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1

Valence band anticrossing in highly mismatched alloys  

Science.gov (United States)

Semiconductor alloys offer the ability to tune certain material parameters such as the band gap or carrier effective mass through precise control of the alloy composition, allowing them to be optimized for specific device requirements. While many alloys demonstrate near linear composition dependencies in these properties, those containing isoelectronic anion species that are significantly mismatched in electronegativity or ionization energy, known as highly mismatched alloys (HMA), exhibit substantial deviation from this trend. Here, the optical and electrical properties of HMAs containing dilute concentrations of large metallic anions are investigated in the context of a valence band anticrossing (VBAC) theory. Minority species with low ionization energies often introduce localized p-states near the valence band edge of the host semiconductor. Hybridization of these localized states with the extended p-states of the host may be described by a 12 x 12 Hamiltonian and produces a splitting of the alloy valence band into E+ and E - states. Photomodulated reflectance studies coupled with the VBAC theory confirm that the band gap bowing observed in GaSbxAs1-x and GaBixAs1-x is caused by an upward movement of the valence band edge as a result of the anticrossing interaction between the E+ and E- states. The valence band restructuring also adversely affects hole transport in these alloys through an increase in the heavy hole effective mass and the addition of an alloy disorder scattering mechanism. Finally, the VBAC theory has been extended to group IV HMAs as well as to the dilute magnetic semiconductor Ga1-x MnxAs, both of which exhibit strong hole localization at the minority species sites.

Alberi, Kirstin Mclean

2

THz valence band polaritons and antipolaritons  

Science.gov (United States)

The coupling of THz and intersubband excitations leads to THz polaritons and antipolaritons and has great potential for device applications. In this paper, based on the dielectric function formalism, we investigate the relevance of controlling cavity resonance and dephasing in the THz polariton and antipolariton dispersions. The input optical dielectric constant stems from numerically exact nonequilibrium many body solutions which are adjusted to a simplified nonlinear dielectric susceptibility. The resulting expression is inserted in the wave equation to describe the coupling of TE- polarized THz radiation and intervalence band transitions in GaAs/Al0.3Ga0.7As multiple quantum well structures with various resonance conditions.

Faragai, I. A.; Pereira, M. F.

2014-09-01

3

Valence band anticrossing in GaBixAs1-x  

Energy Technology Data Exchange (ETDEWEB)

The optical properties of GaBixAs1-x (0.04< x< 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.

Alberi, K.; Dubon, O. D.; Walukiewicz, W.; Yu, K. M.; Bertulis, K.; Krotkus, A.

2007-07-11

4

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

5

Core level and valence band photoemission from UAs  

International Nuclear Information System (INIS)

Ultraviolet and X-ray photoemission spectroscopy is used to study the valence band and core levels of UAs. The valence band consists of a 1.5 eV wide 'f-d' band at Esub(F) = 0. At - 0.5 eV a structure is observed which has mainly d character. The p valence band is centered at - 2.8 eV and consists of 'p-f' and 'p-s' mixing states. The 4f core levels are highly asymmetric and have a satellite 7.8 eV below the main line. The spectra of UAs are compared to the corresponding spectra of ?-U and polycrystalline As obtained under identical experimental conditions. (orig.)

6

Valence band anticrossing in mismatched III-V semiconductor alloys  

Energy Technology Data Exchange (ETDEWEB)

The valence band anticrossing model (VBAC) is applied to explain the composition-dependent trends in dilute GaSb{sub x}As{sub 1-x} and GaBi{sub x}As{sub 1-x} alloys. Photomodulated reflectance spectroscopy (PR) of GaSb{sub x}As{sub 1-x} shows a reduction of the fundamental bandgap energy as well as an increase of the spin-orbit splitting energy with increasing x. The VBAC model predicts that these shifts are the result of a hybridization of the extended valence band states of GaAs with the localized states of the impurity atoms, which induce an upward movement of the valence band edge. Extrapolation of the model to other III-(Sb,Bi)-V alloys is also discussed. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Alberi, K.; Walukiewicz, W.; Yu, K.M. [Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States); Wu, J.; Dubon, O.D. [Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 (United States); Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States); Watkins, S.P.; Wang, C.X. [Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6 (Canada); Liu, X.; Cho, Y.J.; Furdyna, J.K. [Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556 (United States)

2007-04-15

7

Valence band photoemission studies of clean metals  

International Nuclear Information System (INIS)

The application of Angle-Resolved Photoelectron Spectroscopy (ARPES) to crystalline solids and the utilization of such studies to illuminate several questions concerning the detailed electronic structure of such materials, are discussed. Specifically, by construction of a Direct Transition (DT) model and the utilization of energy-dependent angle-resolved normal photoemission in the photon energy range 32 eV < or = h? < or = 200 eV, the bulk band structure of copper is experimentally mapped out along three different directions in the Brillouin Zone; GAMMA to K, GAMMA to L, and GAMMA to X. In addition, various effects which influence the obtainable resolution in vector k-space, namely, thermal disorder, momentum broadening, and band mixing, are discussed and are shown to place severe limitations on the applicability of the DT model. Finally, a model for Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS) based on the symmetry of the initial-state wavefunctions is presented and compared to experimental results obtained from copper single crystals

8

Valence band study of Mg2Si by Auger spectroscopy  

International Nuclear Information System (INIS)

Previous XPS measurements of Mg2Si revealed three valence bands which were all assigned to the Si 3s and 3p states based on a completely ionic model of the bonding in Mg2Si that assumed no occupied Mg 3s or 3p states. Auger spectroscopy is element specific so that the local density of states of Mg and Si can be measured separately. The L1L23V and the L23VV Auger spectra of Mg in Mg2Si were measured from a single crystal of Mg2Si cleaved in UHV. The L23VV spectrum showed the involvement of the Mg 3p state in the upper two valence bands in agreement with recent calculations. More surprisingly the L1L23V spectrum gave evidence of the presence of the Mg 3s state in the lowest energy valence band that had not been predicted. The Auger results gave evidence of covalency in the bonding of Mg2Si

9

Valence-band of cubic semiconductors: Clifford algebra approach II  

International Nuclear Information System (INIS)

Application of Clifford algebra in the analysis of valence-band spin properties in semiconductors is considered. In the first part (Dargys A 2009 Phys. Scr. 80 065701), for this purpose the isomorphism between multivectors and their matrix representations was used to transform the problem to Clifford algebra. Here equivalence rules are established between the spinors of Hilbert space and basis elements of the five-dimensional Clifford algebra Cl4,1. Then, the rules are applied to the total angular momentum components and the two-band hole Hamiltonian. The resulting biquaternionic Schroedinger equation for hole spin is solved as an example.

10

Valence-band photoemission intensities in thorium dioxide  

International Nuclear Information System (INIS)

Resonant photoemission spectra of the O 2p-derived valence band of insulating ThO2 are compared to linear muffin-tin orbital (LMTO) density-of-state (DOS) and XPS intensity calculations. At Th 5d core-level threshold energies (85 ? hv ? 120 eV), resonance is greatest at the bottom of the O 2p band where calculated p/d hybrid states are greatest; p/f hybrid content is weak by comparison. We conclude that the dominant hybridization is between O 2p states and Th 6d. (author)

11

Relaxation and cross section effects in valence band photoemission spectroscopy  

International Nuclear Information System (INIS)

Various problems relating to the interpretation of valence band x-ray photoemission (XPS) spectra of solids are discussed. The experiments and calculations reported herein deal with the following questions: (1) To what extent do many-body effects manifest themselves in an XPS valence band spectrum, and thus invalidate a direct comparison between the photoemission energy distribution, I(E), and the density of states, N(E), calculated on the basis of ground-state one-electron theory. (2) The effect of the binding-energy-dependent photoemission cross section on I(E) at XPS energies. (3) In favorable cases indicated by (1) and (2) we examine the effect of the interaction of the crystal field with the apparent spin-orbit splittings of core levels observed in XPS spectra. (4) The use of tight binding band structure calculations to parameterize the electronic band structure from XPS and other data is described. (5) The use of high energy angle-resolved photoemission on oriented single crystals to gain orbital symmetry information is discussed. (6) The evolution of the shape of the photoemission energy distribution (of polycrystalline Cu) as a function of photon energy from 50 less than or equal h ? less than or equal 175 is discussed

12

Anisotropy of the silicon valence band induced by strain with various orientations  

Science.gov (United States)

Using the first-principles-optimized k•p method and the first-principles density-functional theory in a cooperative manner, we investigate the anisotropy of the silicon valence band induced by uniaxial or biaxial strain with various orientations. Our analysis of the angle dependence of the hole effective mass m* shows that, for all the strain orientations considered, the m* value becomes strikingly small in the strain-parallel direction for uniaxial compression and in the strain-perpendicular direction for biaxial tensility. Our examination also clarifies the equivalence between the effect of uniaxial compression (tensility) and that of biaxial tensility (compression) in the plane perpendicular to the above uniaxial compression (tensility). This equivalence can be elucidated in terms of the ratio of the lattice constant along the uniaxial-strain direction to that in the biaxial-strain plane. We also recognize that the normal compression involved in biaxial tensility under the condition of no normal stress increases the valence-band splitting, and consequently extends the segment of the highest valence-band dispersion with a conspicuously reduced m* value to a lower energy region, so that this segment can accommodate most of the holes.

Inaoka, Takeshi; Kinjyo, YÅ«ki; Yanagisawa, Susumu; Tomori, Kazuya

2013-05-01

13

Electric fields and valence band offsets at strained [111] heterojunctions  

CERN Document Server

[111] ordered common atom strained layer superlattices (in particular the common anion GaSb/InSb system and the common cation InAs/InSb system) are investigated using the ab initio full potential linearized augmented plane wave (FLAPW) method. We have focused our attention on the potential line-up at the two sides of the homopolar isovalent heterojunctions considered, and in particular on its dependence on the strain conditions and on the strain induced electric fields. We propose a procedure to locate the interface plane where the band alignment could be evaluated; furthermore, we suggest that the polarization charges, due to piezoelectric effects, are approximately confined to a narrow region close to the interface and do not affect the potential discontinuity. We find that the interface contribution to the valence band offset is substantially unaffected by strain conditions, whereas the total band line-up is highly tunable, as a function of the strain conditions. Finally, we compare our results with those ...

Picozzi, S; Freeman, A J

1997-01-01

14

Internal-strain effect on the valence band of strained silicon and its correlation with the bond angles  

International Nuclear Information System (INIS)

By means of the first-principles density-functional theory, we investigate the effect of relative atom displacement in the crystal unit cell, namely, internal strain on the valence-band dispersion of strained silicon, and find close correlation of this effect with variation in the specific bond angles due to internal strain. We consider the [111] ([110]) band dispersion for (111) ((110)) biaxial tensility and [111] ([110]) uniaxial compression, because remarkably small values of hole effective mass m* can be obtained in this dispersion. Under the practical condition of no normal stress, biaxial tensility (uniaxial compression) involves additional normal compression (tensility) and internal strain. With an increase in the internal-strain parameter, the energy separation between the highest and second-highest valence bands becomes strikingly larger, and the highest band with conspicuously small m* extends remarkably down to a lower energy region, until it intersects or becomes admixed with the second band. This is closely correlated with the change in the specific bond angles, and this change can reasonably explain the above enlargement of the band separation

15

Internal-strain effect on the valence band of strained silicon and its correlation with the bond angles  

Science.gov (United States)

By means of the first-principles density-functional theory, we investigate the effect of relative atom displacement in the crystal unit cell, namely, internal strain on the valence-band dispersion of strained silicon, and find close correlation of this effect with variation in the specific bond angles due to internal strain. We consider the [111] ([110]) band dispersion for (111) ((110)) biaxial tensility and [111] ([110]) uniaxial compression, because remarkably small values of hole effective mass m* can be obtained in this dispersion. Under the practical condition of no normal stress, biaxial tensility (uniaxial compression) involves additional normal compression (tensility) and internal strain. With an increase in the internal-strain parameter, the energy separation between the highest and second-highest valence bands becomes strikingly larger, and the highest band with conspicuously small m* extends remarkably down to a lower energy region, until it intersects or becomes admixed with the second band. This is closely correlated with the change in the specific bond angles, and this change can reasonably explain the above enlargement of the band separation.

Inaoka, Takeshi; Yanagisawa, Susumu; Kadekawa, Yukihiro

2014-02-01

16

Finite Element Analysis of Valence Band Structure of Square Quantum Well Under the Electric Field  

Science.gov (United States)

Valence band structure with spin-orbit (SO) coupling of GaAs/Ga1-xAlxAs square quantum well (SQW) under the electric field by a calculation procedure based on a finite element method (FEM) is investigated using the multiband effective mass theory (?c {k}?ot ?c {p} method). The validity of the method is confirmed with the results of D. Ahn, S. L. Chuang and Y. C. Chang (J. Appl. Phys. 64 (1998) 4056), who calculated valence band structure, using axial approximation for Luttinger-Kohn Hamiltonian and finite difference method. Our results demonstrated that SO coupling and electric field have significant effects on the valence band structure.

Gunes, M.; Kasapoglu, E.; Sari, H.; Akgungor, K.; Sökmen, I.

17

The absolute energy positions of conduction and valence bands of selected semiconducting minerals  

Energy Technology Data Exchange (ETDEWEB)

The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H{sub 2}O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H{sub 2}O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H{sub 2}O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

Xu, Y.; Schoonen, M.A.A.

2000-04-01

18

Valence band electronic structure of V2O3: identification of V and O bands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a comprehensive study of the photon energy dependence of the valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The analysis of our experimental results, covering an extended photon energy range (20-6000 eV) and combined with GW calculations, allow us to identify the nature of the orbitals contributing to the total spectral weight at different binding energies, and in particular to locate the V 4s at about 8 eV binding energy. From this c...

Papalazarou, E.; Gatti, Matteo; Marsi, M.; Brouet, V.; Iori, F.; Reining, Lucia; Annese, E.; Vobornik, I.; Offi, F.; Fondacaro, A.; Huotari, S.; Lacovig, P.; Tjernberg, O.; Brookes, N. B.; Sacchi, M.

2009-01-01

19

Correlation effects in the valence bands of ferromagnetic semiconductor EuS  

CERN Document Server

We present a many body analysis of the multi-band Kondo lattice model. The study is then combined with the first principles TB-LMTO band structure calculations, in order to investigate the temperature dependent correlation effects in the 3$\\textit{p}$ valence bands of the ferromagnetic semiconductor EuS. Some of the physical properties of interest like the quasi-particle density of states (Q-DOS), spectral density (SD) and quasi-particle band structure (Q-BS) are calculated and discussed. Therewith, we propose a spin resolved ARPES of the valence bands of EuS to be performed.

Sharma, A

2005-01-01

20

Determination of xenon valence and conduction bands by spin-polarized photoemission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Using circularly polarized synchrotron radiation and spin-resolved photoemission, we have studied the valence and conduction bands of Xe(111) up to about 18 eV above the valence-band maximum. Direct information about the symmetry of bands and their hybridization is provided by photoelectron spin polarization. Our data also prove the existence of a gap in the unoccupied bands above the fundamental gap. A comparison of our data with calculated band structures shows the importance of an energy-d...

Kessler, B.; Eyers, A.; Horn, K.; Mu?ller, Norbert; Schmiedeskamp, G.; Scho?nhense, G.; Heinzmann, Ulrich

1987-01-01

 
 
 
 
21

Determination of xenon valence and conduction bands by spin-polarized photoemission  

Science.gov (United States)

Using circularly polarized synchrotron radiation and spin-resolved photoemission, we have studied the valence and conduction bands of Xe(111) up to about 18 eV above the valence-band maximum. Direct information about the symmetry of bands and their hybridization is provided by photoelectron spin polarization. Our data also prove the existence of a gap in the unoccupied bands above the fundamental gap. A comparison of our data with calculated band structures shows the importance of an energy-dependent self-energy correction.

Kessler, B.; Eyers, A.; Horn, K.; Müller, N.; Schmiedeskamp, B.; Schönhense, G.; Heinzmann, U.

1987-07-01

22

A k · p analytical model for valence band of biaxial strained Ge on (001) Si1?xGex  

International Nuclear Information System (INIS)

In this paper, the dispersion relationship is derived by using the k · p method with the help of the perturbation theory, and we obtain the analytical expression in connection with the deformation potential. The calculation of the valence band of the biaxial strained Ge/(001)Si1?xGex is then performed. The results show that the first valence band edge moves up as Ge fraction x decreases, while the second valence band edge moves down. The band structures in the strained Ge/ (001)Si0.4Ge0.6 exhibit significant changes with x decreasing in the relaxed Ge along the [0, 0, k] and the [k, 0, 0] directions. Furthermore, we employ a pseudo-potential total energy package (CASTEP) approach to calculate the band structure with the Ge fraction ranging from x = 0.6 to 1. Our analytical results of the splitting energy accord with the CASTEP-extracted results. The quantitative results obtained in this work can provide some theoretical references to the understanding of the strained Ge materials and the conduction channel design related to stress and orientation in the strained Ge pMOSFET. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

23

XPS valence band spectra and theoretical calculations for investigations on thiogermanate and thiosilicate glasses  

International Nuclear Information System (INIS)

This paper reports on investigations of thiogermanate and thiosilicate crystals and glasses by means of XPS valence band spectra and theoretical calculations (FLAPW method). The calculations were achieved on three crystallized phases GeS2, Na2GeS3 and SiS2 and valence band spectra (visualization of the occupied electronic density of states) were precisely interpreted through modulated density of states and charge density maps. This information was used to go further in the structural investigations of some thiogermanate and thiosilicate glasses. In sodium thiogermanates, an increase in Ge-Ge bonds was revealed as the modifier content (Na2S) increases. In thiosilicates, the evolution of the valence spectra according to the nature of the alkaline atoms (Li, Na) has been interpreted as changes in the local connectivity of units (edge or corner sharing tetrahedra). This study exhibits the potentialities of valence band spectra to provide information on glassy systems

24

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

Science.gov (United States)

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

25

Gd 4f and 5d electrons in Sn0.96Gd0.04Te valence band  

International Nuclear Information System (INIS)

The synchrotron radiation was applied to measure resonant photoemission spectra (Fano-type Gd 4d-4f resonance), constant initial states and constant final states to study the valence band electronic structure of Sn0.96Gd0.4Te crystal. The resonant energy was found to equal to 150.3 eV. The electrons 4f were found to contribute to the valence band of crystal with the maximum located at the crystal valence band edge. (author)

26

Resonating Valence Bond Mechanism of Impurity Band Superconductivity in Diamond  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Superconductivity in an uncompensated boron doped diamond, a very recent observation, is strikingly close to an earlier observation of Anderson-Mott insulator to metal transition, prompting us to suggest an electron correlation driven superconductivity in an impurity band. Random coulomb potential remove a three fold orbital degeneracy of boron acceptor states, resulting in an effective single, narrow, tight binding and half filled band of holes. Singlet coupling between spi...

Baskaran, G.

2004-01-01

27

Composition dependent valence band order in c-oriented wurtzite AlGaN layers  

Science.gov (United States)

The valence band order of polar wurtzite aluminum gallium nitride (AlGaN) layers is analyzed for a dense series of samples, grown heteroepitaxially on sapphire substrates, covering the complete composition range. The excitonic transition energies, found by temperature dependent photoluminescence (PL) spectroscopy, were corrected to the unstrained state using input from X-ray diffraction. k ?p theory yields a critical relative aluminum concentration xc=(0.09±0.05) for the crossing of the uppermost two valence bands for strain free material, shifting to higher values for compressively strained samples, as supported by polarization dependent PL. The analysis of the strain dependent valence band crossing reconciles the findings of other research groups, where sample strain was neglected. We found a bowing for the energy band gap to the valence band with ?9 symmetry of b?9=0.85eV, and propose a possible bowing for the crystal field energy of bcf=-0.12eV. A comparison of the light extraction efficiency perpendicular and parallel to the c axis of AlxGa1-xN/AlyGa1-yN quantum well structures is discussed for different compositions.

Neuschl, B.; Helbing, J.; Knab, M.; Lauer, H.; Madel, M.; Thonke, K.; Meisch, T.; Forghani, K.; Scholz, F.; Feneberg, M.

2014-09-01

28

On the observation of intervalence charge transfer bands in hydrogen-bonded mixed-valence complexes.  

Science.gov (United States)

Ruthenium clusters of the type [Ru3(?3-O)(OAc)6(CO)(L)(nic)], where L = 4-dimethylaminopyridine (dmap) and nic = isonicotinic acid, form hydrogen-bonded mixed-valence dimers upon a single electron reduction. Electrochemical responses show two overlapping reduction waves, indicating the presence of a thermodynamically stable mixed-valence dimer with considerable electronic coupling across the hydrogen bond. Electronic spectra of the singly reduced hydrogen-bonded mixed-valence dimer reveal two intervalence charge transfer bands in the near-infrared region consistent with a Robin-Day class II system. These bands are assigned as metal-to-metal and metal-to-bridge charge transfer, and their behavior is best described by a semiclassical three state model. Infrared spectroscopy suggests localized behavior indicating electron transfer between the two clusters is slower than 10(10) s(-1). PMID:24437710

Canzi, Gabriele; Goeltz, John C; Henderson, Jane S; Park, Roger E; Maruggi, Chiara; Kubiak, Clifford P

2014-02-01

29

Rotational bands terminating at maximal spin in the valence space  

Energy Technology Data Exchange (ETDEWEB)

For nuclei with mass A {le} 120, the spin available in {open_quotes}normal deformation configurations{close_quotes} is experimentally accessible with present detector systems. Of special interest are the nuclei which show collective features at low or medium-high spin and where the corresponding rotational bands with increasing spin can be followed in a continuous way to or close to a non-collective terminating state. Some specific features in this context are discussed for nuclei in the A = 80 region and for {sup 117,118}Xe.

Ragnarsson, I.; Afanasjev, A.V. [Lund Institute of Technology (Sweden)

1996-12-31

30

Discrete-variational Dirac-Slater calculations on the valence band XPS for ?-uranium metal  

International Nuclear Information System (INIS)

We have performed relativistic molecular orbital calculations for the valence band structure of ?-uranium metal, using the discrete-variational Dirac-Slater (DV-DS) method. We have used a U9 model cluster for the calculation of the bulk electronic structure of ?-uranium metal. The calculated valence X-ray photoelectron spectrum (XPS) which is obtained from the U9 model cluster agrees well with the experimental spectrum. The peaks in the experimental spectrum are assigned to atomic components on the basis of the partial density of states (pDOS) for uranium atomic orbitals (AOs)

31

XPS valence bands of La, Ce and Gd and their aluminium alloys  

International Nuclear Information System (INIS)

The XPS valence band spectra of La, Ce and Gd metal and of their aluminium alloys show the 5d band in La metal at 0.8 eV below the Fermi energy with a width of about 1.5 eV; in Ce the 4f band and 5d band are at 0.5 eV and 1.7 eV below the Fermi energy with widths of 0.7 eV and 1.5 eV respectively. The Gd 5d band is very similar to the La 5d band, while the 4f level is well localized at 8.0 eV below the Fermi energy. (author)

32

Theory of the Spin-Splitting of the Valence Band Landau Levels in GaAs  

Science.gov (United States)

We present calculations of the spin-dependent band structure and magneto-optical absorption spectra in bulk GaAs. Our calculations are based on the 8-band Pidgeon-Brown model. Optical properties are calculated within the golden rule approximation and compared with experiments. Although GaAs has a small g-factor leading to nearly spin-degenerate conduction band Landau levels, the valence band Landau levels are spin-split and the spin splitting can be observed in the circularly polarized magneto-absorption spectra. By carefully analyzing the energy band structure and the absorption spectra together, we identify the origins of all the optical transitions. We also separate contributions to the absorption coefficient from spin-up electrons and spin-down electrons to get the conduction band electron spin polarization. This information is used to compute the optically-pumped NMR (OPNMR) signal. We demonstrate that OPNMR can provide unique insight into the spin-dependent valence band electronic states.

Pan, X.; Sanders, G. D.; Stanton, C. J.; Ramaswamy, K.; Mui, S.; Hayes, S. E.; Crooker, S. A.

2009-03-01

33

Valence band structure in boron-zinc oxide films characterized by secondary electron emission  

Science.gov (United States)

Properties of the valence band structure in boron-zinc oxide (BZO) films were investigated using the secondary electron emission due to the Auger neutralization of helium ions, with respect to the application of BZO films to the development of solar cells, in which the conductivity of the BZO films plays a critical role in improving cell performance. The characteristic energy ?0 corresponding to the peak density of states in the valence band showed that BZO film prepared with a 3000 SCCM B2H6 gas flow rate (SCCM denotes cubic centimeters per minute at standard temperature and pressure) had a shallow characteristic energy ?0 = 5 eV, whereas film without boron doping had a deep characteristic energy ?0 = 8.2 eV, suggesting that a high concentration of boron impurity in BZO films might enhance the transition of electrons and holes through the bandgap from the valence to the conduction band in zinc oxide crystals, thereby improving the conductivity of the film. The measurement method developed here demonstrates that the secondary electron emission is very useful in the determination of the band structure in various synthetic films.

Uhm, Han S.; Choi, Joon H.; Yoo, Ha J.; Kwon, Gi C.; Choi, Eun H.

2012-03-01

34

Valence band structure of the icosahedral Ag-In-Yb quasicrystal  

International Nuclear Information System (INIS)

The valence band structure of the icosahedral (i) Ag-In-Yb quasicrystal, which is isostructural to the binary i-Cd-Yb system, is investigated by ultraviolet photoemission spectroscopy (UPS). Experimental results are compared with electronic-structure calculations of a cubic approximant of the same phase. UPS spectra from the fivefold, threefold, and twofold i-Ag-In-Yb surfaces reveal that the valence band near to the Fermi level is dominated by Yb 4f-derived states, in agreement with calculations. The spectra also exhibit peaks which are surface core level shifted, caused by changes in the electronic structure in surface layers. Calculations yield a pseudogap in the density of states due to a hybridization of the Yb 5d band with the Ag 5p and In 5p bands. Both experimental and calculated band features are very similar to those of Cd-Yb. The modification of the band structure after surface treatment by sputtering and by oxidation is also studied. Additionally, the work function of i-Ag-In-Yb measured from the width of UPS spectrum is found to be almost unaffected by surface orientation, but increases after sputtering or oxidation.

35

Radiative transitions between the anion and cation valence bands in CsBr crystals  

International Nuclear Information System (INIS)

6 eV luminescence excitation spectrum for CsBr crystals has the threshold energy of 13.5 eV coinciding with the beginning of photoionization of cesium ions. The luminescence results from electron recombination of the anion valence band with holes autolocalized on cations. Decay of cation excitons (13.2 eV) is followed in CsBr by formation of cation defects manifesting in thermostimulated luminescence

36

Atomic cross-section effects in soft-x-ray photoemission from Ag valence bands  

International Nuclear Information System (INIS)

The relative photoemission intensity from the Ag 4d valence band (VB) was studied as a function of photon energy using 32--250 eV synchrotron radiation. A sharp decrease in intensity of more than an order of magnitude was observed in the range 100 or = 150 eV range in order to bridge the ultraviolet-photomission to x-ray-photoemission gap

37

Valence band study of LaNiO{sub 3-{delta}} thin films  

Energy Technology Data Exchange (ETDEWEB)

The resonant photoemission spectroscopy was used to study the surface electronic structure under La 4d{yields}4f and Ni 3p{yields}3d photo-excitation of thin LaNiO{sub 3-{delta}} films after annealing in ultrahigh vacuum above dehydration temperature. The giant resonance in La 5p and La 5s peaks intensity observed at excitation energy corresponding to a La 4d{yields}4f threshold is accompanied by resonance of the N{sub 4,5}O{sub 2,3}O{sub 2,3} and N{sub 4,5}O{sub 2,3}V Auger peaks. The enhancement in the intensity of valence band maxima (at about 6 eV) may be explained by the small mixing of the La 5d ionic character to the O 2p valence band. The week resonant features observed in the valence band spectra under Ni 3p{yields}3d threshold indicate the loss of nickel species at the LaNiO{sub 3-{delta}} film surface after heat treatment.

Grebinskij, S.; Senulis, M.; Tvardauskas, H.; Bondarenka, V.; Lisauskas, V.; Sliuziene, K.; Vengalis, B. [Institute of Semiconductor Physics, Center for Physical Sciences and Technology, A. Gostauto 11, LT-01108 Vilnius (Lithuania); Orlowski, B.A. [Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw (Poland); Johnson, R.L. [Hamburger Synchrotronstrahlungslabor HASYLAB am Deutschen Elektronen-Synchrotron DESY, Notkestr. 85, D-22603 Hamburg (Germany); Mickevicius, S., E-mail: sigism@pfi.lt [Institute of Semiconductor Physics, Center for Physical Sciences and Technology, A. Gostauto 11, LT-01108 Vilnius (Lithuania)

2011-10-15

38

On the theory of phonoriton in cubic semiconductors with a degenerate valence band  

International Nuclear Information System (INIS)

The ''phonoriton'' is an elementary excitation constructed from an exciton polariton and phonon in semiconductors under intense excitation by an electromagnetic wave near the exciton resonance (L.V. Keldysh and A.L. Ivanov, 1982). In this paper we develop a theory of phonoriton in direct band gap cubic semiconductor with a degenerate valence band using the simple model of J.L. Birman and B.S. Wang (1990). In addition to experimental proofs of the existence of phonoriton we propose an experiment to measure its flight time. (author). 33 refs

39

Band width and multiple-angle valence-state mapping of diamond  

Energy Technology Data Exchange (ETDEWEB)

The band width may be considered the single most important parameter characterizing the electronic structure of a solid. The ratio of band width and Coulomb repulsion determines how correlated or delocalized an electron system is. Some of the most interesting solids straddle the boundary between localized and delocalized, e.g. the high-temperature superconductors. The bulk of the band calculations available today is based on local density functional (DF) theory. Even though the Kohn-Sham eigenvalues from that theory do not represent the outcome of a band-mapping experiment, they are remarkably similar to the bands mapped via photoemission. Strictly speaking, one should use an excited state calculation that takes the solid`s many-body screening response to the hole created in photoemission into account. Diamond is a useful prototype semiconductor because of its low atomic number and large band width, which has made it a long-time favorite for testing band theory. Yet, the two experimental values of the band width of diamond have error bars of {+-}1 eV and differ by 3.2 eV. To obtain an accurate valence band width for diamond, the authors use a band-mapping method that collects momentum distributions instead of the usual energy distributions. This method has undergone extensive experimental and theoretical tests in determining the band width of lithium fluoride. An efficient, imaging photoelectron spectrometer is coupled with a state-of-the-art undulator beam line at the Advanced Light Source to allow collection of a large number of data sets. Since it takes only a few seconds to take a picture of the photoelectrons emitted into a 84{degrees} cone, the authors can use photon energies as high as 350 eV where the cross section for photoemission from the valence band is already quite low, but the emitted photoelectrons behave free-electron-like. This make its much easier to locate the origin of the inter-band transitions in momentum space.

Jimenez, I.; Terminello, L.J.; Sutherland, D.G.J. [Lawrence Berkeley National Lab., CA (United States)] [and others

1997-04-01

40

Valence-skipping and negative-U in the d-band from repulsive local Coulomb interaction  

Science.gov (United States)

We show that repulsive local Coulomb interaction alone can drive valence-skipping charge disproportionation in the degenerate d-band, resulting in effective negative-U. This effect is shown to originate from anisotropic orbital-multipole scattering, and it occurs only for d1,d4,d6, and d9 fillings (and their immediate surroundings). Explicit boundaries for valence-skipping are derived, and the paramagnetic phase diagram for d4 and d6 is calculated. We also establish that the valence-skipping metal is very different, in terms of its local valence distribution, compared to the atomiclike Hund's metal. These findings explain why transition-metal compounds with the aforementioned d-band fillings are more prone to valence-skipping charge order and anomalous superconductivity.

Strand, Hugo U. R.

2014-10-01

 
 
 
 
41

Valence band photoemission studies of the Si(111)-2 x 1 and 7 x 7 surfaces: A comparison  

International Nuclear Information System (INIS)

Soft x-ray photoemission studies of the Si(111) surface at highly surface sensitive photon energies (65eV - 170eV) have revealed the differences in the surface valence band electronic structure as a result of the different surface reconstructions. By annealing the in situ cleaved Si(111) crystal, displaying the 2 x 1 reconstruction, the formed 7 x 7 surface shows significant changes in the valence band. For the 7 x 7 surface, the peak closest to the Fermi level in the valence band EDC, i.e. the most p-like peak, appears as a very sharp structure which to our knowledge has not been observed before. The rest of the valence band looks similar for the two reconstructions except that the intensity of the two deeper lying valence band structures are more suppressed for the 7 x 7 surface at the higher photon energies. This first direct comparison of the valence band EDC's of the Si(111)-2 x 1 and 7 x 7 surfaces from the same cleaved crystal provides clear evidence that the surface electronic structure depends on the surface atomic structure. It also implies that the theoretical bulk structure of Si(111) must be modified for the different surface reconstructions to give an adequate description of the surface electronic structure

42

Pressure variation of the valence band width in Ge: A self-consistent GW study  

DEFF Research Database (Denmark)

Analyzing x-ray emission spectra XES of germanium under pressure Struzhkin et al. [Phys. Rev. Lett. 96, 137402 (2006)] found that the valence band width of diamond Ge does not vary with pressure. This contradicts the usual experience and also what is predicted by density-functional calculations. In the present work we report results of quasiparticle self-consistent GW  (QSGW) band calculations for diamond- as well as ?-tin-type Ge under pressure. For both phases we find that the band width increases with pressure. For ?-tin Ge this agrees with experiment and density-functional theory, but for diamond Ge neither the local density approximation nor the QSGW calculations agree with the conclusions drawn from the XES data.

Modak, Paritosh; Svane, Axel

2009-01-01

43

Exchange interaction between paramagnetic centers and valence band electrons in semiconductors with cubic lattices  

International Nuclear Information System (INIS)

A Hamiltonian is derived for the exchange interaction between paramagnetic ions and electrons from the bands GAMMA6, GAMMA7, GAMMA8, that is the result of the splitting GAMMA'15, GAMMA'25, by the spin-orbit interaction of arbitrary intensity ?sub(so) in semicunductors with cubic lattices. It is shown that, ?sub(so) being sufficiently large, the Hamiltonian of each splitted subzone is of Heisenberg form. Formulas are derived and numerical estimations are carried out for the exchange interaction constants; the relation of the latter to the crystal band structure is discussed. The sign difference which was discovered experimentally earlier for the constants of exchange coupling between 3d ions and electrons of valence and conduction bands in Asup(II)Bsup(VI):Mn2+ crystals is explained by the results of the theory. (author)

44

Valence band offset of wurtzite InN/SrTiO3 heterojunction measured by x-ray photoelectron spectroscopy  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract The valence band offset (VBO of wurtzite indium nitride/strontium titanate (InN/SrTiO3 heterojunction has been directly measured by x-ray photoelectron spectroscopy. The VBO is determined to be 1.26 ± 0.23 eV and the conduction band offset is deduced to be 1.30 ± 0.23 eV, indicating the heterojunction has a type-I band alignment. The accurate determination of the valence and conduction band offsets paves a way to the applications of integrating InN with the functional oxide SrTiO3.

Li Zhiwei

2011-01-01

45

Valence band offset of InN/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract X-ray photoelectron spectroscopy has been used to measure the valence band offset of the InN/BaTiO3 heterojunction. It is found that a type-I band alignment forms at the interface. The valence band offset (VBO and conduction band offset (CBO are determined to be 2.25 ± 0.09 and 0.15 ± 0.09 eV, respectively. The experimental VBO data is well consistent with the value that comes from transitivity rule. The accurate determination of VBO and CBO is important for use of semiconductor/ferrroelectric heterojunction multifunctional devices.

Zhang Weifeng

2011-01-01

46

Valence band offset of wurtzite InN/SrTiO3 heterojunction measured by x-ray photoelectron spectroscopy.  

Science.gov (United States)

The valence band offset (VBO) of wurtzite indium nitride/strontium titanate (InN/SrTiO3) heterojunction has been directly measured by x-ray photoelectron spectroscopy. The VBO is determined to be 1.26 ± 0.23 eV and the conduction band offset is deduced to be 1.30 ± 0.23 eV, indicating the heterojunction has a type-I band alignment. The accurate determination of the valence and conduction band offsets paves a way to the applications of integrating InN with the functional oxide SrTiO3. PMID:21711731

Li, Zhiwei; Zhang, Biao; Wang, Jun; Liu, Jianming; Liu, Xianglin; Yang, Shaoyan; Zhu, Qinsheng; Wang, Zhanguo

2011-01-01

47

Electron-Phonon mechanism for Superconductivity in Na$_{0.35}$CoO$_2$: Valence-Band Suhl-Kondo effect Driven by Shear Phonons  

CERN Document Server

To study the possible mechanism of superconductivity in Na$_{0.35}$CoO$_2$, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and $t_{2g}(a_{1g}+e_g')$-electrons of Co-ions, and study the transition temperature for a s-wave superconductivity. The obtained $T_{\\rm c}$ is very low when the $e_g'$-valence-bands are far below the Fermi level. However, $T_{\\rm c}$ is strongly enhanced when the top of the $e_g'$-valence-bands is close to the Fermi level (say -50meV), thanks to interband hopping of Cooper pairs caused by shear phonons. This ``valence-band Suhl-Kondo mechanism'' due to shear phonons is significant to understand the superconductivity in Na$_{0.35}$CoO$_2$. By the same mechanism, the kink structure of the band-dispersion observed by ARPES, which indicates the strong mass-enhancement ($m^\\ast/m\\sim3$) due to optical phonons, is also explained.

Yada, K; Yada, Keiji; Kontani, Hiroshi

2006-01-01

48

Experimental study of the valence band of Bi2Se3  

Science.gov (United States)

The valence band of Bi2Se3 is investigated by measuring the Shubnikov-de Haas effect as well as galvanomagnetic and thermoelectric transports. At low hole concentration, the hole Fermi surface is closed and boxlike, but at higher carrier concentrations it develops tubelike extensions that are open, in general agreement with our theoretical calculations. However, the experimentally determined density-of-states effective mass is smaller than density-functional-theory calculations predict; although we cannot give a definitive explanation for this, we suspect that the theory may lack sufficient precision to compute room-temperature transport properties, such as the thermopower, in solids with interlayer van der Waals bonds.

Gao, Yi-Bin; He, Bin; Parker, David; Androulakis, Ioannis; Heremans, Joseph P.

2014-09-01

49

Wavefunctions in the valence band of InAs quantum dots  

International Nuclear Information System (INIS)

Magneto-capacitance spectroscopy of InAs self-assembled quantum dots allows the momentum space wavefunctions mapping of conduction and valence band states. To do this, a magnetic field is varied (strength and direction) in the base plane of the quantum dots. The perpendicular magnetic field would also influence the wavefunctions, therefore capacitance-voltage spectroscopy in a tilted field should reveal this effect. We present the result of such experiments performed on self-assembled InAs/GaAs quantum dots embedded in a Schottky diode with a carbon doped p-type back contact. The results cannot be explained by a simple model and possible consequences are discussed.

50

Highly dispersive photonic band-gap-edge optofluidic biosensors  

Science.gov (United States)

Highly dispersive photonic band-gap-edge optofluidic biosensors are studied theoretically. We demonstrate that these structures are strongly sensitive to the refractive index of the liquid, which is used to tune dispersion of the photonic crystal. The upper frequency band-gap edge shifts about 1.8 nm for ? n=0.002, which is quite sensitive. Results from transmission spectra agree well with those obtained from the band structure theory.

Xiao, S.; Mortensen, N. A.

2006-11-01

51

Valence band and core-level photoemission of Au/Ge(001): Band mapping and bonding sites  

Science.gov (United States)

We have used photoemission spectroscopy in order to investigate the electronic states and chemical bonding related to Au induced atomic chains on the Ge(001) surface. Angle-resolved photoemission reveals two types of dispersions around the Fermi level whose intensities strongly depend on the incident photon energy. Around h? =100eV, the band structure is dominated by an electronlike band of mainly one-dimensional (1D) character, which shows Tomonaga-Luttinger-like power-law behavior in the k-integrated spectral function. In contrast, lower photon energies reveal a metallic holelike dispersion which resembles the Ge bulk structure with its heavy-hole, light-hole, and split-off branches. The Au 4f core-level spectra show two doublets indicating two different Au bonding sites, whereas the Ge 3d core-level shows two surface components and one bulk component.

Meyer, S.; Dudy, L.; Schäfer, J.; Blumenstein, C.; Höpfner, P.; Umbach, T. E.; Dollinger, A.; Cui, X. Y.; Patthey, L.; Claessen, R.

2014-09-01

52

Determination of the valence band offset of wurtzite InN/ZnO heterojunction by x-ray photoelectron spectroscopy  

International Nuclear Information System (INIS)

The valence band offset (VBO) of the wurtzite InN/ZnO heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.82±0.23 eV. The conduction band offset is deduced from the known VBO value to be 1.85±0.23 eV, which indicates a type-I band alignment for InN/ZnO heterojunction

53

Valence-band spectra and electronic structure of CuFeO2  

Science.gov (United States)

The delafossite-type CuFeO2 single crystal was studied by means of x-ray emission and x-ray photoelectron spectroscopy. The valence state of Cu ions was found to be 1+, whereas Fe ions were found to be trivalent in the high-spin S=5/2 state. The x-ray emission (Cu L?, Fe L?, and O K?) and photoelectron spectra were compared to the results of the local spin density approximation (LSDA) (full-potential linearized augmented plane wave method and linearized muffin-tin orbitals in atomic sphere approximation method) and LSDA+U calculations. It is found that the maximum of the Cu 3d state distribution is localized closer to the Fermi level than that of the Fe 3d states. The LSDA calculations contradict the experimental results and do not give a correct description of the Cu and Fe 3d positions relative to the Fermi level, and incorrectly predict metallic behaviors (semiconductor observed) and give qualitatively incorrect magnetic properties of CuFeO2. The LSDA+U calculations give a much better agreement with the observed valence-band structure, the measured electrical, and the magnetic properties.

Galakhov, V. R.; Poteryaev, A. I.; Kurmaev, E. Z.; Anisimov, V. I.; Bartkowski, St.; Neumann, M.; Lu, Z. W.; Klein, B. M.; Zhao, Tong-Rong

1997-08-01

54

Character of states near the Fermi level in (Ga,Mn)As: Impurity to valence band crossover  

Science.gov (United States)

We discuss the character of states near the Fermi level in Mn-doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence-band contribution to dc transport, a midinfrared peak at energy ???200meV in the ac conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low-doped ( ?1% Mn) insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity-band ionization energy inferred from the dc transport and the position of the midinfrared peak move to lower energies, and the peak broadens with increasing Mn concentration. In metallic materials with >2% doping, no traces of Mn-related activated contribution can be identified in dc transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher-energy (???250meV) midinfrared feature which appears in the metallic samples is associated with inter-valence-band transitions. Its redshift with increased doping can be interpreted as a consequence of increased screening, which narrows the localized-state valence-band tails and weakens higher-energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence-band picture of high-doped metallic GaAs:Mn material.

Jungwirth, T.; Sinova, Jairo; MacDonald, A. H.; Gallagher, B. L.; Novák, V.; Edmonds, K. W.; Rushforth, A. W.; Campion, R. P.; Foxon, C. T.; Eaves, L.; Olejník, E.; Mašek, J.; Yang, S.-R. Eric; Wunderlich, J.; Gould, C.; Molenkamp, L. W.; Dietl, T.; Ohno, H.

