefore two kinds of MP bands were formed in the connected ring chains, accordingly. These MPs were revealed with positive and negative dispersion for the homo- and anti-connected chain, respectively. Notably, these two MP modes both have wide bandwidth due to the conductive coupling. Moreover, the anti-connected chain is found supporting a novel negative propagating wave with a wide band starting from zero frequency, which is a fancy phenomenon in one-dimensional system.

Li, T; Li, C; Liu, H; Wang, S M; Cao, J X; Zhu, S N; Zhang, X

2009-01-01

360

Band structure effects for dripped neutrons in neutron star crust

The outer layers of a neutron star are supposed to be formed of a solid Coulomb lattice of neutron rich nuclei. At densities above neutron drip density (about one thousandth of nuclear saturation density), this lattice is immersed in a neutron fluid. Bragg scattering of those dripped neutrons by the nuclei which has been usually neglected is investigated, within a simple mean field model with Bloch type boundary conditions. The main purpose of this work is to provide some es...

Chamel, Nicolas

2004-01-01

361

Knowledge about the intrinsic electronic properties of water is imperative for understanding the behaviour of aqueous solutions that are used throughout biology, chemistry, physics, and industry. The calculation of the electronic band gap of liquids is challenging, because the most accurate ab initio approaches can be applied only to small numbers of atoms, while large numbers of atoms are required for having configurations that are representative of a liquid. Here we show that a high-accuracy value for the electronic band gap of water can be obtained by combining beyond-DFT methods and statistical time-averaging. Liquid water is simulated at 300 K using a plane-wave density functional theory molecular dynamics (PW-DFT-MD) simulation and a van der Waals density functional (optB88-vdW). After applying a self-consistent GW correction the band gap of liquid water at 300 K is calculated as 7.3 eV, in good agreement with recent experimental observations in the literature (6.9 eV). For simulations of phase transformations and chemical reactions in water or aqueous solutions whereby an accurate description of the electronic structure is required, we suggest to use these advanced GW corrections in combination with the statistical analysis of quantum mechanical MD simulations. PMID:25388568

Fang, Changming; Li, Wun-Fan; Koster, Rik S; Klimeš, Ji?í; van Blaaderen, Alfons; van Huis, Marijn A

2014-12-01

362

Electronic band structures of graphene nanoribbons with self-passivating edge reconstructions

Energy Technology Data Exchange (ETDEWEB)

Using the nearest-neighbor tight-binding approach we study the electronic band structures of graphene nanoribbons with self-passivating edge reconstructions. For zigzag ribbons the edge reconstruction moves both the Fermi energy and the flat band down by several hundred meV, and the flat band is always found to be below the Fermi energy. The states featured by the flat band are shown to be mainly localized at the atoms belonging to several lattice lines closest to the edges. For armchair ribbons the edge reconstruction strongly modifies the band structure in the region close to the Fermi energy, leading to the appearance of a band gap even for ribbons which were predicted to be metallic in the model of standard armchair edges. The gap widths are, however, strongly different in magnitude and behave in different ways regarding the ribbon width.

Tung Nguyen, L; Huy Pham, C; Lien Nguyen, V, E-mail: nvlien@iop.vast.ac.vn [Theoretical and Computational Physics Department, Institute of Physics, VAST, 10 Dao Tan, Ba Dinh District, Hanoi 10000 (Viet Nam)

2011-07-27

363

We have examined the interaction of a blocked slip band and a grain boundary in deformed titanium using high-resolution electron backscatter diffraction and atomic force microscopy. From these observations, we have deduced the active dislocation types and assessed the dislocation reactions involved within a selected grain. Dislocation sources have been activated on a prism slip plane, producing a planar slip band and a pile-up of dislocations in a near screw alignment at the grain boundary. T...

Britton, Tb; Wilkinson, Aj

2012-01-01

364

Novel band structures in silicene on monolayer zinc sulfide substrate

Opening a sizable band gap in the zero-gap silicene without lowering the carrier mobility is a key issue for its application in nanoelectronics. Based on ?rst-principles calculations, we find that the interaction energies are in the range of -0.09?0.3?eV?per Si atom, indicating a weak interaction between silicene and ZnS monolayer and the ABZn stacking is the most stable pattern. The band gap of silicene can be effectively tuned ranging from 0.025 to 1.05?eV in silicene and ZnS heterobilayer (Si/ZnS HBL). An unexpected indirect-direct band gap crossover is also observed in HBLs, dependent on the stacking pattern, interlayer spacing and external strain effects on silicene. Interestingly, the characteristics of Dirac cone with a nearly linear band dispersion relation of silicene can be preserved in the ABS pattern which is a metastable state, accompanied by a small electron effective mass and thus the carrier mobility is expected not to degrade much. These provide a possible way to design effective FETs out of silicene on a ZnS monolayer.

Li, Sheng-shi; Zhang, Chang-wen; Yan, Shi-shen; Hu, Shu-jun; Ji, Wei-xiao; Wang, Pei-ji; Li, Ping

2014-10-01

365

Electronic transitions in GdN band structure

Using the near-infrared (NIR) absorbance spectroscopy, electronic transitions and spin polarization of the GdN epitaxial film have been investigated; and the GdN epitaxial film was grown by a reactive rf sputtering technique. The GdN film exhibited three broad bands in the NIR frequency regimes; and those bands are attributable primarily to the minority and majority spin transitions at the X-point and an indirect transition along the ?-X symmetric direction of GdN Brillouin zone. We experimentally observe a pronounced red-shift of the indirect band gap when cooling down below the Curie temperature which is ascribed to the orbital-dependent coulomb interactions of Gd-5dxy electrons, which tend to push-up the N-2p bands. On the other hand, we have evaluated the spin polarization of 0.17 (±0.005), which indicates that the GdN epitaxial film has almost 100% spin-polarized carriers. Furthermore, the experimental result of GdN electronic transitions are consistent with the previous reports and are thus well-reproduced. The Arrott plots evidenced that the Curie temperature of GdN film is 36 K and the large spin moment is explained by the nitrogen vacancies and the intra-atomic exchange interaction.

Vidyasagar, R.; Kita, T.; Sakurai, T.; Ohta, H.

2014-05-01

366

Electronic transitions in GdN band structure

International Nuclear Information System (INIS)

Using the near-infrared (NIR) absorbance spectroscopy, electronic transitions and spin polarization of the GdN epitaxial film have been investigated; and the GdN epitaxial film was grown by a reactive rf sputtering technique. The GdN film exhibited three broad bands in the NIR frequency regimes; and those bands are attributable primarily to the minority and majority spin transitions at the X-point and an indirect transition along the ?-X symmetric direction of GdN Brillouin zone. We experimentally observe a pronounced red-shift of the indirect band gap when cooling down below the Curie temperature which is ascribed to the orbital-dependent coulomb interactions of Gd-5dxy electrons, which tend to push-up the N-2p bands. On the other hand, we have evaluated the spin polarization of 0.17 (±0.005), which indicates that the GdN epitaxial film has almost 100% spin-polarized carriers. Furthermore, the experimental result of GdN electronic transitions are consistent with the previous reports and are thus well-reproduced. The Arrott plots evidenced that the Curie temperature of GdN film is 36?K and the large spin moment is explained by the nitrogen vacancies and the intra-atomic exchange interaction.

367

The structural and electronic properties of CdTe(001), CdSe(001), and ZnSe(001) C(2 x 2) reconstructed surfaces have been investigated through the use of first-principles calculations. To simulate the surface, we employed the slab model. Using the experimentally determined lattice parameters as inputs, we relaxed the internal atomic positions of the outer atomic layers. We demonstrate that our model appropriately reproduces both the surface structural parameters and the known electronic properties found for these semiconductor compounds in bulk. Finally, we discuss our results of the projected bulk bands and the surface and resonance states found for these surfaces.

Rubio-Ponce, A.; Olguín, D.

2015-01-01

368

We derive the equations for approximate particle number projection based on mean field wave functions with finite range density dependent forces. As an application ground bands of even-A superdeformed nuclei in the A=150 and A=190 regions are calculated with the Gogny force. We discuss nuclear properties such as quadrupole moments, moments of inertia and quasiparticle spectra, among others, as a function of the angular momentum. We obtain a good overall description.

Valor, A.; Egido, J. L.; Robledo, L. M.