2007-09-01

55

Magneto-photoluminescence of GaN/AlGaN quantum wells: valence band reordering and excitonic binding energies  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for...

Shields, P. A.; Nicholas, R. J.; Grandjean, N.; Massies, J.

2001-01-01

56

Valence band offsets at oxide/InN interfaces determined by X-ray photoelectron spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

The valence band offset (VBO) at the interface between indium nitride (InN) and selected oxide materials (Al{sub 2}O{sub 3}, TiO{sub 2}, In{sub 2}O{sub 3} and HfO{sub 2}) is determined using X-ray photoelectron spectroscopy (XPS). For exact VBO determination, InN samples with oxide cap layers of varying thickness are investigated. The VBO values are extrapolated by linear regression of the thickness dependent energetic distances {delta}E between the valence band maxima (VBM) at the oxide and InN surface and their corresponding heterointerface. The determined VBO values are (2.7 {+-} 0.2) eV for Al{sub 2}O{sub 3}/InN, (1.8 {+-} 0.2) eV for TiO{sub 2}/InN, (1.5 {+-} 0.2) eV for In{sub 2}O{sub 3}/InN, (1.3 {+-} 0.2) eV for e-beam evaporated HfO{sub 2} on InN, and (2.0 {+-} 0.2) eV for atomic layer deposited HfO{sub 2} on InN. In some cases the oxide deposition process leads to an oxidation of the InN film at the oxide/InN interface (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Eisenhardt, Anja; Eichapfel, Georg; Himmerlich, Marcel; Krischok, Stefan [Institut fuer Physik and Institut fuer Mikro- und Nanotechnologien, TU Ilmenau, PF 100565, 98684 Ilmenau (Germany); Knuebel, Andreas; Passow, Thorsten; Wang, Chunyu; Benkhelifa, Fouad; Aidam, Rolf [Fraunhofer-Institut fuer Angewandte Festkoerperphysik, Tullastr. 72, 79108 Freiburg (Germany)

2012-03-15

57

Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)  

International Nuclear Information System (INIS)

Auger lineshapes of the Ge M1M4sub(,)5V and M3M4sub(,)5V and Se M1M4sub(,)5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at - 2eV, - 8eV, and - 13eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at approx. - 8 eV and at approx. - 13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge. (orig.)

58

X-ray photoemission spectra of the valence bands of the 3d transition metals, Sc to Fe  

International Nuclear Information System (INIS)

High-resolution x-ray-photoemission valence-band (VB) spectra are presented of the 3d transition metals, Sc to Fe. The measurements were conducted under ultra-high-vacuum conditions using polycrystalline samples. In the hcp elements Sc and Ti the main peaks in the valence bands are identified and compared with the results of band-structure calculations. Some rigidity is observed in the Sc and Ti bands as well as in the V and Cr bands. The expected two-peak structure for V and Cr is observed. In chromiun we also observed a relatively low density of states at the Fermi energy. The VB spectrum for chromium is compared to theoretical predictions, and we present an assignment of the Cr symmetry-point energies. Our valence-band spectrum of Mn is quite similar to that of Huefner and Wertheim and our spectrum of Fe is in good agreement with that observed by Hoechst, Goldman, and Huefner. These iron VB spectra agree qualitatively with expectations based on band models of ferromagnetism. It is observed that the VB spectrum of bcc iron may be constructed to agree within an energy scale factor from two bcc Cr VB's shifted to account for spin polarization. We also deduce the symmetry-point energies in iron

59

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy  

International Nuclear Information System (INIS)

The valence band offset (VBO) of MgO/TiO2 (rutile) heterojunction has been directly measured by X-ray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 ± 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 ± 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO2 based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO2 based field-effect transistors and dye-sensitized solar cells.

60

Valence band offset of MgO/TiO{sub 2} (rutile) heterojunction measured by X-ray photoelectron spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

The valence band offset (VBO) of MgO/TiO{sub 2} (rutile) heterojunction has been directly measured by X-ray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 {+-} 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 {+-} 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO{sub 2} based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO{sub 2} based field-effect transistors and dye-sensitized solar cells.

Zheng Gaolin, E-mail: zhenggl@semi.ac.cn [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China); Wang Jun [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China); Liu Xianglin, E-mail: xlliu@semi.ac.cn [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China); Yang Anli; Song Huaping; Guo Yan; Wei Hongyuan; Jiao Chunmei; Yang Shaoyan; Zhu Qinsheng; Wang Zhanguo [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China)

2010-09-15

 
 
 
 
61

Investigation of the valence band in the case of thin films of RNI5 (R=Ce, Y, La) compounds  

International Nuclear Information System (INIS)

The paper presents the evaluation by optical methods of the valence band shape in the case of inoxidized thin films of RNi5 (R=Ce, Y, La) compounds. The results were correlated with those obtained by XPS spectroscopy for same compounds yet not as thin films but bulk. (Author)

62

Valence and conduction band offsets at amorphous hexagonal boron nitride interfaces with silicon network dielectrics  

International Nuclear Information System (INIS)

To facilitate the design of heterostructure devices employing hexagonal/sp2 boron nitride, x-ray photoelectron spectroscopy has been utilized in conjunction with prior reflection electron energy loss spectroscopy measurements to determine the valence and conduction band offsets (VBOs and CBOs) present at interfaces formed between amorphous hydrogenated sp2 boron nitride (a-BN:H) and various low- and high-dielectric-constant (k) amorphous hydrogenated silicon network dielectric materials (a-SiX:H, X?=?O, N, C). For a-BN:H interfaces formed with wide-band-gap a-SiO2 and low-k a-SiOC:H materials (Eg???8.2?8.8?eV), a type I band alignment was observed where the a-BN:H band gap (Eg?=?5.5?±?0.2?eV) was bracketed by a relatively large VBO and CBO of ?1.9 and 1.2?eV, respectively. Similarly, a type I alignment was observed between a-BN:H and high-k a-SiC:H where the a-SiC:H band gap (Eg?=?2.6?±?0.2?eV) was bracketed by a-BN:H with VBO and CBO of 1.0?±?0.1 and 1.9?±?0.2?eV, respectively. The addition of O or N to a-SiC:H was observed to decrease the VBO and increase the CBO with a-BN:H. For high-k a-SiN:H (Eg?=?3.3?±?0.2?eV) interfaces with a-BN:H, a slightly staggered type II band alignment was observed with VBO and CBO of 0.1?±?0.1 and ?2.3?±?0.2?eV, respectively. The measured a-BN:H VBOs were found to be consistent with those deduced via application of the commutative and transitive rules to VBOs reported for a-BN:H, a-SiC:H, a-SiN:H, and a-SiO2 interfaces with Si (100)

63

Effects of lattice-mismatch induced built-in strain on the valence band properties of wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O quantum well heterostructures  

Energy Technology Data Exchange (ETDEWEB)

We present a theoretical study of the effects of lattice-mismatch induced built-in strain on the electronic properties of valence band states in wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O Quantum Well (QW) heterostructures. In this purpose, a 6x6 k.p method has been used to incorporate the effects of strain and nonparabolicity. The energies corresponding to the transitions between conduction band (C), heavy hole (HH), light hole (LH) and crystal-field split-off hole (CH) bands have been calculated as a function of Mg composition and strain. We have also calculated the energy dispersions and wave functions of strained wurtzite ZnO. It is found that ZnO is always under a biaxial tensile strain, in the whole Mg composition range investigated (x<40%). As a consequence, the light hole valence subband is shifted upwards with respect to the corresponding heavy hole valence subband, resulting in a reduction of ZnO direct band gap by almost 6% when x=35%. This is found to result in turn in a significantly reduced in-plane hole effective mass at the top of the valence band which is always LH-like. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Zitouni, K.; Kadri, A. [Laboratoire d' Etude des Materiaux Optoelectronique and Polymeres, L.E.M.O.P., Departement de Physique, Universite d' Oran (Es-Senia), 31100 Oran (Algeria)

2007-07-01

64

Resonant 5f valence-band satellite at the 5d threshold in uranium metal  

International Nuclear Information System (INIS)

Photoelectron energy distribution curves for polycrystalline ?-uranium show a giant Fano-type resonance for the intensity of the 5f valence-band features at the Fermi energy E/sub F/, when the photon energy is tuned through the 5d core absorption edge at around h?roughly-equal94 to 103 eV. Further, a sharp (full width at half maximum = 2.2 eV) 5f satellite excitation at an initial energy E/sub i/ = -2.3 eV below E/sub F/ has been observed, which has a close analogy to the ''6-eV satellite'' in Ni. This shakeup satellite shows also a strong resonant enhancement (> or = 6 times) at the 5d core-level threshold, but a distinctly different cross-section dependence. The origin of the resonances of the 5f main line and the 2.3-eV satellite is discussed in an atomic framework, taking 5f and 6d screening into account

65

Determination of the valence band offset at selected oxide/InN interfaces  

International Nuclear Information System (INIS)

The valence band offsets (VBO) at different oxide/InN(0001) interfaces are investigated for TiO2, HfO2, Al2O3 and In2O3 using X-ray photoelectron spectroscopy. These oxide materials might be potential candidates for the use as barrier material in InN based transistors. The precise knowledge of the band alignment at the oxide/InN interface is relevant to understand the carrier transport characteristics in electronic devices. InN films with a thickness of 1 ?m were grown by PAMBE on GaN(0001)/Al2O3 templates. Thin oxide films were grown on top of these InN layers, within a series of varying thickness (1-5 nm). TiO2 and HfO2 were deposited by plasma-assisted e-beam evaporation, while for Al2O3 a remote plasma ALD process was used and In2O3 was grown by MOCVD. Thickness dependent changes of the barrier could only be found for TiO2 which exhibited the strongest degree of process-induced InN interface oxidation. The VBO values, which were determined by linear extrapolation of the thickness dependence, are 1.8 eV, 1.2 eV, 2.65 eV and 1.5 eV for the TiO2/InN, HfO2/InN, Al2O3/InN and In2O3/InN heterointerface, respectively.

66

Comparison of the background corrected valence band XPS spectra of Fe and Co aluminides and silicides with their electronic structures  

International Nuclear Information System (INIS)

The background corrected valence band XPS spectra and the electronic structures of FeAl, FeSi, CoAl and CoSi were studied. Clean surfaces of the polycrystalline samples were obtained by in situ fracturing of the samples in an XPS spectrometer. The energy loss parts of the Fe 2p, Co 2p and valence band spectra were removed by the deconvolution method using Al 2s or Si 2s spectra as response functions. CoAl exhibited a satellite peak in the Co 2p region, but the other compounds had no clear satellite peaks in the Co 2p and Fe 2p regions. The experimentally background corrected valence band spectra were compared with the calculated spectra using the first-principle band calculation. There were large discrepancies between the spectra above the binding energy of 5 eV. These indicated that the experimental spectra could not be explained by the electronic structures of the ground states alone

67

Exchange-Mediated Anisotropy of (Ga,Mn)As Valence-Band Probed by Resonant Tunneling Spectroscopy  

Science.gov (United States)

We report on experiments and theory of resonant tunneling anisotropic magnetoresistance (TAMR) in AlAs/GaAs/AlAs quantum wells (QW) contacted by a (Ga,Mn)As ferromagnetic electrode. Such resonance effects manifest themselves by bias-dependent oscillations of the TAMR signal correlated to the successive positions of heavy (HH) and light (LH) quantized hole energy levels in GaAs QW. We have modeled the experimental data by calculating the spin-dependent resonant tunneling transmission in the frame of the 6×6 valence-band k·p theory. The calculations emphasize the opposite contributions of the (Ga,Mn)As HH and LH subbands near the ? point, unraveling the anatomy of the diluted magnetic semiconductor valence band.

Elsen, M.; Jaffrès, H.; Mattana, R.; Tran, M.; George, J.-M.; Miard, A.; Lemaître, A.

2007-09-01

68

CuPc molecules adsorbed on Au(110)-(1x2) growth morphology and evolution of valence band states  

CERN Document Server

We present the growth morphology, the long range ordering, and the evolution of the valence band electronic states of ultra-thin films of copper phthalocyanine (CuPc) deposited on the Au(110)-(1x2) reconstructed surface, as a function of the organic molecule coverage. The Low Energy Electron Diffraction (LEED) patterns present a (5x3) reconstruction from the early adsorption stages. High-Resolution UV photoelectron spectroscopy (HR-UPS) data show the disappearance of the Au surface states related to the (1x2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital (HOMO) gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage.

Evangelista, F; Corradini, V; Donzello, M P; Mariani, C; Betti, M G

2003-01-01

69

CuPc molecules adsorbed on Au(110)-(1x2): growth morphology and evolution of valence band states  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present the growth morphology, the long range ordering, and the evolution of the valence band electronic states of ultra-thin films of copper phthalocyanine (CuPc) deposited on the Au(110)-(1x2) reconstructed surface, as a function of the organic molecule coverage. The Low Energy Electron Diffraction (LEED) patterns present a (5x3) reconstruction from the early adsorption stages. High-Resolution UV photoelectron spectroscopy (HR-UPS) data show the disappearance of the Au ...

Evangelista, F.; Ruocco, A.; Corradini, V.; Donzello, M. P.; Mariani, C.; Betti, M. G.

2003-01-01

70

Modeling wave dispersion and band gaps in heterogeneous elastic media  

Directory of Open Access Journals (Sweden)

Full Text Available In this paper we report recent developments and results concerning validation of the homogenization approach applied in modeling waves in strongly heterogeneous elastic media. The homogenization limit model is obtained for stationary waves, but can also be used to estimate dispersion properties for long guided waves propagation. Band gaps distribution depends on the material contrast and on the geometrical arrangements in the microstructure. Similarity between discrete structures and heterogeneous continua is used to demonstrate the dispersion phenomena. The modeling approach has been extended to the piezo-phononic materials, which may be useful in designing smart materials. Also problems of optimal shape design at the microscopic level were pursued.

Rohan E.

2007-10-01

71

Evidence of Eu{sup 2+} 4f electrons in the valence band spectra of EuTiO{sub 3} and EuZrO{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

We report on optical band gap and valence electronic structure of two Eu{sup 2+}-based perovskites, EuTiO{sub 3} and EuZrO{sub 3} as revealed by diffuse optical scattering, electron energy loss spectroscopy, and valence-band x-ray photoelectron spectroscopy. The data show good agreement with the first-principles studies in which the top of the valence band structure is formed by the narrow Eu 4f{sup 7} electron band. The O 2p band shows the features similar to those of the Ba(Sr)TiO{sub 3} perovskites except that it is shifted to higher binding energies. Appearance of the Eu{sup 2+} 4f{sup 7} band is a reason for narrowing of the optical band gap in the title compounds as compared to their Sr-based analogues.

Kolodiazhnyi, T. [National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan); Valant, M. [Materials Research Laboratory, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica (Slovenia); Williams, J. R. [International Center for Young Scientists (ICYS), MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan); Bugnet, M.; Botton, G. A. [Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7 (Canada); Ohashi, N. [International Center for Materials Nanoarchitectonics, MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan); Sakka, Y. [National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)

2012-10-15

72

Troposcatter dispersion measurements with angle diversity in two frequency bands  

Science.gov (United States)

Measurements of received signal level as well as of multipath dispersion were carried out over a 278 km troposcatter test link. The tests were performed simultaneously in two frequency bands (UHF: 900 MHz, C-band: 4.478 GHz), using receiving channels and a combined angle diversity antenna with two diversity beams in UHF and four diversity beams in C-band. A correlation factor of 0.3 between the temporal evolution of the signal of both UHF channels was measured. A statistical analysis of the time of flight measurements (delay spread and Doppler spread) was made. The Doppler spread is clearly frequency dependent. The statistical correlation between the relative amplitude variation of the received signal and the delay spread shows a decrease of the amplitude variation with higher delay spread. The delay spread can not be determined from the amplitude variations.

Schmitt, F.

1981-10-01

73

Valence state, hybridization and electronic band structure in the charge ordered AlV2O4.  

Science.gov (United States)

The valence state, hybridization and electronic band structure of charge ordered AlV2O4 are investigated by measuring the electron energy loss spectra (EELS) and performing band structure calculations using the WIEN2k code. White line ratio and O K edges of V2O5, VO2, V2O3 and AlV2O4, obtained using electron energy loss spectroscopy, are analysed specifically to probe systematically the VO6 octahedra in all of them. The systematic decrease of the L2 intensity and the O K edge intensity from V(5+) in V2O5 to AlV2O4 indicates a progressive increase in the occupancy of the hybridized states, which is corroborated by the absence of a transition from O 1s to hybridized 2t(2g). Band structure calculations on the parent charge frustrated cubic phase and the charge ordered rhombohedral phase clearly document a band gap in the charge ordered state. From the structural information obtained after convergence and the spectroscopic information from EELS, it appears that partial orbital occupancy may lead to a deviation from an integral valence state on all the vanadium in this exotic charge ordered spinel system. PMID:24285259

Kalavathi, S; Amirthapandian, S; Chandra, Sharat; Sahu, P Ch; Sahu, H K

2014-01-01

74

The valence band electronic structure of the Cu(111) (?3X?3)R30deg-Si interface  

International Nuclear Information System (INIS)

Full text: The structure and bonding of the copper-silicon interface is of considerable interest from a number of aspects. Firstly as a catalyst in the commercial synthesis of silane polymers, secondly as an anti-corrosion treatment, and thirdly, the formation of a well ordered and reactive silicon layer, which can be oxidised is relevant in the creation of ultra-thin silicon oxide-metal interfaces for electronic devices. Silicon is capable of forming a number of compounds with copper, the most widely studied of which is Cu3Si. Calculations have shown that when silicon impurity atoms are incorporated into a copper solid, there is an interaction between copper 3d levels and the 3s and sp levels of silicon. The silicon 2p orbitals rehybridise with the copper 3d band to form bonding and antibonding states separated by -4 eV. The resulting compounds have metallic, rather than semiconducting nature, there is charge transfer from copper to silicon and there is an increase in electron density into the silicon valence bands, making silicon more reactive. The splitting of the density of states near the Fermi edge has been measured as 4-5 eV in amorphous copper-silicon alloys, using Si K? fluorescence spectroscopy and has also been inferred from the 4 eV splitting of the LV V auger lines in Cu-Si compounds and in copper deposited on Si(100) and Si(111) surfaces. In this study we have used high resolution valence band photoemission spectroscopy to investignd photoemission spectroscopy to investigate the nature of the silicon valence bands in a well ordered silicon-copper interface. By comparing the valence band spectra of the clean surface and those from the silicon interface, we are able to identify three silicon-derived features which are in agreement with other published data. We suggest that these levels are due to emission from the 3s and 3p levels of Si

75

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

76

Band Dispersion During Isoelectric Focusing in a Microchip  

Science.gov (United States)

Ampholyte based isoelectric focusing (IEF) is simulated for a two-dimensional horse shoe microchannel. Mobility correction for proteins and ampholytes are considered in the model because the mobility of both large molecules (proteins) and small molecules (ampholytes) varies with ionic strength in the IEF process. Four model proteins are allowed to focus in the presence of 25 biprotic carrier ampholytes in a horse shoe microchannel. Normalized variances of protein bands are calculated from numerical results using moment method. We particularly show dispersion behavior of proteins in IEF and discuss the differences between linear electrophoretic transports and nonlinear IEF in a horse shoe microchannel. Our numerical results show that protein spreading is induced by a turn during gradient formation stage, but the dispersed bands are rearranged and straighten as double peaks of a protein start to focus at the focal point. The rearrangement of spreading band is very unique compared to other linear electrokinetic phenomena (electroosmotic flow and capillary zone electrophoresis) and is independent of channel position and channel shape. Hence, one can perform the IEF to separate proteins in complex geometry without incorporating hyperturns.

Dutta, Prashanta; Shim, Jaesool; Ivory, Cornelius F.

2008-11-01

77

Bulk electronic structure studied by hard X-ray photoelectron spectroscopy of the valence band: The case of intermetallic compounds  

Energy Technology Data Exchange (ETDEWEB)

Highlights: •Bulk sensitivity of HAXPES was used to explore the electronic structure of several Heusler compounds in form of bulk materials and thin films. •Strong changes of the electronic structure Heusler shape memory Mn{sub 2}NiGa appear at the phase transition. •HAXPES of C1{sub b} Heusler compounds with narrow band gap showed the existence of in gap states close to the Fermi energy. •Linear dichroism in hard X-ray photoelectron spectroscopy (LDAD-HAXPES) was used to study the angular asymmetry in photoemission from the valence states of three kinds of Heusler compounds. -- Abstract: Photoelectron spectroscopy (PES) has evolved into the most relevant, powerful, and nondestructive method for investigating atoms, molecules, and solids. In particular, hard X-ray photoelectron spectroscopy (HAXPES) has emerged as a powerful tool for investigating the bulk electronic structure of materials in a variety of applied fields such as chemistry, physics, and materials science. In addition, PES was used for investigating the symmetries of various materials’ electronic structures. However, thus far, such studies have been restricted to atoms, molecules, adsorbates, and surfaces because low-energy (<1 keV) electrons have limited probing depths. This is disadvantageous because three-dimensional (3D) bulk states cannot be studied. The present work demonstrates that this drawback can be eliminated by using hard X-rays with variable polarization for excitation. In the current study, this issue was investigated using several Heusler compounds, which have been attracting increasing levels of interest. There are more than 2000 Heusler compounds in total. Owing to their tunable electronic structures, Heusler compounds exhibit multifarious properties useful for spintronic, optoelectronic, shape memory, and thermoelectric applications. Herein, we report the results of bulk-sensitive, high energy photoelectron spectroscopy of the valence bands of several Heusler compounds for various applications. It is shown that the measured valence band spectra are clearly resolved and are in good agreement with the first-principles calculations of the compounds’ electronic structures.

Ouardi, Siham; Fecher, Gerhard H., E-mail: fecher@cpfs.mpg.de; Felser, Claudia

2013-10-15

78

Ab-initio calculation of the valence-band offset at strained GaAs/InAs (001) heterojunction  

International Nuclear Information System (INIS)

We present a self consistent pseudopotential calculation of the valence band offset (VBO) at GaAs/InAs (001) strained heterojunction, which is chose as an example of the isovalent polar with common-anion lattice mismatched heterojunctions. The effects of strain are studied by looking at the variation of the VBO versus the in plane lattice constant, which is imposed by the substrate. Our results show that the VBO can be tuned by about 0.17 eV going from GaAs to InAs substrates. Comparison of our work with the available experimental and theoretical results is also discussed. (author). 25 refs, 3 figs, 5 tabs

79

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

Science.gov (United States)

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

80

Spectroscopy of neutron-rich 168,170Dy: Yrast band evolution close to the NpNn valence maximum  

International Nuclear Information System (INIS)

The yrast sequence of the neutron-rich dysprosium isotope 168Dy has been studied using multinucleon transfer reactions following collisions between a 460-MeV 82Se beam and an 170Er target. The reaction products were identified using the PRISMA magnetic spectrometer and the ? rays detected using the CLARA HPGe-detector array. The 2+ and 4+ members of the previously measured ground-state rotational band of 168Dy have been confirmed and the yrast band extended up to 10+. A tentative candidate for the 4+?2+ transition in 170Dy was also identified. The data on these nuclei and on the lighter even-even dysprosium isotopes are interpreted in terms of total Routhian surface calculations and the evolution of collectivity in the vicinity of the proton-neutron valence product maximum is discussed.

 
 
 
 
81

Magneto-photoluminescence of GaN\\/AlGaN quantum wells valence band reordering and excitonic binding energies  

CERN Document Server

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for the observed values for the g-factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wavefunction from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al (phys. stat. sol. (b) 216, 371 (1999)) that uses a variational approach in the envelope function formalism that includes the effect of the electric field in the wells.

Shields, P A; Grandjean, N; Massies, J

2001-01-01

82

Improved model for the stress-induced leakage current in thin silicon dioxide based on conduction-band electron and valence-band electron tunneling  

Science.gov (United States)

This article presents a detailed investigation on the stress-induced leakage current (SILC) conduction mechanism via conduction-band electron (CBE) and valence-band electron (VBE) tunneling in thin oxides. An improved SILC model that is able to reproduce the experimental SILC over a wide range of oxide fields, and yet give a realistic level of extracted neutral trap concentration, is proposed. Calculations performed with the improved SILC model suggest that SILC conduction via neutral traps is accompanied by energy relaxation (i.e., an inelastic mechanism), irrespective of the origin (i.e., whether CBE or VBE) of the tunneling species. For both CBE and VBE tunneling, inelastic tunneling with energy relaxation (Erelax) of 1.5 and 0.8 eV, was found to fit the experimental measurements well. These values of Erelax agree with those reported in the literature.

Chim, W. K.; Lim, P. S.

2002-02-01

83

Valence-band electronic structure of MoS2 and Cs/MoS2(0002) studied by angle-resolved x-ray photoemission spectroscopy  

International Nuclear Information System (INIS)

The angle dependence of the valence-band photoemission from the trigonal prismatic layered MoS2 shows both the forward-scattering features normally observed in core-level photoelectron diffraction and, in addition, the initial-state orbital character associated with partially occupied, nonbonding MoIV(4dz2+4dx2-y2 +4dxy) orbitals near the top of the valence band. The difference in forward scattering between the Mo and S emitters is also used to assess relative contributions from the Mo and S atomic orbitals at specific binding energies within the valence band. Deposition of cesium (0.23 ML Cs with 1 ML equal to the Cs saturation coverage) onto the basal plane of MoS2 introduces a density of states at 1.25 eV above the top of the valence-band maximum. The intensity anisotropy for this Cs-induced valence level is interpreted via the angle dependence of the electric dipole matrix element as due to the initial-state orbital character at the bottom of the conduction band of the Cs/MoS2 heterostructure. copyright 1996 The American Physical Society

84

The quasiparticle band dispersion in epitaxial multilayer silicene  

International Nuclear Information System (INIS)

The growth of multilayer silicene is an exciting challenge for the future of silicon nano-electronics. Here, we use angle-resolved photoemission spectroscopy to map the entire Brillouin zone (BZ) of (?3 × ?3)R30° reconstructed epitaxial multilayer silicene islands, growing on top of the first (3 × 3) reconstructed silicene wetting layer, on Ag(111) substrates. We found ?- and V-shape linear dispersions, which we relate to the ? and ?* bands of massless quasiparticles in multilayer silicene, at the BZ centre ?-bar 0 and at all the ?-bar centres of the (?3 × ?3)R30° Brillouin zones in the extended scheme, due to folding of the Dirac cones at the K-bar and K-bar ' points of the (1 × 1) silicene BZ. The Fermi velocity of ?0.3 × 106 m s?1 obtained is highly promising for potential silicene-based devices. (fast track communication)

85

Doping-induced changes in the valence band edge structure of homoepitaxial B-doped diamond films below Mott's critical density  

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.)

86

XPS core level and valence band spectra of LaH3  

International Nuclear Information System (INIS)

XPS analysis of LaH3 shows that the conduction band of the La metal disappears and that a new hydrogen induced band is formed 5.8 eV below Esub(F). The La 3d core levels of LaH3 are split into two peaks with about 1/3 of the intensity being in the low binding energy peak. It is proposed to describe the observed 3d structure by the screening of the 3d hole by means of a charge transfer from the new hydrogen band to the empty 4f level, the latter being pulled down below th hydrogen band. This would be a shake down process as in La metal. (author)

87

Fingerprints of CuPt ordering in III-V semiconductor alloys: Valence-band splittings, band-gap reduction, and x-ray structure factors  

International Nuclear Information System (INIS)

Spontaneous CuPt ordering induces a band-gap reduction ?Eg relative to the random alloy, a crystal field splitting ?CF at valence-band maximum, as well as an increase of spin-orbit splitting ?SO. We calculate these quantities for AlxIn1-xP, AlxIn1-xAs, GaxIn1-xP, and GaxIn1-xAs using the local density approximation (LDA), as well as the more reliable LDA-corrected formalism. We further provide these values and the valence-band splittings ?E12 (between bar ?4,5v and bar ?6v(1)) and ?E13 (between bar ?4,5v and bar ?6v(2)) for these materials as a function of the degree ? of long range order. In the absence of an independent measurement of ?, experiment is currently able to deduce only the ratio ?Eg/?CF. Our LDA-corrected results for this quantity compare favorably with recent experiments for GaxIn1-xP and GaxIn1-xAs, but not for AlxIn1-xP, where our calculation does not support the experimental assignment. The open-quotes optical LRO parameter ?close quotes can be obtained by fitting our calculated ?Eg(?) to the measured ?Eg(?), and by expressing the measured ?E12(?) and ?E13(?) in terms of our calculated ?CF(?) and ?SO(?). We also provide the calculated x-ray structure factors for ordered alloys that can be used experimentally to deduce ? independently. copyright 1998 The American Physical Society

88

Direct calculation of valence-band Auger emission: Spin polarization of Auger electrons from a potassium (110) surface  

Science.gov (United States)

We report calculations on spin-polarized Auger electron emission from the valence band of a potassium (110) film. In treating this process we use an N-electron scheme that is based on a generalized version of density-functional theory. The transition rate is determined by explicitly evaluating the transition matrix elements that contain the four states involved. The core and valence states are obtained from a self-consistent full-potential linearized augmented plane wave calculation on a K(110) multilayer. In the experiments that the present calculations refer to, the particular oriented core-hole state is created by photoexcitation using circularly polarized light. The observed energy spectrum and the angular dependence of the spin polarization of the emitted Auger electrons can be simply related to the character of the spinor-hole state. We compare our results for normal incidence of the light to the pertinent experiments on the Auger spin polarization referenced to the spin of the incoming photons. The results are in fair agreement with each other.

Yuan, Jianmin; Fritsche, L.; Noffke, J.

1997-10-01

89

Anomalous temperature dependence in valence band spectra: A resonant photoemission study of layered perovskite Sr2CoO4  

Science.gov (United States)

Valence band spectra (VBS) and its modification across Curie temperature (TC) of Sr2CoO4 thin film are studied using resonant photoemission spectroscopy. It is found that VBS mainly consists of hybridized states of Co-3d t2geg and O-2p; however, Co-3d eg states show its prominence only in the ferromagnetic temperature regime. Below TC, spectral weight transfer takes place anomalously from high binding energy (B.E.) region to low B.E. region, signifying the enhanced intermediate or low spin state Co4+ ions. It is suggested that spin-lattice coupling and many-body effects in Sr2CoO4 derived from the strong electron correlations lead to such temperature dependence of VBS.

Pandey, Pankaj K.; Choudhary, R. J.; Phase, D. M.

2014-05-01

90

Anomalous temperature dependence in valence band spectra: A resonant photoemission study of layered perovskite Sr2CoO4  

International Nuclear Information System (INIS)

Valence band spectra (VBS) and its modification across Curie temperature (TC) of Sr2CoO4 thin film are studied using resonant photoemission spectroscopy. It is found that VBS mainly consists of hybridized states of Co-3d t2geg and O-2p; however, Co-3d eg states show its prominence only in the ferromagnetic temperature regime. Below TC, spectral weight transfer takes place anomalously from high binding energy (B.E.) region to low B.E. region, signifying the enhanced intermediate or low spin state Co4+ ions. It is suggested that spin-lattice coupling and many-body effects in Sr2CoO4 derived from the strong electron correlations lead to such temperature dependence of VBS

91

Determination of the valence-band offset of CdS/CIS solar cell devices by target factor analysis  

Energy Technology Data Exchange (ETDEWEB)

X-ray photoemission spectroscopy (XPS) is used to determine and compare the valence-band offsets ({Delta}E{sub v}) for CdS grown by chemical bath deposition on single-crystal and thin-film CuInSe{sub 2} (CIS). The thin-film CIS device was suitable for photovoltaic energy production. By sputtering through the CdS/CIS interface and reducing the depth profile with target factor analysis, the magnitude of {Delta}E{sub v} was determined to be {Delta}E{sub v} = 1.06 {+-} 0.15 eV for both the single-crystal and thin-film interfaces. This determination of {Delta}E{sub v} is about 0.25 eV larger than many previously reported estimations CdS grown by physical vapor deposition on CIS and helps explain the record performance of CdS/CIS photovoltaic devices.

Niles, D.W.; Contreras, M.; Ramanathan, K.; Noufi, R. [National Renewable Energy Lab., Golden, CO (United States)

1996-05-01

92

Temperature-dependent valence-band photoemission spectra of La1-xSrxMnO3  

International Nuclear Information System (INIS)

We have studied the temperature dependence of the photoemission spectra of La1-xSrxMnO3 (x=0.0, 0.2, and 0.4) and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for x=0.2 and 0.4. By contrast, the spectra of La0.6Sr0.4CoO3 show no significant temperature dependence. From comparison between the temperature- and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La1-xSrxMnO3, we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations. copyright 1997 The American Physical Society

93

Valence-band structure of highly efficient p-type thermoelectric PbTe-PbS alloys  

Science.gov (United States)

Experimental evidence is given relevant to the temperature dependence of the valence band structure of PbTe and PbTe1-xSx alloys (0.04?x?0.12), and its effect on the thermoelectric figure of merit ZT. The x=0.08 sample has ZT˜1.55 at 773 K. The magnetic field dependence of the high-temperature Hall resistivity of heavily p-type (>1019 cm-3) Na-doped PbTe1-xSx reveals the presence of high-mobility electrons. This casts doubts on prior analyses of the Hall coefficient suggesting that temperature induces a rapid rise in energy of the “heavy” hole relative to the “light” hole bands. The electron-like behavior is likely induced by the topology of the Fermi surface when the L- and ?-bands merge. Negative values for the low-temperature thermopower are also observed. The data show that PbTe continues to be a direct-gap semiconductor at temperatures where the ZT and S2? of p-type PbTe are optimal, e.g., 700-800 K.

Jaworski, Christopher M.; Nielsen, Michele D.; Wang, Hsin; Girard, Steven N.; Cai, Wei; Porter, Wally D.; Kanatzidis, Mercouri G.; Heremans, Joseph P.

2013-01-01

94

Impact of cation-based localized electronic states on the conduction and valence band structure of Al{sub 1?x}In{sub x}N alloys  

Energy Technology Data Exchange (ETDEWEB)

We demonstrate that cation-related localized states strongly perturb the band structure of Al{sub 1?x}In{sub x}N leading to a strong band gap bowing at low In content. Our first-principles calculations show that In-related localized states are formed both in the conduction and the valence band in Al{sub 1?x}In{sub x}N for low In composition, x, and that these localized states dominate the evolution of the band structure with increasing x. Therefore, the commonly used assumption of a single composition-independent bowing parameter breaks down when describing the evolution both of the conduction and of the valence band edge in Al{sub 1?x}In{sub x}N.

Schulz, S., E-mail: stefan.schulz@tyndall.ie [Photonics Theory Group, Tyndall National Institute, Dyke Parade, Cork (Ireland); Caro, M. A.; O' Reilly, E. P. [Photonics Theory Group, Tyndall National Institute, Dyke Parade, Cork (Ireland); Department of Physics, University College Cork, Cork (Ireland)

2014-04-28

95

Quasiparticle excitations in valence-fluctuation materials: effects of band structure and crystal fields  

International Nuclear Information System (INIS)

Evidence is now quite strong that the elementary hybridization model is the correct way to understand the lattice-coherent Fermi liquid regime at very low temperatures. Many-body theory leads to significant renormalizations of the input parameters, and many of the band-theoretic channels for hybridization are suppressed by the combined effects of Hund's-rule coupling, crystal-field splitting, and the f-f Coulomb repulsion U. Some exploratory calculations based on this picture are described, and some inferences are drawn about the band structures of several heavy-fermion materials. These inferences can and should be tested by suitably modified band-theoretic calculations. We find evidence for a significant Baber-scattering contribution in the very-low-temperature resistivity. A new mechanism is proposed for crossover from the coherent Fermi-liquid regime to the incoherent dense-Kondo regime. 28 refs

96

Influence of Cu(In,Ga)Se2 band gap on the valence band offset with CdS  

International Nuclear Information System (INIS)

The absorber/buffer interface is essential for high-performance thin film chalcopyrite solar cells. In this contribution we present studies of interface formation between Cu(In,Ga)Se2 with 30 and 100% Ga content and the II-VI semiconductor CdS using in situ photoelectron spectroscopy. Clean Cu(In,Ga)Se2 surfaces with a Cu-deficient surface composition were prepared by heating-off of Se layers, which were deposited onto the absorber layers in the deposition chamber directly after absorber deposition. Interfaces with CdS were prepared by stepwise evaporation. The determined band alignments are compared to theoretical calculations

97

Spectra and Spin Polarization of the Valence Band Auger Emission from Cr (100) Surface  

Science.gov (United States)

Calculations have been carried out on spin-polarized Auger electron emission from a seven-layer chromium (100)-film. The core and valence states concerning the Auger transition are obtained from a self-consistent full-potential linearized augmented plane wave calculation by using a repeated slab geometry. The calculations refer to experiments on the L3M23V- and L3VV-transitions that have recently been carried out by Heinzmann et al. The Auger spectrum obtained for the L3VV transition agrees relatively well with our calculations, whereas the observed L3M23V-Auger structure is considerably wider than that predicted by our theory. Nevertheless, the spin-polarization in the latter case, which is about -13%, is in fair agreement with the experiment, different from the L3VV-transition where the experiment yields -10% as opposed to the theoretical value of -25%. We give possible reasons for the origin of these discrepancies.

Yuan, Jian-Min; Fritsche, L.; Noffke, J.

2003-10-01

98

Electron correlation effects in the UPS valence band spectra of Au and Ag  

International Nuclear Information System (INIS)

Modulations of the relative peak heights of the d band maxima previously observed in UPS spectra of Au and Ag are shown to correspond to the thresholds of excitations from inner levels with equal principal quantum number. Contrary to earlier proposed interpretations the correspondence leads to an explanation of the modulations as an effect of atomic electron correlation interaction. (orig.)

99

Conduction band/valence band kinetic modeling of the LiF:Mg,Ti system incorporating creation of defects in the irradiation stage  

Energy Technology Data Exchange (ETDEWEB)

A conduction band/valence band kinetic model is described for the irradiation stage of the thermoluminescent LiF:Mg,Ti system. Unlike previous investigations the model incorporates creation of fluorine vacancies via irradiation and their subsequent partial filling by electrons in the relaxation stage leading to the creation of F centers. The radiation induced vacancies thus operate as additional competitors to the 4 eV electron trapping center (TC) associated with composite glow peak 5 in the glow curve of LiF:Mg,Ti. With the appropriate choice of parameter-values the model successfully predicts the experimentally measured, linear/exponentially saturating dose response of the optical absorption (OA) bands of the 4 eV TC and the 5.45 eV competitive center (CC) which serves as well as the recombination stage competitor. The increased competition due to the creation of negative ion vacancies during irradiation is found to lead to changes in the population dose response characteristics of the various centers taking part in the TL mechanism. The implication of these changes on the relative thermoluminescence (TL) efficiency following heavy charged particle (HCP) irradiation is discussed.

Eliyahu, I., E-mail: ilan.eliyahu@gmail.com [Ben Gurion University of the Negev, Beersheva 84105 (Israel); Soreq Nuclear Research Center, Yavne 81800 (Israel); Horowitz, Y.S. [Ben Gurion University of the Negev, Beersheva 84105 (Israel); Oster, L. [Sami Shamoon College of Engineering, Beersheva 84100 (Israel)

2012-12-15

100

Spin exchange energy for a pair of valence band holes in artificial molecules  

Science.gov (United States)

We study the exchange energy for two interacting holes in vertically stacked InGaAs quantum dots using the configuration interaction method and an axially symmetric 4-band Kohn-Luttinger Hamiltonian that introduces heavy-hole and light-hole subbands mixing due to the spin–orbit interaction. We demonstrate that, as a consequence of the band mixing, a singlet–triplet degeneracy appears for a single specific value of the interdot barrier, for which the bonding and antibonding heavy hole have identical energy. We explain this degeneracy using a simple model matrix Hamiltonian as due to the interaction effects for a degenerate ground state of the hole. We demonstrate that, in conditions of the singlet–triplet degeneracy due to hybridization of the bonding and antibonding energy levels, the spin exchange energy becomes insensitive to external electric fields.

Pasek, W. J.; Szafran, B.; Nowak, M. P.

2014-11-01

 
 
 
 
101

Observation of monolayer valence band spin-orbit effect and induced quantum well states in MoX2  

Science.gov (United States)

Transition metal dichalcogenides transition metal dichalcogenides have attracted much attention recently due to their potential applications in spintronics and photonics because of the indirect to direct band gap transition and the emergence of the spin-valley coupling phenomenon upon moving from the bulk to monolayer limit. Here, we report high-resolution angle-resolved photoemission spectroscopy on MoSe2 single crystals and monolayer films of MoS2 grown on highly ordered pyrolytic graphite substrate. Our experimental results resolve the Fermi surface trigonal warping of bulk MoSe2, and provide evidence for the critically important spin-orbit split valence bands of monolayer MoS2. Moreover, we systematically image the formation of quantum well states on the surfaces of these materials, and present a theoretical model to account for these experimental observations. Our findings provide important insights into future applications of transition metal dichalcogenides in nanoelectronics, spintronics and photonics devices as they critically depend on the spin-orbit physics of these materials.

Alidoust, Nasser; Bian, Guang; Xu, Su-Yang; Sankar, Raman; Neupane, Madhab; Liu, Chang; Belopolski, Ilya; Qu, Dong-Xia; Denlinger, Jonathan D.; Chou, Fang-Cheng; Hasan, M. Zahid

2014-08-01

102

Valence Band Circular Dichroism in non-magnetic Ag/Ru(0001) at normal emission  

Digital Repository Infrastructure Vision for European Research (DRIVER)

For the non-magnetic system of Ag films on Ru(0001), we have measured the circular dichroism of photoelectrons emitted along the surface normal, the geometry typically used in photoemission electron microscopy (PEEM). Photoemission spectra were acquired from micrometer-sized regions having uniformly thick Ag films on a single, atomically flat Ru terrace. For a single Ag layer, we find a circular dichroism that exceeds 6% at the d-derived band region around 4.5 eV binding ene...

Mascaraque, Arantzazu; Mentes?, T. Onur; Mccarty, Kevin F.; Marco, Jose F.; Schmid, Andreas K.; Locatelli, Andrea; La Figuera, Juan

2013-01-01

103

Valence-Band UPS, 6$p$ Core-Level XPS Photoemission Spectroscopy, and Low-Energy Electron Diffraction of a Uranium (001) Single Crystal  

CERN Document Server

Valence-band ultraviolet photoemission spectroscopy (UPS) at 173K and 6p core-level X-ray photoemission spectroscopy (XPS) at room temperature were performed on a high quality uranium single crystal. Significant agreement is found with first-principles electronic band-structure calculations, using a generalized gradient approximation (GGA). In addition, using Low Energy Electron Diffraction (LEED) for the (001) surface, we find a well-ordered orthorhombic crystallographic structure representative of the bulk material.