1999-01-01

369

Superconducting and optical properties of ?-zirconium from its augmented-plane-wave band structure

International Nuclear Information System (INIS)

The detailed electronic energy band structure of hexagonal close-packed ?-zirconium, corresponding to the atomic configuration of 4d25s2 of its four outermost valence electrons, has been computed by the composite-wave variational version of the augmented-plane-wave(APW) method in conjunction with the X?(? = 0.70424) exchange approximation for obtaining the potentials. From these data the electronic density of states and its angular-momentum-decomposed components have been obtained by the Raubenheimer-Gilat method. These quantities are required in order to calculate the electron-phonon interaction parameter (lambda) and the superconducting transition temperature (Tsub(c)) within the framework of the theories of Gaspari and Gyorffy and McMillan. A study of the variation of Tsub(c) with the Coulomb pseudopotential (?*) revealed that ?* = 0.1 yields the best agreement between theory and experiment for ?-Zr. Also studied from the energy band data are the optical properties of ?-Zr via the variation of the imaginary part of the interband dielectric function for bound electrons and the corresponding optical conductivity (?) as a function of the photon energy (?). The results indicate five peaks in the ?-? curve, the positions of which on the energy scale are in fair agreement with experiment. (author)

370

Energy Technology Data Exchange (ETDEWEB)

The detailed electronic energy band structure of hexagonal close-packed ..cap alpha..-zirconium, corresponding to the atomic configuration of 4d/sup 2/5s/sup 2/ of its four outermost valence electrons, has been computed by the composite-wave variational version of the augmented-plane-wave(APW) method in conjunction with the X..cap alpha..(..cap alpha.. = 0.70424) exchange approximation for obtaining the potentials. From these data the electronic density of states and its angular-momentum-decomposed components have been obtained by the Raubenheimer-Gilat method. These quantities are required in order to calculate the electron-phonon interaction parameter (lambda) and the superconducting transition temperature (Tsub(c)) within the framework of the theories of Gaspari and Gyorffy and McMillan. A study of the variation of Tsub(c) with the Coulomb pseudopotential (..mu..*) revealed that ..mu..* = 0.1 yields the best agreement between theory and experiment for ..cap alpha..-Zr. Also studied from the energy band data are the optical properties of ..cap alpha..-Zr via the variation of the imaginary part of the interband dielectric function for bound electrons and the corresponding optical conductivity (sigma) as a function of the photon energy (..omega..). The results indicate five peaks in the sigma-..omega.. curve, the positions of which on the energy scale are in fair agreement with experiment.

Chatterjee, D. (Indian Association for the Cultivation of Science, Jadavpur. Director' s Research Unit)

1984-09-01

371

InN/GaN Superlattices: Band Structures and Their Pressure Dependence

DEFF Research Database (Denmark)

Creation of short-period InN/GaN superlattices is one of the possible ways of conducting band gap engineering in the green-blue range of the spectrum. The present paper reports results of photoluminescence experiments, including pressure effects, on a superlattice sample consisting of unit cells with one monolayer of InN and 40 monolayers of GaN. The results are compared with calculations performed for different types of superlattices: InN/GaN, InGaN/GaN, and InN/InGaN/GaN with single monolayers of InN and/or InGaN. The superlattices are simulated by band structure calculations based on the local density approximation (LDA) with a semi-empirical correction for the ‘‘LDA gap error’’. A similarity is observed between the results of calculations for an InGaN/GaN superlattice (with one monolayer of InGaN) and the experimental results. This indicates that the fabricated InN quantum wells may contain some Ga atoms due to interdiffusion

Gorczyca, Iza; Suski, Tadek

2013-01-01

372

Full-band-structure calculation of Shockley-Read-Hall recombination rates in InAs

We report a calculational procedure to obtain the rate of electron-hole recombination, mediated by the Shockley-Read-Hall (SRH) mechanism. Our method uses a combination of first-principles calculations and accurate empirical band structures. First, we use ab initio calculations to identify the point defects, their densities and energy levels in the gap. Then we parametrize the tight-binding interaction between defect and the host atoms in a Green's function approach to obtain the defect levels as identified by the first-principles calculations. Finally, the resulting tight-binding Hamiltonian is used to obtain the dipole matrix element between the conduction and valence band states, mediated through the defect levels in the gap, in second-order perturbation theory. The states are integrated over the entire Brillioun zone, subject to energy and momentum conservation, to obtain the limiting lifetimes of the carriers. This method is applied to study the minority carrier lifetimes in n-doped InAs. The calculated effective lifetimes that include Auger and SRH recombinations as functions of temperature agree reasonably well with experiment. Our calculation of lifetimes in 3.5×1016 and 2.0×1016cm-3 n-doped InAs indicate that SRH is dominant at low temperatures and that the lifetimes vary between 10-8 and 10-7 s.

Krishnamurthy, Srinivasan; Berding, M. A.

2001-07-01

373

Valence band structure of HgTe/Hg1-xCdxTe single quantum wells

Properties of the valence-band structure of modulation-doped type-III HgTe/Hg1-xCdxTe(001) quantum wells (QW's) have been studied by means of magneto-transport experiments and self-consistent Hartree calculations using the full 8×8 k.p Hamiltonian in the envelope-function approximation. A metallic top gate was used in order to investigate the band structure by varying the hole concentration or the Fermi energy. This resulted in direct experimental evidence of an indirect band gap in a quantum well with an inverted band structure. The Kramers degeneracy for finite k is removed, and only one spin-orbit split valence subband is occupied due to the indirect band structure. The fourfold symmetry of the H2 valence band at finite values of k|| is also reflected in a QW with a normal band structure, whose H1 valence band has four occupied secondary maxima at finite values of k|| in addition to the central maximum.

Ortner, K.; Zhang, X. C.; Pfeuffer-Jeschke, A.; Becker, C. R.; Landwehr, G.; Molenkamp, L. W.

2002-08-01

374

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. PMID:25607045

Liu, Wei-Sheng; Chu, Ting-Fu; Huang, Tien-Hao

2014-12-15

375

Band structure and optical properties of hexagonal In-rich InxAl1-xN alloys

International Nuclear Information System (INIS)

Full potential linear augmented plane wave calculations have been performed to study the electronic and optical properties of In-rich InxAl1-xN alloys in the hexagonal wurtzite structure. Compositions of x = 0.9375, 0.8125 and 0.6875 are considered which follow from replacing one, three and five In atoms by Al in the 32-atom supercell. The new form of exchange correlation, i.e. Engel-Vosko’s generalized gradient approximation within density functional theory, is employed. The calculations yield the band structure and total density of states as well as the imaginary part ?2(?) of the ordinary and extraordinary dielectric function. The calculated dependence of the bandgap on the composition is in good agreement with recent experimental studies. A reversal of the valence band ordering is found between x = 0.8125 and 0.6875. The absorption features in the high-energy range of ?2(?) are related to critical points of the band structure. The transition energies for these van Hove singularities are determined and their bowing parameters are discussed. (paper)

376

Energy Technology Data Exchange (ETDEWEB)

The (/sup 14/C)2-deoxyglucose (2DG) technique has been used to map the effects of increasing intensities of wide band noise on 2DG uptake in mongolian gerbil brain auditory structures. Animals were injected with (/sup 14/C)2DG and exposed to silence or continuous wide band noise at 25 dB, 45 dB, 65 dB, 85 dB or 105dB SPL. Brains were removed, frozen-sectioned, and autoradiographed on X-ray film. The ratio of the optical density of gray matter structures to the optical density of cerebellar peduncles in each animal was used to semiquantitate the results. The dorsal and ventral cochlear nuclei, superior olive/trapezoid body, inferior colliculus, and the dorsal and ventral nuclei of the lateral lemniscus all showed increases in 2DG uptake during exposure to wide band noise (WBN). As noise intensity increased from 0 to 105 dB SPL, 2DG uptake increased regularly to a maximum at 85 or 105 dB SPL. As WBN intensity increased, deeper layers of inferior colliculus were activated. The medial geniculate nucleus and auditory cortex showed a lesser increase in 2DG uptake during noise exposure. Non-auditory structures, including the cerebellar cortex and the medullary reticular nuclei, showed no increase in 2DG uptake during noise exposure at any intensity tested.

Sharp, F.R.; Ryan, A.F.; Goodwin, P.; Woolf, N.K. (California Univ., San Diego (USA))

1981-12-28

377

International Nuclear Information System (INIS)

The [14C]2-deoxyglucose (2DG) technique has been used to map the effects of increasing intensities of wide band noise on 2DG uptake in mongolian gerbil brain auditory structures. Animals were injected with [14C]2DG and exposed to silence or continuous wide band noise at 25 dB, 45 dB, 65 dB, 85 dB or 105dB SPL. Brains were removed, frozen-sectioned, and autoradiographed on X-ray film. The ratio of the optical density of gray matter structures to the optical density of cerebellar peduncles in each animal was used to semiquantitate the results. The dorsal and ventral cochlear nuclei, superior olive/trapezoid body, inferior colliculus, and the dorsal and ventral nuclei of the lateral lemniscus all showed increases in 2DG uptake during exposure to wide band noise (WBN). As noise intensity increased from 0 to 105 dB SPL, 2DG uptake increased regularly to a maximum at 85 or 105 dB SPL. As WBN intensity increased, deeper layers of inferior colliculus were activated. The medial geniculate nucleus and auditory cortex showed a lesser increase in 2DG uptake during noise exposure. Non-auditory structures, including the cerebellar cortex and the medullary reticular nuclei, showed no increase in 2DG uptake during noise exposure at any intensity tested. (Auth.)

378

The band structure and electronic transport property of indium nitride nanoribbon are investigated by employing first-principles calculation using density functional theory. The nanoribbon of pure InN, oxygen, fluorine and gallium substituted InN nanoribbon and defect InN nanoribbon are studied and analyzed. The band structure of pure InN, oxygen, fluorine and gallium substituted InN shows semiconducting nature whereas defect InN nanoribbon shows a metallic nature. The density of states of InN nanoribbon provides insight for the localization of charges in valence and conduction bands. The transport property are studied in terms of transmission spectrum, pure and gallium substituted InN has almost same transmission, in contrast oxygen and fluorine substituted InN shows a different fashion in transmission spectrum. The presence of defect modifies the transmission that depends on the geometry of the structure and position of the defect in the nanostructure. The information provided in the present study will enhance the electronic transport property to tailor new material with improved performance in optoelectronic devices.

Chandiramouli, R.; Sriram, S.

2014-01-01

379

The effects of pressure on the structural and vibrational properties of the layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored by first principles calculations. We observe significant changes in the calculated structural properties with different corrections for treating van der Waals interactions to Density Functional Theory (DFT), as compared with standard DFT functionals. In particular, the calculated ground state lattice parameters, volume and bulk modulus obtained with Grimme's scheme, are found to agree well with experiments. The calculated vibrational frequencies demonstrate the dependence of the intra and inter-molecular interactions on FOX-7 under pressure. In addition, we also found a significant increment in the N-H...O hydrogen bond strength under compression. This is explained by the change in bond lengths between nitrogen, hydrogen, and oxygen atoms, as well as calculated IR spectra under pressure. Finally, the computed band gap is about 2.3 eV with generalized gradient approximation, and is enhanced to 5.1 eV with the GW approximation, which reveals the importance of performing quasiparticle calculations in high energy density materials. PMID:24410219

Appalakondaiah, S; Vaitheeswaran, G; Lebègue, S

2014-01-01

380

Bonding nature as well as structural, optoelectronic and thermal properties of the cubic XMg2O4(X = Si, Ge) spinel compounds have been calculated using a full-potential augmented plane-wave plus local orbitals (FP-APW+lo) method within the density functional theory. The exchange-correlation potential was treated with the PBE-GGA approximation to calculate the total energy. Moreover, the modified Becke-Johnson potential (TB-mBJ) was also applied to improve the electronic band structure calculations. The computed ground-state parameters (a, B, B? and u) are in excellent agreements with the available theoretical data. Calculations of the electronic band structure and bonding properties show that these compounds have a direct energy band gap (?-?) with a dominated ionic character and the TB-mBJ approximation yields larger fundamental band gaps compared to those obtained using the PBE-GGA. Optical properties such as the complex dielectric function ?(?), reflectivity R(?) and energy loss function L(?), for incident photon energy up to 40 eV, have been predicted. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the effects of pressure P and temperature T on the thermal expansion coefficient, Debye temperature and heat capacity for the considered compounds are investigated for the first time.

Semari, F.; Ouahrani, T.; Khachai, H.; Khenata, R.; Rabah, M.; Bouhemadou, A.; Murtaza, G.; Amin, B.; Rached, D.

2013-07-01

381

Acoustic band pinning in the phononic crystal plates of anti-symmetric structure

International Nuclear Information System (INIS)

Acoustic bands are studied numerically for a Lamb wave propagating in an anti-symmetric structure of a one-dimensional 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. (condensed matter: structural, mechanical, and thermal properties)

382

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

Energy Technology Data Exchange (ETDEWEB)

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.

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

383

Steps are taken to unite theoretical predictions of electronic band structure with direct measurements from high-resolution angle-resolved photoemission spectroscopy (ARPES). A new process of accessing data from self-consistent eld calculations allows for calculations of band structure along any path in momentum space. Using the inner potential model, a primitive model is developed for direct comparison of photoemission measurements and augmented plane-wave + local orbitals (APW+lo) band structure calculations close to the Fermi energy, Ef. This model is then applied to ARPES measurements for URu2Si2.

Hristov, Alexander

2009-08-15

384

Compact Electromagnetic Bandgap Structures for Notch Band in Ultra-Wideband Applications

This paper introduces a novel approach to create notch band filters in the front-end of ultra-wideband (UWB) communication systems based on electromagnetic bandgap (EBG) structures. The concept presented here can be implemented in any structure that has a microstrip in its configuration. The EBG structure is first analyzed using a full wave electromagnetic solver and then optimized to work at WLAN band (5.15–5.825 GHz). Two UWB passband filters are used to demonstrate the applicability and effectiveness of the novel EBG notch band feature. Simulation results are provided for two cases studied. PMID:22163430

Rotaru, Mihai; Sykulski, Jan

2010-01-01

385

Structural properties of low density liquid selenium

International Nuclear Information System (INIS)

We present extensive results of Reverse Monte Carlo (RMC) simulations of the static structural properties of liquid selenium from 573K to 1773K based on the diffraction data of Inui et al. (Inui M, Tamura K, Oh'ishi Y, Nakaso I, Funakoshi K and Utsumi W 1999 J. Non. - Cryst. Solids 250-252, 519). In all cases, the positions of 5000 atoms in a configuration box were altered until the resulting static structure factor S(q) was in agreement with the experimental one. Rom the simulations, we were able to extract information on the temperature dependence of the pair distribution function g(r), bond angle correlation b(cos ?) and coordination number distribution P(N). From our calculations, we deduce that with an increase in temperature, there is a decrease in the fraction of two fold coordinated atoms indicative of a destruction of the chain like structure of Se. Furthermore, our results indicate that the transition from a semiconducting to a metallic state with increasing temperature is accompanied by structural changes. (author)

386

Band structure approach to resonant x-ray scattering

International Nuclear Information System (INIS)

We study the resonance behavior of the forbidden 600 and 222 x-ray Bragg peaks in Ge. These peaks remain forbidden in the resonant dipole scattering approximation, even taking into account the nonlocal 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 behavior, including the phase of the scattering, is a direct measure of this p-d hybridization and obtain quantitative agreement with experiment. We discuss the implications of this to other materials like V2O 3 in which the resonating atom is not at a center of inversion symmetry

387

Structures of High Density Molecular Fluids

Energy Technology Data Exchange (ETDEWEB)

The goal of this proposal is to develop an in-situ probe for high density molecular fluids. We will, therefore, use Coherent Anti-Stokes Raman Spectroscopy (CARS) applied to laser heated samples in a diamond-anvil cell (DAC) to investigate molecular fluids at simultaneous conditions of high temperatures (T > 2000K) and high pressures (P > 10 GPa.) Temperatures sufficient to populate vibrational levels above the ground state will allow the vibrational potential to be mapped by CARS. A system capable of heating and probing these samples will be constructed. Furthermore, the techniques that enable a sample to be sufficiently heated and probed while held at static high pressure in a diamond-anvil-cell will be developed. This will be an in-situ investigation of simple molecules under conditions relevant to the study of detonation chemistry and the Jovain planet interiors using state of the art non-linear spectroscopy, diamond-anvil-cells, and laser heating technology.

Baer, B; Cynn, H; Iota, V; Yoo, C-S

2002-02-01

388

Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed.