Opeil, C P; Manley, M E; Lashley, J C; Hults, W L; Hanrahan, R J; Smith, J L; Mihaila, B; Blagoev, K B; Albers, R C; Littlewood, P B

2006-01-01

104

Magnetic susceptibility in weak magnetic field and valence band structure of tin telluride  

International Nuclear Information System (INIS)

Measurement results of magnetic susceptibility (MS) dependence in Sn1-xTe on hole concentration and temperature within p and T wide range are given. Abalysis of experimental curves shows that in ?(p,T=0) curve there are some peculiarities at p=1.1x1020, 2.3x1020 and 4.9x1020 cm-3 which are identified as Van Hove peculiarities corresponding to ?-extremum, to saddle point in ?L direction and to ?-extremum of valent band. Effect of high nonparabolicity of heavy hole spectrum in ?- and ?-subbonds on contribution of heavy holes into MS is considered and estimations of some characteristics of hole spectrum are obtained

105

Valence band circular dichroism in non-magnetic Ag/Ru(0001) at normal emission  

International Nuclear Information System (INIS)

For the non-magnetic system of Ag films on Ru(0001), we have measured the circular dichroism of photoelectrons emitted along the surface normal, the geometry typically used in photoemission electron microscopy. Photoemission spectra were acquired from micrometer-sized regions having uniformly thick Ag films on a single, atomically flat Ru terrace. For a single Ag layer, we find a circular dichroism that exceeds 6% at the d-derived band region around 4.5 eV binding energy. The dichroism decreases as the Ag film thickness increases to three atomic layers. We discuss the origin of the circular dichroism in terms of the symmetry lowering that can occur even in normal emission.

106

On the interpretation of valence band photoemission spectra at organic-metal interfaces  

CERN Document Server

Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as "interface states" having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.

Giovanelli, L; Amsalem, P; Lee, H -L; Abel, M; Clair, S; Koudia, M; Faury, T; Petaccia, L; Topwal, D; Salomon, E; Angot, T; Cafolla, A A; Koch, N; Porte, L; Goldoni, A; Themlin, J -M

2012-01-01

107

UV photoelectron spectroscopy at near ambient pressures: mapping valence band electronic structure changes from Cu to CuO.  

Science.gov (United States)

Valence band (VB) changes and hence electronic structure evolution was directly observed with low kinetic energy (KE) electrons at near ambient pressure (NAP) conditions with He I photon source in a custom built laboratory ambient pressure photoelectron spectrometer (Lab-APPES). Polycrystalline Cu surfaces were gradually oxidized in O2 to Cu2O, to a mixture of Cu2O + CuO, and finally to CuO between 300 and 625 K and at NAP. Typical VB features for Cu, Cu2O, and CuO were observed, and the results corroborate well with core level and Auger spectral changes. High mean free path associated with low KE electrons, very low or no inelastic scattering, and effective pumping and the design of electrostatic lens regime help to minimize the electron attenuation at NAP conditions. The present results extend the capabilities of the APPES tool to explore the in situ evolution of electronic structure of materials at NAP and high temperatures. PMID:24601627

Roy, Kanak; Gopinath, Chinnakonda S

2014-04-15

108

Valence band states and polarized optical emission from nonpolar and semipolar III–nitride quantum well optoelectronic devices  

Science.gov (United States)

Nonpolar and semipolar III–nitride quantum wells (QWs) and devices have been extensively studied due to their unique valence band (VB) structure and polarized optical emission. Unlike conventional c-plane oriented III–nitride QWs, the low crystal symmetry and unbalanced biaxial stress in nonpolar and semipolar QWs separates the topmost VBs and gives rise to polarized optical emission. Since the first experimental reports on nonpolar devices, research on this topic has progressed very rapidly and has covered nonpolar m-plane and a-plane QWs and devices as well as semipolar (11\\bar{2}2), (20\\bar{2}1), and (20\\bar{2}\\bar{1}) QWs and devices. Issues such as strain, plane inclination angle (with respect to the c-plane), indium composition, temperature, and their impact on QW VB structure and device performance have been extensively studied. In this paper we review the physical background and theoretical analysis of the VB states and polarized optical emission of nonpolar and semipolar structures and discuss their potential impacts on optoelectronic devices. Experimental results for nonpolar and semipolar light-emitting diodes and laser diodes will be covered along with additional discussions on the potential applications and challenges related to their unique physical properties.

Zhao, Yuji; Farrell, Robert M.; Wu, Yuh-Renn; Speck, James S.

2014-10-01

109

Valence one-electron and shake-up ionisation bands of polycyclic aromatic hydrocarbons. IV. The dibenzanthracene species  

Energy Technology Data Exchange (ETDEWEB)

A comprehensive study of the He (I) ultra-violet photoelectron spectra of the 1.2,3.4; 1.2,5.6 and 1.2,7.8 isomers of dibenzanthracene up to the double ionisation threshold at {approx}18 eV is presented with the aid of one-particle Green's Function calculations performed using the outer-valence Green's Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with basis sets of improving quality. Suited extrapolations of the ADC(3) results for the one-electron energies characterising the {pi}-band system ({epsilon} {sub b} < 10 eV) to Dunning's correlation consistent basis set of triple zeta quality (cc-pVTZ) enable theoretical insights into HeI measurements which approach chemical accuracy (1 kcal/mol or 43.4 meV). In contrast, a confrontation of simulated spectral envelopes with high-resolution He I photoelectron spectra indicates that polycyclic aromatic molecules with sterically overcrowded bay regions are more susceptible to undergo vibronic coupling complications at the {sigma}-ionisation onset. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital (or one-electron) picture of ionisation at the ADC(3)/6-31G levels. The extent of shake-up bands is correspondingly related to topological, structural and magnetic criteria of aromaticity. Comparison is made with calculations of the lowest doublet-doublet excitation energies of the related radical cations, by means of time-dependent density functional theory (TDDFT)

Deleuze, Michael S. [Theoretische Chemie, Departement SBG, Universiteit Hasselt, Agoralaan, Gebouw D, B-3590 Diepenbeek (Belgium)], E-mail: michael.deleuze@uhasselt.be

2006-10-26

110

Characterisation of MoSe/sub x/Te/sub 2-x/ (0 <= x <= 2) electrodes in terms of energetic location of valence and conduction bands  

International Nuclear Information System (INIS)

The layer type MoSe/sub x/Te/sub 2-x/ (0 <= x <= 2) has been grown by chemical vapour transport technique. The photoelectrochemical solar cells have been fabricated using the grown crystals as photoelectrodes and platinum grid as counter electrode in aqueous iodine/iodide solution. The optical band gap determination has been done from the study of the spectral response of the cells. The location of valence and conduction band edges and flat band potentials have been evaluated using Mott-Schottky plots. (author)

111

Valence band offset, strain and shape effects on confined states in self-assembled InAs/InP and InAs/GaAs quantum dots  

International Nuclear Information System (INIS)

I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dot single-particle and many-body properties as a function of the quantum dot–surrounding matrix valence band offset. I use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million-atom nanosystems. I demonstrate that the overall confinement in quantum dots is a non-trivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dots and the characteristic ‘shell-like’ structure of confined hole states in the commonly considered ‘low-strain’ lens type InAs/InP quantum dot. I also demonstrate that strain leads to single-band-like behavior of hole states of disk type (‘indium flushed’) InAs/GaAs and InAs/InP quantum dots. I show how strain and valence band offset affect quantum dot many-body properties: the excitonic fine structure, an important factor for efficient entangled photon pair generation, and the biexciton and charged exciton binding energies. (paper)

112

Valence band offset at GaN/?-Si3N4 and ?-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy  

International Nuclear Information System (INIS)

Ultra thin films of pure ?-Si3N4 (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using ?-Si3N4 layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/?-Si3N4/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the ?-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of ?-Si3N4 and a type-II heterojunction. The conduction band offset was deduced to be ? 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/?-Si3N4 interface formation.

113

Valence-Band Offset of m-Plane GaN(1bar 100) Films Grown on LiAlO2(100) Substrates  

Science.gov (United States)

We present first-principles calculations on the heterojunction between a wurtzite GaN(1bar 100) film and a tetragonal LiAlO2(100) substrate. The relative barrier heights of different models of the GaN(1bar 100)?LiAlO2(100) interface are examined as a function of the valence-band offset. The most favorable interface consists of fourfold-coordinated N and Ga, which has the lowest valence-band offset of 0.534 eV. This interface structure indicates that the nitrided LiAlO2(100) surface stabilizes the GaN/LiAlO2 heteroepitaxial structures. Our findings explain a variety of experiments and may prove useful for improving the internal quantum efficiency of nonpolar GaN-based optoelectronic devices.

Liu, Po-Liang; Chou, Yen-Ting; Hong, Jia-Yang

2013-07-01

114

Valence-band and core-level photoemission study of single-crystal Bi2CaSr2Cu2O8 superconductors  

International Nuclear Information System (INIS)

High-quality single crystals of Bi2CaSr2Cu2O8 superconductors have been prepared and cleaved in ultrahigh vacuum. Low-energy electron diffraction measurements show that the surface structure is consistent with the bulk crystal structure. Ultraviolet photoemission and x-ray photoemission experiments were performed on these well-characterized sample surfaces. The valence-band and the core-level spectra obtained from the single-crystal surfaces are in agreement with spectra recorded from polycrystalline samples, justifying earlier results from polycrystalline samples. Cu satellites are observed both in the valence band and Cu 2p core level, signaling the strong correlation among the Cu 3d electrons. The O 1s core-level data exhibit a sharp, single peak at 529-eV binding energy without any clear satellite structures

115

X-Ray Spectroscopic Investigation of the Valence-Band Structure of Layered Compounds MPS 3 ( M = Mn, Fe, Ni, Zn, Mg)  

Science.gov (United States)

The P K? emission and S K? emission spectra influorescence from the layered compounds MPS3 ( M = Mn, Fe, Ni, Zn, Mg)have been measured with a high-resolution two-crystal vacuumspectrometer equipped with Ge(111) crystals. The measured emissionspectra are mainly determined by the [P2S6]4- cluster. TheP K? emissionand S K? emission spectra of ZnPS3, NiPS3 and FePS3are compared with available XPS valence-bandspectra and theoretical band structures on a common energyscale and their valence-band structuresare investigated. It is shown that the P 3p and S 3pstates in their valencebands lie very close to each other and the respective 3p statesare alike in the MPS3 compounds.

Sugiura, Chikara; Kamata, Atsushi; Nakai, Shun-ichi

1996-07-01

116

Theoretical study of influencing factors on the dispersion of bulk band-gap edges and the surface states in topological insulators Bi2Te3 and Bi2Se3  

International Nuclear Information System (INIS)

The dispersion of the band-gap edge states in bulk topological insulators Bi2Te3 and Bi2Se3 is considered within density functional theory. The dependences of this dispersion both on the approximation used for an exchange-correlation functional at fixed unit cell parameters and atomic positions and on these parameters and positions that are obtained upon structural relaxation performed using a certain approximated functional are analyzed. The relative position of the Dirac point of topologically protected surface states and the valence band maximum in the surface electronic structure of the topological insulators is discussed

117

Doping-induced changes in the valence band edge structure of homoepitaxial B-doped diamond films below Mott's critical density  

Energy Technology Data Exchange (ETDEWEB)

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 10{sup 20} cm{sup -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 10{sup 19} cm{sup -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.)

Takeuchi, D.; Ogura, M.; Tokuda, N.; Okushi, H. [Energy Technology Research Institute (ETRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki (Japan); Yamasaki, S. [Energy Technology Research Institute (ETRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki (Japan); Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki (Japan)

2009-09-15

118

Photonic Band Structure of Dispersive Metamaterials Formulated as a Hermitian Eigenvalue Problem  

Science.gov (United States)

We formulate the photonic band structure calculation of any lossless dispersive photonic crystal and optical metamaterial as a Hermitian eigenvalue problem. We further show that the eigenmodes of such lossless systems provide an orthonormal basis, which can be used to rigorously describe the behavior of lossy dispersive systems in general.

Raman, Aaswath; Fan, Shanhui

2010-02-01

119

Plasmon dispersion and quasiparticle band structures for noble metals  

International Nuclear Information System (INIS)

Full text: In density-functional theory the Kohn-Sham eigenvalues cannot be interpreted as excitation energies, but band structures are often qualitatively correct and can be used as a starting point for perturbation-theoretical treatments. The GW approximation for the electronic self-energy is a particularly successful method for accurate quantitative band-structure calculations. The screened interaction W incorporates local field effects from the inversion of the dielectric matrix determined in the Random Phase Approximation. The inverted dielectric matrix combined with non-local empirical pseudopotentials also yields adequately calculated low energy excited state properties of materials. Dielectric calculations based on the pseudo-wavefunctions and energy band structure within a plasmon picture determine mean free path lengths and Fourier components of the inelastic scattering potential. Efficient band structures simulate ballistic transport effects in copper and surface state changes with strain during silver epitaxial film growth on silicon, but care must be taken with convergence, in order to obtain good agreement with first principle calculations and experimental values. Comparisons are made with recent LDA and GW calculations for silver

120

XPS Valence Band and Core Level Spectra of Sputter-Deposited Fe-Cu, Fe-Ag and Fe-Cu-Ag Alloy Films  

Science.gov (United States)

Valence band and core-level characteristics of sputter-deposited Fe-Cu, Fe-Ag and Fe-Cu-Ag alloy films have been studied by X-ray photoelectron spectroscopy using monochromatized Al K? radiation. These films are composed of metastable bcc, fcc and amorphous phases depending on the alloy composition. Their valence band spectra consist of well-resolved peaks representative of Fe-, Cu- and Ag-derived d bands in the respective alloy systems, whose spectral features are similar to those of Ni-Cu and Pd-Ag alloys reported by Hüfner et al. The spectral component of the Fe-derived d band near the Fermi level tends to shrink rapidly in Cu- and Ag-rich fcc alloys. This feature is discussed in relation to the variation of the Curie temperature with the Cu or Ag concentration in these alloys. The core-level spectra of Fe 2p, Cu 2p and Ag 3d states reveal that charge transfer among constituent elements is not appreciable. The effects of thermal annealing and the attendant phase separation upon these XPS spectra are also studied.

Ushida, Masanori; Tanaka, Kazuhide; Sumiyama, Kenji; Nakamura, Yoji

1989-05-01

 
 
 
 
121

Spin orbit splitting in the valence bands of ZrSxSe2?x: Angle resolved photoemission and density functional theory  

International Nuclear Information System (INIS)

Highlights: ? We performed high resolution ARPES on 1T–ZrSxSe2?x. ? A characteristic splitting of the chalcogen p-derived VB along high symmetry directions was observed. ? The splitting size at the A point of the BZ is found to increase from 0.06 to 0.31 eV from ZrS2 towards ZrSe2. ? Electronic structure calculations based on the DFT were performed using the model of TB–MBJ. ? The calculations show that the splitting is due to SO coupling of the valence bands. -- Abstract: Angle-resolved photoelectron spectroscopy using synchrotron radiation has been performed on 1T–ZrSxSe2?x, where x varies from 0 to 2, in order to study the influence of the spin-orbit interaction in the valence bands. The crystals were grown by chemical vapour transport technique using Iodine as transport agent. A characteristic splitting of the chalcogen p-derived valence bands along high symmetry directions has been observed experimentally. The size of the splitting increases with the increase of the atomic number of the chalcogenide, e.g. at the A point of the Brillouin zone from 0.06 eV to 0.31 eV with an almost linear dependence with x, as progressing from ZrS2 towards ZrSe2, respectively. Electronic structure calculations based on the density functional theory have been performed using the model of Tran–Blaha [1] and the modified version of the exchange potential proposed by Becke and Johnson [2] (TB–MBJ) both with and without spin-orbit (SO) coupling. The calculations show that the splitting is mainly due to spin-orbit coupling and the degeneracy of the valance bands is lifted

122

Spatially resolved, energy-filtered imaging of core level and valence band photoemission of highly p and n doped silicon patterns.  

Science.gov (United States)

An accurate description of spatial variations in the energy levels of patterned semiconductor substrates on the micron and sub-micron scale as a function of local doping is an important technological challenge for the microelectronics industry. Spatially resolved surface analysis by photoelectron spectromicroscopy can provide an invaluable contribution thanks to the relatively non-destructive, quantitative analysis. We present results on highly doped n and p type patterns on, respectively, p and n type silicon substrates. Using synchrotron radiation and spherical aberration-corrected energy filtering, we have obtained a spectroscopic image series at the Si 2p core level and across the valence band. Local band alignments are extracted, accounting for doping, band bending and surface photovoltage. PMID:21828576

Barrett, N; Zagonel, L F; Renault, O; Bailly, A

2009-08-01

123

Development of wave length-dispersive soft x-ray emission spectrometers for transmission electron microscopes - an introduction of valence electron spectroscopy for transmission electron microscopy  

International Nuclear Information System (INIS)

Two types of wavelength-dispersive soft X-ray spectrometers, a high-dispersion type and a conventional one, for transmission electron microscopes were constructed. Those spectrometers were used to study the electronic states of valence electrons (bonding electrons). Both spectrometers extended the acceptable energy regions to higher than 2000 eV. The best energy resolution of 0.08 eV was obtained for an Al L-emission spectrum by using the high-dispersion type spectrometer. By using the spectrometer, C K-emission of carbon allotropes, Cu L-emission of Cu1-xZnx alloys and Pt M-emission spectra were presented. The FWHM value of 12 eV was obtained for the Pt M?-emission peak. The performance of the conventional one was also presented for ZnS and a section specimen of a multilayer device. W-M and Si-K emissions were clearly resolved. Soft X-ray emission spectroscopy based on transmission electron microscopy (TEM) has an advantage for obtaining spectra from a single crystalline specimen with a defined crystal setting. As an example of anisotropic soft X-ray emission, C K-emission spectra of single crystalline graphite with different crystal settings were presented. From the spectra, density of states of ?- and ?-bondings were separately derived. These results demonstrated a method to analyse the electronic states of valence electrons of materials in the nanometre scale based on TEM. (author)

124

Superconductivity of thallium-doped lead telluride. Density of states in the valence band of thallium-doped and sodium-doped PbTe  

International Nuclear Information System (INIS)

Measurements of the low-temperature specific heat and Hall coefficient of p-type PbTe doped with the acceptor impurities thallium and sodium were used to find the density of states g in the valence band as a function of the Hall density of holes p77 measured at T = 77 0K. In the range p77>4 x 1019 cm-3 a considerable difference in g(p77) was discovered between PbTe:Tl and PbTe:Na. The experimental results are interpreted in terms of the idea that the dominant perturbation of the band spectrum of PbTe by Na impurities (as distinct from Tl) is a considerable increase in the energy separation of the split-off ''heavy'' extremum in the valence band. The model under discussion gives a qualitative explanation of the reasons for the existence of bulk superconductivity in PbTe:Tl, as recently found by the present authors, and for its absence in PbTe:Na. Consequences of the present experimental results are examined and they are found to be relevant to the study of the properties of heavily doped semiconductors in both normal and superconducting states

125

Valence electron energy loss study of Fe-doped SrTiO3 and a sigma13 boundary: electronic structure and dispersion forces.  

Science.gov (United States)

Valence electron energy loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of a sigma13 tilt grain boundary in SrTiO3. In a first step the electronic structure of bulk SrTiO3 has been analysed quantitatively by comparing VEELS spectra with vacuum ultraviolet spectra and with ab initio density of states calculations. The electronic structure of a near sigma13 grain boundary and the corresponding dispersion forces were then determined by spatially resolved VEELS. Also the effects of delocalization of the inelastic scattering processes were estimated and compared with results from the literature. PMID:11281150

van Benthem, K; French, R H; Sigle, W; Elsässer, C; Rühle, M

2001-02-01

126

Single-carrier impact ionization favored by a limited band dispersion  

CERN Document Server

A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the conduction or valence band resulting in a restriction of kinetic energy for one of the charge carriers. This feature is not common to the majority of technologically relevant semiconductors, but it is observed in chalcogenides, such as Selenium and compound I2-II-IV-VI4 alloys.

Darbandi, A

2012-01-01

127

Symmetry assisted consideration of the dynamic pseudo Jahn-Teller problem in mixed-valence species with square topology: Intervalence optical bands  

Science.gov (United States)

Here we report the evaluation of the electronic and vibronic levels of mixed-valence tetra-ruthenium (2Ru(II) + 2Ru(III)) systems (assembled as two coupled Creutz-Taube complexes) for which possible molecular implementations of quantum-dot cellular automata was proposed. These complexes are representatives of Robin and Day classes I, II, and III and therefore the vibronic coupling is crucially important for the adequate description of the magnetic characteristics and profiles of the intervalence optical bands. We apply recently suggested symmetry-adapted approach to the accurate solution of the dynamic pseudo Jahn-Teller problems (1 B 1 g , 21 A 1 g , 1 B 2 g , 1 E u ) ? ( b 1 g + e u ) and (3 A 2 g , 3 B 1 g , 23 E u ) ? ( b 1 g + e u ) in bi-electronic mixed-valence species with square topology. The algorithm for the solution of the eigen-problem takes full advantage of the point symmetry arguments and therefore the full matrix of the Jahn-Teller JT Hamiltonian is blocked according to the irreps of the point group. The vibronic basis is used for the evaluation of the shape-functions of the intervalence optical absorption in mixed-valence tetra-ruthenium (2Ru(II) + 2Ru(III)) complexes.

Clemente-Juan, Juan Modesto; Palii, Andrew; Coronado, Eugenio; Tsukerblat, Boris

2014-05-01

128

Valence band photoemission Ni{sub x}Pd{sub 1-x} alloy films on Cu{sub 3}Au(100)  

Energy Technology Data Exchange (ETDEWEB)

Spin and angle-resolved photoemission are important experimental methods to investigate the electronic band structure of crystalline materials. Spin-integrated measurements can be performed relatively fast with a high energy and momentum resolution, whereas spin-resolved measurements reveal further information about the band structure. On the compound of Ni{sub x}Pd{sub 1-x}/Cu{sub 3}Au(100) we investigate the electronic and magnetic properties with regard to the thickness and the composition of the ultrathin Ni{sub x}Pd{sub 1-x} layer. The composition differences should be reflected in an inverse spin-reorientation transition. Furthermore we compare the measured spectra to the theoretical calculations of the band structure. The measurement takes place at Beamline 5 at DELTA, Dortmund, with an unique detector setup. We can acquire 2-dimensional angle resolved data for band mapping and spin resolved one-dimensional data quasi-simultaneously. This will provide us with high resolution band structure spectra and images as well as spin resolved valence band spectra. Our samples are prepared in-situ by e-beam evaporation and characterized by LEED and Auger spectroscopy.

Gulik, Matthias [Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse, 52428 Juelich (Germany); Technische Universitaet Dortmund, Maria-Goeppert-Mayer- Str. 2, D-44221 Dortmund (Germany); Plucinski, Lukasz; Schneider, Claus M. [Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse, 52428 Juelich (Germany)

2010-07-01

129

Size and alloying induced shift in core and valence bands of Pd-Ag and Pd-Cu nanoparticles  

International Nuclear Information System (INIS)

In this report, X-ray photoelectron spectroscopy studies have been carried out on Pd, Ag, Cu, Pd-Ag, and Pd-Cu nanoparticles having identical sizes corresponding to mobility equivalent diameters of 60, 40, and 20?nm. The nanoparticles were prepared by the gas phase synthesis method. The effect of size on valence and core levels in metal and alloy nanoparticles has been studied by comparing the values to those with the 60?nm nanoparticles. The effect of alloying has been investigated by comparing the valence and core level binding energies of Pd-Cu and Pd-Ag alloy nanoparticles with the corresponding values for Pd, Ag, and Cu nanoparticles of identical sizes. These effects have been explained in terms of size induced lattice contractions, alloying induced charge transfer, and hybridization effects. The observation of alloying and size induced binding energy shifts in bimetallic nanoparticles is important from the point of view of hydrogen reactivity

130

Size and alloying induced shift in core and valence bands of Pd-Ag and Pd-Cu nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

In this report, X-ray photoelectron spectroscopy studies have been carried out on Pd, Ag, Cu, Pd-Ag, and Pd-Cu nanoparticles having identical sizes corresponding to mobility equivalent diameters of 60, 40, and 20?nm. The nanoparticles were prepared by the gas phase synthesis method. The effect of size on valence and core levels in metal and alloy nanoparticles has been studied by comparing the values to those with the 60?nm nanoparticles. The effect of alloying has been investigated by comparing the valence and core level binding energies of Pd-Cu and Pd-Ag alloy nanoparticles with the corresponding values for Pd, Ag, and Cu nanoparticles of identical sizes. These effects have been explained in terms of size induced lattice contractions, alloying induced charge transfer, and hybridization effects. The observation of alloying and size induced binding energy shifts in bimetallic nanoparticles is important from the point of view of hydrogen reactivity.

Sengar, Saurabh K.; Mehta, B. R., E-mail: brmehta@physics.iitd.ac.in [Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016 (India); Govind [Surface Physics Group, National Physical Laboratory (CSIR), New Delhi 110012 (India)

2014-03-28

131

Measurement of the valence band-offset in a PbSe/ZnO heterojunction by x-ray photoelectron spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

A heterojunction of PbSe/ZnO has been grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was used to directly measure the valence-band offset (VBO) of the heterojunction. The VBO, {Delta}E{sub V}, was determined as 2.51 {+-} 0.05 eV using the Pb 4p{sup 3/2} and Zn 2p{sup 3/2} core levels as a reference. The conduction-band offset, {Delta}E{sub C}, was, therefore, determined to be 0.59 {+-} 0.05 eV based on the above {Delta}E{sub V} value. This analysis indicates that the PbSe/ZnO heterojunction forms a type I (Straddling Gap) heterostructure.

Li Lin; Qiu Jijun; Weng Binbin; Yuan Zijian; Shi Zhisheng [School of Electrical and Computer Engineering, University of Oklahoma, Norman, Oklahoma 73019 (United States); Li Xiaomin; Gan Xiaoyan [State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 (China); Sellers, Ian R. [Deparment of Physics, University of Oklahoma, Norman, Oklahoma 73019 (United States)

2012-12-24

132

Band dispersion of MgB sub 2 , graphite and diamond from resonant inelastic scattering  

CERN Document Server

The quantitative band mapping for MgB sub 2 , graphite and diamond are realized using resonant inelastic x-ray scattering (RIXS) measurements. RIXS shows distinct dispersive features when the excitation energy is tuned near B 1s and C 1s thresholds, which are assigned to the calculated energy bands using k sup->-momentum conservation. The agreement between experiment and theory suggests that electron-electron interactions are not important for MgB sub 2 , which behaves like a conventional metal and is well described by band theory.

Sokolov, A V; Leitch, S; Moewes, A; Kortus, J; Finkelstein, L D; Skorikov, N A; Xiao, C; Hirose, A

2003-01-01

133

4f hybridization and band dispersion in gadolinium thin films and compounds  

International Nuclear Information System (INIS)

There is interplay between intra-atomic orbital hybridization and extra-atomic hybridization in various gadolinium systems, which affects magnetic coupling and electron itinerancy (localization). The results do not always follow expectation. The experimental band structure of thin Gd(0001) films, grown on the Mo(112) surface, along the anti ?- anti M does not agree with expectations even qualitatively. In particular, the dispersion of the gadolinium band, with strong 5d weight near 2 eV binding energy provides considerable evidence to support the case for 4f-5d hybridization, with increasing 5d localization. On the other hand, there is also evidence of extra-atomic Gd 4f hybridization leading band dispersion in the occupied 4f levels in Gd2O3. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

134

Exporting superconductivity across the gap: Proximity effect for semiconductor valence-band states due to contact with a simple-metal superconductor  

Science.gov (United States)

The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nanostructured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductor's charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductor's conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of interband coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of interband coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.

Moghaddam, A. G.; Kernreiter, T.; Governale, M.; Zülicke, U.

2014-05-01

135

A new circular photonic crystal fiber for effective dispersion compensation over E to L wavelength bands  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english This paper presents a new circular photonic crystal fiber (C-PCF) for effective dispersion compensation covering E to L wavelength bands ranging from 1360-1625 nm. To investigate its guiding properties, finite element method (FEM) with a perfectly matched layer absorbing boundary condition is used. [...] From our numerical simulation, it is found that the designed C-PCF simultaneously shows a large negative dispersion of about -248.65 to -1069 ps/(nm.km) over E to L wavelength bands and a relative dispersion slope (RDS) exactly equal to that of a single mode fiber (SMF) at 1.55 µm wavelength. It is also found that residual dispersion after compesating 40 km long SMF is within ±62 ps/nm which ensures application of C-PCF in high speed WDM system. Besides, dispersion slope, slope compensation ratio, effective area and confinement loss of the proposed C-PCF are also evaluated and discussed.

M. M., Haque; M. S., Rahman; M. Samiul, Habib; M. Selim, Habib; S. M. A., Razzak.

2013-12-01

136

A new circular photonic crystal fiber for effective dispersion compensation over E to L wavelength bands  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english This paper presents a new circular photonic crystal fiber (C-PCF) for effective dispersion compensation covering E to L wavelength bands ranging from 1360-1625 nm. To investigate its guiding properties, finite element method (FEM) with a perfectly matched layer absorbing boundary condition is used. [...] From our numerical simulation, it is found that the designed C-PCF simultaneously shows a large negative dispersion of about -248.65 to -1069 ps/(nm.km) over E to L wavelength bands and a relative dispersion slope (RDS) exactly equal to that of a single mode fiber (SMF) at 1.55 µm wavelength. It is also found that residual dispersion after compesating 40 km long SMF is within ±62 ps/nm which ensures application of C-PCF in high speed WDM system. Besides, dispersion slope, slope compensation ratio, effective area and confinement loss of the proposed C-PCF are also evaluated and discussed.

M. M., Haque; M. S., Rahman; M. Samiul, Habib; M. Selim, Habib; S. M. A., Razzak.

137

Combining GW calculations with exact-exchange density-functional theory: An analysis of valence-band photoemission for compound semiconductors  

CERN Document Server

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-electrons that are explicitly included as valence electrons in our pseudopotential approach. Unlike in the majority of previous GW studies, which are almost exlusively based on ground state calculations in the local-density approximation (LDA), we combine GW with exact-exchange DFT calculations in the optimised-effective potential approach (OEPx). This is a much more elaborate and computationally expensive approach. However, we show that applying the OEPx approach leads to an improved description of the d-electron hybridisation compared to t...

Rinke, P; Neugebauer, J; Freysoldt, C; Scheffler, M; Rinke, Patrick; Qteish, Abdallah; Neugebauer, Joerg; Freysoldt, Christoph; Scheffler, Matthias

2005-01-01

138

Verification of ?7 symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A exciton state  

International Nuclear Information System (INIS)

Circularly-polarized and angular-resolved magneto-photoluminescence spectroscopy was carried out to study the free A exciton 1S state in wurtzite ZnO at 5 K. The ? 7 symmetry of the top valence band symmetry is confirmed according to the unique selection rules of Zeeman splitting lines. The out-of-plane component B? of the magnetic field, which is parallel to ZnO's c-axis, leads to linear Zeeman splitting of both the dipole-allowed ? 5 exciton state and the weakly allowed ? 1 /? 2 exciton states. The in-plane field B?, which is perpendicular to the c-axis, increases the oscillator strength of the weak ? 1 /? 2 states by forming a mixed exciton state. For the ? 7 symmetry, the lower energy Zeeman splitting peak of the weak ? 1 /? 2 can only be ?+ polarization. (paper)

139

Anomalous temperature dependence in valence band spectra: A resonant photoemission study of layered perovskite Sr{sub 2}CoO{sub 4}  

Energy Technology Data Exchange (ETDEWEB)

Valence band spectra (VBS) and its modification across Curie temperature (T{sub C}) of Sr{sub 2}CoO{sub 4} thin film are studied using resonant photoemission spectroscopy. It is found that VBS mainly consists of hybridized states of Co-3d t{sub 2g}e{sub g} and O-2p; however, Co-3d e{sub g} states show its prominence only in the ferromagnetic temperature regime. Below T{sub C}, spectral weight transfer takes place anomalously from high binding energy (B.E.) region to low B.E. region, signifying the enhanced intermediate or low spin state Co{sup 4+} ions. It is suggested that spin-lattice coupling and many-body effects in Sr{sub 2}CoO{sub 4} derived from the strong electron correlations lead to such temperature dependence of VBS.

Pandey, Pankaj K.; Choudhary, R. J., E-mail: ram@csr.res.in; Phase, D. M. [UGC DAE Consortium for Scientific Research, University Campus, Indore 452001 (India)

2014-05-05

140

Temperature-dependent valence-band photoemission spectra of La{sub 1{minus}x}Sr{sub x}MnO{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

We have studied the temperature dependence of the photoemission spectra of La{sub 1{minus}x}Sr{sub x}MnO{sub 3} (x=0.0, 0.2, and 0.4) and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for x=0.2 and 0.4. By contrast, the spectra of La{sub 0.6}Sr{sub 0.4}CoO{sub 3} show no significant temperature dependence. From comparison between the temperature- and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La{sub 1{minus}x}Sr{sub x}MnO{sub 3}, we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations. {copyright} {ital 1997} {ital The American Physical Society}

Saitoh, T.; Sekiyama, A.; Kobayashi, K.; Mizokawa, T.; Fujimori, A. [Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan); Sarma, D.D. [Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012 (India); Takeda, Y. [Department of Chemistry, Faculty of Engineering, Mie University, Tsu 514 (Japan); Takano, M. [Institute for Chemical Research, Kyoto University, Uji, Kyoto 611 (Japan)

1997-10-01

 
 
 
 
141

Thermoelectric properties of GeTe at high hydrostatic pressure up to 8.5 GPa and its valence-band structure  

International Nuclear Information System (INIS)

The thermopower and the relative electrical resistance of rhombohedral ?-GeTe and orthorhombic #betta#-GeTe polycrystalline samples with hole density between 3.9 x 1020 and 2 x 1021 cm-3 are measured under hydrostatic pressure up to 8.5 GPa at room temperature. The phase transition of ?-GeTe to the cubic rocksalt structure (#betta#-GeTe) reported earlier at 3.5 GPa under nonhydrostatic pressure is not observed up to 8.5 GPa. Orthorhombic #betta#-GeTe also exhibits no phase transition. The pressure dependences of the thermopower provide support for the multiple valence-band model of GeTe. (author)

142

Valence and conduction band alignment at ScN interfaces with 3C-SiC (111) and 2H-GaN (0001)  

Science.gov (United States)

In order to understand and predict the behavior of future scandium nitride (ScN) semiconductor heterostructure devices, we have utilized in situ x-ray and ultra-violet photoelectron spectroscopy to determine the valence band offset (VBO) present at ScN/3C-SiC (111) and 2H-GaN (0001)/ScN (111) interfaces formed by ammonia gas source molecular beam epitaxy. The ScN/3C-SiC (111) VBO was dependent on the ScN growth temperature and resistivity. VBOs of 0.4 ± 0.1 and 0.1 ± 0.1 eV were, respectively, determined for ScN grown at 925 °C (low resistivity) and 800 °C (high resistivity). Using the band-gaps of 1.6 ± 0.2 and 1.4 ± 0.2 eV previously determined by reflection electron energy loss spectroscopy for the 925 and 800 °C ScN films, the respective conduction band offsets (CBO) for these interfaces were 0.4 ± 0.2 and 0.9 ± 0.2 eV. For a GaN (0001) interface with 925 °C ScN (111), the VBO and CBO were similarly determined to be 0.9 ± 0.1 and 0.9 ± 0.2 eV, respectively.

King, Sean W.; Nemanich, Robert J.; Davis, Robert F.

2014-08-01

143

Dynamics of Impurity and Valence Bands in Ga1-xMnxAs Within the Dynamical Mean-Field Approximation  

International Nuclear Information System (INIS)

We calculate the density-of-states and the spectral function of Ga1-xMnxAs within the dynamical mean-field approximation. Our model includes the competing effects of the strong spin-orbit coupling on the J=3/2 GaAs hole bands and the exchange interaction between the magnetic ions and the itinerant holes. We study the quasiparticle and impurity bands in the paramagnetic and ferromagnetic phases for different values of impurity-hole coupling Jc at a Mn doping of x=0.05. By analyzing the anisotropic angular distribution of the impurity band carriers at T=0, we conclude that the carrier polarization is optimal when the carriers move along the direction parallel to the average magnetization.

144

Spiral phase plate based on polymer dispersed liquid crystal for wide visible band applications.  

Science.gov (United States)

This study demonstrates helical wave fronts via a spiral phase plate based on polymer dispersed liquid crystals (PDLCs). Because the PDLC is electric tunable, the plate can be used in a wide visible band. In addition, if the probe beam deviates from the center of the sample, some of the light propagates out of the sectors. We propose some of the applications for the results. PMID:25321351

Wu, Shing-Trong; Fuh, Andy Ying-Guey

2014-09-01

145

Monovalent-copper (I; 3d10, S = O) induced oxygen-2p valence band holes with concomitant attractive interaction inside low-dimensional oxygen networks  

International Nuclear Information System (INIS)

Results of electrical transport and magnetic measurements on coldpressed compacts of La2CuO4 and La1.70Ba0.30CuO4 provide strong evidence for their p-type metallic and Pauli paramagnetic character at room temperature connected with the existence of low density-of-states oxygen-2p valence band holes induced by ground state monovalent copper (3d10, S = O). The host La2CuO4 exhibits an electronic phase transition at ? 85 K from a p-type broad-band metal (p = 2.5 x 10-2 holes/La2CuO4, ?h = 10-4 m2/Vs, mh* = 0.5m0, EF = 0.26 eV, N(EF) = 0.15 states/eV-La2CuO4) to a p-type polaron-mediated ICDW state (p ? 2.5 x 10-2 holes/La2CuO4, ?h ? 10-4 m2/Vs, mh* ? 10m0, EF = 11 meV, N(EF) = 3.4 states/eV-La2CuO4). The attractive interaction attending the O-2p VB holes inside low-dimensional oxygen networks is discussed in the framework of recent papers on structural distortions, electronic structures, and phase transitions in copper containing oxidic hosts and doped superconducting derivatives. 61 refs. (author)

146

Degeneracy and effective mass in the valence band of two-dimensional (100)-GaAs quantum well systems  

International Nuclear Information System (INIS)

Quantum Hall measurement of two-dimensional high-mobility [??2×106?cm2/(V·s)] hole systems confined in a 20?nm wide (100)-GaAs quantum well have been performed for charge densities between 4 and 5?×?1010?cm?2 in a temperature range of 10–160 mK. The Fourier analysis of the Shubnikov-de Haas (SdH) oscillations of the magnetoresistance vs. the inverse of the magnetic field 1/B reveals a single peak, indicating a degenerate heavy hole (HH) band. The hole density p=(e/h)·f agrees with the Hall measurement result within 3%. The HH band degeneracy is understood through the diminishing spin-orbit interaction due to the low charge density and the nearly symmetric confinement. SdH oscillations fitted for 0.08?T???B???0.24?T to the Dingle parameters yield an effective mass between 0.30 and 0.50?me in good agreement with previous cyclotron resonance results

147

Degeneracy and effective mass in the valence band of two-dimensional (100)-GaAs quantum well systems  

Energy Technology Data Exchange (ETDEWEB)

Quantum Hall measurement of two-dimensional high-mobility [??2×10{sup 6}?cm{sup 2}/(V·s)] hole systems confined in a 20?nm wide (100)-GaAs quantum well have been performed for charge densities between 4 and 5?×?10{sup 10}?cm{sup ?2} in a temperature range of 10–160 mK. The Fourier analysis of the Shubnikov-de Haas (SdH) oscillations of the magnetoresistance vs. the inverse of the magnetic field 1/B reveals a single peak, indicating a degenerate heavy hole (HH) band. The hole density p=(e/h)·f agrees with the Hall measurement result within 3%. The HH band degeneracy is understood through the diminishing spin-orbit interaction due to the low charge density and the nearly symmetric confinement. SdH oscillations fitted for 0.08?T???B???0.24?T to the Dingle parameters yield an effective mass between 0.30 and 0.50?m{sub e} in good agreement with previous cyclotron resonance results.

Tarquini, Vinicio, E-mail: vinicio.tarquini@wayne.edu; Knighton, Talbot; Wu, Zhe; Huang, Jian [Department of Physics and Astronomy, Wayne State University, 666 W. Hancock, Detroit, Michigan 48201 (United States); Pfeiffer, Loren; West, Ken [Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 (United States)

2014-03-03

148

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors  

International Nuclear Information System (INIS)

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias

149

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors  

Energy Technology Data Exchange (ETDEWEB)

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Lao, Yan-Feng; Wolde, Seyoum; Unil Perera, A. G., E-mail: uperera@gsu.edu [Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303 (United States); Zhang, Y. H.; Wang, T. M. [Key Laboratory of Artificial Structures and Quantum Control, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240 (China); Kim, J. O.; Schuler-Sandy, Ted; Tian, Zhao-Bing; Krishna, S. S. [Center for High Technology Materials, Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87106 (United States)

2014-04-28

150

Extend the omnidirectional zero-average-index photonic band gap using the band edge formalism: Application to the metamaterial with Drude dispersion  

Science.gov (United States)

The band edge formalism of zero-average-index photonic band gap for the one-dimensional photonic crystals containing dispersion negative-index metamaterial is obtained. By adopting the band edge formalism, the upper and lower frequencies limits of the zero-average-index band gap are analyzed, and the results are in complete agreement with the results got by the projected band structure method. It is shown that the width and the band edge of the zero-average-index photonic band gap are significantly affected by the each constitutive parameter of the component materials of the one-dimensional photonic crystals. Based on the band edge formalism, new method of extending the zero-average-index photonic band gap has been presented by tuning the constitutive parameter of the component materials. Moreover, the conditions for zero width zero-average-index band gap are obtained. The realizability of tuning the constitutive parameter of the Drude dispersion metamaterial constructed by the nonresonant composite right/left-handed transmission structures is also discussed.

Xiang, Yuanjiang; Wen, Shuangchun; Dai, Xiaoyu; Tang, Zhixiang; Fan, Dianyuan

2010-11-01

151

Valence Band Character of NiS2-xSex using 3p-3d Resonant ARPES  

Science.gov (United States)

Understanding the strong correlated system is one of the most challenging tasks in condensed matter physics. Especially, the metal insulator transition (MIT) has been one of the major topics recent few decades. NiS2-xSex is known as one of famous material which has MIT. The cubic pyrite NiS2 is a charge-transfer (CT) insulator. NiS2 attracts particular interest as it easily forms a solid solution with NiSe2 (NiS2-xSex) which, while being isoelectronic and isostructural to NiS2, is nevertheless a good metal. MIT, induced by Se alloying, is observed at low temperature (T) for x=0.45. Perucchi and his collaborators revealed closed relation between MIT and band width through comparison of infrared spectroscopy result and LDA calculation. However, it was only an indirect observation, and is inconsistent with recent proposal that NiS2 is not a CT insulator but an insulator due to the bonding-antibonding splitting in the S -- S (Se -- Se) dimers. To reveal the true mechanism in the MIT in NiS2-xSex, resonant photoemission experiment is essential. According to competing theories (CT insulator and insulator due to bonding-antibonding splitting), it is expected that the character of the main band that is responsible for the MIT should be different. Therefore, we performed 3p-3d resonant ARPES for various Se dopings (x=0.43; insulator, x=0.5, 0.7, 2.0; metal) and observed a significant change between on- and off-resonances near the MIT. Our experimental result supports that the origin of MIT in NiS2-xSex is the CT theory rather than the dimer theory.