Ho, Ching-Hwa; Chen, Hsin-Hung

2014-08-01

389

Energy Technology Data Exchange (ETDEWEB)

The structure and electronic properties of the ZnO(0001) and ZnO(000 anti 1) surfaces as studied by density functional calculations are presented. The stability of the surface has already been investigated by various groups. The electronic surface band structure, however, in particular the existence of surface states and the differences between experimental band dispersion for both terminations, still appears to pose open problems. To address these issues, we compare Kohn Sham band structures and electrostatic potentials close to the surface for the relaxed (1 x 1)-surface, (2 x 2) vacancy reconstructions, and surfaces with pits. In particular the effect of the bending of the electrostatic potential at the surface on the eigenstates is quantified. Comparing the adsorption energies of Fe atoms for various adsorption sites on ZnO(000 anti 1), the fcc hollow position turned out to be energetically favorable. The oxidation state of the Fe atom is derived from the projected density of states.

Pal, Sougata; Jasper-Toennies, Torben; Hack, Michael; Pehlke, Eckhard [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel (Germany)

2011-07-01

390

International Nuclear Information System (INIS)

Highlights: • Electronic structure of TlPb2Cl5 is calculated by the FP-LAPW method. • The valence band is dominated by contributions of Cl 3p states. • Contributions of Pb 6p* states dominate at the bottom of the conduction band. • The FP-LAPW data allow concluding that TlPb2Cl5 is an indirect-gap material. • XPS core-level and valence-band spectra of polycrystalline TlPb2Cl5 are measured. -- Abstract: We report on first-principles calculations of total and partial densities of states of atoms constituting TlPb2Cl5 using the full potential linearized augmented plane wave (FP-LAPW) method. The calculations reveal that the valence band of TlPb2Cl5 is dominated by contributions of the Cl 3p-like states, which contribute mainly at the top of the valence band with also significant contributions throughout the whole valence-band region. In addition, the bottom of the conduction band of TlPb2Cl5 is composed mainly of contributions of the unoccupied Pb 6p-like states. Our FP-LAPW data indicate that the TlPb2Cl5 compound is an indirect-gap material with band gap of 3.42 eV. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar+ ion-irradiated surfaces of a TlPb2Cl5 polycrystalline sample were measured. The measurements reveal high chemical stability and confirm experimentally the low hygroscopicity of TlPb2Cl5 surface

391

Energy Technology Data Exchange (ETDEWEB)

Highlights: • Electronic structure of TlPb{sub 2}Cl{sub 5} is calculated by the FP-LAPW method. • The valence band is dominated by contributions of Cl 3p states. • Contributions of Pb 6p{sup *} states dominate at the bottom of the conduction band. • The FP-LAPW data allow concluding that TlPb{sub 2}Cl{sub 5} is an indirect-gap material. • XPS core-level and valence-band spectra of polycrystalline TlPb{sub 2}Cl{sub 5} are measured. -- Abstract: We report on first-principles calculations of total and partial densities of states of atoms constituting TlPb{sub 2}Cl{sub 5} using the full potential linearized augmented plane wave (FP-LAPW) method. The calculations reveal that the valence band of TlPb{sub 2}Cl{sub 5} is dominated by contributions of the Cl 3p-like states, which contribute mainly at the top of the valence band with also significant contributions throughout the whole valence-band region. In addition, the bottom of the conduction band of TlPb{sub 2}Cl{sub 5} is composed mainly of contributions of the unoccupied Pb 6p-like states. Our FP-LAPW data indicate that the TlPb{sub 2}Cl{sub 5} compound is an indirect-gap material with band gap of 3.42 eV. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar{sup +} ion-irradiated surfaces of a TlPb{sub 2}Cl{sub 5} polycrystalline sample were measured. The measurements reveal high chemical stability and confirm experimentally the low hygroscopicity of TlPb{sub 2}Cl{sub 5} surface.

Khyzhun, O.Y., E-mail: khyzhun@ipms.kiev.ua [Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, Kyiv 03142 (Ukraine); Bekenev, V.L.; Denysyuk, N.M. [Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, Kyiv 03142 (Ukraine); Parasyuk, O.V. [Department of Inorganic and Physical Chemistry, Eastern European National University, 13 Voli Avenue, Lutsk 43025 (Ukraine); Fedorchuk, A.O. [Department of Inorganic and Organic Chemistry, Lviv National University of Veterinary Medicine and Biotechnologies, Pekarska St., 50, 79010 Lviv (Ukraine)

2014-01-05

392

International Nuclear Information System (INIS)

The influence of arrangement and content of substituents (B, In) in BGaAs, InGaAs, and BInGaAs alloys on the stabilities and band gaps is investigated using density-functional supercell calculations. The stability of ternary alloys decreases from InGaAs over BGaAs to GaAsN. Typical substituent structures show the following stability order: isolated substituents - [110] chains - clusters - twisted [111] chains - (200/211) arrangements (most stable). This is valid for both the In- and B-poor as well as the In- and B-rich alloys. From the fact that grown InGaAs provides a different gap than the most stable arrangement one can conclude that other structures (isolated indium atoms or InmAs clusters) are formed during the growth. Simultaneous substitutions (BInGaAs) of larger (In) and smaller (B) atoms prefer arrangements in larger distances (220) for isovalent boron substitution and in In-B bonds for antisite boron substitution. The high degree boron antisite substitution induces partially occupied acceptor bands which lead to a strong reduction of the band gap in comparison to the isovalent substitution. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

393

Anomalous density dependence of structural relaxation time in water

Abstract The density dependence of water structural relaxation time has been determined by inelastic ultraviolet scattering (IUVS). The observed trend is opposite with respect to the expected one. This striking experimental result calls for further investigations.

Bencivenga, Filippo; Cimatoribus, Andrea; Gessini, Alessandro; Izzo, Maria Grazia; Masciovecchio, Claudio

2008-01-01

394

Relationship of Aerial Broad Band Reflectance to Meloidogyne incognita Density in Cotton

Aerial images were obtained on 22 July 1999 and 4 August 2000 from five cotton sites infested with Meloidogyne incognita. Images contained three broad bands representing the green (500-600 nm), red (600-700 nm), and near-infrared (700-900 nm) spectrum. Soil samples were collected and assayed for nematodes in the fall at these sites. Sampling locations were identified from images, by locating the coordinates of a wide range of light intensity (measured as a digital number) for each single band...

Wheeler, T. A.; Kaufman, H. W.

2003-01-01

395

Evaluation of the possibility of increasing the downlink E.I.R.P. density limits in the Ku band

The purpose of this paper is to examine the possibility of increasing the downlink E.I.R.P. density limits for operation of Fixed Satellite Service (F.S.S.) links in the Ku band. The Ku band is extensively used for voice, video, data, remote sensing applications and for transmission to VSAT (Very Small Aperture Terminals) networks. However, the limit on downlink E.I.R.P. density is considered to be conservative and it's assumed that much higher E.I.R.P. densities should be possible without causing significant interference to adjacent users. If this is proved, it would be possible to achieve lower Bit Error Rates and thereby increase reliability of satellite links. The increased power transmission capability would also allow transmission of more signals with the available bandwidth or lead to reduction in size of receiving antennas. With the help of existing F.C.C. limits on antenna gain patterns and the I.T.U. interference criterion, an increase in downlink E.I.R.P. density per carrier by more than 66% is shown to be possible. This increase in power would dramatically affect Fixed Satellite Services.

Kumar, Shriram; Schmitt, Neil M.; Friedman, Robert

2005-08-01

396

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.

Senkovskiy, B. V.; Usachev, D. Yu.; Fedorov, A. V.; Shelyakov, A. V.; Adamchuk, V. K.

2012-08-01

397

Surface structures on wide band gap insulators and their use as templates for the growth of adsorbates are reviewed. Surface structures include evaporation structures, vicinal surfaces, facetted surfaces, epitaxial structures, or structures transferred to or induced by the growth of thin films. Most structures have been realized so far on Al2O3 and on alkali halide crystals. The guided growth of adsorbates is discussed, considering the examples of metallic clusters or wires and ordered films of organic molecules.