Han, Garam; Kim, Yeongkwan; Koh, Yoonyoung; Kim, Beomyoung; Song, Dongjoon; Seo, Jungjin; Kyung, Wonshik; Lee, Kyungdong; Kim, Changyoung

2013-03-01

152

Proposal for highly residual dispersion compensating defected core decagonal photonic crystal fiber over S+C+L+U wavelength bands  

Science.gov (United States)

A defected core decagonal photonic crystal fiber is designed and numerically optimized to obtain its residual chromatic dispersion compensation in the wavelength range of 1460 to 1675 nm i.e., over S+C+L+U wavelength bands having an average dispersion of about -390 ps/(nm km) with a dispersion variation of 7 ps/(nm km). The designed fiber, with a flattened dispersion profile, has four rings of holes in the cladding region, which results in low confinement loss and small effective mode area at wavelength 1550 nm. For residual chromatic dispersion compensation, the proposed fiber can be used in wavelength division multiplexing optical fiber data communication systems.

Islam, Md. Aminul; Ahmad, Redwan; Ali, Md. Sharafat; Nasim, K. M.

2014-07-01

153

Energy Dispersion Compensation and Beam Loading in X-Band Linacs for the JLC/NLC  

International Nuclear Information System (INIS)

The shape of an RF pulse is distorted upon propagating through an X-band accelerator structure due to dispersive effects. This distortion together with beam loading introduce energy spread between 192 bunches. In order to minimize this energy spread we modify the input RF pulse shape. The pulse propagation, energy gain, and beam loading are modeled with a mode-matching computer code and a circuit model. A 2D model and a circuit model of a complete 60 cm structure, consisting of 55 cells and input and output couplers is analyzed. This structure operates with a 5?/6 phase advance per cell. Dispersive effects for this structure are more significant than for previously studied 2?/3 phase advance accelerating structures. Experimental results are compared with the theoretical model and excellent agreement is obtained for the propagation of an RF pulse through the structure

154

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

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

155

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

Science.gov (United States)

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; Mankad, Venu; Jha, Prafulla K.

2014-04-01

156

Valence band electronic structure of V2O5 as determined by resonant soft X-ray emission spectroscopy  

International Nuclear Information System (INIS)

Resonant soft X-ray emission (SXE) spectra have been measured for a series of excitation energies at the V L? and O K? bands of divanadium pentoxide, V2O5. In addition, a near-edge X-ray absorption fine structure (NEXAFS) spectrum at the V 2p and O 1s edges has been also recorded to aid the assignment of the features seen in the SXE data. The V L? SXE spectrum of V2O5 consists of two distinct components that can be assigned to the V 3deg-like states strongly hybridized with the O 2p?-like states and to the V 3dt2g-like states weakly hybridized with the O 2p?-like states. The resonant SXE spectra recorded at photon energies close to the position of the V LIII edge revealed Raman-like features with an energy loss of 7 eV similar to charge transfer states previously observed by Schmitt et al. [T. Schmitt, L.-C. Duda, M. Matsubara, M. Mattesini, M. Klemm, A. Augustsson, J.-H. Guo, T. Uozumi, S. Horn, R. Ahuja, A. Kotani, J. Nordgren, Phys. Rev. B 69 (2004) 125103] for the V6O13 oxide

157

Band structure of two-dimensional square lattice photonic crystals of circular dispersive metamaterial rods  

Science.gov (United States)

By virtue of the efficiency of the Dirichlet-to-Neumann map method, the details of the band structure of a two-dimensional square lattice photonic crystal composed of dispersive metamaterial circular rods in air background has been studied. We show that there are two flat bands at the band structure of the system for both H-polarization and E-polarization. These flat bands are created around the magnetic resonance frequency, surface plasmon frequency and magnetic surface plasmon frequency. We realized that the modes with frequencies lying above the resonance frequency behave like resonant cavity modes created in a single metallic cylindrical waveguide. While, due to the relatively large and imaginary refractive index of the metamaterial rods at the frequencies lying below the resonance frequency, the modes are localized modes with negligible penetration into the rods. Moreover, the modes are localized at the interface of the cylindrical metamaterial rods and the air background for the frequencies around the surface plasmon frequency and the magnetic surface plasmon frequency.

Soltani Vala, A.; Roshan Entezar, S.; Sedghi, A. A.

2011-06-01

158

On the absence of resonance in the valence band photoemission at L2 resonance edge and the delayed onset of the normal Auger-decay  

International Nuclear Information System (INIS)

Graphical abstract: IR as a function of the CK-decay width (FWHM) (?CK) of the resonantly excited L2-hole state. Here the Auger-decay width (FWHM) is 0.3 eV. Highlights: ? The absence of VB resonance at the L2 resonance in a Mn6 is explained. ? The resonant Mn Auger-electron spectroscopy spectra of MnO are analyzed. ? The delayed onset of normal Mn Auger-decay in MnO is explained. - Abstract: No clear resonance is present in the resonant valence band photoemission spectrum of a Mn6 (S = 4) single molecule magnet grafted on Au (1 1 1) for photon energy at the L2 resonance edge. This is due to the L2-hole’s dominant decay by the L2–L3V Coster–Kronig (CK) decay. The excess energy of the photon at which the Mn resonant Raman scattering regime transits to the normal Mn Auger-decay regime in MnO depends on the Mn Auger-decay channels. This is because of the localization and delocalization of two holes in the Auger final states. The Mn resonant Auger-electron spectroscopy spectra of MnO are analyzed.

159

On the absence of resonance in the valence band photoemission at L{sub 2} resonance edge and the delayed onset of the normal Auger-decay  

Energy Technology Data Exchange (ETDEWEB)

Graphical abstract: I{sub R} as a function of the CK-decay width (FWHM) ({Gamma}{sub CK}) of the resonantly excited L{sub 2}-hole state. Here the Auger-decay width (FWHM) is 0.3 eV. Highlights: Black-Right-Pointing-Pointer The absence of VB resonance at the L{sub 2} resonance in a Mn{sub 6} is explained. Black-Right-Pointing-Pointer The resonant Mn Auger-electron spectroscopy spectra of MnO are analyzed. Black-Right-Pointing-Pointer The delayed onset of normal Mn Auger-decay in MnO is explained. - Abstract: No clear resonance is present in the resonant valence band photoemission spectrum of a Mn{sub 6} (S = 4) single molecule magnet grafted on Au (1 1 1) for photon energy at the L{sub 2} resonance edge. This is due to the L{sub 2}-hole's dominant decay by the L{sub 2}-L{sub 3}V Coster-Kronig (CK) decay. The excess energy of the photon at which the Mn resonant Raman scattering regime transits to the normal Mn Auger-decay regime in MnO depends on the Mn Auger-decay channels. This is because of the localization and delocalization of two holes in the Auger final states. The Mn resonant Auger-electron spectroscopy spectra of MnO are analyzed.

Ohno, Masahide [Quantum Science Research, 2-8-5 Tokiwadai, Itabashi-ku, Tokyo 174 0071 (Japan)

2012-06-19

160

Valence band electronic structure of Nd1?xYxMnO3 using X-ray absorption, photoemission and GGA + U calculations  

International Nuclear Information System (INIS)

Highlights: •Decrease in the occupancy of Mn 3d orbitals with doping. •Greater splitting of the eg orbitals due to the increased Jahn–Teller distortion with doping. •Decrease in O 2p–Mn 3d charge transfer character with doping. •Increase in charge transfer energy and band gap with doping. •Calculations hint a subtle change from a charge transfer to Mott–Hubbard type insulator with doping. -- Abstract: The electronic structures of Nd1?xYxMnO3 (x = 0–0.5) were studied using X-ray absorption near-edge structure (XANES) at the Mn L3,2- and O K-edge along with valence-band photoemission spectroscopy (VB-PES). The systematic increase in white-line intensity of the Mn L3,2-edge with doping, suggests a decrease in the occupancy of Mn 3d orbitals. The O K-edge XANES shows a depletion of unoccupied states above the Fermi energy. The changes in the O K-edge spectra due to doping reflects an increase in the Jahn–Teller distortion. The VB-PES shows broadening of the features associated with Mn 3d and O 2p hybridized states and the shift of these features to a slightly higher binding energy in agreement with our GGA + U calculations. The system shows a net shift of the occupied and unoccupied states away from the Fermi energy with doping. The shift in theoretical site-projected density of states of x = 0.5 composition with respect to x = 0 suggest a subtle change from a charg

 
 
 
 
161

Strongly Anisotropic Band Dispersion of an Image State Located above Metallic Nanowires  

Science.gov (United States)

Indium can be grown on Si(111) in a 4×1 pattern that contains rows of In atoms spaced 13.3 Å apart that have quasi-one-dimensional electronic structure. This ordered array of metallic wires produces an image-induced surface state series. We have measured the dispersion of the most tightly bound \\(n = 1\\) image state band and found it to be unconventional because it falls below the free electron parabola perpendicular to the In atom rows. The most straightforward explanation for this is that the electrons feel the surface corrugation potential produced by the rows of In atoms. We were able to infer the form of the potential from our measurements.

Hill, I. G.; McLean, A. B.

1999-03-01

162

Spectral properties of quasi-one-dimensional conductors with a finite transverse band dispersion  

Energy Technology Data Exchange (ETDEWEB)

We determine the one-particle spectral function and the corresponding derived quantities for the conducting chain lattice with finite inter-chain hopping t{sub perpendicular} and three-dimensional long-range Coulomb electron-electron interaction. The standard G{sub 0}W{sub 0} approximation is used. It is shown that, due to the optical character of the anisotropic plasmon dispersion caused by the finite t{sub perpendicular}, a low energy quasi-particle {delta}-peak appears in the spectral function in addition to the hump present at energies of the order of the plasmon energy. Particular attention is devoted to the continuous crossover from the non-Fermi liquid regime to the Fermi liquid regime with increasing t{sub perpendicular}. It is shown that the spectral weight of the hump transfers to the quasi-particle as the optical gap in the plasmon dispersion increases together with t{sub perpendicular}, with the quasi-particle residuum Z behaving like -ln t{sub perpendicular}){sup -1} in the limit t{sub perpendicular} {yields}0. Our approach is appropriate for the wide range of energy scales given by the plasmon energy and the width of the conduction band, and is complementary to the Luttinger liquid techniques that are limited to the low energy regime close to the Fermi surface.

Losic, Z Bonacic; Zupanovic, P [Department of Physics, Faculty of Natural Sciences, Mathematics and Kinesiology, University of Split, Teslina 12, 21000 Split (Croatia); Bjelis, A [Department of Physics, Faculty of Science, University of Zagreb, POB 162, 10001 Zagreb (Croatia)], E-mail: agicz@pmfst.hr, E-mail: bjelis@phy.hr

2008-08-13

163

A Direct Measurement of Atmospheric Dispersion in N-band Spectra: Implications for Mid-IR Systems on ELTs  

CERN Document Server

Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR adaptive optics system, we measure atmospheric dispersion in the N-band with the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate that atmospheric dispersion is generally linear in the N-band, although there is some residual curvature. We compare our measurements to theory, and make predictions for ELT Strehls and image FHWM with and without an atmospheric dispersion corrector (ADC). We find that for many mid-IR applications, an ADC will be necessary on ELTs.

Skemer, Andrew; Hoffmann, William; Close, Laird; Kendrew, Sarah; Mathar, Richard; Stuik, Remko; Greene, Thomas; Woodward, Charles; Kelley, Michael

2009-01-01

164

The additional contribution caused by Coulomb interaction to the exciton dispersion in multiple quantum wells and superlattices for direct band gap cubic semiconductors  

International Nuclear Information System (INIS)

The exciton dispersion in multiple quantum wells and superlattices is studied for direct band gap cubic semiconductors in the three-band-model. The Coulomb interaction between the light and heavy exciton states in different wells is calculated. This leads to a direction dependence and split of the exciton dispersion. (author). 21 refs

165

Luminescent-kinetic parameters of CsPbCl3 nanocrystals dispersed in wide-band perovskite-like matrices  

Directory of Open Access Journals (Sweden)

Full Text Available CsPbCl3 nanocrystals are obtained in perovskite-like CsBCl3 (B=Sr, Ca, Mg matrices doped with Pb ions (CPb=0.05 and 1 mol.%. The luminescent-kinetic parameters of the CsPbCl3 nanocrystals dispersed in CsBCl3 (B=Sr, Ca, Mg matrices are studied under the pulsed UV and X-ray excitation. The conclusion about the formation of CsPbCl3 nanocrystals is confirmed with the data of their luminescence decay kinetics and a short-wavelength shift of the exciton luminescence maximum, when compared to that of an excitonic luminescence in a bulk single CsPbCl3 crystal. Under the pulsed X-ray excitation, re-absorption of core-valence luminescence of CsBCl3 (B=Sr, Ca, Mg matrices with the CsPbCl3 nanocrystals is registered.

S. Myagkota

2003-06-01

166

Highly birefringent broadband-dispersion-compensating photonic crystal fibre over the E + S + C + L + U wavelength bands  

Science.gov (United States)

The key features of this manuscript include a hybrid design of a square-core octagonal-lattice photonic crystal fibre with only circular air holes that exhibits high birefringence and broadband dispersion compensation covering the E, S, C, L, and U communication bands (1360-1675 nm) and a statistical comparison of the proposed design with some other recently proposed designs. A numerical simulation showed a negative dispersion coefficient in the range of approximately -134 to -385 ps/(nm km) over E and U bands with a relative dispersion slope (RDS), which is equal to that of a single-mode fibre (SMF, approximately 0.0036 nm-1) and a birefringence in the order of 2.13 × 10-2 at the operating wavelength of 1.55 ?m. Because sensing applications require a highly birefringent fibre, statistical correlations between the birefringence and different parameters were investigated. Moreover, the effective area, residual dispersion, effective dispersion, and confinement loss associated with the proposed design are also reported.

Habib, Md. Selim; Rana, Md. Shohel; Moniruzzaman, Md.; Ali, Md. Sharafat; Ahmed, N.

2014-10-01

167

Band structure of boron doped carbon nanotubes  

CERN Document Server

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

168

Analysis of photonic band gap in dispersive properties of tunable three-dimensional photonic crystals doped by magnetized plasma  

Energy Technology Data Exchange (ETDEWEB)

In this paper, the magnetooptical effects in dispersive properties for two types of three-dimensional magnetized plasma photonic crystals (MPPCs) containing homogeneous dielectric and magnetized plasma with diamond lattices are theoretically investigated for electromagnetic (EM) wave based on plane wave expansion (PWE) method, as incidence EM wave vector is parallel to the external magnetic field. The equations for two types of MPPCs with diamond lattices (dielectric spheres immersed in magnetized plasma background or vice versa) are theoretically deduced. The influences of dielectric constant, plasma collision frequency, filling factor, the external magnetic field, and plasma frequency on the dispersive properties for both types of structures are studied in detail, respectively, and some corresponding physical explanations are also given. From the numerical results, it has been shown that the photonic band gaps (PBGs) for both types of MPPCs can be manipulated by plasma frequency, filling factor, the external magnetic field, and the relative dielectric constant of dielectric, respectively. Especially, the external magnetic field can enlarge the PBG for type-2 structure (plasma spheres immersed in dielectric background). However, the plasma collision frequency has no effect on the dispersive properties of two types of three-dimensional MPPCs. The locations of flatbands regions for both types of structures cannot be tuned by any parameters except for plasma frequency and the external magnetic field. The analytical results may be informative and of technical use to design the MPPCs devices.

Zhang HaiFeng [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Nanjing Artillery Academy, Nanjing 211132 (China); Liu Shaobin; Yang Huan [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Kong Xiangkun [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Jiangsu Key Laboratory of Meteorological Observation and Information Processing, Nanjing University of Information Science and Technology, Nanjing 210044 (China)

2013-03-15

169

Modulation instability of narrow-band nanosecond pulses propagating in anomalous-dispersion fibre  

International Nuclear Information System (INIS)

We demonstrate that, in the presence of noise, propagation of narrow-band 100-ns pulses through a 6-km long optical fibre sharply reduces their amplitude when a threshold power (?0.2 W) is exceeded. The effect is due to the development of modulation instability and is sensitive to the noise level in the spectral region ?100 GHz in width around the central frequency. (fiber and integrated optics)

170

High gain double-pass L-band EDFA with dispersion compensation as feedback loop  

Science.gov (United States)

We demonstrate an efficient double-pass L-band erbium-doped fiber amplifier (EDFA) incorporating chirped fiber Bragg grating (CFBG). The amplifier structure exploits the characteristics of CFBG to reflect the amplified signal back into the gain medium, filter out the recycled forward amplified spontaneous emission and block the residual 1480 nm pump power. The amplifier configuration has high gain and low noise figures as compared to double-pass EDFA using broadband mirror. The demonstrated amplifier has gain of more than 48 dB and low noise figure of less than 4 dB at low input signal power of -40 dBm.

Haleem, M. R.; Al-Mansoori, M. H.; Jamaludin, M. Z.; Abdullah, F.; Din, N. Md

2011-02-01

171

Energy-expending behaviour in frightened caribou when dispersed singly or in small bands  

Directory of Open Access Journals (Sweden)

Full Text Available The behaviour of single, and small bands of caribou (Rangifer tarandus groenlandicus when confronted by humans was compared with the energy—saving behaviour zoologists have ascribed to caribou in encounters with non-hunting wolves (Canis lupus. When confronted by me, or upon getting my scent, caribou ran away on all occasions. Their flight was occasionally interrupted by short stops to look back in my direction, but would continue on all occasions until they were out of sight. This behaviour is inconsistent with the one ascribed to caribou by zoologists when the intruder is a wolf instead of a human. In their view, the caribou stop their flight soon after the wolf gives up the chase, and accordingly save energy owing to their ability to distinguish between hunting and non-hunting wolves. However, small bands of caribou, as well as single animals, have never been observed to behave in this manner. On the contrary, the behaviour of caribou in such encounters is known to follow the same pattern as in their encounters with humans. Energy—saving behaviour is, however, sometimes observed when caribou become inquisitive about something in their surroundings. They will then readily approach as well as try to get down-wind of the object. When the object does not induce fear, it may simply be ignored, or charged before the caribou calm down. The effect of this "confirming behaviour" is that energy which would otherwise have been spent in needless flights from non-predators is saved.

Otto Blehr

1997-04-01

172

Fermi Surface and Band Dispersion in $La_{2-x}Sr_{x}CuO_4$  

CERN Document Server

Using angle-resolved photoemission spectroscopy (ARPES), we have observed the band structure, Fermi surface and their doping dependences in La2-xSrxCuO4. The results reveal that the Fermi surface undergoes a dramatic change: it is hole-like and centered at k=(pi,pi) in underdoped (x = 0.1) and optimally doped (x = 0.15) samples as in other cuprates while it is electron-like and centered at (0,0) in heavily overdoped (x = 0.3) ones. The peak in the ARPES spectra near (pi/2,pi/2) is broad and weak unlike in other cuprates. In the underdoped and optimally doped samples, a superconducting gap (Delta = 10 - 15 meV) is observed on the Fermi surface near (pi,0).

Ino, A; Mizokawa, T; Shen, Z X; Fujimori, A; Takaba, M; Tamasaku, K; Eisaki, H; Uchida, S I

1999-01-01

173

A multiple-relaxation-time lattice-boltzmann model for bacterial chemotaxis: effects of initial concentration, diffusion, and hydrodynamic dispersion on traveling bacterial bands.  

Science.gov (United States)

Bacterial chemotaxis can enhance the bioremediation of contaminants in aqueous and subsurface environments if the contaminant is a chemoattractant that the bacteria degrade. The process can be promoted by traveling bands of chemotactic bacteria that form due to metabolism-generated gradients in chemoattractant concentration. We developed a multiple-relaxation-time (MRT) lattice-Boltzmann method (LBM) to model chemotaxis, because LBMs are well suited to model reactive transport in the complex geometries that are typical for subsurface porous media. This MRT-LBM can attain a better numerical stability than its corresponding single-relaxation-time LBM. We performed simulations to investigate the effects of substrate diffusion, initial bacterial concentration, and hydrodynamic dispersion on the formation, shape, and propagation of bacterial bands. Band formation requires a sufficiently high initial number of bacteria and a small substrate diffusion coefficient. Uniform flow does not affect the bands while shear flow does. Bacterial bands can move both upstream and downstream when the flow velocity is small. However, the bands disappear once the velocity becomes too large due to hydrodynamic dispersion. Generally bands can only be observed if the dimensionless ratio between the chemotactic sensitivity coefficient and the effective diffusion coefficient of the bacteria exceeds a critical value, that is, when the biased movement due to chemotaxis overcomes the diffusion-like movement due to the random motility and hydrodynamic dispersion. PMID:25223537

Yan, Zhifeng; Hilpert, Markus

2014-10-01

174

High resolution broad-band spectroscopy in the NIR using the Triplespec externally dispersed interferometer at the Hale telescope  

Science.gov (United States)

High resolution broad-band spectroscopy at near-infrared wavelengths has been performed using externally dis- persed interferometry (EDI) at the Hale telescope at Mt. Palomar. The EDI technique uses a field-widened Michelson interferometer in series with a dispersive spectrograph, and is able to recover a spectrum with a resolution 4 to 10 times higher than the existing grating spectrograph. This method increases the resolution well beyond the classical limits enforced by the slit width and the detector pixel Nyquist limit and, in principle, decreases the effect of pupil variation on the instrument line-shape function. The EDI technique permits arbi- trarily higher resolution measurements using the higher throughput, lower weight, size, and expense of a lower resolution spectrograph. Observations of many stars were performed with the TEDI interferometer mounted within the central hole of the 200 inch primary mirror. Light from the interferometer was then dispersed by the TripleSpec near-infrared echelle spectrograph. Continuous spectra between 950 and 2450 nm with a resolution as high as ~27,000 were recovered from data taken with TripleSpec at a native resolution of ˜2,700. Aspects of data analysis for interferometric spectral reconstruction are described. This technique has applications in im- proving measurements of high-resolution stellar template spectra, critical for precision Doppler velocimetry using conventional spectroscopic methods. A new interferometer to be applied for this purpose at visible wavelengths is under construction.

Erskine, David J.; Edelstein, J.; Sirk, M.; Wishnow, E.; Ishikawa, Y.; McDonald, E.; Shourt, W. V.

2014-07-01

175

Analysis of dispersion and attenuation of surface waves in poroelastic media in the exploration-seismic frequency band  

Science.gov (United States)

We analyse dispersion and attenuation of surface waves at free surfaces of possible vacuum/poroelastic media: permeable-'open pore', impermeable-'closed pore' and partially permeable boundaries, which have not been previously reported in detail by researchers, under different surface-permeable, viscous-damping, elastic and fluid-flowing conditions. Our discussion is focused on their characteristics in the exploration-seismic frequency band (a few through 200 Hz) for near-surface applications. We find two surface-wave modes exist, R1 waves for all conditions, and R2 waves for closed-pore and partially permeable conditions. For R1 waves, velocities disperse most under partially permeable conditions and least under the open-pore condition. High-coupling damping coefficients move the main dispersion frequency range to high frequencies. There is an f1 frequency dependence as a constant-Q model for attenuation at high frequencies. R1 waves for the open pore are most sensitive to elastic modulus variation, but least sensitive to tortuosities variation. R1 waves for partially permeable surface radiate as non-physical waves (Im(k) waves, velocities are slightly lower than the bulk slow P2 waves. At low frequencies, both velocity and attenuation are diffusive of f1/2 frequency dependence, as P2 waves. It is found that for partially permeable surfaces, the attenuation displays -f1 frequency dependence as frequency increasing. High surface permeability, low-coupling damping coefficients, low Poisson's ratios, and low tortuosities increase the slope of the -f1 dependence. When the attenuation coefficients reach 0, R2 waves for partially permeable surface begin to radiate as non-physical waves. ?? 2011 The Authors Geophysical Journal International ?? 2011 RAS.

Zhang, Y.; Xu, Y.; Xia, J.

2011-01-01

176

SFI++ I: A New I-band Tully-Fisher Template, the Cluster Peculiar Velocity Dispersion and H0  

CERN Document Server

The SFI++ consists of ~5000 spiral galaxies which have measurements suitable for the application of the I-band Tully-Fisher (TF) relation. This sample builds on the SCI and SFI samples published in the 1990s but includes significant amounts of new data as well as improved methods for parameter determination. We derive a new I-band TF relation from a subset of this sample which consists of 807 galaxies in the fields of 31 nearby clusters and groups. This sample constitutes the largest ever available for the calibration of the TF template and extends the range of line-widths over which the template is reliably measured. Careful accounting is made of observational and sample biases such as incompleteness, finite cluster size, galaxy morphology and environment. We find evidence for a type-dependent TF slope which is shallower for early type than for late type spirals. The line-of-sight cluster peculiar velocity dispersion is measured for the sample of 31 clusters. This value is directly related to the spectrum of...

Masters, K L; Haynes, M P; Giovanelli, R; Masters, Karen L.; Springob, Christopher M.; Haynes, Martha P.; Giovanelli, Riccardo

2006-01-01

177

Photoinduced electron accumulation in colloidally dispersed wide band-gap semiconductor nanosheets.  

Science.gov (United States)

We investigated photoinduced electron accumulation in a colloidal system of layered hexaniobate that is known as a photocatalytically active wide band-gap semiconductor, and attempted to control the photoresponse by introducing additives into the colloid. The inorganic nanosheets were obtained by exfoliation of the layered oxide. UV-irradiation of the colloids led to electron accumulation in the nanosheets to generate reduced niobate species. Propylammonium ions introduced as the exfoliating reagent and present as the counter ions of niobate nanosheets were indicated as the electron donor that stabilized the electron-accumulating state. Yield and half-life of the reduced niobate species greatly increased by adding an appropriate amount of photochemically inert clay nanosheets, while they increased only a little by the addition of molecular electron donors such as EDTA and triethanolamine. Moreover, the molecular species diminished the enhancement effect of the clay nanosheets. The results suggested that the photochemical event was not explained by direct interactions between the semiconductor nanosheets and the additives at molecular level but governed by indirect interactions between the colloid components regulated by the colloid structure. PMID:21055763

Nakato, Teruyuki; Yamada, Yoshimi; Nakamura, Mari; Takahashi, Atsushi

2011-02-01

178

Valence band-edge engineering of nickel oxide nanoparticles via cobalt doping for application in p-type dye-sensitized solar cells.  

Science.gov (United States)

We have systematically studied the effects of substitutional doping of p-type nanoparticulate NiO with cobalt ions. Thin films of pure and Co-doped NiO nanoparticles with nominal compositions Co(x)Ni(1-x)O(y) (0 ? x ? 0.1) were fabricated using sol-gel method. X-ray photoelectron spectroscopy revealed a surface enrichment of divalent cobalt ions in the Co(x)Ni(1-x)O(y) nanoparticles. Mott-Schottky analysis in aqueous solutions was used to determine the space charge capacitance values of the films against aqueous electrolytes, which yielded acceptor state densities (N(A)) and apparent flat-band potentials (E(fb)). Both N(A) and E(fb) values of the doped NiO were found to gradually increase with increasing amount of doping; thus the Fermi energy level of the charge carriers decreased with Co-doping. The photovoltage of p-DSCs constructed using the Co(x)Ni(1-x)O(y) films increased with increasing amount of cobalt, as expected from the trend in the E(fb). Co-doping increased both carrier lifetimes within the p-DSCs and the carrier transport times within the nanoparticulate semiconductor network. The nominal composition of Co?.??Ni?.??O(y) was found to be optimal for use in p-DSCs. PMID:23054373

Natu, Gayatri; Hasin, Panitat; Huang, Zhongjie; Ji, Zhiqiang; He, Mingfu; Wu, Yiying

2012-11-01

179

Valence band electronic structure of Nd{sub 1?x}Y{sub x}MnO{sub 3} using X-ray absorption, photoemission and GGA + U calculations  

Energy Technology Data Exchange (ETDEWEB)

Highlights: •Decrease in the occupancy of Mn 3d orbitals with doping. •Greater splitting of the e{sub g} orbitals due to the increased Jahn–Teller distortion with doping. •Decrease in O 2p–Mn 3d charge transfer character with doping. •Increase in charge transfer energy and band gap with doping. •Calculations hint a subtle change from a charge transfer to Mott–Hubbard type insulator with doping. -- Abstract: The electronic structures of Nd{sub 1?x}Y{sub x}MnO{sub 3} (x = 0–0.5) were studied using X-ray absorption near-edge structure (XANES) at the Mn L{sub 3,2}- and O K-edge along with valence-band photoemission spectroscopy (VB-PES). The systematic increase in white-line intensity of the Mn L{sub 3,2}-edge with doping, suggests a decrease in the occupancy of Mn 3d orbitals. The O K-edge XANES shows a depletion of unoccupied states above the Fermi energy. The changes in the O K-edge spectra due to doping reflects an increase in the Jahn–Teller distortion. The VB-PES shows broadening of the features associated with Mn 3d and O 2p hybridized states and the shift of these features to a slightly higher binding energy in agreement with our GGA + U calculations. The system shows a net shift of the occupied and unoccupied states away from the Fermi energy with doping. The shift in theoretical site-projected density of states of x = 0.5 composition with respect to x = 0 suggest a subtle change from a charge transfer to Mott–Hubbard type insulator.

Balasubramanian, Padmanabhan, E-mail: padmanabhan@iopb.res.in [Department of Physics, Tamkang University, Tamsui 251, Taiwan (China); Institute of Physics, Bhubaneshwar 751005 (India); Nair, Harikrishnan S. [J?lich Center for Neutron Sciences, Forschungszentrum J?lich, Outstation at FRM II, LichtenberGstr. 1, Garching 85747 (Germany); Tsai, H.M. [Department of Physics, Tamkang University, Tamsui 251, Taiwan (China); National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan (China); Bhattacharjee, S. [Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala (Sweden); Liu, M.T. [Department of Physics, Tamkang University, Tamsui 251, Taiwan (China); Yadav, Ruchika [Department of Physics, Indian Institute of Science, C.V. Raman Avenue, Bangalore 560012 (India); Chiou, J.W. [Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan (China); Lin, H.J.; Pi, T.W. [National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan (China); Tsai, M.H. [Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan (China); Elizabeth, Suja [Department of Physics, Indian Institute of Science, C.V. Raman Avenue, Bangalore 560012 (India); Pao, C.W.; Wang, B.Y.; Chuang, C.H.; Pong, W.F. [Department of Physics, Tamkang University, Tamsui 251, Taiwan (China)

2013-08-15

180

The valence electronic structure of polycrystalline SiC as observed by (e,2e) spectroscopy  

International Nuclear Information System (INIS)

The spectral momentum density of the valence electrons of a thin polycrystalline silicon carbide (SiC) film has been measured using an (e,2e) spectrometer employing a non-coplanar asymmetric geometry with estimated energy and momentum resolutions of about 2.0 eV and 0.15 a.u., respectively. Well-defined valence band dispersion has been observed from the measured momentum density which resembles a parabola, but deviates from what is expected for a free electron near the top to the band and at the boundary of the Brillouin zone, where the antisymmetric gap due to the unequal potentials between the Si and C sites in SiC is clearly visible. Based on the assumption that the special momentum density of polycrystalline materials is a spherical average of the spectral momentum density of the corresponding single crystalline phase of the materials, ab initio linear muffin-tin orbital (LMTO) calculations have been performed using the zinc-blende structure of ?-SiC. The measured dispersion of the energy band is in excellent agreement with theory. Reasonable agreement is also obtained for the energy-integrated momentum density, although the measured momentum density exceeds considerably the calculated one at high momenta. The theoretical implication of using crystal band structure calculations for studying disordered materials is also discussed. 21 refs., 8 figs

 
 
 
 
181

Electronic band structure of beryllium oxide  

CERN Document Server

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

182

Strong Energy-momentum Dispersion of Phonon Dressed Carriers in the Lightly Doped Band Insulator SrTiO3  

Energy Technology Data Exchange (ETDEWEB)

Much progress has been made recently in the study of the effects of electron-phonon (el-ph) coupling in doped insulators using angle resolved photoemission (ARPES), yielding evidence for the dominant role of el-ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is the perovskite SrTiO{sub 3} (STO), well known for its giant dielectric constant of 10000 at low temperature, exceeding that of La{sub 2}CuO{sub 4} by a factor of 500. Based on this fact, it has been suggested that doped STO should be the archetypical bipolaron superconductor. Here we report an ARPES study from high-quality surfaces of lightly doped SrTiO{sub 3}. Comparing to lightly doped Mott insulators, we find the signatures of only moderate electron-phonon coupling: a dispersion anomaly associated with the low frequency optical phonon with a {lambda}{prime} {approx} 0.3 and an overall bandwidth renormalization suggesting an overall {lambda}{prime} {approx} 0.7 coming from the higher frequency phonons. Further, we find no clear signatures of the large pseudogap or small polaron phenomena. These findings demonstrate that a large dielectric constant itself is not a good indicator of el-ph coupling and highlight the unusually strong effects of the el-ph coupling in doped Mott insulators.

Meevasana, Warawat

2010-05-26

183

The aggregation effect on the absorption band shift of disperse red-13 attached on the surface of silica spheres  

Science.gov (United States)

Optical properties of disperse red-13 (DR-13) attached on the surface of the silica spheres have been investigated. Two step synthetic processes have been used to attach the DR-13 on the surface of the silica spheres. The first step is a urethane bond formation between a DR-13 (OH) and a 3-isocyanatopropyl triethoxysilane (ICPTES, NCO) in pyridine at 50 °C. The second step is the hydrolysis and condensation reactions between the resulting compound of the first step reaction (ICPDR) and the silica spheres (ICPDRSS). The absence of a characteristic absorption peak at 2270 cm-1 representing the NCO asymmetric stretching vibration and the existence of a new absorption peat at 1700 cm-1 corresponding the CO stretching vibration indicate the urethane bond formation. The absorption intensity of the DR-13 in methanol, ethanol and 2-propanol linearly increased with the increase of the amount of the DR-13. The ICPDRSS has weak brownish color when it is dried. The field emission scanning electron microscope shows the aggregated ICPDR on the surface of the silica spheres. The average diameter of the spheres is approximately 380 nm. The color of the ICPDRSS changed to intense red when it was wetted in methanol, ethanol and 2-propanol. The absorption peak of the dried ICPDRSS film is at 475 nm. The stopband appears at 788 nm and disappears when the methanol is filled between the spheres due to the low refractive index contrast. New absorption band appears at 718 nm when the methanol is absorbed due to the large aggregation of the ICPDR on the surface of the spheres.

Kang, Kwang-Sun

2013-12-01

184

MANIFESTATION OF DEFORMATION EFFECT IN BAND SPECTRA IN CRYSTALS WITH INHOMOGENEOUS BONDING  

Directory of Open Access Journals (Sweden)

Full Text Available The effect of the shear strains on the energy spectrum of the strongly anisotropic SbSI crystal has been investigated by group-theoretical method in combination with the Pikus method of invariants. The first-principles local density approximation has been implemented to determine the band structure of the crystal. Ab initio calculations of the band structure have shown an exact localization of twofold degenerate maximum of the valence band in the T point. It turned out that the shear strains result in the band topology changes in the vicinity of the T point and the k-linear term appears in the corresponding dispersion law.

D.M.Bercha

2003-01-01

185

Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow band-pass filtering.  

Science.gov (United States)

Fiber lasers mode-locked with normal cavity dispersion have recently attracted great attention due to large output pulse energy and femtosecond pulse duration. Here we accurately characterized the timing jitter of normal-dispersion fiber lasers using a balanced cross-correlation method. The timing jitter characterization experiments show that the timing jitter of normal-dispersion mode-locked fiber lasers can be significantly reduced by using narrow band-pass filtering (e.g., 7-nm bandwidth filtering in this work). We further identify that the timing jitter of the fiber laser is confined in a limited range, which is almost independent of cavity dispersion map due to the amplifier-similariton formation by insertion of the narrow bandpass filter. The lowest observed timing jitter reaches 0.57 fs (rms) integrated from 10 kHz to 10 MHz Fourier frequency. The rms relative intensity noise (RIN) is also reduced from 0.37% to 0.02% (integrated from 1 kHz to 5 MHz Fourier frequency) by the insertion of narrow band-pass filter. PMID:25402069

Qin, Peng; Song, Youjian; Kim, Hyoji; Shin, Junho; Kwon, Dohyeon; Hu, Minglie; Wang, Chingyue; Kim, Jungwon

2014-11-17

186

Band structure parameters for quantum wells: Magnetoluminescence determinations  

Energy Technology Data Exchange (ETDEWEB)

We report on low-temperature magnetoluminescence determinations of bandstructure parameters for an 8.5nm-wide n-type In{sub 0.20}Ga{sub 0.80}As/GaAs strained single-quantum well structure. We find that the conduction-band is almost parabolic, with its mass varying from 0.067m{sub 0} at zone center to 0.069m{sub 0} at a Fermi energy E{sub f} {approx} 50 meV, and that the valence-bands are highly non-parabolic with the valence band masses m, varying from about 0.1 m{sub 0} at zone center to about 0.3m{sub 0} for large k-vectors. A comparison with a k x p calculation for the valence-band mass and dispersion curve show good agreement. An accurate and unambiguous determination for the band-gap energy E{sub gap} is made by extrapolating the magnetic field dependent Landau level transition energies to zero field.

Jones, E.D.

1997-04-01

187

Valency and molecular structure  

CERN Document Server

Valency and Molecular Structure, Fourth Edition provides a comprehensive historical background and experimental foundations of theories and methods relating to valency and molecular structures. In this edition, the chapter on Bohr theory has been removed while some sections, such as structures of crystalline solids, have been expanded. Details of structures have also been revised and extended using the best available values for bond lengths and bond angles. Recent developments are mostly noted in the chapter on complex compounds, while a new chapter has been added to serve as an introduction t

Cartmell, E

1977-01-01

188

Electronic band structure of magnetic bilayer graphene superlattices  

Science.gov (United States)

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

189

The effect of temperature and angle of incidence on photonic band gap in a dispersive Si-based one dimensional photonic crystal  

Energy Technology Data Exchange (ETDEWEB)

The effect of temperature and angle of incidence on photonic band gaps (PBGs) in a dispersive Si-based one dimensional photonic crystal consisting of alternate layers of silicon and air has been investigated. More physically realistic situation is considered by taking the refractive index of silicon layers as a function of temperature and wavelength. The effect of temperature and angle of incidence on reflection spectra of proposed structure for TE polarization has also been studied. The PBG can be tuned by varying the temperature of the geometry or by changing the angle of incidence. The propagation characteristics of the proposed structure are analyzed using transfer matrix method.

Kumar, Vipin [Department of Physics, Digamber Jain (P.G.) College, Baraut 250611 (India); Suthar, B., E-mail: bhuvneshwer@gmail.com [Department of Physics, Government Engineering College, Bikaner 334004 (India); Kumar, Arun [AITTM, Amity University, Noida (India); Singh, Kh.S. [Department of Physics, Digamber Jain (P.G.) College, Baraut 250611 (India); Bhargava, A. [Nanophysics Laboratory, Department of Physics, Government Dungar College, Bikaner 334001 (India)

2013-05-01

190

Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals  

Energy Technology Data Exchange (ETDEWEB)

We report the first-principles electronic structure of BiVO{sub 4}, a promising photocatalyst for hydrogen generation. BiVO{sub 4} is found to be a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band minimum. These interactions result in symmetric hole and electron masses. Implications for the design of ambipolar metal oxides are discussed.

Walsh, A.; Yan,Y.; Huda, M. N.; Al-Jassim, M. M.; Wei, S. H.

2009-01-01

191

Design Of Near Zero Ultra-Flattened Chromatic Dispersion Highly Non Linear Holey Fiber In Tele Communication Band  

Directory of Open Access Journals (Sweden)

Full Text Available We propose a four-ring hexagonal holey fiber (HF which exhibits near zero ultra-flattened chromatic dispersion and nonlinear property simultaneously in a modest number of rings. The finite element method with perfectly matched layers boundary condition is used to investigate the guiding properties. A four ring HF with flattened dispersion of 0.85ps/nm/km from 1.14 to 1.60 µm wavelength range, 21.34W-1km-1 nonlinear coefficient and splice loss 3.82 dB at 1.55µm is numerically demonstrated.

Mahbubur Rahman

2012-11-01

192

Valence-band offset of n-Zn0.8Mg0.2O/p-Ni0.8Mg0.2O heterojunction with tunable bandgaps of both sides measured by X-ray photoelectron spectroscopy  

Science.gov (United States)

The valence-band offset (VBO) of n-Zn0.8Mg0.2O/p-Ni0.8Mg0.2O heterojunction grown by pulsed laser deposition was investigated by X-ray photoelectron spectroscopy. Core levels of Zn 2p and Ni 2p were used to align the VBO of n-Zn0.8Mg0.2O/p-Ni0.8Mg0.2O heterojunction. It was found that n-Zn0.8Mg0.2O/p-Ni0.8Mg0.2O heterojunction has a type-II band alignment and its VBO is determined to be 1.88 ± 0.05 eV, and conduction-band offset is deduced to be -1.91 ± 0.05 eV. Alloying with Mg can tune the positions of valence-band maximum (VBM), conduction-band minimum (CBM) and bandgaps of both Zn1-x Mg x O and Ni1-x Mg x O thin films, according to which we can design various heterojunction devices with desired CBM and VBM values of both sides by tuning appropriate Mg composition. Also, there is blue shift of absorption edges of the integral Zn0.8Mg0.2O/Ni0.8Mg0.2O heterojunction, which has a significant impact on the design and application of deep-ultraviolet optoelectronic devices, such as solar-blind UV detectors with high photoresponse performance.

Guo, Yan-Min; Zhu, Li-Ping; Niu, Wen-Zhe; Zhang, Xiang-Yu; Ye, Zhi-Zhen

2014-09-01

193

Band structure and optical transitions in atomic layers of hexagonal gallium chalcogenides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga$_2$X$_2$ (X=S, Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a Mexican-hat dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions---changes in the Fermi-surface topology of hole-doped Ga$_2$X$...

Zolyomi, Viktor; Drummond, Neil; Falko, Vladimir

2013-01-01

194

Valence mirror nuclei  

Energy Technology Data Exchange (ETDEWEB)

A comparison of examples of 'valence mirror nuclei' (nuclei of the type N = 82, ZETA = 60 + k and N = 60 + k, ZETA 50) /sup 142/Nd (/sup 144/Sm) and /sup 110/Sn (/sup 112/Sn) reveals a remarkable similarity of the level energies of the yrast and yrare states. This is valid for states up to an energy of 5 MeV which have an unbroken magic core for the other kinds of particle.

Wirowski, R.; Yan, J.; Brentano, P. von; Dewald, A.; Gelberg, A.

1988-09-01

195

Valence force field and phonon spectrum of ZnS  

International Nuclear Information System (INIS)

Valence force fields have been extensively used in the study of the vibrational analysis of spectra of molecules. The application of the valence force field to the study of the lattice vibration of solids is limited. The study of the lattice vibration of the solid with the application of valence force model involves the transformation of the valence coordinates into atomic displacement coordinates. This transformation is performed to set up the secular equation of the ZnS lattice using Urey Bradley Valence Force field. The elements of the dynamical matrix of the lattice were obtained in terms of the valence force parameters. The parameters have been evaluated with the help of experimental phonon frequencies and measured values of elastic constants. The parameters have been used to calculate phonon dispersion curves along(100), (111) and (110) directions. The agreement between the theory and experiment is found to be satisfactory. This reflects the applicability of valence force field to the study of dynamical behaviour of ZnS crystal lattice. (author)

196

Ferromagnetism in the t-t' Hubbard model: interplay of lattice, band dispersion, and interaction effects studied within a Goldstone-mode preserving scheme  

CERN Document Server

Ferromagnetism in the Hubbard model is investigated on sc, bcc, and fcc lattices using a systematic inverse-degeneracy ($1/{\\cal N}$) expansion which incorporates self-energy and vertex corrections such that spin-rotation symmetry and the Goldstone mode are explicitly preserved. First-order quantum corrections to magnon energies are evaluated for several cases, providing a comprehensive picture of the interplay of lattice, band dispersion, and interaction effects on the stability of the ferromagnetic state with respect to both long- and short-wavelength fluctuations. Our results support the belief that ferromagnetism is a generic feature of the Hubbard model at intermediate and strong coupling provided the DOS is sufficiently asymmetric and strongly peaked near band edge, as for fcc lattice with finite $t'$. For short-wavelength modes, behavior of a characteristic energy scale $\\omega^* \\sim T_c$ (magnon-DOS-peak energy) is in excellent agreement with the $T_c$ vs. $n$ behavior within DMFT, both with respect ...

Pandey, S; Pandey, Sudhakar; Singh, Avinash

2006-01-01

197

??????????? ? ??????????(Valency and isomorphism  

Directory of Open Access Journals (Sweden)

Full Text Available Valency relations in a paraphrase should match those of the source expres- sion. This kind of isomorphism is investigated in the present article.In the passive sentence The plant was visited by Putin the relation between Putin and plant is the same as in the corresponding active sentence. It is not equally easy to see how isomorphism can be preserved in, for example, Who is the author of these lines?, derived from Who wrote these lines? This is achieved by introducing a new concept of basic semantic unit. Obviously, the predicate write, which is explicit in the source expression, must somehow be introduced in the semantic representation of the para- phrase. There are two ways of accounting for this "missing" predicate. It can either be extracted (computed from the words author and lines and assume the form of an implicit predicate, or the corresponding meaning can be car- ried by an agreement morpheme attached to the noun author (in the Russian paraphrase this noun is marked with the so-called agreement case. The first actant of write needs to be represented only once - through the word who - and therefore the noun author is assigned syntactic status, which means that it does not occupy a node of its own in the semantic graph.A number of further examples - not only sentences, but also phrases and words - are analysed in order to illustrate different cases of isomor- phism.

Lennart Lönngren

2008-01-01

198

Visualizing Valence Electron Structure  

Science.gov (United States)

The purpose of this research is to effectively model and understand Density Functional Theory (DFT). DFT uses the electron density to model the ground state electronic structure of atoms, molecules, and solids. We calculate and visualize the gradient of the electron density, its Laplacian, and the kinetic energy density, also derived from the electron density, for the AE6 test set of molecules. This set of six molecules accurately represents the DFT prediction of atomization energies of a plethora of molecules. Calculations were done using the ABINIT DFT code. Pseudopotentials were used to represent the individual atom cores, keeping an accurate representation of valence interactions. By using these density- derived functions we can better visualize and interpret intermolecular phenomenon caused by electron interactions, like the effects of a triple bond on the entire structure of the molecule, or the effect of electron affinity of an individual atom within a molecule. These physical variables are used in DFT to help predict the electron-electron interaction energy of a molecule, and knowing how they relate to each other may help better model this. Ultimately, we hope to effectively use our results to understand and improve the Density Functional method for modeling electronic structure.

Kuna, Aeryk; Cancio, Antonio

2011-10-01

199

Expression of the optical constants of chalcogenide thin films using the new parameterization dispersion model  

Science.gov (United States)

In this paper, a new dispersion model of the optical constants of amorphous solids enabling us to perform an efficient parameterization of the spectral dependences of the optical constants of chalcogenide thin films will be presented. This dispersion model is based on mathematical modeling the density of electronic states (DOS) corresponding to both the valence and conduction bands. The imaginary part of the dielectric function is then calculated by the numerical convolution of the DOS. The real part of the dielectric function is calculated using the corresponding Kramers-Kronig (KK) relation in a suitable numerical way. Moreover, the existence of the transitions between the localized states inside the band gap and the extended states inside both the valence and conduction bands is also taken into account using the corresponding convolutions. Thus, the dispersion model presented includes the absorption corresponding to the Urbach and Tauc regions. Then the dispersion model described allows to interpret spectroellipsometric and spectrophotometric data measured for the chalcogenide thin films within the wide spectral region (200-900 nm).

Franta, Daniel; Ohlídal, Ivan; Frumar, Miloslav; Jedelský, Jaroslav

2003-05-01

200

Actinide valences in borosilicate glass  

International Nuclear Information System (INIS)

The valence states of 238U, 237Np, 239Pu, and 241Am have been studied over a period of time after initial incorporation into a low-titanium borosilicate glass. Under the air-firing conditions used, only the following valence states were observed: U(VI), Np(IV), Pu(IV), and Am(III). Over a period of one year, no actinide valence changes were observed. Preliminary EXAFS studies on uranium- and thorium-doped glasses indicate ordering in the first coordination shell of the actinide ions, with apparent local structure resembling that existing in model crystalline compounds. (orig.)

 
 
 
 
201

The band structure of BeTe a combined experimental and theoretical study  

CERN Document Server

Using angle-resolved synchrotron-radiation photoemission spectroscopy we have determined the dispersion of the valence bands of BeTe(100) along $\\Gamma X$, i.e. the [100] direction. The measurements are analyzed with the aid of a first-principles calculation of the BeTe bulk band structure as well as of the photoemission peaks as given by the momentum conserving bulk transitions. Taking the calculated unoccupied bands as final states of the photoemission process, we obtain an excellent agreement between experimental and calculated spectra and a clear interpretation of almost all measured bands. In contrast, the free electron approximation for the final states fails to describe the BeTe bulk band structure along $\\Gamma X$ properly.

Nagelstrasser, M; Fischer, F; Litz, T; Waag, A; Fleszar, A; Hanke, W; Steinrück, H P; Landwehr, Gottfried

1998-01-01

202

Ductility Enhancement of Molybdenum Phase by Nano-sizedd Oxide Dispersions  

Energy Technology Data Exchange (ETDEWEB)

The present research is focused on ductility enhancement of molybdenum (Mo) alloys by adding nano-sized oxide particles to the alloy system. The research approach includes: (1) determination of microscopic mechanisms responsible for the macroscopic ductility enhancement effects through atomistic modeling of the metal-ceramic interface; (2) subsequent computer simulation-aided optimization of composition and nanoparticle size of the dispersion for improved performance; (3) synthesis and characterization of nanoparticle dispersion following the guidance from atomistic computational modeling analyses (e.g., by processing a small sample of Mo alloy for evaluation); and (4) experimental testing of the mechanical properties to determine optimal ductility enhancement.Through atomistic modeling and electronic structure analysis using full-potential linearized muffin-tin orbital (FP-LMTO) techniques, research to date has been performed on a number of selected chromium (Cr) systems containing nitrogen (N) and/or magnesium oxide (MgO) impurities. The emphasis has been on determining the properties of the valence electrons and the characteristics of the chemical bonds they formed. It was found that the brittle/ductile behavior of this transitional metal system is controlled by the relative population of valence charges: bonds formed by s valence electrons yield metallic, ductile behavior, whereas bonds formed by d valence electrons lead to covalent, brittle behavior. The presence of valence bands from impurities also affects the metal bonding, thereby explaining the detrimental and beneficial effects induced by the inclusion of N impurities and MgO dispersions. These understandings are useful for optimizing ductility enhancement effects on the dispersion materials.

Bruce Kang

2008-07-31

203

Pressure Induced Valence Transitions in f-Electron Systems  

CERN Document Server

A review is given of pressure induced valence transitions in f-electron systems calculated with the self-interaction corrected local spin density (SIC-LSD) approximation. These calculations show that the SIC-LSD is able to describe valence changes as a function of pressure or chemical composition. An important finding is the dual character of the f-electrons as either localized or band-like. A finite temperature generalisation is presented and applied to the study of the p-T phase diagram of the alpha to gamma phase transition in Ce.

Temmerman, W M; Petit, L; Lueders, M; Strange, P; Szotek, Z

2006-01-01

204

Strong energy-momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO3  

International Nuclear Information System (INIS)

Much progress has been made recently in the study of the effects of electron-phonon (el-ph) coupling in doped insulators using angle-resolved photoemission (ARPES), yielding evidence for the dominant role of el-ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises as to how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is that of perovskite SrTiO3 (STO), well known for its giant dielectric constant of 10 000 at low temperatures, exceeding that of La2CuO4 by a factor of 500. Based on this fact, it has been suggested that doped STO should be the archetypical bipolaron superconductor. Here we report an ARPES study from high-quality surfaces of lightly doped STO. In comparison to lightly doped Mott insulators, we find the signatures of only moderate el-ph coupling; a dispersion anomaly associated with the low-frequency optical phonon with a ? ' ?0.3 and an overall bandwidth renormalization suggesting an overall ? ' ?0.7 coming from the higher frequency phonons. Furthermore, we find no clear signatures of the large pseudogap or small-polaron phenomena. These findings demonstrate that a large dielectric constant itself is not a good indicator of el-ph coupling and highlight the unusually strong effects of the el-ph coupling in doped Mott insulators.

205

Valence transversities: the collinear extraction  

CERN Document Server

In these proceedings, we propose an extraction of the valence transversity parton distributions. Based on an analysis of pion-pair production in deep-inelastic scattering off transversely polarized targets, this extraction of transversity is performed in the framework of collinear factorization. The recently released data for proton and deuteron targets at HERMES and COMPASS allow for a flavor separation of the valence transversities, for which we give a complete statistical study.

Courtoy, A; Radici, Marco

2013-01-01

206

Ocular dispersion  

Science.gov (United States)

Spectrally resolved white-light interferometry (SRWLI) was used to measure the wavelength dependence of refractive index (i.e., dispersion) for various ocular components. The accuracy of the technique was assessed by measurement of fused silica and water, the refractive indices of which have been measured at several different wavelengths. The dispersion of bovine and rabbit aqueous and vitreous humor was measured from 400 to 1100 nm. Also, the dispersion was measured from 400 to 700 nm for aqueous and vitreous humor extracted from goat and rhesus monkey eyes. For the humors, the dispersion did not deviate significantly from water. In an additional experiment, the dispersion of aqueous and vitreous humor that had aged up to a month was compared to freshly harvested material. No difference was found between the fresh and aged media. An unsuccessful attempt was also made to use the technique for dispersion measurement of bovine cornea and lens. Future refinement may allow measurement of the dispersion of cornea and lens across the entire visible and near-infrared wavelength band. The principles of white- light interferometry including image analysis, measurement accuracy, and limitations of the technique, are discussed. In addition, alternate techniques and previous measurements of ocular dispersion are reviewed.

Hammer, Daniel X.; Noojin, Gary D.; Thomas, Robert J.; Stolarski, David J.; Rockwell, Benjamin A.; Welch, Ashley J.

1999-06-01

207

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

Science.gov (United States)

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. Project supported by the National Natural Science Foundation of China (Grant No. 60636030).

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

2011-03-01

208

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)

209

Ab initio study of valence and Rydberg states of CH3Br  

Science.gov (United States)

We performed configuration interaction ab initio calculations on the valence and 5s, 5pa1, and 5pe Rydberg bands of the CH3Br molecule as a function of the methyl-bromide distance for frozen C3v geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the CH3Br+ ion with a minimum at short distance. One state emerging from the 5pe band has valence and ion-pair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ion-pair production experiments. In each band, there is a single state, which has a non-negligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the 5pe band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methyl-bromide distance decreases due to a mixing between valence and 5s Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the 1Q1 state is dominant. In each Rydberg band, however, spin-orbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state (1Q0) with significant parallel transition moment induced by spin-orbit interaction with the ground state at large distance.

Escure, Christelle; Leininger, Thierry; Lepetit, Bruno

2009-06-01

210

Event valence and unrealistic optimism.  

Science.gov (United States)

The effect of event valence on unrealistic optimism was studied. 94 Deakin University students rated the comparative likelihood that they would experience either a controllable or an uncontrollable health-related event. Valence was manipulated to be positive (outcome was desirable) or negative (outcome was undesirable) by varying the way a given event was framed. Participants either were told the conditions which promote the event and rated the comparative likelihood they would experience it or were told the conditions which prevent the event and rated the comparative likelihood they would avoid it. For both the controllable and the uncontrollable events, unrealistic optimism was greater for negative than positive valence. It is suggested that a combination of the 'motivational account' of unrealistic optimism and prospect theory provides a good explanation of the results. PMID:12931926

Gold, Ron S; Martyn, Kate

2003-06-01

211

Valency Configuration of Transition Metal Impurities in ZnO  

CERN Document Server

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM = Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn1-xTMxO, the localized TM2+ configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy eF close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with eF close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

Petit, L; Svane, A; Temmerman, W M; Szotek, Z; Janotti, A

2006-01-01

212

LIII x-ray absorption in Yb compounds: Temperature dependence of the valence  

International Nuclear Information System (INIS)

We report x-ray-absorption measurements near the Yb LIII absorption edge of YbAl3, YbCu2Si2, YbAgCu4, and related compounds. Our results suggest that the bulk Yb valence changes with temperature in a manner consistent with predictions of the single impurity Anderson model. We discuss the relationship between these results and recent valence-band photoemission results in the same compounds where the existence of temperature dependence has been disputed

213

Ab initio many-body effects in TiSe?: A possible excitonic insulator scenario from GW band-shape renormalization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe?. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ?0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k² parabolic dispersion toward a k?...

Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, Claude; Aebi, Philipp; Onida, Giovanni; Olevano, V.

2012-01-01

214

Unusual competition of structural phases and semi-conducting behaviour of bands in superheavy Copernicium  

Science.gov (United States)

In this letter we address the problem of phase stability in the relatively new element 112, namely Copernicium (Cn). The ground state properties as well as the thermodynamic quantities were computed from a first-principles approach based on the local density approximation with spin orbit (SO) consideration. We found that relativistic effects play a vital role in the phase stability and electronic properties of Cn. The obtained results reveal some unusual energetic competition between the crystallographic forms that Cn may adopt, i.e., sc, bcc, fcc and hcp. Nevertheless, relativistic SO coupling increases the energetic stability of the bcc phase over the other structures; moreover dynamical calculations via phonon dispersion provide strong support for our findings. Band structure results indicate that Copernicium is at least a semiconductor in all possible phases. The valence bands of Cn have strong 6d character, with a significant mixing between the fully occupied 7s and 6d bands.

Zaoui, A.; Ferhat, M.

2012-03-01

215

Ab initio many-body effects in TiSe2: A possible excitonic insulator scenario from GW band-shape renormalization  

Science.gov (United States)

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ?0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k2 parabolic dispersion toward a k4 shape. In particular, GW moves the top of valence moved toward a circle of points away from ?, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.

Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, C.; Aebi, P.; Onida, G.; Olevano, V.

2012-05-01

216

Extensive long-distance pollen dispersal and highly outcrossed mating in historically small and disjunct populations of Acacia woodmaniorum (Fabaceae), a rare banded iron formation endemic  

Science.gov (United States)

Background and Aims Understanding patterns of pollen dispersal and variation in mating systems provides insights into the evolutionary potential of plant species and how historically rare species with small disjunct populations persist over long time frames. This study aims to quantify the role of pollen dispersal and the mating system in maintaining contemporary levels of connectivity and facilitating persistence of small populations of the historically rare Acacia woodmaniorum. Methods Progeny arrays of A. woodmaniorum were genotyped with nine polymorphic microsatellite markers. A low number of fathers contributed to seed within single pods; therefore, sampling to remove bias of correlated paternity was implemented for further analysis. Pollen immigration and mating system parameters were then assessed in eight populations of varying size and degree of isolation. Key Results Pollen immigration into small disjunct populations was extensive (mean minimum estimate 40 % and mean maximum estimate 57 % of progeny) and dispersal occurred over large distances (?1870m). Pollen immigration resulted in large effective population sizes and was sufficient to ensure adaptive and inbreeding connectivity in small disjunct populations. High outcrossing (mean tm = 0·975) and a lack of apparent inbreeding suggested that a self-incompatibility mechanism is operating. Population parameters, including size and degree of geographic disjunction, were not useful predictors of pollen dispersal or components of the mating system. Conclusions Extensive long-distance pollen dispersal and a highly outcrossed mating system are likely to play a key role in maintaining genetic diversity and limiting negative genetic effects of inbreeding and drift in small disjunct populations of A. woodmaniorum. It is proposed that maintenance of genetic connectivity through habitat and pollinator conservation will be a key factor in the persistence of this and other historically rare species with similar extensive long-distance pollen dispersal and highly outcrossed mating systems. PMID:25100675

Millar, Melissa A.; Coates, David J.; Byrne, Margaret

2014-01-01

217

The method of thermodynamic perturbation in the intermediate valence problem  

International Nuclear Information System (INIS)

The effect of hybridization on the free energy and the magnetic susceptibility X of intermediate valence compounds is studied employing the method of thermodynamic perturbation in second order. This method is applied to a lattice with a variable concentration c of impurities that are described by the Anderson model. A non-divergent logarithmic correction appears when the chemical potential ? sub(e) is not in the middle of the rectangular conduction band, and it is shown by a different method that this correction is also present in the exactly soluble case of independent electrons with hybridization. When c=1 the logarithmic term is cancelled by a term proportional to (?2? sub(e)/?H2). By thermodynamic arguments it is shown that this concentration dependent contribution coincides with a different expression that was recently obtained by another approach. The contribution of the third state of charge to X is discussed; this state is usually neglected in intermediate valence models. (Author)

218

Thermal expansion contributions to band-gap and band offset temperature dependencies  

Energy Technology Data Exchange (ETDEWEB)

Using a thermodynamic approach, the authors examine the role thermal expansion plays in determining the temperature dependence of band gaps and band offsets. Because of the large variation in the standard volume of electrons (or, equivalently, variation in conduction-band deformation potential) with symmetry of the conduction band, most of the band-gap temperature variation can be ascribed to the valence band for the X gap, while the variation is more equitably divided between the conduction and valence band for the {Gamma} band gap. The particulars of the AlAs/GaAs system are examined in detail, where they find evidence that X electrons have a larger electron-phonon entropy than {Gamma} electrons, and that the lowest conduction-band offset exhibits considerable temperature variation.

Malloy, K.J. mitev. (Univ. of New Mexico, Albuquerque (United States)); Vechten, J.A. van (Oregon State Univ., Corvallis (United States))

219

Reentrant valence transition in EuO at high pressures : beyond the bond-valence model  

Energy Technology Data Exchange (ETDEWEB)

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at {approx}45 GPa is accompanied by a reentrant Eu valence transition into a lower valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.

Souza-Neto, N.M.; Zhao, J.; Alp, E.; Shen, G.; Sinogeikin, S. V.; Lapertot, G.; Haskel, D. (X-Ray Science Division); (Laboratorio Nacional de Luz Suncrotron-LNLS); (Carnegie Institution of Washington); (Commissariat a l' Energie Atomique)

2012-07-12

220

Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model  

Science.gov (United States)

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at ?45GPa is accompanied by a reentrant Eu valence transition into a lower valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.

Souza-Neto, N. M.; Zhao, J.; Alp, E. E.; Shen, G.; Sinogeikin, S. V.; Lapertot, G.; Haskel, D.

2012-07-01

 
 
 
 
221

Mixed valence model for superconductivity  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english Superconductivity often occurs in crystals with one active electron per site with charge density wave (CDW) or spin density wave (SDW) as 'mother state'. It is proposed that superconductivity is possible when the differences in equilibrium geometry and energy between the diabatic CDW and SDW states [...] are so small that there is interaction between them via the zero point vibrations. Electron pairing in real space is directly related to oxidation states being different in two units. Three valence states in succession have to be stable (ground state or low-energy excited states) and we therefore refer to this mixed valence model as the MV-3 model. Examples are chosen from bismuthates, cuprates, and fullerides. The theory is simple and straightforward and offers solutions to other important problems as well, for example for A3C(6)0(A = K; Rb), that (1) there are no magnetic moments in crystal phase, and (2) that these systems are superconducting metals while A4C(6)0 are insulators.

Sven, Larsson.

2003-12-01

222

Band structure of superlattice with ?-like potential  

International Nuclear Information System (INIS)

Band structure of superlattice with ?-like potential has been calculated taking into account interaction of carriers of different kinds. Superlattices of semiconductors with degenerated valence band and zero-gap semiconductors have been considered. For the latter semimetal-semiconductor transition has been obtained. (author). 8 refs, 1 fig

223

Generation of 3.6???m radiation and telecom-band amplification by four-wave mixing in a silicon waveguide with normal group velocity dispersion.  

Science.gov (United States)

Mid-infrared light generation through four-wave mixing-based frequency down-conversion in a normal group velocity dispersion silicon waveguide is demonstrated. A telecom-wavelength signal is down-converted across more than 1.2 octaves using a pump at 2190 nm in a 1 cm-long waveguide. At the same time, a 13 dB on-chip parametric gain of the telecom signal is obtained. PMID:24690784

Kuyken, B; Verheyen, P; Tannouri, P; Liu, X; Van Campenhout, J; Baets, R; Green, W M J; Roelkens, G

2014-03-15

224

Reentrant valence transition in EuO at high pressures: beyond the bond-valence model  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $\\approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $\\emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent s...

Souza-neto, N. M.; Zhao, J.; Alp, E. E.; Shen, G.; Sinogeikin, S. V.; Lapertot, G.; Haskel, D.

2012-01-01

225

Calculation of Confined Phonon Spectrum in Narrow Silicon Nanowires Using the Valence Force Field Method  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We study the effect of confinement on the phonon properties of ultra-narrow silicon nanowires of side sizes of 1-10nm . We use the modified valence force field method to compute the phononic dispersion, and extract the density of states, the transmission function, the sound velocity, the ballistic thermal conductance and boundary scattering-limited diffusive thermal conductivity. We find that the phononic dispersion and the ballistic thermal conductance are functions of the ...

Karamitaheri, Hossein; Neophytou, Neophytos; Taheri, Mohsen Karami; Faez, Rahim; Kosina, Hans

2013-01-01

226

Thermoelectric, band structure, chemical bonding and dispersion of optical constants of new metal chalcogenides Ba4CuGa5Q12 (Q=S, Se)  

Science.gov (United States)

The electronic structure and dispersion of optical constants of the Ba4CuGa5S12 and Ba4CuGa5Se12 compounds were calculated by the first-principles full-potential linearized augmented plane wave (FPLAPW) method. We employed the local density approximation (LDA), generalized gradient approximation (GGA) and Engel-Vosko GGA (EVGGA) to calculate the electronic structures, Fermi surface, thermoelectric, chemical bonding and dispersion of optical constants of these compounds. By investigating the influence of replacing S by Se, it has been found that the charge density around 'Ga' is greater in Ba4CuGa5Se12 than Ba4CuGa5S12. Fermi surface of Ba4CuGa5S12 consists of an electronic sheet only because there is no empty region while Ba4CuGa5Se12 contains both holes and electronic sheets because this compound contains both empty and shaded region. As we replace S by Se the heights of the peaks decreases as a results the reflectivity also decreases. It is noticed that the reflectivity is over 68% (60%) for Ba4CuGa5S12 (Ba4CuGa5Se12) compounds within the energy range studied. This implies that the material will serve as a good reflector. By replacing S by Se the figure of merit values increases from 0.97 to 1.0, which shows the good thermoelectric behavior of both compounds.

Reshak, A. H.; Azam, Sikander

2014-08-01

227

Indirect-direct band gap transition through electric tuning in bilayer MoS{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

We investigate the electronic properties of bilayer MoS{sub 2} exposed to an external electric field by using first-principles calculations. It is found that a larger interlayer distance, referring to that by standard density functional theory (DFT) with respect to that by DFT with empirical dispersion corrections, makes indirect-direct band gap transition possible by electric control. We show that external electric field effectively manipulates the valence band contrast between the K- and ?-valleys by forming built-in electric dipole fields, which realizes an indirect-direct transition before a semiconductor-metal transition happens. Our results provide a novel efficient access to tune the electronic properties of two-dimensional layered materials.

Zhang, Z. Y.; Si, M. S., E-mail: sims@lzu.edu.cn; Wang, Y. H.; Gao, X. P. [Key laboratory for Magnetism and Magnetic Materials of the Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730 000 (China); Sung, Dongchul; Hong, Suklyun [Graphene Research Institute, Sejong University, Seoul 143 747 (Korea, Republic of); He, Junjie [Department of Physics, Xiangtan University, Hunan 411 105 (China)

2014-05-07

228

Valence instabilities in cerium intermetallics  

International Nuclear Information System (INIS)

The primary purpose of this investigation was to study the magnetic behaviour of cerium in intermetallic compounds, that show an IV behaviour, e.g. CeSn3. In the progress of the investigations, it became of interest to study the effect of changes in the lattice of the IV compound by substituting La or Y for Ce, thus constituting the Cesub(1-x)Lasub(x)Sn3 and Cesub(1-x)Ysub(x)Sn3 quasibinary systems. A second purpose was to examine the possibility of introducing instabilities in the valency of a trivalent intermetallic cerium compound: CeIn3, also by La and Y-substitutions in the lattice. Measurements on the resulting Cesub(1-x)Lasub(x)In3 and Cesub(1-x)Ysub(x)In3 quasibinaries are described. A third purpose was to study the (gradual) transition from a trivalent cerium compound into an IV cerium compound. This was done by examining the magnetic properties of the CeInsub(x)Snsub(3-x) and CePbsub(x)Snsub(3-x) systems. Finally a new possibility was investigated: that of the occurrence of IV behaviour in CeSi2, CeSi, and in CeGa2. (Auth.)

229

Ab initio valence calculations in chemistry  

CERN Document Server

Ab Initio Valence Calculations in Chemistry describes the theory and practice of ab initio valence calculations in chemistry and applies the ideas to a specific example, linear BeH2. Topics covered include the Schrödinger equation and the orbital approximation to atomic orbitals; molecular orbital and valence bond methods; practical molecular wave functions; and molecular integrals. Open shell systems, molecular symmetry, and localized descriptions of electronic structure are also discussed. This book is comprised of 13 chapters and begins by introducing the reader to the use of the Schrödinge

Cook, D B

1974-01-01

230

Mixed valency in cerium oxide crystallographic phases: Determination of valence of the different cerium sites by the bond valence method  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxid...

Shoko, E.; Smith, M. F.; Mckenzie, Ross H.

2008-01-01

231

Event valence and unrealistic optimism: further evidence.  

Science.gov (United States)

Unrealistic optimism is assessed using either a single question, the rating of own likelihood of experiencing an event compared to that of the average person, or two questions, separate rating of own likelihood and that of the average person. The effect of event valence on unrealistic optimism was studied in a sample of 175 students using the two-question measure. Valence was manipulated to be positive, i.e., outcome was desirable, or negative, i.e., outcome was undesirable, by 'framing' the same event appropriately. Unrealistic optimism was greater for negative than positive valence. The effect was of the same direction and magnitude as that found by Gold and Martyn (2003) using the single-question measure. The relationship between event valence and unrealistic optimism does not depend on whether unrealistic optimism is assessed with one or two questions. PMID:15587209

Gold, Ron S; Martyn, Kate

2004-10-01

232

Calculation of valence electron momentum densities using the projector augmented-wave method  

CERN Document Server

We present valence electron Compton profiles calculated within the density-functional theory using the all-electron full-potential projector augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al) and hydrogen-bonded ((H_2O)_2) systems agree well with experiments and computational results obtained with other band-structure and basis set schemes. The PAW basis set describes the high-momentum Fourier components of the valence wave functions accurately when compared with other basis set schemes and previous all-electron calculations.

Makkonen, I; Puska, M J

2004-01-01

233

Formation of an unconventional Ag valence state in Ag2NiO2  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The Ag ion in the recently synthesized novel material Ag2NiO2 adopts an extremely unusual valency of 1/2, leaving the Ni ion as 3+, rather than the expected 2+. Using first principles calculations, we show that this mysterious subvalent state emerges due to a strong bonding-antibonding interaction between the two Ag layers which drives the lower band beneath the O p complex, eliminating the possibility of a conventional Ag 1+ valence state. The strong renormalization of the ...

Johannes, M. D.; Streltsov, S.; Mazin, I. I.; Khomskii, D. I.

2006-01-01

234

Valence fluctuation in CeMo2Si2C  

International Nuclear Information System (INIS)

Highlights: •Evidence for valence fluctuation of Ce ions. •XAS provides average formal LIII valence of Ce. •Kadowaki Woods ratio and Sommerfeld Wilson ratio indicate Fermi-liquid behavior. •DFT calculations reveal strong hybridization between Ce 4f and Mo 4d states. -- Abstract: We report on the valence fluctuation of Ce in CeMo2Si2C as studied by means of magnetic susceptibility ?(T), specific heat C(T), electrical resistivity ?(T) and X-ray absorption spectroscopy. Powder X-ray diffraction revealed that CeMo2Si2C crystallizes in CeCr2Si2C-type layered tetragonal crystal structure (space group P4/mmm). The unit cell volume of CeMo2Si2C deviates from the expected lanthanide contraction, indicating non-trivalent state of Ce ions in this compound. The observed weak temperature dependence of the magnetic susceptibility and its low value indicate that Ce ions are in valence fluctuating state. The formal LIII Ce valence in CeMo2Si2C????=3.14 as determined from X-ray absorption spectroscopy measurement is well below the value ?????3.4 in tetravalent Ce compound CeO2. The temperature dependence of specific heat does not show any anomaly down to 1.8 K which rules out any magnetic ordering in the system. The Sommerfeld coefficient obtained from the specific heat data is ? = 23.4 mJ/mol K2. The electrical resistivity follows the T2 behavior in the low temperature range below 35 K confirming a Fermi liquid behavior. Accordingly both the Kadowaki Woods ratio A/?2 and the Sommerfeld Wilson ratio ?(0)/? are in the range expected for Fermi-liquid systems. In order to get some information on the electronic states, we calculated the band structure within the density functional theory, eventhough this approach is not able to treat 4f electrons accurately. The non-f electron states crossing the Fermi level have mostly Mo 4d character. They provide the states with which the 4f sates are strongly hybridized, leading to the intermediate valent state

235

Coupling of THz radiation with intervalence band transitions in microcavities.  

Science.gov (United States)

The strong coupling of THz radiation and material excitations can improve the quantum efficiency of THz emitters. In this paper, we investigate THz polaritons and antipolaritons based on valence band transitions, which allow TE coupling in a simple configuration. The approach can improve the quantum efficiency of THz based devices based on TE mode in the strong coupling regime of THz radiations and intervalence bands transitions in a GaAs/AlGaAs quantum wells. A Nonequilibrium Many Body Approach for the optical response beyond the Hartree-Fock approximation is used as input to the effective dielectric function formalism for the polariton/antipolariton problem. The energy dispersion relations in the THz range are obtained by adjusting the full numerical solutions to simple analytical expressions, which can be used for non specialists in a wide number of new structures and material systems. The combination of manybody and nonparabolicity at high densities leads to dramatic changes in the polariton spectra in a nonequilibrium configuration, which is only possible for intervalence band transitions. PMID:24663634

Pereira, M F; Faragai, I A

2014-02-10

236

Band offsets in transition-metal oxide heterostructures  

Science.gov (United States)

We measured valence band offsets in Ta2O5-WO3, Ta2O5-Nb2O5 and WO3-Nb2O5 heterostructure couples by in situ x-ray photoelectron spectroscopy, immediately following the bi-layer growth in ultra-high vacuum. Conduction band offsets were estimated using the measured valence band offsets in conjunction with the literature values for the respective band gaps. The offsets between Ta2O5 and WO3 and between Ta2O5 and Nb2O5 layers were strongly asymmetric, with 0.8-1.1 eV (0.1-0.2 eV) barriers for the conduction (valence) bands, depending on the particular couple and the stacking sequence. Such asymmetry can be very useful in switching devices.

Goldfarb, I.; Ohlberg, D. A. A.; Strachan, J. P.; Pickett, M. D.; Yang, J. Joshua; Medeiros-Ribeiro, G.; Williams, R. S.

2013-07-01

237

Subliminal Affect Valence Words Change Conscious Mood Potency but Not Valence: Is This Evidence for Unconscious Valence Affect?  

Directory of Open Access Journals (Sweden)

Full Text Available Whether or not affect can be unconscious remains controversial. Research claiming to demonstrate unconscious affect fails to establish clearly unconscious stimulus conditions. The few investigations that have established unconscious conditions fail to rule out conscious affect changes. We report two studies in which unconscious stimulus conditions were met and conscious mood changes measured. The subliminal stimuli were positive and negative affect words presented at the objective detection threshold; conscious mood changes were measured with standard manikin valence, potency, and arousal scales. We found and replicated that unconscious emotional stimuli produced conscious mood changes on the potency scale but not on the valence scale. Were positive and negative affects aroused unconsciously, but reflected consciously in potency changes? Or were the valence words unconscious cognitive causes of conscious mood changes being activated without unconscious affect? A thought experiment is offered as a way to resolve this dilemma.

Michael Snodgrass

2012-10-01

238

Valence changes associated with the metal-insulator transition in Bi1-xLaxNiO3  

Science.gov (United States)

Perovskite-type BiNiO3 is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray-absorption (XAS) spectra of Bi1-xLaxNiO3 to investigate how the electronic structure changes with La doping. From Ni 2p XAS, we observed an increase of the valence of Ni from 2+ toward 3+ . Combined with the core-level photoemission study, it was found that the average valence of Bi remains ˜4+ and that the Ni valence behaves as ˜(2+x)+ , that is, La substitution results in a hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for x>0 , consistent with the metallic behavior of the La-doped compounds. The Ni 2p XAS, Ni 2p core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni2+?Ni3+ valence change.

Wadati, H.; Takizawa, M.; Tran, T. T.; Tanaka, K.; Mizokawa, T.; Fujimori, A.; Chikamatsu, A.; Kumigashira, H.; Oshima, M.; Ishiwata, S.; Azuma, M.; Takano, M.

2005-10-01

239

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

International Nuclear Information System (INIS)

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

240

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

 
 
 
 
241

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles  

International Nuclear Information System (INIS)

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations

242

Laser-induced defect centers and valence state change of Mn ions in sodium borate glasses  

International Nuclear Information System (INIS)

In this report, an ultrafast laser was used to produce persistent change of valence states of Mn ions in sodium borate glasses. Glass composition of sodium borate in this study was 85B2O3-15Na2O. We observed a broad emission at 600 nm of the transition 4T1-6A1 of doped Mn2+ ions with excitation at 325 nm in an unirradiated area. Femtosecond laser irradiation produced additional strong blue emission at 440 nm and reduced the emission intensity from Mn2+ ions. The irradiation produced the absorption band of Mn3+ at 550 nm, resulting in color change of the glass. The blue band was measured as a function of irradiation exposure for samples with various concentrations of Mn ions. The valence state change of Mn2+ to Mn3+ was suppressed in Mn and Cr codoped glasses

243

Band-gap and the Electronic structure of BN nanotubes  

Science.gov (United States)

A verity of BN nanotubes were studied via high resolution electron energy loss spectroscopy. Careful examination of low energy loss spectra (0-50 eV) the band gap of the single tube can be extracted. Moreover the spectral features give insight into the BN nanotube valence band structure.

Okawa, David; Aloni, Shaul; Mickelson, William

2005-03-01

244

When does event valence affect unrealistic optimism?  

Science.gov (United States)

"Unrealistic optimism" (UO) has been found to be greater for negative (i.e. undesirable) than for positive (i.e. desirable) events. In two studies, we tested whether this "valence effect" is explicable in terms of motivational processes. In each case, participants were students (n = 159 in Study 1, n = 90 in Study 2), UO for the same health event was measured, and valence was manipulated by framing the event either negatively or positively. The degree to which the event would be expected to evoke concern was varied by manipulating the ease with which it could be avoided (Study 1) or the severity of its consequences (Study 2). A similar pattern of results was found in each study: the valence effect was exhibited in the condition where the event would have evoked concern, but not in the condition where it would have evoked little concern. The findings are in accord with an explanation of the valence effect in motivational terms. Implications for health education are discussed. PMID:21745029

Gold, Ron S; de Sousa, Phillip N

2012-01-01

245

Valence electronic state density in thorium dioxide  

Directory of Open Access Journals (Sweden)

Full Text Available This work analyses the fine low energy (0-40 eV X-ray photoelectron spectra of ThO2, taking into account relativistic X?-discrete variation electronic structure calculations for the ThO8 (D4h cluster reflecting thorium's close environment in ThO2. As a result, it was theoretically shown and experimentally confirmed that Th5f electrons in ThO2 can participate directly (~0.6 Th5f electrons in chemical bond formation.Th6p electrons were shown to be a significant part (~0.44 Th6p electrons not only of inner valence molecular orbitals, but to play a significant role in outer valence molecular orbitals formation, as well. Inner valence molecular orbitals composition and sequent order were established to belong to the binding energy range of 13 eV to 40 eV. The valence electronic state density in the range of 0-40 eV in ThO2 was also calculated. For the first time, these data allowed an interpretation of the fine X-ray photoelectron spectra (0-40 eV and high resolution O4,5(Th X-ray emition spectral structure (~60 - ~85 eV of ThO2.

Teterin Anton Yu.

2008-01-01

246

Valence Bond Mapping of Antiferromagnetic Spin Chains  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Boson mapping techniques are developed to describe valence bond correlations in quantum spin chains. Applying the method to the alternating bond hamiltonian for a generic spin chain, we derive an analytic expression for the transition points which gives perfect agreement with existing Density Matrix Renormalization Group (DMRG) and Quantum Monte Carlo (QMC) calculations.

Dukelsky, Jorge; Pittel, Stuart

1997-01-01

247

Photoionization cross-sections of ground and excited valence levels of actinides  

Directory of Open Access Journals (Sweden)

Full Text Available The photoionization cross-sections of ground and excited atomic states of actinide atoms were calculated by the Dirac-Fock-Slater method for two excitation energies of X-ray radiation (1253.6 eV and 1486.6 eV. These data are required for calculations of intensities of X-ray photoelectron spectra of actinide compound valence bands and interpretation of experimental spectra.

Yarzhemsky Victor G.

2012-01-01

248

Observation of Rydberg transitions from the inner valence shell of ethane  

International Nuclear Information System (INIS)

The electron impact spectrum of ethane has been examined in a region that includes ionization out of the inner valence shell. One diffuse structure and a progression of ten vibrational bands have been found in a 4 eV range below and to some degree overlapping the 2A2/sub u/ ion threshold. Evidence indicates that the observed transitions belong to the symmetry forbidden Rydberg series (2a2/sub u/)2?(2a2/sub u/, np? or np?)

249

Optoelectronic properties of InAlN/GaN distributed bragg reflector heterostructure examined by valence electron energy loss spectroscopy.  

Science.gov (United States)

High-resolution monochromated electron energy loss spectroscopy (EELS) at subnanometric spatial resolution and Kramers-Kronig analysis of the Valence-EELS data, allowing band gap energy to be measured and an insight on the polytypism of the GaN layers. PMID:23058502

Eljarrat, A; Estradé, S; Ga?evi?, Z; Fernández-Garrido, S; Calleja, E; Magén, C; Peiró, F

2012-10-01

250

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex.  

Science.gov (United States)

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes. PMID:15360240

Shaikh, Nizamuddin; Goswami, Sanchita; Panja, Anangamohan; Wang, Xin-Yi; Gao, Song; Butcher, Ray J; Banerjee, Pradyot

2004-09-20

251

Electronic band structure of the layered dichalcogenide MoTe2  

International Nuclear Information System (INIS)

The transition metal dichalcogenide ?-MoTe2 grown by chemical vapour transport (CVT) at temperatures above 1175 K and cooled to room temperature shows a layered structure that can be described as stacked sandwiches of the tree layers Te-Mo-Te. One molybdenum atom is surrounded by six tellurium atoms forming an octahedron which is slightly distorted because of the molybdenum atom being displaced from the centre. Therefore strong metal-metal bonds are found which form zigzag chains along the crystallographic b-direction. Upon cooling below 250 K the material undergoes a structural phase transition where the ? phase (monoclinic) changes to an orthorhombic Td-structure. In this contribution we present a detailed temperature dependent study of electronic band structure and Fermi-surface of ?-MoTe2 performed by angle-resolved photoemission spectroscopy (ARPES). The measurements were carried out at BESSY II. It results that the dispersion of the valence bands along the different high-symmetric directions of the Brillouin zone are extremely weak. These bands reveal close insight into the low temperature and phase transition behaviour of quasi-two dimensional materials like ?-MoTe2. Fermi level crossings are studied by Fermi-surface maps. The experimental findings will be compared with tight-binding band structure calculations

252

Valence changes associated with the metal-insulator transition in Bi$_{1-x}$La$_x$NiO$_3$  

CERN Document Server

Perovskite-type BiNiO$_3$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray absorption (XAS) spectra of Bi$_{1-x}$La$_{x}$NiO$_{3}$ to investigate how the electronic structure changes with La doping. From Ni $2p$ XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains $\\sim 4+$ and that the Ni valence behaves as $\\sim (2+x)+$, that is, La substitution results in hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for $x>0$, consistent with the metallic behavior of the La-doped compounds. The Ni $2p$ XAS, Ni $2p$ core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shap...

Wadati, H; Tran, T T; Tanaka, K; Mizokawa, T; Fujimori, A; Chikamatsu, A; Kumigashira, H; Oshima, M; Ishiwata, S; Azuma, M; Takano, M

2005-01-01

253

Band structures of ZnTe:O alloys with isolated oxygen and with clustered oxygen impurities  

Energy Technology Data Exchange (ETDEWEB)

Highlights: • Band structures of ZnTe:O alloy highly depends on the status of oxygen. • Clustered oxygen lowers the bandgap while isolated oxygen increases the bandgap. • The solar adsorption efficiency of ZnTe:O can be improved by oxygen clustering. -- Abstract: First-principles calculations reveal that band structures of ZnTe:O alloys highly depend on the configuration of oxygen in the alloy. For alloys with isolated oxygen, the calculated band structure shows the formation of intermediate states between valence and conduction band and the shift of conduction band to higher energy level. It expands the gap between valence and conduction band. For alloys with clustered oxygen, the formation of intermediate band is still observed, while the gap between valence and conduction band is decreased. For alloys with oxygen impurities adjacent to Zn vacancy, the band structure only shows the decrease of the gap between valence and conduction band without the formation of any intermediate band. These results suggest the critical role of Zn–O bonding in determining the energy level of the impurity states. On the basis of our results, a possible band engineering approach is suggested in order to improve the performance of ZnTe:O alloy as intermediate band solar adsorbent.

Ling, Chen, E-mail: chen.ling@tema.toyota.com; Zhou, Li Qin; Banerjee, Debasish; Jia, Hongfei

2014-01-25

254

Theoretical investigations on valence vibronic transitions  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english This article reviews previously employed methods to study several valence electronic transitions, optically forbidden or not, enhancing intensity through vibronic coupling. Electronic transition dipole moments were calculated using several ab initio methods including electron correlation. In this me [...] thod the square of the electronic transition dipole moments are directly calculated along the normal coordinates of vibration and then expanded with a polynomial function. Afterwards, analytical vibrational integration using harmonic wave functions, of the square of the transition moments function, allows us to obtain partial (i.e. for each vibrational mode) and total optical oscillator strengths (OOS), for the vibronic transition of interest. We illustrate the accuracy of the method through valence transitions of benzene (C6H6), formaldehyde (H2CO), acetone (C3H6O) and formic acid (HCOOH).

Itamar, Borges Jr.; Alexandre B., Rocha; Carlos Eduardo, Bielschowsky.