Bennewitz, Roland

2006-07-01

398

Energy Technology Data Exchange (ETDEWEB)

The modifications of the electronic band structure of solids due to electron-phonon interactions (temperature and zero-point motion effects) have been explored by Manuel Cardona from both the experimental and theoretical sides. In the present contribution, we focus on the theoretical approaches to such effects. Although the situation has improved since the seventies, the wish for a fully developed theory (and associated efficient implementations) is not yet fulfilled. We review noticeable semi-empirical and first-principle studies, with a special emphasis on the Allen-Heine-Cardona (AHC) approach. We then focus on the non-diagonal Debye-Waller contribution, appearing beyond the rigid-ion approximation, in a Density-Functional Theory (DFT) approach. A numerical study shows that they can be sizeable (10%-50%) for diatomic molecules. We also present the basic idea of a new formalism, based on Density-Functional Perturbation Theory, that allows one to avoid the sums over a large number of empty states, and speed up the calculation by one order of magnitude, compared to the straightforward implementation of the AHC approach within DFT. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Gonze, X. [European Theoretical Spectroscopy Facility, Institute of Condensed Matter and Nanosciences, Universite Catholique de Louvain, 1, Place Croix du Sud, 1348 Louvain-la-neuve (Belgium); Boulanger, P. [European Theoretical Spectroscopy Facility, Institute of Condensed Matter and Nanosciences, Universite Catholique de Louvain, 1, Place Croix du Sud, 1348 Louvain-la-neuve (Belgium); Departement de physique, Universite de Montreal, Montreal (Canada); Cote, M. [Departement de physique, Universite de Montreal, Montreal (Canada)

2011-01-15

399

Band structure engineering through orbital interaction for enhanced thermoelectric power factor

International Nuclear Information System (INIS)

Band structure engineering for specific electronic or optical properties is essential for the further development of many important technologies including thermoelectrics, optoelectronics, and microelectronics. In this work, we report orbital interaction as a powerful tool to finetune the band structure and the transport properties of charge carriers in bulk crystalline semiconductors. The proposed mechanism of orbital interaction on band structure is demonstrated for IV-VI thermoelectric semiconductors. For IV-VI materials, we find that the convergence of multiple carrier pockets not only displays a strong correlation with the s-p and spin-orbit coupling but also coincides with the enhancement of power factor. Our results suggest a useful path to engineer the band structure and an enticing solid-solution design principle to enhance thermoelectric performance

400

Band structure engineering through orbital interaction for enhanced thermoelectric power factor

Energy Technology Data Exchange (ETDEWEB)

Band structure engineering for specific electronic or optical properties is essential for the further development of many important technologies including thermoelectrics, optoelectronics, and microelectronics. In this work, we report orbital interaction as a powerful tool to finetune the band structure and the transport properties of charge carriers in bulk crystalline semiconductors. The proposed mechanism of orbital interaction on band structure is demonstrated for IV-VI thermoelectric semiconductors. For IV-VI materials, we find that the convergence of multiple carrier pockets not only displays a strong correlation with the s-p and spin-orbit coupling but also coincides with the enhancement of power factor. Our results suggest a useful path to engineer the band structure and an enticing solid-solution design principle to enhance thermoelectric performance.

Zhu, Hong; Sun, Wenhao; Ceder, Gerbrand [Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (United States); Armiento, Rickard [Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183 Linköping (Sweden); Lazic, Predrag [Theoretical Physics Division, Rudjer Boskovic Institute, Bijenicka Cesta 54, Zagreb (Croatia)

2014-02-24

401

Quadrant-type X-band single-cell structure for high gradient tests

International Nuclear Information System (INIS)

We are building a new high-gradient X-band (11.424 GHz) testbench, called “Shield-B,” for basic studies to establish a acceleration technology with 100MV/m or higher, where various single-cell structures are to be tested. We focus on quadrant-type structures, on which no surface currents associated with magnetic fields flow across cell-to-cell junctions unlike disk-type structures. In this study, we propose a quadrant-type X-band single-cell structure with a heavy damped structure by waveguides, overcoming its issues. (author)

402

Band structure of Heusler compounds studied by photoemission and tunneling spectroscopy

Energy Technology Data Exchange (ETDEWEB)

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{sub 2}MnGa no high TMR was achieved. Unpolarized tunneling spectroscopy reveals contribution of interface states close to the Fermi energy. Additionally magnon excitations due to magnetic impurities at the interface are observed. Such contributions can be the reason of a reduced TMR compared to the theoretical predictions. Nevertheless, for energies close to the Fermi energy and for Co{sub 2}MnGa, the validity of the band structure calculations is demonstrated with this technique as well.

Arbelo Jorge, Elena

2011-07-01

403

Probing the graphite band structure with resonant soft-x-ray fluorescence

International Nuclear Information System (INIS)

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 (?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

404

Ab initio calculations of quasiparticle band structure in correlated systems: LDA++ approach

We discuss a general approach to a realistic theory of the electronic structure in materials containing correlated d or f electrons. The main feature of this approach is the taking into account of the energy dependence of the electron self-energy with the momentum dependence being neglected (local approximation). It allows us to consider such correlation effects as the non-Fermi-step form of the distribution function, the enhancement of the effective mass including Kondo resonances,'' the appearance of the satellites in the electron spectra, etc. To specify the form of the self-energy, it is useful to distinguish (according to the ratio of the on-site Coulomb energy U to the bandwidth W) three regimes-strong, moderate, and weak correlations. In the case of strong interactions (U/W>1-rare-earth system) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix nmm' the band-structure problem is formulated as the functional problem on nmm' for f electrons and the standard local-denisty-approximation functional for delocalized electrons. In the case of moderate correlations (U/W~1-metal-insulator regime, Kondo systems) we start from the d=? dynamical mean-field iterative perturbation scheme of Kotliar and co-workers and also make use of our multiband atomic Green function for constrained nmm'. Finally for the weak interactions (U/WBickers and Scalapino is generalized to the realistic multiband case. We present two-band, two-dimensional model calculations for all three regimes. A realistic calculation in the Hubbard-I scheme with the exact solution of the on-site multielectron problem for f(d) shells was performed for mixed-valence 4f compound TmSe, and for the classical Mott insulator NiO.

Lichtenstein, A. I.; Katsnelson, M. I.

1998-03-01

405

Probing the graphite band structure with resonant soft-x-ray fluorescence

Energy Technology Data Exchange (ETDEWEB)

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.

Carlisle, J.A.; Shirley, E.L.; Hudson, E.A. [Lawrence Berkeley National Lab., CA (United States)] [and others

1997-04-01

406

The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

International Nuclear Information System (INIS)

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba3(VO4)2, Pb3(VO4)2, YVO4, BiVO4, CeVO4 and Ag3VO4 were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba3(VO4)2 and YVO4 have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb3(VO4)2 and BiVO4 the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag3VO4 mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (Eg=2.2 eV). In CeVO4 excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1ion band lower the effective band gap to 1.8 eV. In the Ce1-xBixVO4 (0?x?0.5) and Ce1-xYxVO4 (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi3+ or Y3+ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals. - Graphical abstract: The electronic structures of six vanadate salts, Ba3(VO4)2, Pb3(VO4)2, YVO4, BiVO4, Ag3VO4 and CeVO4, are studied. The results show that the oxygen to vanadium charge transfer, which is largely responsible for the electronic structure near the Fermi level, can be altered significantly through interactions with the surrounding cations.

407

Energy Band Analysis of MQW Structure Based on Kronig-Penny Model

Directory of Open Access Journals (Sweden)

Full Text Available The effects of different potential well depths, well widths and barrier widths on energy band of multiple quantum well (MQW structures are discussed in detail based on Kronig-Penny model. The results show that if the well and barrier width stay unchanged, the first and second band gaps increase linearly with the well depth. When the well depth is constant, the first and second band gaps increase exponentially with the barrier width in a wide well. However, in narrow well one, the second band gap saturates when the barrier width is wide enough. On condition that the well and barrier have equal width, the first band gap decreases exponentially with well-barrier width while the second gap still shows an exponential increase with the width. These results are insightful for the design of MQW structure optoelectronic devices.

Yu Zhang

2013-07-01

408

Electronic structure of BaIrO3: A first principle study using local spin-density approximations

We investigate the electronic structure of BaIrO3, an interesting compound exhibiting charge density wave transition in its insulating phase and ferromagnetic transition at the same temperature, using full potential linearized augmented plane wave method within the local spin density approximations. The ferromagnetic ground state could exactly be described in these calculations and the calculated spin magnetic moment is found to be small as observed in the magnetic measurements. Interestingly, no signature of exchange splitting is observed in the density of states corresponding to Ir 5d and/or any other electronic states. The small spin moment appears essentially due to unequal population of the up- and down-spin Ir 5d bands. Comparison of the valence band density of states with the experimental spectral functions suggests that a rigid shift of the Fermi level towards higher energies in the calculated density of states provides a good description of the experimental spectra. This indicates that the intrinsic ...

Maiti, K

2006-01-01

409

Heisenberg Model and Supersymmetric t-J Model with Two-Band Structure in One Dimension

We study critical properties of the one-dimensional integrable Heisenberg model and supersymmetric t-J model with two-band structure. We apply conformal field theory analysis to the Bethe-ansatz solution of these models, and compute bulk susceptibilities and critical exponents. The universal formulas obtained for critical exponents can be applied to nonintegrable spin and electron models with two-band structure.