255

Pion Valence-quark Parton Distribution Function  

CERN Document Server

Within the Dyson-Schwinger equation formulation of QCD, a rainbow ladder truncation is used to calculate the pion valence-quark distribution function(PDF). The gap equation is renormalized at a typical hadronic scale, of order 0.5GeV, which is also set as the default initial scale for the pion PDF. We implement a corrected leading-order expression for the PDF which ensures that the valence-quarks carry all of the pion's light-front momentum at the initial scale. The scaling behavior of the pion PDF at a typical partonic scale of order 5.2GeV is found to be $(1-x)^{\

Chang, Lei

2014-01-01

256

Evaluative conditioning induces changes in sound valence  

Directory of Open Access Journals (Sweden)

Full Text Available Evaluative Conditioning (EC has hardly been tested in the auditory domain, but it is a potentially valuable research tool. In Experiment 1 we investigated whether the affective evaluation of short environmental sounds can be changed using affective words as unconditioned stimuli (US. Congruence effects on an affective priming task (APT for conditioned sounds demonstrated successful EC. Subjective ratings for sounds paired with negative words changed accordingly. In Experiment 2 we investigated whether the acquired valence remains stable after repeated presentation of the conditioned sound without the US or whether extinction occurs. The acquired affective value remained present, albeit weaker, even after 40 extinction trials. These results warrant the use of EC to study processing of short environmental sounds with acquired valence, even if this requires repeated stimulus presentations. This paves the way for studying processing of affective environmental sounds while effectively controlling low level-stimulus properties.

AnnaC.Bolders

2012-04-01

257

Janus Nematic Colloids with Designable Valence  

Directory of Open Access Journals (Sweden)

Full Text Available Generalized Janus nematic colloids based on various morphologies of particle surface patches imposing homeotropic and planar surface anchoring are demonstrated. By using mesoscopic numerical modeling, multiple types of Janus particles are explored, demonstrating a variety of novel complex colloidal structures. We also show binding of Janus particles to a fixed Janus post in the nematic cell, which acts as a seed and a micro-anchor for the colloidal structure. Janus colloidal structures reveal diverse topological defect configurations, which are effectively combinations of surface boojum and bulk defects. Topological analysis is applied to defects, importantly showing that topological charge is not a well determined topological invariant in such patchy nematic Janus colloids. Finally, this work demonstrates colloidal structures with designable valence, which could allow for targeted and valence-conditioned self-assembly at micro- and nano-scale.

Simon ?opar

2014-05-01

258

The influence of electron correlation and relativistic effects on the valence shell photoelectron spectrum of iodothiophene  

International Nuclear Information System (INIS)

The valence shell photoelectron spectrum of 2-iodothiophene (2-I-Th) has been studied experimentally and calculations have been carried out to characterize the main bands due to single-hole ionized states for both 2- and 3-iodothiophene. The importance of electron correlation in the formation of satellite states has also been assessed. The vertical ionization energies and spectral intensities of the entire valence shell photoelectron spectrum have been computed using the third-order algebraic-diagrammatic construction approximation for the one-particle Green function. These theoretical predictions have allowed assignments to be proposed for all the prominent structure observed in the experimental spectra, and have highlighted the breakdown of the molecular orbital model of ionization for the ?1-orbital. By comparing the present theoretical and experimental results for iodothiophene with similar data for chloro-and bromothiophene, it has been found that the binding energies and intensities of the satellites associated with the ?1-orbital form a pattern which is common to all three monohalothiophenes. Relativistic Hartree-Fock calculations have been carried out to investigate the importance of such effects on the valence shell ionization of iodothiophene. Synchrotron radiation has been used to measure photoelectron angular distributions and branching ratios of 2-I-Th. The spectral behaviour of the asymmetry parameters has provided an experimental means of distinguishing photoelectron bands due to ?-orbitals from those associated with ?-orbitals. A high-resolution photoelectron spectrum of the outer valence shell of 2-I-Th has been recorded using HeI radiation, and vibrational structure has been observed and analysed in the ##tildeX SRC=http://ej.iop.org/icons/Entities/tildeX.gif ALT=tilde X ALIGN=BOTTOM/>## 2A'', A-tilde 2A'', B-tilde 2A' and ##tildeC SRC=http://ej.iop.org/icons/Entities/tildeC.gif ALT=tilde C ALIGN=MIDDLE/>## 2A'' state bands

259

Intramolecular electron transfer on the vibrational timescale in mixed valence ruthenium clusters  

International Nuclear Information System (INIS)

The thermodynamic stability of the mixed valence (one electron reduced) state between linked Ru3 units was studied by means of electrochemical methods for the series of the ligand-bridged triruthenium cluster dimer, [Ru3(?3-O)(?-CH3CO2)6(CO)(L)(?-BL)Ru3(?3-O)(?-CH3CO2)6(CO)(L)] (BL = 1,4 pyrazine: L = 4-dimethylaminopyridine (dmap) (1a), pyridine (py) (1b), 4-cyanopyridine (cpy) (1c), 1-azabicyclo[2.2.2]octane (1d); BL = 4,4'-bipyridine: L= dmap (2a), py(2b), cpy (2c); BL 2,7-diazapyrene; L = dmap (3a); BL = 1,4-diazabicyclo[2.2.2]octane: L = dmap (4a), py(4b), cpy (4c). The mixed valence states undergoing rapid intermolecular electron transfers were observed by IR spectro-electrochemistry. By simulating dynamical effects on the observed ?(CO) absorption band shapes, the rate constants ke for electron transfer in the mixed valence states of 1a, 1b, 1c and 1d were estimated to be 9x1011 s-1 (at room temperature (rt)), 5x1011 s-1 (at rt), ca. 1x1011 s-1 (at rt), and 1x1012 s-1 (at -18 oC), respectively. Possible applications of this approach to asymmetric mixed valence systems were discussed. (author)

260

A study of the valence shell spectroscopic and thermodynamic properties of trifluoronitrosomethane cations  

Science.gov (United States)

A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in trifluoronitrosomethane (CF3NO) as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range ˜10-42 eV, and appearance energies have been determined for 10 fragment ions. At high excitation energies, singly charged atomic fragments have been observed. For the main dissociation channels, leading to the formation of NO+, CF2+ or CF3+, the experimental appearance energies have been compared with thermochemical estimates, and a satisfactory agreement has been found. Structure observed in the total ion yield curve has been interpreted with the aid of excited state transition energies and oscillator strengths obtained in a time-dependent Hartree Fock calculation. The theoretical results show that configuration interaction strongly affects many of the valence states. A HeI excited photoelectron spectrum of CF3NO has been measured and the orbital ionisation energies have been compared with theoretical values computed using the Outer Valence Green's Function approach. A large Franck-Condon gap is observed between the X˜A and the A˜A state bands, in accord with the calculated vertical ionisation energies of 10.87 and 16.32 eV for the 12a' (n-) and the 11a' (n+) orbitals, respectively. In the ion yield curve, the corresponding energy range is strongly influenced by autoionising valence states.

Graham, D. M.; Powis, I.; Underwood, J. G.; Shaw, D. A.; Holland, D. M. P.

2012-02-01

 
 
 
 
261

Electron magnetic scattering on valence nucleon orbits  

International Nuclear Information System (INIS)

Cross-sections for elastic electron scattering by the magnetization distribution of 49Ti, 51V, 59Co, 87Sr and 93Nb have been measured in the range of momentum transfer 1.7 - 3.3 fm-1. The results are interpreted in terms of radial distribution of the unpaired proton or neutron. Meson exchange and core polarisation effects are investigated. The valence nucleon radii obtained are compared with Hartree-Fock predictions

262

Valence bond solid states with symplectic symmetry  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We introduce a one-dimensional valence bond solid (VBS) state with symplectic symmetry SP(n) and construct the corresponding parent Hamiltonian. We argue that there is a gap in the spectrum. We calculate exactly the static correlation functions, which fall off exponentially. Hence the model introduced here shares all properties of the Haldane scenario for integer-spin quantum antiferromagnets. We further show that the VBS state possesses string order and discuss its generali...

Schuricht, Dirk; Rachel, Stephan

2008-01-01

263

Synthetic selectivity through avoidance of valence frustration.  

Science.gov (United States)

A series of di-copper(I) complexes has been prepared via the reaction of copper(I) tetrafluoroborate, 2,6-diformylpyridine, 8-aminoquinoline, and a series of aliphatic diamines and 4-substituted anilines. To avoid a "valence-frustrated" state, involving a mismatch between the number of ligand donor atoms and the number of metal acceptor sites, the product structures formed selectively: One of the formyl groups of the diformylpyridine reacted specifically with the aminoquinoline, whereas the other formyl group reacted with the diamine or aniline. The observed selectivity was demonstrated to be thermodynamic in nature: When two dicopper complexes that were stable yet "valence-frustrated" were mixed, an imine metathesis reaction was observed to occur spontaneously to generate a "valence-satisfied" structure. In addition to control over the constitution of the ligands, we were able to exercise control over their relative orientations within the complex. Diamines exclusively gave structures in which the ligand exhibited a head-to-head orientation along the copper-copper axis to avoid stretching. Anilines gave predominantly head-to-tail structures, with the proportion of head-to-head isomer decreasing in complexes that incorporate more electron-deficient anilines and disappearing in less polar solvents. We also demonstrated the removal of the metals and the hydrogenation of the imine bonds to generate a molecule containing nonexchanging secondary amines, suggesting potential uses of this technique in the domain of organic synthesis. PMID:17098865

Hutin, Marie; Bernardinelli, Gérald; Nitschke, Jonathan R

2006-11-21

264

Semionic resonating valence-bond states  

Science.gov (United States)

The nature of the kagome Heisenberg antiferromagnet (HAFM) is under ongoing debate. While recent evidence points towards a Z2 topological spin liquid, the exact nature of the topological phase is still unclear. In this paper, we introduce semionic resonating valence-bond (RVB) states, this is, resonating valence-bond states which are in the Z2-ordered double-semion phase, and study them using projected entangled pair states. We investigate their physics and study their suitability as an ansatz for the HAFM, as compared to a conventional RVB state which is in the toric-code Z2 topological phase. In particular, we find that a suitably optimized "semionic simplex RVB" outperforms the equally optimized conventional "simplex RVB" state, and that the entanglement spectrum (ES) of the semionic RVB behaves very differently from the ES of the conventional RVB, which suggests using the ES to discriminate the two phases. Finally, we also discuss the possible relevance of space-group symmetry breaking in valence-bond wave functions with double-semion topological order.

Iqbal, Mohsin; Poilblanc, Didier; Schuch, Norbert

2014-09-01

265

Actinides, the narrowwest bands  

International Nuclear Information System (INIS)

A table of elements is shown that demonstrates the crossover from superconductivity to magnetism as well as regions of mixed valence. In particular, the actinides must eventually show 4f-electron like mixed valence, after the 5f-electrons become localized. There also seems to be an adiabatic continuation between heavy fermion and mixed valence behavior

266

Multidimensional X-Ray Spectroscopy of Valence and Core Excitations in Cysteine  

CERN Document Server

Several nonlinear spectroscopy experiments which employ broadband x-ray pulses to probe the coupling between localized core and delocalized valence excitation are simulated for the amino acid cysteine at the K-edges of oxygen and nitrogen and the K and L-edges of sulfur. We focus on two dimensional (2D) and 3D signals generated by two- and three-pulse stimulated x-ray Raman spectroscopy (SXRS) with frequency-dispersed probe. We show how the four-pulse x-ray signals $\\boldsymbol{k}_\\mathrm{I}=-\\boldsymbol{k}_1+\\boldsymbol{k}_2+\\boldsymbol{k}_3$ and $\\boldsymbol{k}_\\mathrm{II}=\\boldsymbol{k}_1-\\boldsymbol{k}_2+\\boldsymbol{k}_3$ can give new 3D insight into the SXRS signals. The coupling between valence- and core-excited states can be visualized in three dimensional plots, revealing the origin of the polarizability that controls the simpler pump-probe SXRS signals.

Biggs, Jason D; Healion, Daniel; Mukamel, Shaul

2013-01-01

267

Full-Band Monte Carlo Simulation of HgCdTe APDs  

Science.gov (United States)

A full-band Monte Carlo model has been developed for understanding the carrier multiplication process in HgCdTe infrared avalanche photodiodes. The proposed model is based on a realistic electronic structure obtained by pseudopotential calculations and a phonon dispersion relation determined by ab initio techniques. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation, thus removing adjustable parameters such as deformation potential coefficients. The computation of the impact ionization transition rate is based on the calculated electronic structure and the corresponding wavevector-dependent dielectric function. The Monte Carlo model is applied to investigate key performance figures of long-wavelength infrared (LWIR) and mid-wavelength infrared (MWIR) HgCdTe avalanche photodetectors such as carrier multiplication and noise properties. Good agreement is achieved between simulations and experimental results. The multiplication process in LWIR ( ? c = 9.0 ?m at 80 K) and MWIR ( ? c = 5.1 ?m at 80 K) devices is found to be initiated only by electrons, as expected from excess noise measurements. This single-carrier multiplication behavior can be traced back to the details of the computed valence-band structure and phonon scattering rates.

Bertazzi, Francesco; Moresco, Michele; Penna, Michele; Goano, Michele; Bellotti, Enrico

2010-07-01

268

Persistence of the three-state description of mixed valency at the localized-to-delocalized transition.  

Science.gov (United States)

Application of a semiclassical three-state model of mixed valency to complexes of the type [Ru(3)(?(3)-O)(OAc)(6)(CO)(py)-(?(2)-BL)-Ru(3)(?(3)-O)(OAc)(6)(CO)(py)](-1), where BL = 1,4-pyrazine or 4,4'-bipyridine and py = 4-dimethylaminopyridine, pyridine, or 4-cyanopyridine is described. The appearance of two intervalence charge transfer (IVCT) bands in the near-infrared (NIR) region of the electronic spectra of these complexes is explained well by the three-state model. An important feature of the three-state model is that the IVCT band evolves into two bands: one that is metal-to-bridging-ligand-charge-transfer (MBCT) in character and another that is metal-to-metal-charge-transfer (MMCT) in character. The three-state model also fully captures the observed spectroscopic behavior in which the MBCT transition increases in energy and the MMCT band decreases in energy with increasing electronic communication in a series of mixed valence ions. The appearance of both the MBCT and MMCT bands is found to persist as coalescence of infrared (IR) vibrational spectra suggest a ground state delocalized on the picosecond time scale. The solvent and temperature dependence of the MBCT and MMCT electronic transitions defines the mixed valence complexes reported here as lying on the borderline of delocalization. PMID:21561112

Glover, Starla D; Kubiak, Clifford P

2011-06-01

269

Angle-resolved ultraviolet photoelectron spectroscopy and theoretical simulation of a well-ordered ultrathin film of tetratetracontane (n-C44H90) on Cu(100): Molecular orientation and intramolecular energy-band dispersion  

Science.gov (United States)

The electronic structure and molecular orientation of a tetratetracontane (n-C44H90; TTC) ultrathin film on a Cu(100) surface were studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. A well-oriented thin film of TTC was successfully prepared by vacuum evaporation in ultrahigh vacuum at room temperature. We observed a (2×1)-like low-energy electron-diffraction (LEED) pattern for the deposited TTC film. This result indicates that the TTC molecules lie on the Cu(100) surface in two types of domains, rectangular to each other, in which the alkyl-chain axes are along the [110] and [11¯0] directions of the Cu(100) surface. The application of the dipole selection rules to the normal-emission ARUPS spectrum revealed that the C-C-C plane of TTC is parallel to the Cu(100) surface plane (flat-on orientation). The intramolecular energy-band dispersion of TTC was examined by changing the take-off angle of emitted electron along the [110] direction of the Cu(100) surface. The observed results support the conclusion about the direction of alkyl-chain axes by LEED observation. In order to analyze the molecular orientation more quantitatively, we also performed theoretical simulations of the angle-resolved photoemission spectra using the independent-atomic-center (IAC) approximation combined with ab initio molecular-orbital (MO) calculations for various molecular orientations. The simulated spectra for flat-on orientation are in excellent agreement with the observed spectra. These results once again verify the deduced molecular orientation, and also demonstrate the reliability of theoretical simulation with the IAC/MO approximation for compounds without a ?-electron system. Furthermore, we observed a work function change of about -0.3 eV by adsorption of TTC. Such a decrease of the work function indicates the formation of a dipole layer at the interface, in contrast to the traditional picture of energy-level alignment assuming a common vacuum level at the organic/metal interface.

Yoshimura, Daisuke; Ishii, Hisao; Ouchi, Yukio; Ito, Eisuke; Miyamae, Takayuki; Hasegawa, Shinji; Okudaira, Koji Kamiya; Ueno, Nobuo; Seki, Kazuhiko

1999-09-01

270

Quest for chiral doublet bands in A?110 mass region  

International Nuclear Information System (INIS)

High spin states of 106,108Ag were studied by using fusion evaporation reaction on Beijing HI-13 Tandem Accelerator. The negative-parity bands were updated for both nuclei. Doublet bands similar to chiral bands in 104Rh were identified in 106,108Ag respectively. But there is much difference of the moment of inertia and alignment between the yrast and yrare bands in both of them. It may result from the different deformations of the core and angular momentum coupling mode between valence particles and the inner-core for the yrast and yrare bands. (authors)

271

Valence skipping driven superconductivity and charge Kondo effect  

International Nuclear Information System (INIS)

Highlights: •Valence skipping in metallic compounds can give rise to an unconventional superconductivity. •Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. •The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. •We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. •There is a high temperature region near the boundary. -- Abstract: Valence skipping in metallic compounds can give rise to an unconventional superconductivity. Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. The superconducting state is changed into a metallic state with a local singlet as the attractive interaction |U| increases. There is a high temperature region near the boundary

272

Valence skipping driven superconductivity and charge Kondo effect  

Energy Technology Data Exchange (ETDEWEB)

Highlights: •Valence skipping in metallic compounds can give rise to an unconventional superconductivity. •Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. •The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. •We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. •There is a high temperature region near the boundary. -- Abstract: Valence skipping in metallic compounds can give rise to an unconventional superconductivity. Several elements in the periodic table show valence skipping (or valence missing), for example, Bi forms the compounds in valence states +3 and +5. The doping of valence skipping elements will induce superconductivity and this will lead to a possibility of high temperature superconductivity. We consider the Wolf model with negative-U impurities, and show a phase diagram including superconducting phase. The superconducting state is changed into a metallic state with a local singlet as the attractive interaction |U| increases. There is a high temperature region near the boundary.

Yanagisawa, Takashi, E-mail: t-yanagisawa@aist.go.jp; Hase, Izumi

2013-11-15

273

SmB6: Topological insulator or semiconductor with valence-fluctuation induced hopping transport?  

Science.gov (United States)

We advert to the fact that the presence of valence fluctuations (VFs) in semiconductors with in-gap impurity bands unconditionally leads to dynamical changes (fluctuations) of energies of localized impurity states. We provide arguments that in the impurity subnetwork consisting of centers having energy levels fluctuating around the Fermi energy there exist favorable conditions for hops from occupied states to empty states of less energy. Consequently, we propose original valence-fluctuation induced hopping mechanism as a new possibility to explain unusual metallic-like conduction of SmB6 and other Kondo insulators experimentally observed at lowest temperatures. Interestingly, the proposed mechanism infers enhanced metallic-like surface conductivity of SmB6, what resembles a characteristic property of topological insulator, and is in agreement with experimental observations attempting to prove the existence of topologically protected surface state in SmB6.

Batko, I.; Batkova, M.

2014-10-01

274

Local magnetic moments and intermediate valence state of cerium impurities in ferromagnetic rare-earth metals  

Science.gov (United States)

Using experimental data for magnetic hyperfine fields obtained through the technique of time differential perturbed ?-? angular correlations (TDPAC) in the highly diluted system CeR (R=Gd, Tb), we theoretically discuss the formation of the local moments at the Ce impurity site, adopting an intermediate valence model. The ferromagnetic rare-earth host is considered as a transition metal in the beginning of a 5d series and the CeR alloys are described as strongly d-f correlated and hybridized systems. One calculates the hyperfine field which reproduces quite well the experimental data. The Ce valence in these systems is also obtained, giving values in good agreement with experimental results and theoretical claims involving Gd and Tb band calculations. Some comments about the diluted systems YbR (R=Gd, Tb) are also made.

Leal, C. E.; Troper, A.

1994-05-01

275

A study of the valence shell electronic structure of uracil and the methyluracils  

Energy Technology Data Exchange (ETDEWEB)

The valence shell photoelectron spectra of uracil, 1-methyluracil and 6-methyluracil have been studied experimentally and theoretically. Synchrotron radiation has been used to record spectra at photon energies of 40 and 80 eV. Photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between {sigma}- and {pi}-type orbitals. Vertical ionization energies and spectral intensities have been evaluated using the many-body Green's function approach, thereby enabling theoretical photoelectron spectra to be derived. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Two of the outer-valence vertical ionization energies are similar to one another and the vibrational progressions associated with these transitions overlap strongly. Vibronic interaction between these states, induced through the excitation of out-of-plane vibrational modes, may lead to nonadiabatic effects. Preliminary theoretical investigation of this interaction has been performed.

Holland, D.M.P. [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom)], E-mail: d.m.p.holland@dl.ac.uk; Potts, A.W. [Department of Physics, King' s College, Strand, London WC2R 2LS (United Kingdom); Karlsson, L. [Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala (Sweden); Zaytseva, I.L.; Trofimov, A.B. [Laboratory of Quantum Chemistry, Irkutsk State University, Karl-Marx Street 1, 664003 Irkutsk (Russian Federation); Schirmer, J. [Theoretische Chemie, Physikalisch-Chemisches Institut, Universitaet Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg (Germany)

2008-11-03

276

Formation of an unconventional Ag valence state in Ag2NiO2  

Science.gov (United States)

The Ag ion in the recently synthesized novel material Ag2NiO2 adopts an extremely unusual valency of (1)/(2) , leaving the Ni ion as 3+ , rather than the expected 2+ . Using first-principles calculations, we show that this mysterious subvalent state emerges due to a strong bonding-antibonding interaction between the two Ag layers that drives the lower band beneath the O p complex, eliminating the possibility of a conventional Ag 1+ valence state. The strong renormalization of the specific heat coefficient ? is likely due to strong spin fluctuations that stem from nearly complete compensation of the ferromagnetic (metallic double exchange and 90° superexchange) and antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path) interactions.

Johannes, M. D.; Streltsov, S.; Mazin, I. I.; Khomskii, D. I.

2007-05-01

277

XANES study of rare-earth valency in $LRu_{4}P_{12}$ (L = Ce and Pr)  

CERN Document Server

Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES spectrum suggests that Ce is mainly trivalent, but the 4f state strongly hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We find that the metal-insulator (MI) transition at TMI = 60 K in PrRu4P12 does not originate from Pr valence fluctuation.

Lee, C H; Sekine, C; Shirotani, I; Ishii, M

1999-01-01

278

Valence electron momentum distributions in cadmium  

International Nuclear Information System (INIS)

The valence 5s and 4d electron momentum distributions in cadmium have been measured using noncoplanar symmetric (e, 2e) electron coincidence spectroscopy at a total energy of 1200eV. They are in close agreement with Hartree-Fock momentum distributions both in shape and relative magnitudes. Some satellite lines of very low intensity have been detected. A CI calculation of the Cd ground state and several Cd+ ion states has been carried out to predict cross reactions for the ground state and various satellite transitions. The predictions are in agreement with the data

279

Reply to Isgur's comments on valence QCD  

International Nuclear Information System (INIS)

With the goal of understanding the complexity of QCD and the role of symmetry in dynamics, the authors studied a field theory called Valence QCD (VQCD) in which the Z graphs are forbidden so that the Fock space is limited to the valence quarks. The authors calculated nucleon form factors, matrix elements, and hadron masses both with this theory and with quenched QCD on a set of lattices with the same gauge background. Comparing the results of the lattice calculations in these two theories, the authors drew conclusions regarding the SU(6) valence quark model and chiral symmetry. While recognizing the goal of VQCD, Nathan Isgur disagrees on some of the conclusions the authors have drawn. The foremost objection raised in section 2 is to their suggestion that the major part of the hyperfine splittings in baryons is due to Goldstone boson exchange and not one-gluon-exchange (OGE) interactions. The logic of Isgur's objection is that VQCD yields a spectroscopy vastly different from quenched QCD and therefore the structure of the hadrons (to which hyperfine splittings in a quark model are intimately tied) is also suspect so no definite conclusions are possible. To put this into perspective it should be emphasized at the outset that spectroscopy is only one aspect of hadron physics examined in section 1. The authors have studied the axial and scalar couplings of nucleon in terms of FA/DA and FS/DS, the neutron to proton magnetic mome/sub>, the neutron to proton magnetic moment ratio ?n/?p, and various form factors. None of these results reveal any pathologies of hadron structure and turn out to be close to the SU(6) relations, as expected. In fact this is what motivated the study of valence degrees of freedom via VQCD. In section 2 the authors address specific issues related to spectroscopy in VQCD. Isgur also presented more general arguments against the idea of boson exchange as a contributor to hyperfine effects. A cornerstone of his discussion is the unifying aspect of OGE in a quark model picture. The authors believe that it is also natural and economical to identify chiral symmetry as the common origin for much of the physics being discussed here. Therefore in section 3 the authors take the opportunity to sketch out an effective theory that may serve as a framework to interpret the numerical results of VQCD

280

Bulk band gaps in divalent hexaborides  

Energy Technology Data Exchange (ETDEWEB)

Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.

Denlinger, Jonathan; Clack, Jules A.; Allen, James W.; Gweon, Gey-Hong; Poirier, Derek M.; Olson, Cliff G.; Sarrao, John L.; Bianchi, Andrea D.; Fisk, Zachary

2002-08-01

 
 
 
 
281

Large, dispersive photoelectron Fermi edge and the electronic structure of YBa2Cu3O6.9 single crystals measured at 20 K  

International Nuclear Information System (INIS)

We have performed angle-integrated photoemission measurements at 20 K on well-oxygenated (Tc=92 K) single crystals of YBa2Cu3O6.9 cleaved n situ, and find a relatively large, resolution-limited Fermi edge which shows large amplitude variations with photon energy, indicative of band-structure final-state effects. Some dispersion is seen even in our angle-integrated measurements. Our best estimate of N(EF) per Cu atom is that it is about 20% that of Cu metal with about a 20-80 mix of Cu 3d and O 2p orbitals. Dispersive and final-state effects are seen throughout the valence bands. The line shapes of the spectra as a function of photon energy are very well reproduced by band-structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peak agree with calculated bands to within ?0.5 eV. We conclude that a Fermi-liquid approach to conductivity is appropriate

282

A study of the valence shell electronic structure and photoionisation dynamics of para-dichlorobenzene and para-bromochlorobenzene  

International Nuclear Information System (INIS)

Highlights: ? Electronic structure and photoionisation dynamics of pDCB and pBCB have been studied. ? Dynamics affected by halogen atom Cooper minimum. ? Many-body effects influence inner valence shell ionisation. - Abstract: The valence shell electronic structure and photoionisation dynamics of para-dichlorobenzene and para-bromochlorobenzene have been investigated both experimentally and theoretically. High resolution photoelectron spectra of the outer valence orbitals have been recorded with HeI radiation and the observed structure has been interpreted using calculated ionisation energies and spectral intensities. The theoretical predictions for the single-hole ionic states due to outer valence ionisation agree satisfactorily with the experimental results. Ionisation from the inner valence orbitals is strongly influenced by many-body effects and the with a particular orbital is spread amongst numerous satellites. Some of the photoelectron bands exhibit vibrational progressions and tentative assignments have been proposed. The photoionisation dynamics of the outer valence orbitals of para-dichlorobenzene have been investigated theoretically by using the continuum multiple scattering approach to calculate photoionisation partial cross-sections and photoelectron anisotropy parameters. The results show that ionisation from some of the orbitals is affected by the Cooper minimum associated with the chlorine atom. Synchrotron radiation has been used to record angle resolved photoelectron spectra of the entire valence shell, for photon energies between threshold and ?100 eV, and these have allowed the corresponding experimental data to be derived. A comparison between the predicted and measured anisotropy parameters confirms the influence of the Cooper minimum in those orbitals related to the chlorine lone-pairs

283

Valence Instability and Superconductivity in Heavy Fermion Systems  

CERN Document Server

Many cerium-based heavy fermion (HF) compounds have pressure-temperature phase diagrams in which a superconducting region extends far from a magnetic quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an enhancement of the superconducting transition temperature was found to coincide with an abrupt valence change, with strong circumstantial evidence for pairing mediated by critical valence, or charge transfer, fluctuations. This pairing mechanism, and the valence instability, is a consequence of a f-c Coulomb repulsion term U_fc in the hamiltonian. While some non-superconducting Ce compounds show a clear first order valence instability, analogous to the Ce alpha-gamma transition, we argue that a weakly first order valence transition may be a general feature of Ce-based HF systems, and both magnetic and critical valence fluctuations may be responsible for the superconductivity in these systems.

Holmes, A T; Miyake, K; Holmes, Alexander T.; Jaccard, Didier; Miyake, Kazumasa

2006-01-01

284

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells  

Directory of Open Access Journals (Sweden)

Full Text Available A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k·p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells.

Alexander Mellor

2013-02-01

285

Valence modulations in CeRuSn  

Science.gov (United States)

CeRuSn exhibits an extraordinary room temperature structure at 300 K with the coexistence of two types of Ce ions, namely trivalent Ce 3+ and intermediate-valent Ce(4-? )+, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the c axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl-type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the c axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L3 absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to our previous report, the new data exhibit small but significant variations as a function of temperature that are consistent with a transition of a fraction of Ce3+ ions to the intermediate valence state, analogous to the ? ?? transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.

Feyerherm, R.; Dudzik, E.; Prokeš, K.; Mydosh, J. A.; Huang, Y.-K.; Pöttgen, R.

2014-07-01

286

Proton-neutron symmetry in valence mirror nuclei  

Science.gov (United States)

Nuclear states of yrast and yrare type have been investigated in pairs of valence mirror nuclei: 110-142Nd, 114-146Gd, 43-51V, 45-53Mn, as well as in some even calcium isotopes with their corresponding N=28 valence mirror isotones. The observed similarities can be related to an approximate proton-neutron symmetry in valence mirror nuclear states.

Yan, J.; Wirowski, R.; von Brentano, P.; Dewald, A.; Gelberg, A.

1990-08-01

287

Proton-neutron symmetry in valence mirror nuclei  

Energy Technology Data Exchange (ETDEWEB)

Nuclear states of yrast and yrare type have been investigated in pairs of valence mirror nuclei: {sup 110}Sn-{sup 142}Nd, {sup 114}Sn-{sup 146}Gd, {sup 43}Ca-{sup 51}V, {sup 45}Ca-{sup 53}Mn, as well as in some even calcium isotopes with their corresponding {ital N}=28 valence mirror isotones. The observed similarities can be related to an approximate proton-neutron symmetry in valence mirror nuclear states.

Yan, J.; Wirowski, R.; von Brentano, P.; Dewald, A.; Gelberg, A. (Institut fuer Kernphysik der Universitaet zu Koeln, 5000 Koeln 41 (Federal Republic of Germany))

1990-08-01

288

Valence photoelectron spectrum of KBr: Effects of electron correlation  

International Nuclear Information System (INIS)

The valence photoelectron spectrum has been measured for molecular KBr. Experimental energies of the main and satellite structures have been compared with the results of ab initio calculations based on molecular orbital theory including configuration and multiconfiguration interaction approaches. Comparison between the experimental KBr spectrum and previously reported Kr valence photoelectron spectrum has also been performed in order to find out if electron correlation is of the same importance in the valence ionized state of KBr as in the corresponding state of Kr

289

Valence quark distributions of the proton from maximum entropy approach  

CERN Document Server

We present an attempt of maximum entropy principle to determine valence quark distributions in the proton at very low resolution scale $Q_0^2$. The initial three valence quark distributions are obtained with limited dynamical information from quark model and QCD theory. Valence quark distributions from this method are compared to the lepton deep inelastic scattering data, and the widely used CT10 and MSTW08 data sets. The obtained valence quark distributions are consistent with experimental observations and the latest global fits of PDFs. Maximum entropy method is expected to be particularly useful in the case where relatively little information from QCD calculation is given.

Wang, Rong

2014-01-01

290

Hole Doping Evolution of the Quasiparticle Band in Models of Strongly Correlated Electrons for the High-T$_c$ Cuprates  

CERN Document Server

Quantum Monte Carlo (QMC) and Maximum Entropy (ME) techniques are used to study the spectral function $A({\\bf p},\\omega)$ of the one band Hubbard model in strong coupling including a next-nearest-neighbor electronic hopping with amplitude $t'/t= -0.35$. These values of parameters are chosen to improve the comparison of the Hubbard model with angle-resolved photoemission (ARPES) data for high-$T_c$ cuprates in the antiferromagnetic insulating regime. A quasiparticle (q.p.) band is clearly observed in the QMC analysis at the temperature of the simulation $T=t/3$, both at and away from half-filling. At a small hole density the quasiparticle dispersion resembles the result in the non-interacting limit $U/t=0$, although with reduced effective hopping amplitudes. Such narrow q.p. band produces a large accumulation of weight in the density of states at the top of the valence band. As the electronic density $$ decreases further away from half-filling, the chemical potential travels through this energy window with a l...

Duffy, D; Haas, S W; Moreo, A; Riera, J A; Dagotto, E; Duffy, Daniel; Nazarenko, Alexander; Haas, Stephan; Moreo, Adriana; Riera, Jose; Dagotto, Elbio

1997-01-01

291

Correlation-induced corrections to the band structure of boron nitride: a wave-function-based approach.  

Science.gov (United States)

We present a systematic study of the correlation-induced corrections to the electronic band structure of zinc-blende BN. Our investigation employs an ab initio wave-function-based local Hamiltonian approach which offers a rigorous theoretical framework for the calculation of the polarization and local charge redistribution effects around an extra electron or hole placed into the conduction or valence bands of semiconducting and insulating materials. Moreover, electron correlations beyond relaxation and polarization can be readily incorporated. The electron correlation treatment is performed on finite clusters. In conducting our study, we make use of localized Wannier functions and embedding potentials derived explicitly from prior periodic Hartree-Fock calculations. The on-site and nearest-neighbor charge relaxations bring corrections of several eV to the Hartree-Fock band gap. Additional corrections are caused by long-range polarization effects. In contrast, the dispersion of the Hartree-Fock bands is marginally affected by electron correlations. Our final result for the fundamental gap of zinc-blende BN compares well with that derived from soft x-ray experiments at the B and N K-edges. PMID:19655849

Stoyanova, A; Hozoi, L; Fulde, P; Stoll, H

2009-07-28

292

Dispersion Forces  

CERN Document Server

In this book, a modern unified theory of dispersion forces on atoms and bodies is presented which covers a broad range of advanced aspects and scenarios. Macroscopic quantum electrodynamics is shown to provide a powerful framework for dispersion forces which allows for discussing general properties like their non-additivity and the relation between microscopic and macroscopic interactions. It is demonstrated how the general results can be used to obtain dispersion forces on atoms in the presence of bodies of various shapes and materials. Starting with a brief recapitulation of volume I, this volume II deals especially with bodies of irregular shapes, universal scaling laws, dynamical forces on excited atoms, enhanced forces in cavity quantum electrodynamics, non-equilibrium forces in thermal environments and quantum friction. The book gives both the specialist and those new to the field a thorough overview over recent results in the field. It provides a toolbox for studying dispersion forces in various contex...

Buhmann, Stefan Yoshi

2012-01-01

293

BCS theory on a flat band lattice  

International Nuclear Information System (INIS)

It is important to study the many-body effects of the electrons on the flat band lattice, since the flatness of the dispersion may lead to instabilities in the presence of interactions. To clarify the effects of flat dispersion for superconductivity, we investigate a simple two-band BCS Hamiltonian, where one of the bands has a flat dispersion, in the mean field approximation. A superconducting gap ? and a critical temperature Tc strongly depend on the pairing coupling constant V. Especially, for small coupling constant, ? and Tc are proportional to V, which are quite different from the single band BCS cases. Higher Tc, compared to the normal BCS system, are realized due to the existence of the flat band

294

Robustness of Quantum Criticality of Valence Fluctuations  

Science.gov (United States)

The mechanism of emergence of robust quantum criticality in Yb- and Ce-based heavy electron systems under pressure is analyzed theoretically. By constructing a minimal model for quasicrystal Yb15Al34Au51 and its approximant, we show that quantum critical points of the first-order valence transition of Yb appear in the ground-state phase diagram with their critical regimes being overlapped to be unified, giving rise to a wide quantum critical regime. This well explains the robust unconventional criticality observed in Yb15Al34Au51 under pressure. We also discuss broader applicability of this mechanism to other Yb- and Ce-based systems such as ?-YbAlB4 showing unconventional quantum criticality.

Watanabe, Shinji; Miyake, Kazumasa

2013-08-01

295

Awaking of ferromagnetism in GaMnN through control of Mn valence  

CERN Document Server

Room temperature ferromagnetism of GaMnN thin films is awaked by a mild hydrogenation treatment of samples synthesized by molecular beam epitaxy. Local environment of Mn atoms is monitored by Mn-L2,3 near edge x-ray absorption fine structure (NEXAFS) technique. Doped Mn ions are present at substitutional sites of Ga both before and after the hydrogenation. No secondary phase can be detected. Major valency of Mn changes from +3 to +2 by the hydrogenation. The present result supports the model that the ferromagnetism occurs when Mn2+ and Mn3+ are coexistent and holes in the mid- gap Mn band mediate the magnetic coupling.

Sonoda, S; Oba, F; Ikeno, H; Hayashi, H; Yamamoto, T; Yuba, Y; Yoshida, K; Aoki, M; Asari, M; Akasaka, Y; Kindo, K; Hori, H; Sonoda, Saki; Tanaka, Isao; Oba, Fumiyasu; Ikeno, Hidekazu; Hayashi, Hiroyuki; Yamamoto, Tomoyuki; Yuba, Yoshihiko; Yoshida, Ken'ichi; Aoki, Masahiko; Asari, Masatoshi; Akasaka, Yoichi; Kindo, Koichi; Hori, Hidenobu

2006-01-01

296

XANES study of rare-earth valency in LRu4P12 (L = Ce and Pr)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES spectrum suggests that Ce is mainly trivalent, but the 4f state strongly hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We find that the metal-insu...

Lee, C. H.; Oyanagi, H.; Sekine, C.; Shirotani, I.; Ishii, M.

2000-01-01

297

On the electrical conductivity for the mixed-valence model with d-f correlations  

International Nuclear Information System (INIS)

The static electrical conductivity of mixed-valence systems is calculated in the model of Matlak and Nolting [Solid State Commun., 47, 11 (1983); Z. Phys., B55, 103 (1984)]. The method takes into account the atomic properties more exactly than those connected with bands, and hence emphasizes the ionic aspect of the problem in some way; indeed, the calculations overestimate the atomic properties. Some results are presented in a graph. It is found that the electrical conductivity depends strongly on temperature and the electron-hole attraction constant

298

Mixed valence europium nitridosilicate Eu2SiN3.  

Science.gov (United States)

The mixed valence europium nitridosilicate Eu(2)SiN(3) has been synthesized at 900 degrees C in welded tantalum ampules starting from europium and silicon diimide Si(NH)(2) in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a = 542.3(11) pm, b = 1061.0(2) pm, c = 1162.9(2) pm, Z = 8, 767 independent reflections, 37 parameters, R1 = 0.017, wR2 = 0.032). Eu(2)SiN(3) is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN(4) tetrahedra running parallel [100] with a maximum stretching factor f(s) = 1.0. The compound is isostructural with Ca(2)PN(3) and Rb(2)TiO(3), and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu(2+) and Eu(3+), respectively. (151)Eu Mössbauer spectroscopy of Eu(2)SiN(3) differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu(2+) and Eu(3+) signals over the whole temperature range. The Eu(2+) site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 mu(B)/f.u. and a small contribution of Eu(3+), in accordance with Eu(2+) and Eu(3+) in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T(C) = 24 K. DFT calculations of Eu(2)SiN(3) reveal a band gap of approximately 0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu(2+) and Eu(3+). PMID:19610643

Zeuner, Martin; Pagano, Sandro; Matthes, Philipp; Bichler, Daniel; Johrendt, Dirk; Harmening, Thomas; Pöttgen, Rainer; Schnick, Wolfgang

2009-08-12

299

Band anticrossing in ZnOSe highly mismatched alloy  

Science.gov (United States)

ZnOxSe1-x layers with x ? 1.35% were studied by photoreflectance at 80 K. Careful analysis of the PR spectra allowed the identification of the optical transitions from the valence band to the E- and E+ subbands originating from the band anticrossing interaction between the resonant oxygen level and the conduction band of the ZnSe host. In addition, it was possible to resolve a strain-induced splitting of the valence band into the heavy- and light-hole subbands. The strain changes from compressive to tensile with increasing oxygen concentration for these ZnOxSe1-x layers grown on a GaAs substrate.

Welna, Monika; Kudrawiec, Robert; Nabetani, Yu; Walukiewicz, Wladyslaw

2014-07-01

300

Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4.  

Science.gov (United States)

We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) Å, c = 32.4330(6) Å, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond valence sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, while Eu(2) has an average valence of ??+2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mössbauer and magnetization measurements. The Eu(3+) components donate electrons into the conduction bands that are mainly composed of Bi 6px and 6py states. Consequently, the material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping. PMID:25314008

Zhai, Hui-Fei; Zhang, Pan; Wu, Si-Qi; He, Chao-Yang; Tang, Zhang-Tu; Jiang, Hao; Sun, Yun-Lei; Bao, Jin-Ke; Nowik, Israel; Felner, Israel; Zeng, Yue-Wu; Li, Yu-Ke; Xu, Xiao-Feng; Tao, Qian; Xu, Zhu-An; Cao, Guang-Han

2014-10-29

 
 
 
 
301

Studies of valence-quark shadowing at HERA  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We propose to measure valence-quark shadowing by observing charged pion productions in electron-nucleus scattering at HERA. Studies of shadowing phenomena in the valence-quark distribution could be useful in discriminating among various models, which produce similar results in the $F_2$ shadowing.

Kumano, S.

1995-01-01

302

Valence instabilities as a possible source of actinide system inconsistencies  

International Nuclear Information System (INIS)

The presence of a mixed-valence state in light actinides appears evident from the crystal structures of certain U, Np, and Pu phases. As supporting evidence, the physical property response of these actinide elements (and some of their alloys) is compared with that of rare-earth metallic compounds known to have an unstable valence. Impurities may stabilize an intermediate (different) valence state locally in rare-earth compounds in the presence of the valence state of the bulk phase. Impurity elements from different periodic table groupings may likewise stabilize different intermediate valence states in light actinide elements, thus contributing to inconsistencies in results reported by different experimentalists. Any model (theory) advanced for explaining the physical property behavior of U, Np, and Pu may also require consideration of a configurational limit. A phenomenological connection could exist between a martensitic transformation and the fluctuation temperature in both rare earth and actinide systems

303

A study of the valence shell spectroscopic and thermodynamic properties of trifluoronitrosomethane cations  

International Nuclear Information System (INIS)

Highlights: ? Fragmentation processes in CF3NO have been studied using mass spectrometry. ? Singly charged atomic fragments have been observed. ? Experimental appearance energies have been compared to thermochemical estimates. ? Hartree Fock transition energies and oscillator strengths have been calculated. - Abstract: A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in trifluoronitrosomethane (CF3NO) as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range ?10–42 eV, and appearance energies have been determined for 10 fragment ions. At high excitation energies, singly charged atomic fragments have been observed. For the main dissociation channels, leading to the formation of NO+, CF2+ or CF3+, the experimental appearance energies have been compared with thermochemical estimates, and a satisfactory agreement has been found. Structure observed in the total ion yield curve has been interpreted with the aid of excited state transition energies and oscillator strengths obtained in a time-dependent Hartree Fock calculation. The theoretical results show that configuration interaction strongly affects many of the valence states. A HeI excited photoelectron spectrum of CF3NO has been measured and the orbital ionisation energies have been compared with theoretical values computed using the Outer Valence Green’s Function approach. A large Franck–Condon gap is observed between the 12a? (n-) and the 11a? state bands, in accord with the calculated vertical ionisation energies of 10.87 and 16.32 eV for the 12a? (n?) and the 11a? (n+) orbitals, respectively. In the ion yield curve, the corresponding energy range is strongly influenced by autoionising valence states.

304

Substrate-controlled band positions in CH3NH3PbI3 perovskite films.  

Science.gov (United States)

Using X-ray and ultraviolet photoelectron spectroscopy, the surface band positions of solution-processed CH3NH3PbI3 perovskite thin films deposited on an insulating substrate (Al2O3), various n-type (TiO2, ZrO2, ZnO, and F:SnO2 (FTO)) substrates, and various p-type (PEDOT:PSS, NiO, and Cu2O) substrates are studied. Many-body GW calculations of the valence band density of states, with spin-orbit interactions included, show a clear correspondence with our experimental spectra and are used to confirm our assignment of the valence band maximum. These surface-sensitive photoelectron spectroscopy measurements result in shifting of the CH3NH3PbI3 valence band position relative to the Fermi energy as a function of substrate type, where the valence band to Fermi energy difference reflects the substrate type (insulating-, n-, or p-type). Specifically, the insulating- and n-type substrates increase the CH3NH3PbI3 valence band to Fermi energy difference to the extent of pinning the conduction band to the Fermi level; whereas, the p-type substrates decrease the valence band to Fermi energy difference. This observation implies that the substrate's properties enable control over the band alignment of CH3NH3PbI3 perovskite thin-film devices, potentially allowing for new device architectures as well as more efficient devices. PMID:25209217

Miller, Elisa M; Zhao, Yixin; Mercado, Candy C; Saha, Sudip K; Luther, Joseph M; Zhu, Kai; Stevanovi?, Vladan; Perkins, Craig L; van de Lagemaat, Jao

2014-10-28

305

Qualitative feature of the low-lying spectrum of intrashell states of 4-valence-electron atoms derived from symmetry consideration  

CERN Document Server

Inherent nodal surfaces existing in the wavefunctions of intrashell states of 4-valence-electron atoms have been investigated. The decisive effect of these surfaces has been demonstrated, the ordering of low-lying levels has been predicted, a primary classification scheme has been proposed, the existence of three rotation bands has been suggested.

Bao, C G

1998-01-01

306

A study of the valence shell electronic structure and photoionisation dynamics of meta-dichlorobenzene and meta-bromochlorobenzene  

International Nuclear Information System (INIS)

Highlights: ? Electronic structure and photoionisation dynamics of mDCB and mBCB have been studied. ? Dynamics affected by halogen atom Cooper minimum. ? Many-body effects influence inner valence shell ionisation. - Abstract: A combined experimental and theoretical investigation has been performed to study the valence shell electronic structure and photoionisation dynamics of meta-dichlorobenzene and meta-bromochlorobenzene. Angle resolved photoelectron spectra of meta-dichlorobenzene have been recorded using synchrotron radiation in the photon energy range from close to threshold to 100 eV. These have enabled photoelectron anisotropy parameters and branching ratios to be derived. The continuum multiple scattering approach has been employed to calculate photoionisation partial cross-sections and photoelectron angular distributions of the outer valence orbitals of meta-dichlorobenzene. A comparison between the corresponding experimental and theoretical results has demonstrated that ionisation from some of the orbitals is influenced by the Cooper minimum associated with the chlorine atom. Ionisation energies and spectral intensities evaluated with the third-order algebraic diagrammatic construction approximation for the one-particle Green’s function and the outer valence Green’s function approaches have allowed the features observed in the complete valence shell photoelectron spectra of meta-dichlorobenzene and meta-bromochlorobenzene to be interpreted. Many-body phenomena strongly influence ionisation from the inner valence orbitals and lead to the intensity associated with a particular orbital being redistributed amongst numerous satellites. High resolution photoelectron spectra have been recorded with HeI radiation. Vibrational structure has been observed in some of the photoelectron bands and tentative assignments have been proposed

307

Excited-state dynamics of highly strained 1,2,3-tri-tert-butylanthraquinone undergoing valence isomerization  

Science.gov (United States)

Sub-picosecond laser photolysis of the title compound (TBAQ) reveals that a broad absorption band (at 450-750 nm) due to the lowest excited singlet state ( 1TBAQ ?) decreases with a very short lifetime of 1.7 ps and then an absorption band due to the lowest excited triplet state ( 3TBAQ ?) remains as the residual absorption. The fast decay channel of 1TBAQ ? is probably ascribed to the valence isomerization to yield a hemi-Dewar type isomer reported by Miki et al., considering the distorted molecular structure of TBAQ estimated by molecular mechanics calculations.

Nakayama, Toshihiro; Amijima, Yutaka; Ushida, Kiminori; Hamanoue, Kumao

1996-08-01

308

Substrate-induced band gap renormalization in semiconducting carbon nanotubes.  

Science.gov (United States)

The quasiparticle band gaps of semiconducting carbon nanotubes (CNTs) supported on a weakly-interacting hexagonal boron nitride (h-BN) substrate are computed using density functional theory and the GW Approximation. We find that the direct band gaps of the (7,0), (8,0) and (10,0) carbon nanotubes are renormalized to smaller values in the presence of the dielectric h-BN substrate. The decrease in the band gap is the result of a polarization-induced screening effect, which alters the correlation energy of the frontier CNT orbitals and stabilizes valence band maximum and conduction band minimum. The value of the band gap renormalization is on the order of 0.25 to 0.5 eV in each case. Accounting for polarization-induced band gap changes is crucial in comparing computed values with experiment, since nanotubes are almost always grown on substrates. PMID:24402238

Lanzillo, Nicholas A; Kharche, Neerav; Nayak, Saroj K

2014-01-01

309

Valence density of states of group IVA transition-metal dichalcogenides  

International Nuclear Information System (INIS)

The valence densities of states (VDOS) of the IVA transition-metal dichalcogenides ZrS2, ZrSe2, TiSe2 are calculated using the Gilat-Raubenheimer method and analysed in detail VDOS based on quadratic Lagrangian interpolation (QLI) of the energies evaluated in the final self-consistent symmetrised OPW (SCSOPW) potential at 131 symmetry independent k points are found to show close resemblance to XPS measurements and recent LCAO VDOS. Using an analysis based on the division of the SCSOPW QLI VDOS into partial VDOS from individual bands we find that four pairs of valence bands (1-2, 3-4, 5-6 and 7-8) give rise to four main peaks of SCSOPW QLI VDOS. A similar analysis shows that the use of the Slater-Koster interpolation caused some artificial deep valleys into the earlier SCSOPW LCAO VDOS. The methods used to calculate SCSOPW QLI VDOS and SCSOPW LCAO VDOS are also described. (author)

310

Borderline class II/III ligand-centered mixed valency in a porphyrinic molecular rectangle.  

Science.gov (United States)

A molecular rectangle of the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL are cofacially aligned [5,15-bis(4-ethynylpyridyl)-10,20-bis(n-hexyl)-porphyrinato]zinc ligands, has been examined via electrochemical, spectroelectrochemical, and electronic Stark effect methods. The rectangle displays three electrochemically accessible reductions assigned as LL based. The singly reduced rectangles are part of an unusual class of mixed-valence complexes where cofacial ligands, in this case porphyrins, comprise the degenerate redox centers. Absorption spectra for the singly reduced rectangle show two intense and narrow absorption bands in the near-infrared (NIR) region; the lower energy band is assigned as an intervalence transition. Time-dependent density functional theory electronic structure calculations support the assignment. Curiously, the transition displays a non-Marcus-type solvent dependence. NIR region electroabsorbance measurements of the singly reduced rectangle reveal a small but readily measurable dipole moment change of 0.56 +/- 0.05 eA. On the basis of spectroelectrochemical and electroabsorption measurements, the singly reduced rectangle is assigned as a borderline class II/class III mixed-valence species. PMID:16060631

Dinolfo, Peter H; Lee, Suk Joong; Coropceanu, Veaceslav; Brédas, Jean-Luc; Hupp, Joseph T

2005-08-01

311

Controlling the band gap energy of cluster-assembled materials.  

Science.gov (United States)

Cluster-assembled materials combine the nanoscale size and composition-dependent properties of clusters, which have highly tunable magnetic and electronic properties useful for a great variety of potential technologies. To understand the emergent properties as clusters are assembled into hierarchical materials, we have synthesized 23 cluster-assembled materials composed of As7(3-)-based motifs and different countercations and measured their band gap energies. We found that the band gap energy varies from 1.09 to 2.21 eV. In addition, we have carried out first principles electronic structure studies to identify the physical mechanisms that enable control of the band gap edges of the cluster assemblies. The choice of counterion has a profound effect on the band gap energy in ionic cluster assemblies. The top of the valence band is localized on the arsenic cluster, while the conduction band edge is located on the alkali metal counterions. Changing the counterion changes the position of the conduction band edge, enabling control of the band gap energy. We can also vary the architecture of the ionic solid by incorporating cryptates as counterions, which provide charge but are separated from the clusters by bulky ligands. Higher dimensionality typically decreases the band gap energy through band broadening; however band gap energies increased upon moving from zero-dimensional (0D) to two-dimensional (2D) assemblies. This is because internal electric fields generated by the counterion preferentially stabilize the adjacent lone pair orbitals that mark the top of the valence band. Thus, the choice of the counterion can control the position of the conduction band edge of ionic cluster assemblies. In addition, the dimensionality of the solid via internal electric fields can control the valence band edge. Through covalently linking arsenic clusters into composite building blocks, we have also been able to tune the band gap energy. We used a theoretical description based on cluster orbital theory to provide microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy. Also, we have shown how dimeric linkers can be used to control the band gap energy. Lastly, we also investigated the effects of charge transfer complexes of M(CO)3 on the band gap energy. PMID:23734558

Mandal, Sukhendu; Reber, Arthur C; Qian, Meichun; Weiss, Paul S; Khanna, Shiv N; Sen, Ayusman

2013-11-19

312

Association of limited valence patchy particles in two dimensions  

International Nuclear Information System (INIS)

We investigate theoretically the phase behavior of particles with limited valence in two dimensions, by solving the first-order Wertheim theory form. As previously found for three dimensions, in two dimensions also the valence has a strong impact on the phase diagram, controlling the location of the gas-liquid coexistence. On decreasing the valence, the critical density and temperature decrease while the region of gas-liquid instability shrinks and vanishes. At low temperatures, the system reaches its ground state with particles forming a fully bonded network which spans the system.

313

Association of limited valence patchy particles in two dimensions  

Science.gov (United States)

We investigate theoretically the phase behavior of particles with limited valence in two dimensions, by solving the first-order Wertheim theory form. As previously found for three dimensions, in two dimensions also the valence has a strong impact on the phase diagram, controlling the location of the gas-liquid coexistence. On decreasing the valence, the critical density and temperature decrease while the region of gas-liquid instability shrinks and vanishes. At low temperatures, the system reaches its ground state with particles forming a fully bonded network which spans the system.

Tartaglia, Piero; Sciortino, Francesco

2010-03-01

314

Association of limited valence patchy particles in two dimensions  

Energy Technology Data Exchange (ETDEWEB)

We investigate theoretically the phase behavior of particles with limited valence in two dimensions, by solving the first-order Wertheim theory form. As previously found for three dimensions, in two dimensions also the valence has a strong impact on the phase diagram, controlling the location of the gas-liquid coexistence. On decreasing the valence, the critical density and temperature decrease while the region of gas-liquid instability shrinks and vanishes. At low temperatures, the system reaches its ground state with particles forming a fully bonded network which spans the system.

Tartaglia, Piero; Sciortino, Francesco [Dipartimento di Fisica, Universita di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185 Rome (Italy)

2010-03-17

315

Intersite interactions and susceptibility in mixed valence systems  

International Nuclear Information System (INIS)

This paper considers the effect of intersite processes on the susceptibility in mixed valence system. The method of thermodynamical perturbation used in this paper can also be generalized to study other properties of mixed valence system. The general formula of partition function of two-site interactions for the mixed valence system is given. The numerical calculations show that the intersite interaction is large enough to explain the minimum of susceptibility discovered in experiments. The different types of our theoretical curves predict that the susceptibility should exhibit a rich variety of behaviour at low temperature for various materials. (author)

316

Band to band tunneling in III-V semiconductors: Implications of complex band structure, strain, orientation, and off-zone center contribution  

Energy Technology Data Exchange (ETDEWEB)

In this paper, we use a tight binding Hamiltonian with spin orbit coupling to study the real and complex band structures of relaxed and strained GaAs. A simple d orbital on-site energy shift coupled with appropriate scaling of the off-diagonal terms is found to correctly reproduce the band-edge shifts with strain. Four different ?100? strain combinations, namely, uniaxial compressive, uniaxial tensile, biaxial compressive, and biaxial tensile strain are studied, revealing rich valence band structure and strong relative orientation dependent tunneling. It is found that complex bands are unable to provide unambiguous tunneling paths away from the Brillouin zone center. Tunneling current density distribution over the Brillouin zone is computed using non-equilibrium Green's function approach elucidating a physical picture of band to band tunneling.

Majumdar, Kausik, E-mail: kausik.majumdar@sematech.org [SEMATECH, 257 Fuller Road, STE 2200, Albany, New York 12203 (United States)

2014-05-07

317

Band to band tunneling in III-V semiconductors: Implications of complex band structure, strain, orientation, and off-zone center contribution  

International Nuclear Information System (INIS)

In this paper, we use a tight binding Hamiltonian with spin orbit coupling to study the real and complex band structures of relaxed and strained GaAs. A simple d orbital on-site energy shift coupled with appropriate scaling of the off-diagonal terms is found to correctly reproduce the band-edge shifts with strain. Four different ?100? strain combinations, namely, uniaxial compressive, uniaxial tensile, biaxial compressive, and biaxial tensile strain are studied, revealing rich valence band structure and strong relative orientation dependent tunneling. It is found that complex bands are unable to provide unambiguous tunneling paths away from the Brillouin zone center. Tunneling current density distribution over the Brillouin zone is computed using non-equilibrium Green's function approach elucidating a physical picture of band to band tunneling

318

Valence, magnétisme et conduction dans les composés à valence intermédiaire : Le cas SmB6  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Le composé SmB6 est un exemple typique de la physique étrange qui peut résulter d'un équilibre de valence. La première configuration du Samarium (Sm2+) correspond à un état isolant non magnétique alors que la seconde (Sm3+) donnerait théoriquement un état magnétique et conducteur. Des mesures de microcalorimétrie sous pression ont permis d'établir le diagramme de phase magnétique de SmB6 : une nouvelle phase magnétique ordonnée à longue distance a été mise en évidence pour...

Derr, Julien

2006-01-01

319

Valence, magnetism and conduction in the intermediate valence compounds: the case SmB6  

International Nuclear Information System (INIS)

In some rare earth based compounds, the 4f level is situated so close to the Fermi level that the valence of the compound can become intermediate between two integer values. The so called 'intermediate valence' compound of Samarium hexaboride (SmB6) is one typical example of the exciting physics which can result from this quantum equilibrium between two valence configurations. The first configuration (Sm2+) corresponds to an insulating and non magnetic state whereas the second one (Sm3+) would theoretically give a magnetic and metallic ground state. This dissertation deals with the influence of pressure on this equilibrium. Specific heat measurements under pressure evidenced a new long range magnetic ordering for pressures higher than pc ? 10 GPa. On another hand, transport measurements measured for the first time in good conditions of hydrostatics found a reliable and reproducible critical pressure for the insulator to metal transition equal to pc. The phase diagram of SmB6 is now well known and the observation for the first time of a magnetic anomaly in the high pressure resistivity curves certifies that the onset of the magnetic phase really coincide with the closure of the gap. This change at the critical pressure pc is discussed in a general frame taking into account the Kondo lattice temperature as a key parameter for the renormalization of the wavefunction from one integer configuration to the other whereas the valence itself is still intermediate. This general idea seems to be valid also for other systems studied in this dissertation like SmS or TmSe and could even be valid for more general cases (Ytterbium, Cerium). In the same time, resistivity measurements under uniaxial stress were undertaken. The result is a strong anisotropy effect observed on the pressure dependence of the residual resistivity in the compound SmB6. The comparison with the transport under hydrostatic conditions enables us to consider a new idea for the nature of the gap, considering only one anisotropic gap which would present anyway two energy scales. (author)

320

Photodissociation of carbon dioxide in singlet valence electronic states. II. Five state absorption spectrum and vibronic assignment  

CERN Document Server

The absorption spectrum of CO$_2$ in the wavelength range 120\\,nm --- 160\\,nm is analyzed by means of quantum mechanical calculations performed using vibronically coupled PESs of five singlet valence electronic states and the coordinate dependent transition dipole moment vectors. The thermally averaged spectrum, calculated for T=190\\,K via Boltzmann averaging of optical transitions from many initial rotational states, accurtely reproduces the experimental spectral envelope, consisting of a low and a high energy band, the positions of the absorption maxima, their FWHMs, peak intensities, and frequencies of diffuse structures in each band. Contributions of the vibronic interactions due to Renner-Teller coupling, conical intersections, and the Herzberg-Teller effect are isolated and the calculated bands are assigned in terms of adiabatic electronic states. Finally, diffuse structures in the calculated bands are vibronically assigned using wave functions of the underlying resonance states. It is demonstrated that...

Grebenshchikov, Sergy Yu

2013-01-01

 
 
 
 
321

Inelastic neutron scattering in valence fluctuation compounds  

Energy Technology Data Exchange (ETDEWEB)

The valence fluctuation compounds are rare earth intermetallics where hybridization of the nearly-localized 4f electrons with the conduction electrons leads to incorporation of the 4f's into the itinerant states. This hybridization slows down the conduction electrons and hence gives them a heavy effective mass, justifying application of the term 'heavy Fermion' (HF) to these materials. During the project period, we grew large single crystals of several such compounds and measured their properties using both standard thermodynamic probes and state-of-the-art inelastic neutron scattering. We obtained three main results. For the intermediate valence compounds CePd{sub 3} and YbAl{sub 3}, we showed that the scattering of neutrons by the fluctuations of the 4f magnetic moment does not have the momentum dependence expected for the itinerant heavy mass state; rather, the scattering is more typical of a localized spin fluctuation. We believe that incoherent scattering localizes the excitation. For the heavy Fermion compound Ce(Ni{sub 0.935}Pd{sub 0.065}){sub 2}Ge{sub 2}, which sits at a T = 0 critical point for transformation into an antiferromagnetic (AF) phase, we showed that the scattering from the AF fluctuations does not exhibit any of the divergences that are expected at a phase transition. We speculate that alloy disorder profoundly suppresses the growth of the fluctuating AF regions, leading to short range clusters rather than regions of infinite size. Finally, we explored the applicability of key concepts used to describe the behavior of rare earth heavy Fermions to uranium based HF compounds where the 5f electrons are itinerant as opposed to localized. We found that scaling laws relating the spin fluctuation energy measured in neutron scattering to the low temperature specific heat and susceptibility are valid for the uranium compounds, once corrections are made for AF fluctuations; however, the degeneracy of the high temperature moment is smaller than expected for rare-earth-like Hund's rule behavior, essentially because the orbital moment is suppressed for itinerant 5f electrons. We also found that the standard local-moment-based theory of the temperature dependence of the specific heat, susceptibility and neutron scattering fails badly for URu{sub 2}Zn{sub 20} and UCo{sub 2}Zn{sub 20}, even though the theory is phenomenally successful for the closely related rare earth compound YbFe{sub 2}Zn{sub 20}. Both these results highlight the distinction between the itineracy of the 5f's and the localization of the 4f's. It is our hope that these results are sufficiently significant as to stimulate deeper investigation of these compounds.

Jon M Lawrence

2011-02-15

322

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.

323

On the valence and lattice parameter in cerium intermetallic compounds  

International Nuclear Information System (INIS)

Experimental data concerning valence and lattice parameter in cerium intermetallic compounds are discussed using the concepts of electronic charge screening and charge transfer between the rare earth and the transition element. (author)

324

Stimulus affective valence reverses spatial compatibility effect  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english In spatial compatibility tasks, the Reaction Time to right-side stimuli is shorter for right key responses (compatible condition) than for left key responses (incompatible condition) and vice-versa for left-side stimuli. Similar results have been found when the stimulus location is not relevant for [...] response selection, such as in the Simon task. The Simon effect is the difference between the reaction times for non-corresponding and corresponding conditions. The Simon effect and its variants may be modulated by using emotional stimuli. However, until now, no work has studied how the affective valence of a stimulus influences spatial compatibility effects along the horizontal dimension. The present study investigated this issue by using small lateralized figures of soccer team players as stimuli. In the experiment, a compatible or incompatible response was chosen according to the team shirt. In one block, for the Favorite team, the volunteers had to press the key on the same side as the stimulus hemifield but the opposite-side key for the Rival team. In the other block, a reverse code had to be used. Fourteen right-handed volunteers were tested. Mean reaction times were subjected to analysis of variance with the following variables: Preference (Favorite/Rival), Hemifield (Left/Right), and Response Key (Left/Right). A three-way interaction was found (F1,13 = 6.60, p = .023), showing that the spatial compatibility effects depended on Preference. The Favorite team player elicited the usual spatial compatibility pattern, but for the Rival team player, the reverse effect was found, with incompatible responses being faster than compatible responses. We propose that this modulation may result from approach/avoidance reactions to the Favorite and Rival teams, respectively. Moreover, we suggest as a corollary that the classic spatial compatibility task is a powerful tool for investigating approach/avoidance effects.

Erick Francisco Quintas, Conde; Fernanda, Jazenko; Roberto Sena, Fraga Filho; Daniella Harth da, Costa; Nelson, Torro-Alves; Mikael, Cavallet; Luiz G, Gawryszewski.

2011-06-01

325

Stimulus affective valence reverses spatial compatibility effect  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english In spatial compatibility tasks, the Reaction Time to right-side stimuli is shorter for right key responses (compatible condition) than for left key responses (incompatible condition) and vice-versa for left-side stimuli. Similar results have been found when the stimulus location is not relevant for [...] response selection, such as in the Simon task. The Simon effect is the difference between the reaction times for non-corresponding and corresponding conditions. The Simon effect and its variants may be modulated by using emotional stimuli. However, until now, no work has studied how the affective valence of a stimulus influences spatial compatibility effects along the horizontal dimension. The present study investigated this issue by using small lateralized figures of soccer team players as stimuli. In the experiment, a compatible or incompatible response was chosen according to the team shirt. In one block, for the Favorite team, the volunteers had to press the key on the same side as the stimulus hemifield but the opposite-side key for the Rival team. In the other block, a reverse code had to be used. Fourteen right-handed volunteers were tested. Mean reaction times were subjected to analysis of variance with the following variables: Preference (Favorite/Rival), Hemifield (Left/Right), and Response Key (Left/Right). A three-way interaction was found (F1,13 = 6.60, p = .023), showing that the spatial compatibility effects depended on Preference. The Favorite team player elicited the usual spatial compatibility pattern, but for the Rival team player, the reverse effect was found, with incompatible responses being faster than compatible responses. We propose that this modulation may result from approach/avoidance reactions to the Favorite and Rival teams, respectively. Moreover, we suggest as a corollary that the classic spatial compatibility task is a powerful tool for investigating approach/avoidance effects.

Erick Francisco Quintas, Conde; Fernanda, Jazenko; Roberto Sena, Fraga Filho; Daniella Harth da, Costa; Nelson, Torro-Alves; Mikael, Cavallet; Luiz G, Gawryszewski.

326

Photonic band structure and omnidirectional band gap in anisotropic superlattice  

Energy Technology Data Exchange (ETDEWEB)

We investigate theoretically the photonic band structure of one-dimensional superlattices (SL) composed of alternating anisotropic layers with their principal axes oriented at arbitrary directions. The dispersion relation of second order is calculated analytically by using the 4 x 4 matrix method which is based on the boundary conditions of the electric and magnetic fields at each interface. It is shown that such structures can exhibit coupled electromagnetic modes between transverse magnetic TM and transverse electric TE modes, and dispersion curves that do not exist in superlattices composed only of isotropic layers. For a given value of the wave vector k {sub ?} (parallel to the layers), the dispersion curves (frequency ?) versus k {sub B} (where k {sub B} is the Bloch wave vector of the periodic system along the axis of the superlattice) are illustrated. Specific applications of these results are given for the case of the biaxial superlattice. We show that with an appropriate choice of the superlattice parameters an absolute (or omnidirectional) band gap for these coupled electromagnetic waves can be obtained. The band gap width depends on the anisotropic parameters of the media forming the SL. (author)

Ouchani, N.; Bria, D.; Nougaoui, A. [Laboratoire de Dynamique et d' Optique des Materiaux, Departement de Physique, Faculte des Sciences, Universite Mohamed I, B.P. 524, 60000 Oujda (Morocco); Djafari-Rouhani, B. [Laboratoire de Dynamique et Structure des Materiaux Moleculaires, UMR CNRS 8024, UFR de Physique, Universite de Lille 1, 59655 Villeneuve d' Ascq (France)

2006-06-15

327

Phenomenology of dihadron FF: Collinear extraction of the valence transversities  

International Nuclear Information System (INIS)

In this paper, we propose an extraction of the valence transversity parton distributions. Based on an analysis of pion-pair production in deep-inelastic scattering off transversely polarized targets, this extraction of transversity is performed in the framework of collinear factorization. The recently released data for proton and deuteron targets at HERMES and COMPASS allow for a flavor separation of the valence transversities, for which we give a complete statistical study.

328

Space-Valence Priming with Subliminal and Supraliminal Words  

Digital Repository Infrastructure Vision for European Research (DRIVER)

To date it is unclear whether (1) awareness-independent non-evaluative semantic processes influence affective semantics and whether (2) awareness-independent affective semantics influence non-evaluative semantic processing. In the current study, we investigated these questions with the help of subliminal (masked) primes and visible targets in a space-valence across-category congruence effect. In line with (1), we found that subliminal space prime words influenced valence classification of sup...

Ansorge, Ulrich; Khalid, Shah; Ko?nig, Peter

2013-01-01

329

Fluctuation valency in inorganic compounds of rare earth elements  

International Nuclear Information System (INIS)

The possibility to display the intermediate valence based on crystallochemical and chemical properties of big group of inorganic compounds of rare earth elements was considered. The classification on groups of periodic table was carried out. The obtained regularities were confirmed by means of electron paramagnetic resonance, L11-spectroscopy of absorption spectrum, X-ray electron analysis, neutron diffraction analysis, and Moessbauer spectrometry. The influence of synthesis conditions, including thermal treatment, pressure, and volume on intermediate valence was considered as well.

330

Fine structure and energy spectrum of exciton in direct band gap cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The influence of the cubic structure on the energy spectrum of direct exciton is investigated, using the new method suggested by Nguyen Van Hieu and co-workers. Explicit expressions of the exciton energy levels 1S, 2S and 2P are derived. A comparison with the experiments and the other theory is done for ZnSe. (author). 10 refs, 1 fig., 2 tabs

331

Explaining the effect of event valence on unrealistic optimism.  

Science.gov (United States)

People typically exhibit 'unrealistic optimism' (UO): they believe they have a lower chance of experiencing negative events and a higher chance of experiencing positive events than does the average person. UO has been found to be greater for negative than positive events. This 'valence effect' has been explained in terms of motivational processes. An alternative explanation is provided by the 'numerosity model', which views the valence effect simply as a by-product of a tendency for likelihood estimates pertaining to the average member of a group to increase with the size of the group. Predictions made by the numerosity model were tested in two studies. In each, UO for a single event was assessed. In Study 1 (n = 115 students), valence was manipulated by framing the event either negatively or positively, and participants estimated their own likelihood and that of the average student at their university. In Study 2 (n = 139 students), valence was again manipulated and participants again estimated their own likelihood; additionally, group size was manipulated by having participants estimate the likelihood of the average student in a small, medium-sized, or large group. In each study, the valence effect was found, but was due to an effect on estimates of own likelihood, not the average person's likelihood. In Study 2, valence did not interact with group size. The findings contradict the numerosity model, but are in accord with the motivational explanation. Implications for health education are discussed. PMID:19444704

Gold, Ron S; Brown, Mark G

2009-05-01

332

Pressure-induced valence change in YbAl3: a combined high pressure inelastic x-ray scattering and theoretical investigation  

Energy Technology Data Exchange (ETDEWEB)

High resolution x-ray absorption (XAS) experiments in the partial fluorescence yield mode (PFY) and resonant inelastic x-ray emission (RXES) measurements under pressure were performed on the intermediate valence compound YbAl{sub 3} up to 38 GPa. The results of the Yb L{sub 3} PFY-XAS and RXES studies show a smooth valence increase in YbAl{sub 3} from 2.75 to 2.93 at ambient to 38 GPa. In-situ angle dispersive synchrotron high pressure x-ray diffraction experiments carried out using a diamond cell at room temperature to study the equation of state showed the ambient cubic phase stable up to 40 GPa. The results obtained from self-interaction corrected local spin density functional calculations to understand the pressure effect on the Yb valence and compressibility are in good agreement with the experimental results.

Bauer, E D [Los Alamos National Laboratory; Kumar, R S [UNIV OF NV; Svane, A [UNIV OF AARHUS; Vaitheeswaran, G [ROYAL INST. OF TECHNOLOGY; Nicol, M F [UNIV OF NV; Kanchana, V [ROYAL INST OF TECHNOLOGY; Hu, M [HPCAT; Cornelius, A L [UNIV OF NV

2008-01-01

333

Possible charge-density wave, superconductivity, and f-electron valence instability in EuBiS2F  

Science.gov (United States)

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, EuBiS2F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi6p states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ˜50 times, owing to the significant hybridizations between Eu4f and Bi6p electrons, as verified by band-structure calculations. Thus EuBiS2F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and f-electron valence instability.

Zhai, Hui-Fei; Tang, Zhang-Tu; Jiang, Hao; Xu, Kai; Zhang, Ke; Zhang, Pan; Bao, Jin-Ke; Sun, Yun-Lei; Jiao, Wen-He; Nowik, I.; Felner, I.; Li, Yu-Ke; Xu, Xiao-Feng; Tao, Qian; Feng, Chun-Mu; Xu, Zhu-An; Cao, Guang-Han

2014-08-01

334

Valence shell photoelectron spectroscopy of pyrene and fluorene: photon energy dependence in the far-ultraviolet region.  

Science.gov (United States)

Inner and outer valence photoelectron spectra (PES) of pyrene and fluorene, two members of the polycyclic aromatic hydrocarbon (PAH) family, were recorded with a high-resolution synchrotron photoelectron spectrometer. Relative photoelectron emission cross sections were measured at photon energies between 15 and 40 eV. Several bands observed in the experimental PES were assigned with the help of OVGF/cc-pVDZ calculations. The first ionization potentials were estimated to be 7.436 ± 0.015 and 7.944 ± 0.055 eV for pyrene and fluorene, respectively. Photoelectron emission cross sections show a clear difference in trend for inner (?-dominated) and outer (?-dominated) bands. Contrary to the expectation from the trend observed in benzene, the inner bands significantly dominate in the photon energy region from 15 to 27 eV and then are found to contribute uniformly. A pronounced peak in the cross sections is observed at a photon energy of approximately 17 eV for both molecules (irrespective of the difference in symmetry and structure), particularly for the inner valence bands. The feature is observed to be independent of the details of the molecular orbital associated with the outgoing electron. These observations are correlated to a collective excitation in the far-ultraviolet region. PMID:24708028

Mishra, Preeti Manjari; Avaldi, Lorenzo; Bolognesi, Paola; Prince, Kevin C; Richter, Robert; Kadhane, Umesh R

2014-05-01

335

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

336

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

Energy Technology Data Exchange (ETDEWEB)

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{sup ?}) 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, Hg{sub 1?x}Cd{sub x}Te, and In{sub 1?x}Ga{sub x}As{sub y}P{sub 1?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.

Dey, Anup, E-mail: a-dey2002@yahoo.com [Electronics and Communication Engineering Department, Kalyani Government Engineering College, Kalyani 741235 (India); Maiti, Biswajit [Physics Department, Kalyani Government Engineering College, Kalyani 741235 (India); Chanda, Debasree [Department of Engineering and Technological Studies, Kalyani University, Kalyani 741235 (India)

2014-04-14

337

XANES study of rare-earth valency in LRu4P12 (L=Ce and Pr)  

Science.gov (United States)

Valency of Ce and Pr in LRu4P12 (L=Ce and Pr) was studied by L2,3-edge x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES spectrum suggests that Ce is mainly trivalent, but the 4f state strongly hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr exists in trivalent state over a wide range in temperature, 20 K<=T<=300 K. We find that the metal-insulator (MI) transition at TMI=60 K in PrRu4P12 does not originate from Pr valence fluctuation.

Lee, C. H.; Oyanagi, H.; Sekine, C.; Shirotani, I.; Ishii, M.

1999-11-01

338

Absence of superconductivity and valence bond order in the Hubbard–Heisenberg model for organic charge-transfer solids  

International Nuclear Information System (INIS)

A frustrated, effective 1/2 -filled band Hubbard–Heisenberg model has been proposed for describing the strongly dimerized charge-transfer solid families ?-(ET)2X and Z[Pd(dmit)2]2. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and, in the case of Z=EtMe3P, a spin-gapped phase that has sometimes been referred to as a valence bond solid. We show that neither superconductivity nor the valence bond order phase occurs within the Hubbard–Heisenberg model. We suggest that a description based on 1/4 -filling, that is reached when the carrier concentration per molecule instead of per dimer is considered, thus may be appropriate. (paper)

339

Origin of valence and core excitations in LiFePO{sub 4} and FePO{sub 4}  

Energy Technology Data Exchange (ETDEWEB)

Electronic structures of LiFePO{sub 4} and FePO{sub 4} have been investigated using valence and core electron energy loss spectroscopy (EELS) supported by ab initio calculations. Valence electron energy loss spectra of FePO{sub 4} are characterized by interband transitions found between 0 and 20 eV, which are not observed in LiFePO{sub 4}. Spectra are fully analysed using band structure calculations and calculated dielectric functions. In particular, we show that interband transitions observed in FePO{sub 4} spectra originate from the states at the top of the valence band, which have mainly oxygen p character. From core-loss EELS, it is observed that the O-K edge in FePO{sub 4} has a pre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-K spectra of LiFePO{sub 4}. The position of the pre-edge peak is determined by a charge transfer process, which shifts the position of the iron 3d bands with respect to the conduction band. The intensity of the pre-edge peak is also determined by the changes in the hybridization of iron 3d and oxygen states as a result of extraction of lithium ions from the LiFePO{sub 4} lattice. We show that the extraction of lithium ions from LiFePO{sub 4} results in large changes in the electronic structure, such that FePO{sub 4} can be considered to be a charge transfer insulator while LiFePO{sub 4} is a typical Mott-Hubbard insulator.

Kinyanjui, M K; Kaiser, U [University of Ulm, Albert Einstein Allee 11, 89081 Ulm (Germany); Axmann, P; Wohlfahrt-Mehrens, M [Centre for Solar Energy and Hydrogen Research, Helmholtzstrasse 8, 89081 Ulm (Germany); Moreau, P; Boucher, F, E-mail: michael.kinyanjui@uni-ulm.d [Institut des Materiaux Jean Rouxel (IMN), Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes cedex 3 (France)

2010-07-14

340

Origin of valence and core excitations in LiFePO(4) and FePO(4).  

Science.gov (United States)

Electronic structures of LiFePO(4) and FePO(4) have been investigated using valence and core electron energy loss spectroscopy (EELS) supported by ab initio calculations. Valence electron energy loss spectra of FePO(4) are characterized by interband transitions found between 0 and 20 eV, which are not observed in LiFePO(4). Spectra are fully analysed using band structure calculations and calculated dielectric functions. In particular, we show that interband transitions observed in FePO(4) spectra originate from the states at the top of the valence band, which have mainly oxygen p character. From core-loss EELS, it is observed that the O-K edge in FePO(4) has a pre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-K spectra of LiFePO(4). The position of the pre-edge peak is determined by a charge transfer process, which shifts the position of the iron 3d bands with respect to the conduction band. The intensity of the pre-edge peak is also determined by the changes in the hybridization of iron 3d and oxygen states as a result of extraction of lithium ions from the LiFePO(4) lattice. We show that the extraction of lithium ions from LiFePO(4) results in large changes in the electronic structure, such that FePO(4) can be considered to be a charge transfer insulator while LiFePO(4) is a typical Mott-Hubbard insulator. PMID:21399256

Kinyanjui, M K; Axmann, P; Wohlfahrt-Mehrens, M; Moreau, P; Boucher, F; Kaiser, U

2010-07-14

 
 
 
 
341

Average replicated T-matrix approximation: valence band of non-crystalline metallic alloys  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Here we present a new approximation for calculation of density of states of multi-component metallic alloys using Lloyd formula. We consider the average square of T-matrix as the pair replication of scattering system. The analytical expressions are probed on TixMn1-x and AlxNi1-x binary alloys. The results are in good agreement with the predicted behavior of density of states for such systems.

Yakibchuk, P.; Volkov, O.; Vakarchuk, S.

2007-01-01

342

State density of valence-band tail and photoconductivity amorphous hydrogenated silicon  

International Nuclear Information System (INIS)

Relation between photoconductivity and g(?) mobility gap within the range adjoining to the top (mobility end) of valent zone (VZ tail) in a-Si:H film is studied. Stationary photoconductivity within spectral maximum range (?=0.63?m) at ?=1017 photxcm-2s-1 flow is measured. Density of g(?) states are controlled using ultrasoft X-ray emission spectroscopy. It is shown, that correlation between photoconductivity and width of VZ tail may reflect the fact of their similar dependence o film heterogeneity: at the increase of share of microholes there occur both expansion of VZ tail and growth of number of respective hydrogen complexes and torn relations which results in drop of photoconductivity

343

Average replicated T-matrix approximation: valence band of non-crystalline metallic alloys  

Directory of Open Access Journals (Sweden)

Full Text Available Here we present a new approximation for calculation of density of states of multi-component metallic alloys using Lloyd formula. We consider the average square of T-matrix as the pair replication of scattering system. The analytical expressions are probed on TixMn1-x and AlxNi1-x binary alloys. The results are in good agreement with the predicted behavior of density of states for such systems.

P.Yakibchuk

2007-01-01

344

Spin-dependent terahertz nonlinearities at inter-valence-band absorption in p-Ge  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Far-infrared absorption and photon drag effect have been investigated in p-type germanium as a function of intensity of a high-power pulsed far-infrared molecular laser. Optical transmission shows saturation independent of the state of polarization of the radiation whereas in the photon drag effect a pronounced linear–circular dichroism occurs. Linear–circular dichroism is caused by spin-selective excitations as a result of selection rules and, equally important, multi-photon transitions ...

Ganichev, Sergey; Ketterl, Hermann; Prettl, Wilhelm

1999-01-01

345

Electronic states in valence and conduction bands of group-III nitrides: Experiment and theory  

International Nuclear Information System (INIS)

A comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented. A large set of the x-ray emission and absorption spectra was collected at the several synchrotron radiation facilities at installations offering the highest possible energy resolution. By taking advantage of the linear polarization of the synchrotron radiation and making careful crystallographic orientation of the samples, the bonds along c axis (?) and ''in plane'' (?) in the wurtzite structure could be separately examined. Particularly for AlN we found pronounced anisotropy of the studied bonds. The experimental spectra are compared directly with ab initio calculations of the partial density of states projected on the cation and anion atomic sites. For the GaN, AlN, and InN the agreement between structures observed in the calculated density of states (DOS) and structures observed in the experimental spectra is very good. In the case of hexagonal BN we have found an important influence of insufficient core screening in the x-ray spectra that influences the DOS distribution. The ionicity of the considered nitrides is also discussed. (c) 2000 The American Physical Society

346

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu 1- xNi x) 4Ga  

Science.gov (United States)

The compounds Ce(Cu 1- xNi x) 4Ga crystallize in the hexagonal CaCu 5-type structure for the whole doping range 0? x?1. The border compounds CeCu 4Ga and CeNi 4Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu 1- xNi x) 4Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f 1 states for up to x=0.5.

Toli?ski, T.; Che?kowska, G.; Falkowski, M.; Kowalczyk, A.

2011-06-01

347

Band alignment of InAs1-xSbx (0.05  

Science.gov (United States)

We determined the unstrained conduction-band and valence-band edge energies of InAs1-xSbx (0.05VBAC) model to explain the bowing of the valence band in InAsSb. Moreover, the spin-orbit splitting energy of InAsSb calculated by our VBAC model fits well with the experimental results reported in previous studies.

Wu, Chen-Jun; Tsai, Gene; Lin, Hao-Hsiung

2009-05-01

348

Calculations for the band lineup of strained InxGa1-xN/GaN quantum wells: Effects of strain on the band offsets  

Science.gov (United States)

Band lineup is one of the most important parameters associated with carrier confinement in heterostructures. Relations for computing the band lineups of InxGa1-xN based heterostructures have been developed. The band positions for InxGa1-xN/GaN heterointerfaces are calculated from the equations developed, which directly corelate the positions of the bands with the band gap of InN and strain at the interface. The strains are calculated from the In mole fractions and lattice constants. The parameters implicitly involved are the elastic stiffness constants (C11 and C12), the hydrostatic deformation potential of the conduction band (a'), and the hydrostatic deformation potential (a) and shear deformation potential (b) for the valence band. Computations have been carried out for different reported band gaps of InN. The effects of strain become prominent as the mole fraction of In increases, changing the band offset ratio.