Itakura, Toshifumi; Kawakami, Norio

1995-07-01

410

Band structure and UV optical spectra of TGS crystals in the range of 4-10eV

Energy Technology Data Exchange (ETDEWEB)

Theoretical and experimental studies of the band energy structure and optical spectra for triglycine sulphate crystal (TGS) (NH{sub 2}CH{sub 2}COOH){sub 3}.H{sub 2}SO{sub 4}, in the ferroelectric phase have been performed for the first time. First principal DFT calculations of the band structure, density of states and dielectric functions spectra {epsilon}'(E) and {epsilon}{sup '}'(E) of TGS crystal have been done using the computer package Cambridge Serial Total Energy Package (CASTEP) code. Experimental spectral dispersions of the complex reflection ratio {rho}(E) have been measured using the synchrotron radiation at BESSY synchrotron source in the spectral range of 4-10eV and the pseudo-dielectric functions <{epsilon}>=<{epsilon}{sup '}>+i<{epsilon}{sup '}'> were evaluated. Experimental data and theoretically calculated dielectric functions have demonstrated a good agreement. The band energy dispersion of valence and conducting bands have been analyzed and were used to identify the dielectric functions peculiarities.

Andriyevsky, B. [Department of Electronics and Computer Sciences, Koszalin University of Technology, 2 Sniadeckich Street, PL-75-453 Koszalin, West Pomeranian (Poland)]. E-mail: bandri@tu.koszalin.pl; Esser, N. [ISAS-Institute for Analytical Sciences Department Berlin, Albert-Einstein-Street 9, D-12489 Berlin (Germany); Patryn, A. [Department of Electronics and Computer Sciences, Koszalin University of Technology, 2 Sniadeckich Street, PL-75-453 Koszalin, West Pomeranian (Poland); Cobet, C. [ISAS-Institute for Analytical Sciences Department Berlin, Albert-Einstein-Street 9, D-12489 Berlin (Germany); Ciepluch-Trojanek, W. [Department of Electronics and Computer Sciences, Koszalin University of Technology, 2 Sniadeckich Street, PL-75-453 Koszalin, West Pomeranian (Poland); Romanyuk, M. [Ivan Franko National University of L' viv, Kyrylo-and-Mefodii Street 8, UA-79005 L' viv (Ukraine)

2006-03-15

411

Modulation of charge-density waves by superlattice structures

We discuss the interplay between electronic correlations and an underlying superlattice structure in determining the period of charge density waves (CDW's), by considering a one-dimensional Hubbard model with a repeated (non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix renormalization group diagonalization of finite systems (up to 120 sites) is used to calculate the charge-density correlation function and structure factor in the ground state. The modulation period can still be predicted through effective Fermi wavevectors, k_F*, and densities, and we have found that it is much more sensitive to electron (or hole) doping, both because of the narrow range of densities needed to go from q*=0 to \\pi, but also due to sharp 2k_F*-4k_F* transitions; these features render CDW's more versatile for actual applications in heterostructures than in homogeneous systems.

Malvezzi, A L; Dos Santos, R R; Malvezzi, Andre L; Paiva, Thereza; Santos, Raimundo R dos

2006-01-01

412

Banded spherulites of PEA, PBS and their PBS-PEA blends were studied by optical and atomic force microscopies. It was found that PEA forms a regular banded structure within the temperature range 23 degrees C to 36 degrees C, otherwise only Maltese-cross spherulites are observed. The banded PEA spherulites exhibit a double ring feature with band spacing between two equivalent birefringent rings increasing with temperature. The ring extinction in PEA banded spherulites is caused by a lamellar twist. There exist two equivalent positions in one twisting period, which show no birefringence. The PBS can also grow in ring-banded spherulites in a wide temperature window with the high temperature threshold for forming banded structures that are not sharply delineated, as in the case of PEA. Also the double ring feature is not so pronounced for the spherulites of pure PBS. Blending PBS with PEA is found to favor the formation of PBS banded structures. During the crystallization process, the early growing PBS spherulites at 75 degrees C show weak birefringence with an evident ring-banded structure. The crystallization of PEA at 0 degrees C makes a great contribution to the birefringence increment of the initially-birefringent rings. This is caused by the analogic crystal orientation of PBS and PEA based on interlamellar phase separation as revealed by AFM observation. The band spacing of PBS spherulites is found to be increase with both increasing temperature and increasing PEA content. This stems from an increment in chain mobility of PBS both with temperature and addition of the PEA component. PMID:19240940

Wang, Tianchang; Wang, Haijun; Li, Huihui; Gan, Zhihua; Yan, Shouke

2009-03-14

413

Application of Hyperspectral Band Elimiation Technique to PVT Images of Composite Structures

Directory of Open Access Journals (Sweden)

Full Text Available A new approach to NDT of composite structures using Band Elimination of the analyzed image index by Hyperspectral image analysis approach is presented and discussed. The matrix Band Elimination technique allows the monitoring and analysis of a components structure based on Filtering of bands and correlation between sequentially pulsed thermal images and their indices. The technique produces several matrices resulting from frame deviation and pixel redistribution calculations for intelligent classification and property prediction. The obtained results proved the technique to be capable of identifying damaged components with ability to model various types of damage under different conditions.

Mahmoud Zaki Iskandarani

2012-10-01

414

Band structure and reflectance for a nonlinear one-dimensional photonic crystal

We consider a model for a one-dimensional photonic crystal formed by a succession of Kerr-type equidistant spaceless interfaces immersed in a linear medium. We calculate the band structure and reflectance of this structure as a function of the incident wave intensity, and find two main behaviors: the appearance of prohibited bands, and the separation and narrowing of these bands. A system with these features is obtained by alternating very thin slabs of a soft matter material with thicker solid films, which can be used to design a device to control light propagation for specific wavelength intervals and light intensities.

Gutiérrez-López, Sergio; Corella-Madueño, Adalberto; Rosas-Burgos, Rodrigo; Reyes, Juan Adrian

2011-01-01

415

Analysis of frequency band structure in one-dimensional sonic crystal using Webster horn equation

Sound propagation through periodic arrangement of scatterers lead to formation of bands of frequencies, known as band gaps, where sound cannot propagate though the structure. We propose a method based on Webster horn equation, along with Floquet theorem, to predict the band gap of a one-dimensional periodic structure made of hard sound-scatterers. The method is further modified to obtain the complex wave numbers, which give the decay constants. The decay constant is used to predict the sound attenuation of the evanescent wave in the finite sonic crystal. The theoretical prediction is verified with experimental measurements.

Gupta, A.; Lim, K. M.; Chew, C. H.

2011-05-01

416

Realization of Band-Notch UWB Monopole Antenna Using AMC Structure

Directory of Open Access Journals (Sweden)

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 ?r = 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.

Pradeep Kumar

2013-06-01

417

Constraints on lunar structure. [propagation velocity and density distribution

A brief review is given of the constraints placed on lunar structure and composition by seismic data and density models. Bounds on the crustal velocity structure in Mare Cognitum are derived using travel-time data from artificial impacts, and a velocity model is determined on the basis of synthetic seismograms. It is shown that the velocities of P- and S-waves in the mantle can be fixed by a least-squares analysis of arrival times from meteor impacts and moonquakes, and that lunar density can be determined from the seismic structure, mean density, and moment of inertia. Olivine-pyroxene mixtures and certain olivine-rich compositions are found to be consistent with the seismic-velocity and density limits. Maximum radii are calculated for pure Fe and pure FeS cores, and it is concluded that the possibility of an ancient lunar magnetic dynamo may have to be reevaluated in the light of these figures.

Dainty, A. M.; Toksoz, M. N.; Solomon, S. C.; Anderson, K. R.; Goins, N. R.

1974-01-01

418

Synthesis, structure, band gap, and electronic structure of CsAgSb4S7

International Nuclear Information System (INIS)

The compound CsAgSb4S7 has been synthesized by the reaction of the elements in a Cs2S3 flux at 773K. The compound crystallizes in a new structure type with eight formula units in space group C2/c of the monoclinic system in a cell at 153K of dimensions a=26.127(2)A, b=8.8599(7)A, c=12.100(1)A, ?=97.650(1)o, and V=2776.0(4)A3. The structure contains two-dimensional ?2[AgSb4S7] layers separated by Cs atoms. Each ?2[AgSb4S7] layer is built from edge-sharing one-dimensional ?1[AgS3] and ?1[Sb4S7] chains. Each Ag atom is tetrahedrally coordinated to four S atoms. Each Sb3+ center is pyramidally coordinated to three S atoms to form an SbS3 group. CsAgSb4S7 is insulating with an optical band gap of 2.04eV. Extended Huckel calculations indicate that the band gap in CsAgSb4S7 is dominated by the Sb 5s and S 3p states above and below the Fermi level

419

International Nuclear Information System (INIS)

Highlights: ? Near Edge X-ray Absorption Fine Structure investigated as tool for probing the conduction band in Cu(In,Ga)S2. ? Absorption edge of anion contains most pertinent electronic information. ? Development of Cu(In,Ga)2 band gap with increased Ga content reflected in anion absorption edge positions. ? Correspondence with theory found. -- Abstract: A non-optimized interface band alignment in a heterojunction-based solar cell can have negative effects on the current and voltage characteristics of the resulting device. To evaluate the use of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) as a means to measure the conduction band position, Cu(In,Ga)S2 chalcopyrite thin film surfaces were investigated as these form the absorber layer in solar cells with the structure ZnO/buffer/Cu(In,Ga)S2/Mo/glass. The composition dependence of the structure of the conduction bands of CuInxGa1?xS2 has been revealed for x = 0, 0.67 and 1 with both hard and soft NEXAFS and the resulting changes in conduction band offset at the junction with the buffer layer discussed. A comprehensive study of the positions of the absorption edges of all elements was carried out and the development of the conduction band with Ga content was observed, also with respect to calculated densities of states

420

Plasma density measurements using chirped pulse broad-band Raman amplification

Stimulated Raman backscattering is used as a non-destructive method to determine the density of plasma media at localized positions in space and time. By colliding two counter-propagating, ultra-short laser pulses with a spectral bandwidth larger than twice the plasma frequency, amplification occurs at the Stokes wavelengths, which results in regions of gain and loss separated by twice the plasma frequency, from which the plasma density can be deduced. By varying the relative delay between the laser pulses, and therefore the position and timing of the interaction, the spatio-temporal distribution of the plasma density can be mapped out.

Vieux, G; Farmer, J P; Hur, M S; Issac, R C; Jaraszynski, D A

2013-01-01

421

Tunneling density of states of MgB2: Evidence for a dominant ?-band contribution

International Nuclear Information System (INIS)

Weakly-transmitting junctions usable for tunneling spectroscopy experiments were fabricated from polycrystalline bulk samples of magnesium diboride with a niobium counter-electrode. A strong nonlinear conductance-versus-voltage behavior for voltages above the values corresponding to the energy gaps of the electrodes was interpreted as a fingerprint of the electron-phonon interaction in superconducting MgB2. A partial Eliashberg spectral function ?2(?)F(?) was extracted from the data by the numeric inversion of the standard strong-coupling Eliashberg equations. In a full agreement with recent theoretical simulations for magnesium diboride, we have observed a dominant contribution from the ?-band of this two-band superconductor. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

422

Indian Ocean Dipole index recorded in Kenyan coral annual density bands

The Indian Ocean Dipole (IOD), which governs climate variability not only over the Indian Ocean but also globally, produces precipitation anomalies in the East African short rain period from October to November. These precipitation anomalies were recorded in annual bands from a coral core obtained in the Malindi Marine Park, Kenya. A signature of the IOD in precipitation was detected in the coral core by luminescence intensity under UV light and by oxygen isotope values dated to January, a few months after the short rain period. Anomalies of the oxygen isotope values correlated well with high and low precipitation, which were related to positive and negative IOD indices, respectively. This study suggests that the coral annual band record is effective for reconstructing IOD events and should be able to reconstruct the IOD record prior to instrumental observations.

Kayanne, Hajime; Iijima, Hiroko; Nakamura, Nobuko; McClanahan, Timothy R.; Behera, Swadhin; Yamagata, Toshio

2006-10-01

423

International Nuclear Information System (INIS)

The high spin states of the neutron-rich 109Tc nucleus are reinvestigated by observing prompt ?-rays from the spontaneous fission of 252Cf. The previously known yrast band based on the 7/2+ state is updated. A side band built on the 11/2+ state is expanded and a new band based on the 15/2+ state is identified. Band crossing in the yrast band occurs around ?? ? 0.36MeV. This band crossing is associated with the alignment of two h11/2 neutrons according to the cranked shell model calculations. The band based on the 11/2+ state is proposed as a candidate for the one-phonon ?-vibrational band, and the band built on the 15/2+ state is proposed as a candidate for the two-phonon ?-vibrational band. Other characteristics for the observed bands are discussed. (nuclear physics)

424

Ab initio calculations of quasiparticle band structure in correlated systems LDA++ approach

We discuss a general approach to a realistic theory of the electronic structure in materials containing correlated d- or f- electrons. The main feature of this approach is the taking into account the energy dependence of the electron self-energy with the momentum dependence being neglected (local approximation). In the case of strong interactions (U/W>>1 - rare-earth system) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix the band structure problem is formulated as the functional problem on Nmm' for f-electrons and the standard LDA-functional for delocalized electrons. In the case of moderate correlations (U/W=1 metal-insulator regime) we start from the dynamical mean field iterative perturbation scheme (IPS) of G. Kotliar et. al. and also make use of our multiband atomic Green function. Finally for the weak interactions (U/W<1 -transition metals) the self-consistent diagrammatic fluctuation- exchange (FLEX)-approach of N. Bicker...

Lichtenstein, A I

1997-01-01

425

Thermospheric density structures over the polar regions observed with CHAMP

Directory of Open Access Journals (Sweden)

Full Text Available We report on the unexpected detection of considerable structure in high latitude thermospheric densities, as derived from an accelerometer onboard the CHAMP satellite. The width of the structures, which can either be maxima or minima, varies between a few hundred km and 2000 km. The amplitudes of these density extrema can reach 50% of ambient. Maxima cluster around 75° (N and S, while minima are found closer to the poles. In a magnetic latitude-magnetic local time frame the maxima are found mainly around the cusp region. Overall, the observed structures somewhat resemble so-called density cells previously found in model calculations. However the models generate their cells around 140–300 km altitude and show little, if any remnant at 400 km or above. This has to be contrasted with the fact that the CHAMP observations were obtained near 430 km altitude. We have explored Joule heating as a possible mechanism for the generation of the structures, at least in density enhancement regions, using Hall currents measured on CHAMP and simultaneous incoherent scatter measurements with EISCAT. However, the electric fields were usually quite small during the period of observation, making the quest for an explanation for the structures all the more challenging.

Keywords. Meteorology and atmospheric dynamics (Thermospheric dynamics – Magnetospheric physics (Polar cap phenomena – Atmospheric composition and structure (Pressure, density, and temperature

K. Schlegel

2005-07-01

426

Band structure in 180Ta studied with transfer reactions

International Nuclear Information System (INIS)

The differential cross sections of emitted particles have been measured in the reactions 181Ta(p,d)180Ta at 19 MeV, 179Hf(3He,d)180Ta at 36 MeV, and 179Hf(?,t)180Ta at 62.2 MeV. The energy resolution was 15 to 40 keV depending on the reaction. Differential cross sections were fitted using a distorted wave program and level characteristics of 180Ta were deduced in the framework of the Nilsson model for odd-odd deformed nuclei including Coriolis coupling. Members of rotational bands have been identified up to 1 MeV excitation energy. The short-lived isomer in 180Ta with spin 1+ was found to be the ground state whereas the long-lived isomer at 82 +- 20 keV excitation energy has spin 9-

427

Band structure in neutron-deficient 103Cd

International Nuclear Information System (INIS)

A considerable amount of experimental works has been recently done on the light Cadmium isotopes. The main purpose of these studies was to search for Coriolis decoupling mechanism in a transitional region. Negative and positive parity bands occuring in 105Cd, 107Cd, 109Cd, 111Cd have been correctly reproduced within the rotor-plus-particle model using self-consistent prolate core single-particle states. The aim of the present work is using HI reactions, to search for the high spin states in 103Cd where only two low-lying levels are known so far, from on-line radioactivity studies, and to check whether this theoretical description remains still satisfactory

428

Shaping topological properties of the band structures in a shaken optical lattice

The realization of band structures with nontrivial topological properties in an optical lattice is an exciting topic in current studies of ultracold atoms. Here we point out that this lofty goal can be achieved by using a simple scheme of shaking an optical lattice, which is directly applicable in current experiments. The photon-assisted band hybridization leads to the production of an effective spin-orbit coupling, in which the band index represents the pseudospin. When this spin-orbit coupling has finite strengths in multiple directions, nontrivial topological structures emerge in the Brillouin zone, such as topological defects with a winding number of 1 or 2 in a shaken square lattice. The shaken lattice also allows one to study the transition between two band structures with distinct topological properties.

Zhang, Shao-Liang; Zhou, Qi

2014-11-01

429

Lifetimes and band structure of electroluminescence of ZnS:Mn based cells

Spectral and kinetic properties of the electroluminescent thin film cells containing ZnS:Mn and CuxS layers were investigated. The cells were produced by thermal evaporation and chemical dipping. Kinetic measurements indicate that, for high manganese concentration the decay curve can be expanded in two exponentials. These exponentials are due to two kinds of manganese centers. Mn2+ ions and Mn2+-Mn2+ pairs. For low cells exhibit a reach structure in the blue and green spectral regions. Further investigation disclosed the presence of two types of the spectra. We also obtained a structure of manganese band of the electroluminescence of the cells for high manganese concentration. Similar to the short-wavelength bands, there are two types of the manganese bands. To our knowledge, this is the first observation of such structure of manganese bands at room temperature.

Chimczak, Eugeniusz; Bertrandt-Zytkowiak, Miroslawa

1997-06-01

430

Fine structure of the red luminescence band in undoped GaN

Energy Technology Data Exchange (ETDEWEB)

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8?eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36?eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50?K) to an unknown deep acceptor having an energy level 1.130?eV above the valence band.

Reshchikov, M. A., E-mail: mreshchi@vcu.edu [Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284 (United States); Usikov, A. [Nitride Crystals, Inc., 181E Industry Ct., Ste. B, Deer Park, New York 11729 (United States); Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, 49 Kronverkskiy Ave., 197101 Saint Petersburg (Russian Federation); Helava, H.; Makarov, Yu. [Nitride Crystals, Inc., 181E Industry Ct., Ste. B, Deer Park, New York 11729 (United States)

2014-01-20

431

Fine structure of the red luminescence band in undoped GaN

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.

Reshchikov, M. A.; Usikov, A.; Helava, H.; Makarov, Yu.

2014-01-01

432

International Nuclear Information System (INIS)

The band edge structure of Pbsub(1-x)Snsub(x)Te is derived in detail using a two band ellipsoidal model and compared with a more rigorous calculation based on six bands. A quantitative comparison is made for two values of the energy gap, corresponding to the cases where x=0 and x=0.17. It was found that, for the occupied states in nondegenerate materials, both models are practically equivalent. Discrepancies may occur only in high degeneracies or deep inversion layers. The agreement between both models was significantly improved by introducing an effective energy gap in the two band model. It is suggested that the use of the effective energy gap may improve the agreement between the two band model and experiment whenever the details of the band edge structure enter the interpretation of the experimental results. (author)

433

The theoretically suggested band structure of the novel p-type semiconductor lithium niobite (LiNbO2), the direct coupling of photons to ion motion, and optically induced band structure modifications are investigated by temperature dependent photoluminescence. LiNbO2 has previously been used as a memristor material but is shown here to be useful as a sensor owing to the electrical, optical, and chemical ease of lithium removal and insertion. Despite the high concentration of vacancies present in lithium niobite due to the intentional removal of lithium atoms, strong photoluminescence spectra are observed even at room temperature that experimentally confirm the suggested band structure implying transitions from a flat conduction band to a degenerate valence band. Removal of small amounts of lithium significantly modifies the photoluminescence spectra including additional larger than stoichiometric-band gap features. Sufficient removal of lithium results in the elimination of the photoluminescence response supporting the predicted transition from a direct to indirect band gap semiconductor. In addition, non-thermal coupling between the incident laser and lithium ions is observed and results in modulation of the electrical impedance.

Shank, Joshua C.; Tellekamp, M. Brooks; Doolittle, W. Alan

2015-01-01

434

Electronic band structure and effective mass parameters of Ge1-xSnx alloys

This work investigates the electronic band structures of bulk Ge1-xSnx alloys using the empirical pseudopotential method (EPM) for Sn composition x varying from 0 to 0.2. The adjustable form factors of EPM were tuned in order to reproduce the band features that agree well with the reported experimental data. Based on the adjusted pseudopotential form factors, the band structures of Ge1-xSnx alloys were calculated along high symmetry lines in the Brillouin zone. The effective masses at the band edges were extracted by using a parabolic line fit. The bowing parameters of hole and electron effective masses were then derived by fitting the effective mass at different Sn compositions by a quadratic polynomial. The hole and electron effective mass were examined for bulk Ge1-xSnx alloys along specific directions or orientations on various crystal planes. In addition, employing the effective-mass Hamiltonian for diamond semiconductor, band edge dispersion at the ?-point calculated by 8-band k.p. method was fitted to that obtained from EPM approach. The Luttinger-like parameters were also derived for Ge1-xSnx alloys. They were obtained by adjusting the effective-mass parameters of k.p method to fit the k.p band structure to that of the EPM. These effective masses and derived Luttinger parameters are useful for the design of optical and electronic devices based on Ge1-xSnx alloys.

Lu Low, Kain; Yang, Yue; Han, Genquan; Fan, Weijun; Yeo, Yee-Chia

2012-11-01

435

We have found that the local density of states fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to SdH and dHvA effects) oscillations in strong magnetic fields, omega _c\\tau > 1. We attribute this to the dominant role of the states near bottoms of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate...

Holder, J. P.; Savchenko, A. K.; Fal Ko, Vladimir I.; Jouault, B.; Faini, G.; Laruelle, F.; Bedel, E.

1999-01-01

436

We have found that the local density of state fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to Shubnikov-de Haas and de Haas-van Alphen effects) oscillations in strong magnetic fields, omega(c)tau>/=1. We attribute this to the dominant role of the states near the bottom of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with field B in accordance with the results obtained in the diffusion approximation. PMID:11017568

Holder; Savchenko; Fal'ko; Jouault; Faini; Laruelle; Bedel

2000-02-14

437

We have found that the local density of states fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to SdH and dHvA effects) oscillations in strong magnetic fields, omega _c\\tau > 1. We attribute this to the dominant role of the states near bottoms of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with B in accordance with the results obtained in the diffusion approximation.

Holder, J P; Fal'ko, V I; Jouault, B; Faini, G; Laruelle, F; Bedel, E; Fal'ko, Vladimir I.

1999-01-01

438

DAMPING OF ELECTRON DENSITY STRUCTURES AND IMPLICATIONS FOR INTERSTELLAR SCINTILLATION

International Nuclear Information System (INIS)

The forms of electron density structures in kinetic Alfven wave (KAW) turbulence are studied in connection with scintillation. The focus is on small scales L ? 108-1010 cm where the KAW regime is active in the interstellar medium, principally within turbulent H II regions. Scales at 10 times the ion gyroradius and smaller are inferred to dominate scintillation in the theory of Boldyrev et al. From numerical solutions of a decaying KAW turbulence model, structure morphology reveals two types of localized structures, filaments and sheets, and shows that they arise in different regimes of resistive and diffusive damping. Minimal resistive damping yields localized current filaments that form out of Gaussian-distributed initial conditions. When resistive damping is large relative to diffusive damping, sheet-like structures form. In the filamentary regime, each filament is associated with a non-localized magnetic and density structure, circularly symmetric in cross section. Density and magnetic fields have Gaussian statistics (as inferred from Gaussian-valued kurtosis) while density gradients are strongly non-Gaussian, more so than current. This enhancement of non-Gaussian statistics in a derivative field is expected since gradient operations enhance small-scale fluctuations. The enhancement of density gradient kurtosis over current kurtosis is not obvious, yet it suggests that modest density fluctuations may yield large scintillation events during pield large scintillation events during pulsar signal propagation. In the sheet regime the same statistical observations hold, despite the absence of localized filamentary structures. Probability density functions are constructed from statistical ensembles in both regimes, showing clear formation of long, highly non-Gaussian tails.

439

Special features of CdCr2Se4 magnetic semiconductor band structure

International Nuclear Information System (INIS)

Data on band spectrum of CdCr2Se4 magnetic semiconductor, obtained using the new method of modulation spectroscopy - the method of cathode absorption - are presented. Temperature dependence of effective density of states in conductivity zone of the given semiconductor is shown