Das, Tapas; Kabi, Sanjib; Biswas, Dipankar

2009-02-01

349

Orbital momentum profiles and binding energy spectra for the complete valence shell of molecular fluorine  

International Nuclear Information System (INIS)

The first electronic structural study of the complete valence shell binding energy spectrum of molecular fluorine, encompassing both the outer and inner valence regions, is reported. These binding energy spectra as well as the individual orbital momentum profiles have been measured using an energy dispersive multichannel electron momentum spectrometer at a total energy of 1500 eV, with an energy resolution of 1.5 eV and a momentum resolution of 0.1 a.u. The measured binding energy spectra in the energy range of 14-60 eV are compared with the results of ADC(4) many-body Green's function and also direct-Configuration Interaction (CI) and MRSD-CI calculations. The experimental orbital electron momentum profiles are compared with SCF theoretical profiles calculated using the target Hartree-Fock approximation with a range of basis sets and with Density Functional Theory predictions in the target Kohn-Sham approximation with non-local potentials. The truncated (aug-cc-pv5z) Dunning basis sets were used for the Density Functional Theory calculations which also include some treatment of correlation via the exchange and correlation potentials. Comparisons are also made with the full ion-neutral overlap amplitude calculated with MRSD-CI wave functions. Large, saturated basis sets (199-GTO) were employed for both the high level SCF near Hartree-Fock limit and MRSD-CI calculations to investigate the effects of electron correlation and relaxation. 66 refs., 9 tabs., 9 figs

350

Valence quark distributions in nucleon at low Q sup 2 in QCD  

CERN Document Server

Valence u- and d-quarks distributions in proton are calculated in QCD at low Q sup 2 and intermediate x, basing on the operator product expansion (OPE). The imaginary part of the virtual photon scattering amplitude on quark current with proton quantum numbers is considered. The initial and final virtualities p sup 2 sub 1 and p sup 2 sub 2 of the currents are assumed to be large, negative and different, p sup 2 sub 1 not =p sup 2 sub 2. The OPE in p sup 2 sub 1 , p sup 2 sub 2 up to dimension 6 operators was performed. Double dispersion representations in p sup 2 sub 1 , p sup 2 sub 2 of the amplitudes in terms of physical states contributions are used. Putting them to be equal to those calculated in QCD, the sum rules for quark distributions are found. The double Borel transformations are applied to the sum rules. Leading order perturbative corrections are accounted. Valence quark distributions are found: u(x) sub v at 0.15

Ioffe, B L

2003-01-01

351

The effect of the electronic and structural environment on the valence of cerium  

International Nuclear Information System (INIS)

By means of a semiempirical analysis the electrostatic energy Esub(psi) of a charge immersed in the screened Coulomb potential of the total charge distribution of the metal was selected as the most appropriate parameter for correlating different cerium environments. The choice was made by determining the smallest dispersion in plots of various physicochemical parameters versus the rate of depression ? of the superconducting transition temperature Tsub(c) of thorium-based alloys containing cerium impurities. The magnetic moment of cerium, which depends on the environment, was found to have the greatest effect on the depression of Tsub(c). Esub(psi) was found to be proportional to ?sup(-1/2). The analysis was extended to some ABsub(j) compounds (A equivalent to elements of groups II - V including cerium and thorium; Bsub(j) equivalent to N, Sn3, Tl3, Rh3, Co2, Ir2, B6 and Be13). An empirical relationship between Esub(psi) and the volume contraction ?V/V due to the formation of the compound was found. The cerium valence for any CeBsub(j) compound is deduced from the Esub(psi)(Ce) value compatible with the curve of ?V/V versus Esub(psi)(A), taking into account the fact that the valence and atomic radius of cerium are related. (Auth.)

352

Dispersion-Enhanced Laser Gyroscope  

Science.gov (United States)

We analyze the effect of a highly dispersive element placed inside a modulated optical cavity on the frequency and amplitude of the output modulation to determine the conditions for enhanced gyroscopic sensitivities. The element is treated as both a phase and amplitude filter, and the time-dependence of the cavity field is considered. Both atomic gases (two-level and multi-level) and optical resonators (single and coupled) are considered and compared as dispersive elements. We find that it is possible to simultaneously enhance the gyro scale factor sensitivity and suppress the dead band by using an element with anomalous dispersion that has greater loss at the carrier frequency than at the side-band frequencies, i.e., an element that simultaneously pushes and intensifies the perturbed cavity modes, e.g. a two-level absorber or an under-coupled optical resonator. The sensitivity enhancement is inversely proportional to the effective group index, becoming infinite at a group index of zero. However, the number of round trips required to reach a steady-state also becomes infinite when the group index is zero (or two). For even larger dispersions a steady-state cannot be achieved, and nonlinear dynamic effects such as bistability and periodic oscillations are predicted in the gyro response.

Smith, David D.; Chang, Hongrok; Arissian, L.; Diels, J. C.

2008-01-01

353

Electronic structures of narrow-band materials  

International Nuclear Information System (INIS)

This thesis investigates the electronic structures of narrow-band actinide, rare earth, and transition metal materials, using the experimental techniques of photoemission (PES) and inverse photoemission (BIS) spectroscopy. Narrow-band materials exhibit many interesting phenomena such as mixed-valence, heavy-fermion properties, and high temperature superconductivity. This is primarily due to the correlated narrow band electrons and their interaction with conduction or valence band electrons. The theoretical characterization of the electronic structures of a many body system is the spectral weight of the single particle Green's function. The spectral weight of narrow band electrons is determined by measuring the PES/BIS spectrum, the spectrum to remove and add electrons. For Ce compounds, the impurity Anderson Hamiltonian can describe both the ground state properties and the 4f spectrum. In this description, the low-energy properties are determined by a low-energy scale set by the Kondo temperature, which characterizes spin fluctuations. To make a detailed test of this picture, the Ce 3d and 4f spectra are measured and analyzed using this model for Ce(Ru1-xRhx)3B2 and CeCu2Si2. The spectral change during the ?-? phase transition of Ce0.7Th0.3 has been analyzed. The Yb 4f spectrum of YbAl3 is measured and analyzed by applying the impurity Anderson Hamiltonian calculations for Ce to Yb with an electron-hole mapping. The author has studied the 4f spectra of heavy-fermion compounds, YbXCu4, with X = Ag, Au, and Pd. The 5f spectra of uranium materials do not resemble the Ce 4f spectra, in having much more f-weight around EF. However, the measured 5f width of the heavy fermion uranium materials exceeds the 5f width obtained from the local density functional calculations

354

Optical study of archetypical valence-fluctuating Eu systems.  

Science.gov (United States)

We have investigated the optical conductivity of the prominent valence-fluctuating compounds EuIr(2)Si(2) and EuNi(2)P(2) in the infrared energy range to get new insights into the electronic properties of valence-fluctuating systems. For both compounds, we observe upon cooling the formation of a renormalized Drude response, a partial suppression of the optical conductivity below 100 meV, and the appearance of a midinfrared peak at 0.15 eV for EuIr(2)Si(2) and 0.13 eV for EuNi(2)P(2). Most remarkably, our results show a strong similarity with the optical spectra reported for many Ce- or Yb-based heavy-fermion metals and intermediate valence systems, although the phase diagrams and the temperature dependence of the valence differ strongly between Eu systems and Ce- or Yb-based systems. This suggests that the hybridization between 4f and conduction electrons, which is responsible for the properties of Ce and Yb systems, plays an important role in valence-fluctuating Eu systems. PMID:23368375

Guritanu, V; Seiro, S; Sichelschmidt, J; Caroca-Canales, N; Iizuka, T; Kimura, S; Geibel, C; Steglich, F

2012-12-14

355

Space-Valence Priming with Subliminal and Supraliminal Words  

Directory of Open Access Journals (Sweden)

Full Text Available To date it is unclear whether (1 awareness-independent non-evaluative semantic processes influence affective semantics and whether (2 awareness-independent affective semantics influence non-evaluative semantic processing. In the current study, we investigated these questions with the help of subliminal (masked primes and visible targets in a space-valence across-category congruence effect. In line with (1, we found that subliminal space prime words influenced valence classification of supraliminal target words (Experiment 1: Classifications were faster with a congruent prime (e.g., the prime ‘up’ before the target ‘happy’ than with an incongruent prime (e.g., the prime ‘up’ before the target ‘sad’. In contrast to (2, no influence of subliminal valence primes on the classification of supraliminal space targets into up- and down-words was found (Experiment 2. Control conditions showed that standard masked response-priming effects were found with both subliminal prime types, and that an across-category congruence effect was also found with supraliminal valence primes and spatial target words. The final Experiment 3 confirmed that the across-category congruence effect indeed reflected priming of target categorization of a relevant meaning category. Together, the data jointly confirmed prediction (1 that awareness-independent non-evaluative semantic priming influences valence judgments.

UlrichAnsorge

2013-02-01

356

Folded bands in metamaterial photonic crystals  

Science.gov (United States)

The dispersion relation of periodic structures that include metamaterials or materials with large anomalous dispersion can give bands with infinite group velocity points. These bands do not span the entire first Brillouin zone but are instead localized in k-space. We show that these points arise when both positive and negative elements are present, with the group index rather than the refractive index being the controlling quantity. A rigorous condition and two approximations are derived, each showing that an appropriate weighted average of group index being zero leads to infinite group velocity points.

Chen, P. Y.; Poulton, C. G.; Asatryan, A. A.; Steel, M. J.; Botten, L. C.; de Sterke, C. Martijn; McPhedran, R. C.

2011-05-01

357

Folded bands in metamaterial photonic crystals  

International Nuclear Information System (INIS)

The dispersion relation of periodic structures that include metamaterials or materials with large anomalous dispersion can give bands with infinite group velocity points. These bands do not span the entire first Brillouin zone but are instead localized in k-space. We show that these points arise when both positive and negative elements are present, with the group index rather than the refractive index being the controlling quantity. A rigorous condition and two approximations are derived, each showing that an appropriate weighted average of group index being zero leads to infinite group velocity points.

358

Determination of the Uranium Valence State in the Brannerite Structure using EELS, XPS, and EDX  

International Nuclear Information System (INIS)

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS), scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX) and x-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO2, CaUO4, SrCa2UO6, UTi2O6, Y0.5U0.5Ti2O6) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M5/(M4 +M5), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimize the effects of electron beam reduction of U(VI) and U(V)

359

On the valence-bond solid phase of the crossed-chain quantum spin model  

CERN Document Server

Using a series expansion based on the flow-equation method we study the ground state energy and the elementary triplet excitations of a generalized model of crossed spin-1/2 chains starting from the limit of decoupled quadrumers. The triplet dispersion is shown to be very sensitive to the inter-quadrumer frustration, exhibiting a line of almost complete localization as well as lines of quantum phase transitions limiting the stability of the valence-bond solid phase. In the vicinity of the checkerboard-point a finite window of exchange couplings is found with a non-zero spin-gap, consistent with known results from exact diagonalization. The ground state energy is lower than that of the bare quadrumer case for all exchange couplings investigated. In the limiting situation of the fully frustrated checkerboard magnet our results agree with earlier series expansion studies.

Brenig, W; Brenig, Wolfram; Grzeschik, Matthias

2003-01-01

360

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu{sub 1-x}Ni{sub x}){sub 4}Ga  

Energy Technology Data Exchange (ETDEWEB)

The compounds Ce(Cu{sub 1-x}Ni{sub x}){sub 4}Ga crystallize in the hexagonal CaCu{sub 5}-type structure for the whole doping range 0{<=}x{<=}1. The border compounds CeCu{sub 4}Ga and CeNi{sub 4}Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu{sub 1-x}Ni{sub x}){sub 4}Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f{sup 1} states for up to x=0.5. - Research highlights: Ni substitution for Cu in CeCu{sub 4}Ga leads to the loss of Ce 4f states localization. Valence fluctuations visible in the XPS of Ce 3d states. Magnetic susceptibility measured and the valence resolved up to 1000 K. Importance of the temperature independent susceptibility

Tolinski, T., E-mail: tomtol@ifmpan.poznan.p [Institute of Molecular Physics, PAS, Smoluchowskiego 17, 60-179 Poznan (Poland); CheLkowska, G. [Institute of Physics, Silesian University, Katowice (Poland); Falkowski, M.; Kowalczyk, A. [Institute of Molecular Physics, PAS, Smoluchowskiego 17, 60-179 Poznan (Poland)

2011-06-15

 
 
 
 
361

A study of the valence shell electronic states of pyrimidine and pyrazine by photoabsorption spectroscopy and time-dependent density functional theory calculations  

Science.gov (United States)

The valence shell electronic states of pyrimidine and pyrazine have been studied experimentally and theoretically. The absolute photoabsorption cross sections have been measured between 4 and 40 eV, using synchrotron radiation, and are dominated by prominent bands associated with intravalence transitions. In contrast, the structure due to Rydberg excitations is weak, but series have been observed converging onto the \\tilde X{\\,}^2B_2 or \\tilde D{\\,}^2B_1 limits in pyrimidine and the \\tilde X{\\,}^2A_g or \\tilde D{\\,}^2B_{3g} limits in pyrazine. A comparison between the photoabsorption spectrum of pyrazine-h4 and that for pyrazine-d4, together with calculated transition energies, has helped clarify the assignments of the 6ag?npb1u and npb2u Rydberg series. The vibrational progressions associated with these states have been assigned through analogy with those in the corresponding photoelectron band. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet-singlet valence transitions, and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. These theoretical results have allowed many of the experimentally observed bands to be assigned and provide a generally satisfactory description of the valence shell photoabsorption spectrum. Several of the prominent bands appearing above the ionization threshold can be correlated with predicted intense intravalence transitions.

Stener, M.; Decleva, P.; Holland, D. M. P.; Shaw, D. A.

2011-04-01

362

Universality class of non-Fermi liquid behaviour in mixed valence systems  

International Nuclear Information System (INIS)

A generalized Anderson single-impurity model with off-site Coulomb interactions is derived from the extended three-band Hubbard model, originally proposed to describe the physics of the copper-oxides. Using the abelian bosonization technique and canonical transformations, an effective Hamiltonian is derived in the strong coupling limit, which is essentially analogous to the Toulouse limit of the ordinary Kondo problem. In this limit, the effective Hamiltonian can be exactly solved, with a mixed valence quantum critical point separating two different Fermi liquid phases, i.e. the Kondo phase and the empty orbital phase. In the mixed valence quantum critical regime, the local moment is only partially quenched and X-ray edge singularities are generated. Around the quantum critical point, a new type of non-Fermi liquid behaviour is predicted with an extra specific heat Cimp ? T1/4 and a singular spin-susceptibility ?imp ? T-3/4. At the same time, the effective Hamiltonian under single occupancy is transformed into a resonant-level model, from which the correct Kondo physical properties (specific heat, spin susceptibility, and an enhanced Wilson ratio) are easily rederived. Finally, a brief discussion is given to relate these theoretical results to observations in U PdxCu5-x (x=1, 1.5) alloys, which show single-impurity critical behaviour consistent with our predictions. (author). 30 refs

363

Photonic band structure and omnidirectional band gap in anisotropic superlattice  

International Nuclear Information System (INIS)

We investigate theoretically the photonic band structure of one-dimensional superlattice (SL) composed of alternating anisotropic layers with their principal axis oriented at arbitrary directions. The dispersion relation of order two is calculated analytically by using the 4 x 4 matrix method which is based on boundary conditions of the electric and magnetic fields at each interface. It is shown that such structures can exhibit coupled electromagnetic modes between transverse magnetic TM and transverse electric TE modes, and dispersion curves that do not exist in superlattices composed only of isotropic layers. For a given value of the wave vector kparallel (parallel to the layers), the dispersion curves (frequency ?) versus kB (where kB is the Bloch wave vector of the periodic system along the axis of the superlattice) is illustrated. Specific applications of these results are given for the case of biaxial superlattice. With an appropriate choice of the superlattice parameters, we show that it is possible to realise, for these coupled electromagnetic waves, an absolute (or omnidirectional) band gap of width depending on the anisotropic parameters of the media forming the SL. (author)

364

Reversible gels of patchy particles: Role of the valence  

Science.gov (United States)

We simulate a binary mixture of colloidal patchy particles with two and three patches, respectively, for several relative concentrations and hence relative average valences. For these limited-valence systems, it is possible to reach low temperatures, where the lifetime of the patch-patch interactions becomes longer than the observation time without encountering phase separation in a colloid-poor (gas) and a colloid rich (liquid) phase. The resulting arrested state is a fully connected long-lived network where particles with three patches provide the branching points connecting chains of two-patch particles. We investigate the effect of the valence on the structural and dynamic properties of the resulting gel and attempt to provide a theoretical description of the formation and of the resulting gel structure based on a combination of the Wertheim theory for associated liquids and the Flory-Stockmayer approach for modeling chemical gelation.

Russo, John; Tartaglia, Piero; Sciortino, Francesco

2009-07-01

365

Optical implementation and entanglement distribution in Gaussian valence bond states  

Science.gov (United States)

We study Gaussian valence bond states of continuous variable systems obtained as the outputs of projection operations from an ancillary space of M infinitely entangled bonds connecting neighboring sites applied at each ofN sites of a harmonic chain. The entanglement distribution in Gaussian valence bond states can be controlled by varying the input amount of entanglement engineered in a (2M+ 1)-mode Gaussian state known as the building block, which is isomorphic to the projector applied at a given site. We show how this mechanism can be interpreted in terms of multiple entanglement swapping from the chain of ancillary bonds, through the building blocks. We provide optical schemes to produce bisymmetric three-mode Gaussian building blocks (which correspond to a single bond, M = 1), and study the entanglement structure in the output Gaussian valence bond states. Finally, the usefulness of such states for quantum communication protocols with continuous variables, like telecloning and teleportation networks, is discussed.

Adesso, G.; Ericsson, M.

2007-08-01

366

Optical implementation and entanglement distribution in Gaussian valence bond states  

CERN Document Server

We study Gaussian valence bond states of continuous variable systems, obtained as the outputs of projection operations from an ancillary space of M infinitely entangled bonds connecting neighboring sites, applied at each of $N$ sites of an harmonic chain. The entanglement distribution in Gaussian valence bond states can be controlled by varying the input amount of entanglement engineered in a (2M+1)-mode Gaussian state known as the building block, which is isomorphic to the projector applied at a given site. We show how this mechanism can be interpreted in terms of multiple entanglement swapping from the chain of ancillary bonds, through the building blocks. We provide optical schemes to produce bisymmetric three-mode Gaussian building blocks (which correspond to a single bond, M=1), and study the entanglement structure in the output Gaussian valence bond states. The usefulness of such states for quantum communication protocols with continuous variables, like telecloning and teleportation networks, is finally...

Adesso, Gerardo

2007-01-01

367

Contribution to the study of higher valency states of americium  

International Nuclear Information System (INIS)

Study of the chemistry of the higher valencies of americium in aqueous solutions and especially the autoreduction phenomenon. First a purification method of americium solutions is studied by precipitation, solvent extraction and ion exchange chromatography. Studies of higher valency states chemical properties are disturbed by the autoreduction phenomenon changing Am VI and Am V in Am III more stable. Stabilization of higher valency states, characterized by a steady concentration of Am VI in solution, can be done by complexation of Am VI and Am V ions or by a protecting effect of foreign ions. The original medium used has a complexing effect by SO42- ions and a protecting effect by the system S2O82--Ag+ consuming H2O2 main reducing agent produced by water radiolysis. These effects are shown by the study of Am VI in acid and basic solutions. A mechanism of the stabilization effect is given

368

Exploring Scaling Laws of Valence Neutron Distributions for Medium Nuclei  

International Nuclear Information System (INIS)

The root-mean-square radii of the valence neutron distributions for many nuclei in He-Mo mass range are calculated in the framework of the single-particle potential model. The scaling laws of valence neutron distributions are obtained by analyzing the relations between the radii and the binding energies of the valence neutrons. Based on these scaling laws, the necessary conditions for the occurrence of neuron halos in 2s1/2, 1p3/2, 1p1/2, 2p3/2, 2p1/2, 1d5/2 and 1d3/2 states are deduced, respectively. The derived quantitative conditions for halo occurrence can provide reference for the searching of neutron halos up to medium nuclei

369

RKKY interaction in mixed valence system and heavy fermion superconductivity  

International Nuclear Information System (INIS)

The 1-D RKKY interaction of mixed valence system is given by using the thermodynamic perturbation theory. The numerical comparisons of 1-D and 3-D RKKY interaction between systems with localized magnetic moments of mixed valence and non-mixed valence show that the former is much stronger than the latter. From some analyses we propose that the heavy Fermion superconductivity comes from the RKKY interaction between two local f electrons which hop off the impurity site to become two continuum electrons. The source of the two impurity electrons hopping is the Coulomb interaction. It is also emphasized that the RKKY interaction does not disappear for the Kondo lattice, when the temperature is less than the Kondo temperature. (author)

370

Band offsets in cubic GaN/AlN superlattices - Theory and experiment  

Energy Technology Data Exchange (ETDEWEB)

The presently unknown band offset in non-polar cubic GaN/AlN superlattices has been investigated by intersubband and interband spectroscopy as well as ab-initio calculations. On the one hand, the conduction band offset has been determined from the comparison of the measured transition energies with model calculations within the effective mass approximation. On the other hand, the valence and conduction band offset has been accurately simulated by calculating many body corrections within the GW approximation on top of hybrid-functional density functional theory (DFT) calculations. A conduction band offset of (1.4{+-}0.1) eV and a valence band offset of (0.5{+-}0.1) eV has been thus obtained as a result of both approaches.

Landmann, Marc; Mietze, Christian; Rauls, Eva; Lischka, Klaus; As, Donat J.; Schmidt, Wolf Gero [Universitaet Paderborn (Germany)

2011-07-01

371

Hydrodynamic dispersion  

International Nuclear Information System (INIS)

A dominant mechanism contributing to hydrodynamic dispersion in fluid flow through rocks is variation of travel speeds within the channels carrying the fluid, whether these be interstices between grains, in granular rocks, or cracks in fractured crystalline rocks. The complex interconnections of the channels ensure a mixing of those parts of the fluid which travel more slowly and those which travel faster. On a macroscopic scale this can be treated statistically in terms of the distribution of times taken by a particle of fluid to move from one surface of constant hydraulic potential to another, lower, potential. The distributions in the individual channels are such that very long travel times make a very important contribution. Indeed, while the mean travel time is related to distance by a well-defined transport speed, the mean square is effectively infinite. This results in an asymmetrical plume which differs markedly from a gaussian shape. The distribution of microscopic travel times is related to the distribution of apertures in the interstices, or in the microcracks, which in turn are affected in a complex way by the stresses acting on the rock matrix

372

Fermi level stabilization and band edge energies in CdxZn1-xO alloys  

Science.gov (United States)

We have measured the band edge energies of CdxZn1-xO thin films as a function of composition by three independent techniques: we determine the Fermi level stabilization energy by pinning the Fermi level with ion irradiation, measure the binding energy of valence band states and core levels by X-ray photoelectron spectroscopy, and probe shifts in the conduction band and valence band density of states using soft X-ray absorption and emission spectroscopy, respectively. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. Given these band alignments, alloy disorder scattering is found to play a negligible role in decreasing the electron mobility for all alloys. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdZnO alloys attractive candidates for photoelectrochemical water splitting applications.

Detert, Douglas M.; Tom, Kyle B.; Battaglia, Corsin; Denlinger, Jonathan D.; Lim, Sunnie H. N.; Javey, Ali; Anders, André; Dubon, Oscar D.; Yu, Kin M.; Walukiewicz, Wladek

2014-06-01

373

Reinterpretation of bond-valence model with bond-order formalism: an improved bond-valence based interatomic potential for PbTiO$_3$  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a modified bond-valence model of PbTiO$_3$ based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. A new energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are re-optimized. The new model potential can be applied both to canonical ensemble ($NV...

Liu, Shi; Grinberg, Ilya; Takenaka, Hiroyuki; Rappe, Andrew M.

2012-01-01

374

A study of the valence shell spectroscopic and thermodynamic properties of trifluoronitrosomethane cations  

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer Fragmentation processes in CF{sub 3}NO have been studied using mass spectrometry. Black-Right-Pointing-Pointer Singly charged atomic fragments have been observed. Black-Right-Pointing-Pointer Experimental appearance energies have been compared to thermochemical estimates. Black-Right-Pointing-Pointer Hartree Fock transition energies and oscillator strengths have been calculated. - Abstract: A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in trifluoronitrosomethane (CF{sub 3}NO) as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range {approx}10-42 eV, and appearance energies have been determined for 10 fragment ions. At high excitation energies, singly charged atomic fragments have been observed. For the main dissociation channels, leading to the formation of NO{sup +}, CF{sub 2}{sup +} or CF{sub 3}{sup +}, the experimental appearance energies have been compared with thermochemical estimates, and a satisfactory agreement has been found. Structure observed in the total ion yield curve has been interpreted with the aid of excited state transition energies and oscillator strengths obtained in a time-dependent Hartree Fock calculation. The theoretical results show that configuration interaction strongly affects many of the valence states. A HeI excited photoelectron spectrum of CF{sub 3}NO has been measured and the orbital ionisation energies have been compared with theoretical values computed using the Outer Valence Green's Function approach. A large Franck-Condon gap is observed between the 12a Prime (n{sub -}) and the 11a{sup Prime} state bands, in accord with the calculated vertical ionisation energies of 10.87 and 16.32 eV for the 12a Prime (n{sub -}) and the 11a Prime (n{sub +}) orbitals, respectively. In the ion yield curve, the corresponding energy range is strongly influenced by autoionising valence states.

Graham, D.M. [School of Physics and Astronomy, and the Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL (United Kingdom); Powis, I. [School of Chemistry, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Underwood, J.G. [Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT (United Kingdom); Shaw, D.A. [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom); Holland, D.M.P., E-mail: david.holland@stfc.ac.uk [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom)

2012-02-06

375

A theoretical investigation of valence and Rydberg electronic states of acrolein  

International Nuclear Information System (INIS)

The main features of the ultraviolet spectrum of acrolein have been studied by a multireference perturbative treatment and by a time dependent density functional approach. The valence and Rydberg transition energies have been calculated and the assignment of the experimental bands has been clarified. The different relaxation trends of the three lowest singlet and triplet excited states have been analyzed by unconstrained geometry optimizations. This has allowed, in particular, the characterization of a twisted 3(??*) state, which is crucial for the interesting photophysics and photochemistry of the acrolein molecule and, more generally, of the ?,?-enones. Solvatochromic shifts in aqueous solution have been investigated using a combined discrete/continuum approach based on the so called polarizable continuum model. The experimental trends are well reproduced by this approach and a closer degeneracy in the triplet manifold has been detected in solution with respect to gas phase

376

Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy.  

Science.gov (United States)

With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of approximately 0.15 eV energy resolution with an electron probe size of approximately 1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y(1)Ba(2)Cu(3)O(7-)(delta) high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS. PMID:15885909

Erni, Rolf; Browning, Nigel D

2005-10-01

377

Soft X-ray emission spectroscopy study of the valence electron states of ?-rhombohedral boron  

International Nuclear Information System (INIS)

High energy-resolution boron K-emission spectra of single-crystalline ?-rhombohedral-boron (?-r-B) were measured with a newly developed soft X-ray emission spectroscopy (SXES) instrument attached to a conventional transmission electron microscope. The intensity profiles of the spectra, which correspond to the density of states of the valence bands with p-symmetry, were compared with those of amorphous boron (am-B), single-crystalline ?-rhombohedral-boron (?-r-B) and B4C. A characteristic shoulder structure appears in the spectrum of ?-r-B but not of am-B, ?-r-B or B4C. This structure should be due to the inter-cluster three-center bonding states among B12 clusters in ?-r-B.

378

A study of valency changes in the system Al2O3:Mn  

International Nuclear Information System (INIS)

Valency changes of the Mn ions in Al2O3 single crystals have been induced by oxidizing and reducing heat treatments (approx. 1700 deg C) and by #betta#-irradiation at room temperature. The concentration of Mn3+, Mn4+ and Mn2+ ions have been measured using three experimental techniques: optical absorption at 80 K, low temperature thermal conductivity measurements and EPR. The optical absorption of the R-lines of Mn4+ has been observed for the first time in the #betta#-irradiated state. From the quantitative results, an estimation of the oscillator strengths of the lines and bands attributed to Mn4+ and Mn3+ has been made. (author)

379

Core-valence luminescence in ABX3 crystals  

International Nuclear Information System (INIS)

The investigation of the new type of intrinsic emission caused by recombination of core holes and valence electrons, i.e., the core valence luminescence (CVL), is made for perovskite-like ABX3 crystals (A = Li, K, Rb, Cs; B = Mg, Ca, Sr, Ba; X = F, Cl, Br). For this crystals the spectra of CVL cover 9 - 2 eV region, time decay constants change in 1 - 3 ns and bound, emission intensity in some crystals exceeds the intensity in AHC in several times. The possibility of growing ABX3 crystals with different lattice parameters allows us to control spectral and kinetics parameters as well as CVL light yield

380

Valence-differential spectroscopy of Co-Fe cyanide films  

Science.gov (United States)

Electrochromism is extensively investigated for practical application of display and memory devices. To develop the material, reliable information on the optical and electronic properties of the solid film is indispensable. Here, we propose valence-differential spectroscopy that can selectively extract the spectral components related to the oxidized/reduced metal site. We applied the spectroscopy to Co2+-Fe2+? and Co2+?-Fe2+ cyanide films with finely control of averaged valence (?) of the transition metal by external electric pulses. The spectroscopy revealed transition energy E, width ?, and oscillator strength f of the spectral components related to the transition metal.

Moritomo, Y.; Nakada, F.; Kurihara, Y.

2009-03-01

 
 
 
 
381

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys  

Science.gov (United States)

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(x)V(1-x), alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(x),V(1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(x)Te(l-x) and ZnSe(Y)Te(1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(y)Te(1-y) alloys in the entire composition range, 0 less than or equal to y less than or equal to 1. The samples used in this study are bulk ZnSe(y)Te(1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(y)Te(l-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Ramdas, A.; Su, Ching-Hua; Whitaker, Ann F. (Technical Monitor)

2001-01-01

382

Correlation effects in the valence ionization spectra of large conjugated molecules: p-Benzoquinone, anthracenequinone and pentacenequinone  

Energy Technology Data Exchange (ETDEWEB)

A review of an extensive series of theoretical studies of the valence one-electron and shake-up ionization spectra of polycyclic aromatic hydrocarbons is presented, along with new results for three planar quinone derivatives, obtained using one-particle Green's function (1p-GF) theory along with the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green's function (OVGF) approximation. These results confirm both for the pi- and sigma-band systems the rapid spreading, upon increasing system size, of many shake-up lines with significant intensities at outer-valence energies. Linear regressions demonstrate that with large conjugated molecules the location of the shake-up onset in the pi-band system is merely determined by the energy of the frontier (HOMO, LUMO) orbitals. Electron pair removal effects are found to almost compensate the electron relaxation effects induced by ionization of pi-levels, whereas the latter effects strongly dominate the ionization of more localized lone-pair (n) levels, and may lead to inversions of the energy order of Hartree-Fock (HF) orbitals. Therefore, although it increases upon a lowering of the HF band gap, and thus upon an increase of system size, the dependence of the one-electron ionization energies onto the quality of the basis set is lesser for pi-levels than for sigma-levels relating to electron lone pairs (n). Basis sets of triple- and quadruple-zeta quality are therefore required for treatments of the outermost pi- and n-ionization energies approaching chemical accuracy [1 kcal/mol, i.e. 0.04 eV]. When 1p-GF theory invalidates Koopmans' theorem and the energy order of HF orbitals, a comparison with Kohn-Sham orbital energies confirms the validity of the meta-Koopmans' theorem for density functional theory.

Knippenberg, S. [Institut fuer Physikalische und Theoretische Chemie, Johann Wolfgang Goethe Universitaet Frankfurt, Max-von-Laue-Str. 7, 60438 Frankfurt am Main (Germany); Research Group Theoretical Chemistry, Department SBG, Hasselt University, Agoralaan, Gebouw D, B-3590 Diepenbeek (Belgium); Deleuze, M.S., E-mail: michael.deleuze@uhasselt.b [Research Group Theoretical Chemistry, Department SBG, Hasselt University, Agoralaan, Gebouw D, B-3590 Diepenbeek (Belgium)

2010-05-15

383

Correlation effects in the valence ionization spectra of large conjugated molecules: p-Benzoquinone, anthracenequinone and pentacenequinone  

International Nuclear Information System (INIS)

A review of an extensive series of theoretical studies of the valence one-electron and shake-up ionization spectra of polycyclic aromatic hydrocarbons is presented, along with new results for three planar quinone derivatives, obtained using one-particle Green's function (1p-GF) theory along with the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green's function (OVGF) approximation. These results confirm both for the ?- and ?-band systems the rapid spreading, upon increasing system size, of many shake-up lines with significant intensities at outer-valence energies. Linear regressions demonstrate that with large conjugated molecules the location of the shake-up onset in the ?-band system is merely determined by the energy of the frontier (HOMO, LUMO) orbitals. Electron pair removal effects are found to almost compensate the electron relaxation effects induced by ionization of ?-levels, whereas the latter effects strongly dominate the ionization of more localized lone-pair (n) levels, and may lead to inversions of the energy order of Hartree-Fock (HF) orbitals. Therefore, although it increases upon a lowering of the HF band gap, and thus upon an increase of system size, the dependence of the one-electron ionization energies onto the quality of the basis set is lesser for ?-levels than for ?-levels relating to electron lone pairs (n). Basis sets of triple- and quadruple-zeta quality are therefore required for treatmty are therefore required for treatments of the outermost ?- and n-ionization energies approaching chemical accuracy [1 kcal/mol, i.e. 0.04 eV]. When 1p-GF theory invalidates Koopmans' theorem and the energy order of HF orbitals, a comparison with Kohn-Sham orbital energies confirms the validity of the meta-Koopmans' theorem for density functional theory.

384

Optical properties of Eu2+/Eu3+ mixed valence, silicon nitride based materials  

Science.gov (United States)

Eu2SiN3, a mixed valence europium nitridosilicate, has been prepared via solid-state reaction synthesis and its oxidation behavior and optical properties have been determined. Furthermore, the stability of several isostructural compounds of the type M2+L3+SiN3 has been predicted by using the density functional theory calculations, and verified by the actual synthesis of CaLaSiN3, CaEuSiN3 and EuLaSiN3. The band gap of CaLaSiN3 was found around 3.2 eV giving the material its yellow color. Eu2SiN3 on the other hand is black due to a combination of the 4f-5d absorption band of Eu2+ and the charge transfer band of Eu3+. Thermogravimetric analysis and Raman spectroscopic study of Eu2SiN3 revealed that oxidation of this compound in dry air takes place via a nitrogen retention complex.

ten Kate, Otmar M.; Vranken, Thomas; van der Kolk, Erik; Jansen, Antonius P. J.; Hintzen, Hubertus T.

2014-05-01

385

Evidence for hybrid surface metallic band in (4?×?4) silicene on Ag(111)  

Energy Technology Data Exchange (ETDEWEB)

The electronic band structure of monolayer (4?×?4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the K{sup ¯}{sub Ag} point and has a saddle point near the M{sup ¯}{sub Ag} point of Ag(111) resembling the ?-band dispersion in graphene.

Tsoutsou, D., E-mail: dtsoutsou@ims.demokritos.gr; Xenogiannopoulou, E.; Golias, E.; Tsipas, P.; Dimoulas, A. [National Center for Scientific Research “Demokritos,” 15310 Athens (Greece)

2013-12-02

386

Evidence for hybrid surface metallic band in (4?×?4) silicene on Ag(111)  

International Nuclear Information System (INIS)

The electronic band structure of monolayer (4?×?4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the K¯Ag point and has a saddle point near the M¯Ag point of Ag(111) resembling the ?-band dispersion in graphene

387

NMR studies of ordered structures and valence states in the successive valence-transition system EuPtP  

Science.gov (United States)

We have studied EuPtP, which undergoes two successive valence transitions at TA˜240 K and TB˜200 K by 31P-nuclear magnetic resonance (NMR) measurements. From the analysis of NMR spectra, we obtained plausible ordered structures and Eu valence states in three phases divided by TA and TB. These ordered structures well explain observed inequivalent P sites and the intensity ratio of the NMR spectra arising from these P sites. The results are also in good accordance with mean Eu valence measured by the x-ray absorption spectroscopy. We also discuss Eu 4 f states and the origin of the transitions from the measurements of nuclear spin lattice relaxation rate and hyperfine coupling constant.

Mito, T.; Nishitani, K.; Koyama, T.; Muta, H.; Maruyama, T.; Pristáš, G.; Ueda, K.; Kohara, T.; Mitsuda, A.; Sugishima, M.; Wada, H.

2014-11-01

388

Microscopic investigation of magnetic dipole bands in {sup 132}Ba  

Energy Technology Data Exchange (ETDEWEB)

Nuclear structure of {sup 132}Ba is investigated within a framework of the pair-truncated shell model. The model reproduces experimental energy levels of the magnetic dipole band with the {nu}(h{sup 2}{sub 11/2}) x {pi}(h{sub 11/2}g{sub 7/2}) configuration. From the analysis of its structure, it turns out that two angular momenta of valence neutrons and protons gradually close as total spin increases.

Higashiyama, Koji [Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033 (Japan); Yoshinaga, Naotaka [Department of Physics, Saitama University, Saitama City 338-8570 (Japan)

2005-10-01

389

Highly Mismatched Alloys for Intermediate Band Solar Cells  

Energy Technology Data Exchange (ETDEWEB)

It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a ''stepping stone'' for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.

Walukiewicz, W.; Yu, K.M.; Wu, J.; Ager III, J.W.; Shan, W.; Scrapulla, M.A.; Dubon, O.D.; Becla, P.

2005-03-21

390

Solvent dependence of the g-anisotropy in the ESR of cyanide-bridged mixed-valence complexes.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The low temperature (approximately 5 K) X-band ESR spectra are reported of the cyanide-bridged mixed-valence complexes [(OC)5Cr(mu-CN)M(NH3)5]X2 (M = Ru, Os; X = PF6(-)) in frozen matrices formed from nitromethane, acetonitrile and dimethylformamide with toluene. The anisotropy (g paralell-g perpendicular) is greater for the ruthenium than for the osmium complex. It is positive in all cases and is strongly dependent on the hydrogen-bonding interaction between the solvent matrix and the metal-...

Laidlaw, Wm; Denning, Rg; Murphy, Dm; Green, Jc

2008-01-01

391

Q?band loop?gap resonator  

International Nuclear Information System (INIS)

The upper frequency for loop?gap resonators intended for use in electron?spin?resonance spectroscopy has been extended to Q band (35 GHz). A practical structure is described containing sample support, frequency tuning, and variable coupling. A typical sample volume is 39 nL. High?energy densities (15 GW?1/2) were achieved. As found previously at X band, Q?band loop?gap resonators permit observation of the dispersion with minimal demodulation of phase noise originating in the klystron. Theoretical calculations of the resonant frequency, Q, and the filling factor are found to be in good agreement with experiment

392

Peculiarities of core-valence luminescence of CsBr and Rb1-xCsxBr crystals  

International Nuclear Information System (INIS)

The study of the emission and excitation spectra as well as the decay kinetics for the intrinsic core-valence luminescence of CsBr and the impurity core-valence luminescence of Rb1-xCsxBr solid solution under excitation in the 10-20 eV energy range has been carried out in the temperature range of 10-300 K. The change of coordination number for the Cs+ ion from n=8 to n=6 at the transformation of CsBr to Rb0.9Cs0.1Br solid solution is accompanied by significant modification of the core-valence luminescence spectra shape. The temperature dependence of the excitation efficiency in the threshold region observed for Rb0.9Cs0.1Br could be caused by the decay of the 5d3/2 excited state of the Cs+ ion due to thermostimulated electron transition from this state into the conduction band, which leads to the appearance of unbounded 5p Cs+ core holes. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

393

Study of the influence of localized Ce 4f orbitals on band structure and properties of iron pnictide compounds  

International Nuclear Information System (INIS)

Rare-earth iron pnictides pull currently great scientific interest over themselves, because they represent because of the high-temperature superconductivity observed at them a promising alternative to the conventional cuprate superconductors. Beside the superconductivity these systems exhibit a broad spectrum of magnetic properties,which are based on the interaction of iron 3d and rare-earth 4f electrons and occur partly in concurrence to, but partly also in coexistence with the superconductivity. The theoretical understanding of these phenomena lets still many questions be open, because above all the 4f electrons escape largely because of their strong spatial localization and high Coulomb-correlation energies a simple treatment in the framework of band-structure theories. The present thesis deals with angular-resolved photoemission studies on CeFe2P2, CeFePO, and CeFeP0.3As0.7O, in which heavy-fermion behaviour respectively ferromagnetism can be observed. The interaction of the 4f electrons with the valence band predominantly derived from Fe 3d orbitals is reflected in the spectra by the occurrence of dispersive structures in the region of the Kondo resonances. These are discussed in the thesis on the base of LDA band structure calculations and the periodic Anderson model, which is solved for simple case studies by means of the DMFT (dynamical mean field theory). By means of the experimental observations a mechanism for the magnetic transitions observed in the compound class, basing on characteristic differences in the band structure, is proposed as well a possible connection with the superconductivity discussed.

394

Theoretical and experimental studies of electronic band structure for GaSb1-xBix in the dilute Bi regime  

Science.gov (United States)

Photoreflectance (PR) spectroscopy was applied to study the band gap in GaSb1-xBix alloys with Bi GaSb1-xBix band gap determined from PR while the agreement between experimental data and the ab initio calculations of the band gap obtained within the SC-AM approach is excellent. These calculations show that the incorporation of Bi atoms into the GaSb host modifies both the conduction and the valence band. The shift rates found in this work are respectively -26.0 meV per % Bi for the conduction band and 9.6 meV per % Bi for the valence band that consequently leads to a reduction in the band gap by 35.6 meV per % Bi. The shifts found for the conduction and valence band give a ˜27% (73%) valence (conduction) band offset between GaSb1-xBix and GaSb. The rate of the Bi-related shift for the split-off band is -7.0 meV per % Bi and the respective increase in the spin-orbit split-off is 16.6 meV per % Bi.

Polak, M. P.; Scharoch, P.; Kudrawiec, R.; Kopaczek, J.; Winiarski, M. J.; Linhart, W. M.; Rajpalke, M. K.; Yu, K. M.; Jones, T. S.; Ashwin, M. J.; Veal, T. D.

2014-09-01

395

Photodissociation of carbon dioxide in singlet valence electronic states. II. Five state absorption spectrum and vibronic assignment  

Science.gov (United States)

The absorption spectrum of CO2 in the wavelength range 120-160 nm is analyzed by means of quantum mechanical calculations performed using vibronically coupled potential energy surfaces of five singlet valence electronic states and the coordinate dependent transition dipole moment vectors. The thermally averaged spectrum, calculated for T = 190 K via Boltzmann averaging of optical transitions from many initial rotational states, accurately reproduces the experimental spectral envelope, consisting of a low and a high energy band, the positions of the absorption maxima, their FWHMs, peak intensities, and frequencies of diffuse structures in each band. Contributions of the vibronic interactions due to Renner-Teller coupling, conical intersections, and the Herzberg-Teller effect are isolated and the calculated bands are assigned in terms of adiabatic electronic states. Finally, diffuse structures in the calculated bands are vibronically assigned using wave functions of the underlying resonance states. It is demonstrated that the main progressions in the high energy band correspond to consecutive excitations of the pseudorotational motion along the closed loop of the CI seam, and progressions differ in the number of nodes along the radial mode perpendicular to the closed seam. Irregularity of the diffuse peaks in the low energy band is interpreted as a manifestation of the carbene-type "cyclic" OCO minimum.

Grebenshchikov, Sergy Yu.

2013-06-01

396

Suppression of valence-quark recombination in ?+ fragmentation into ?+  

International Nuclear Information System (INIS)

We present inclusive ?±-,0 and ? cross sections in the beam fragmentation region of ?+p interactions at 250 GeV/c. The near equality of the ?+ and ?0 cross sections implies suppression of valence quark recombination in forward vector meson production. (orig.)

397

Is samarium metal in an intermediate valence state  

International Nuclear Information System (INIS)

X-ray photoemission spectra of the 4d and 4f electrons strongly suggest that either the bulk or surface of metallic Sm is in an intermediate valence state or else that Sm is divalent at the metal-vacuum interface. Experiments are suggested which may distinguish between these alternatives. (Auth.)

398

Mobile Linkers on DNA-Coated Colloids: Valency without Patches  

Science.gov (United States)

Colloids coated with single-stranded DNA (ssDNA) can bind selectively to other colloids coated with complementary ssDNA. The fact that DNA-coated colloids (DNACCs) can bind to specific partners opens the prospect of making colloidal "molecules." However, in order to design DNACC-based molecules, we must be able to control the valency of the colloids, i.e., the number of partners to which a given DNACC can bind. One obvious, but not very simple approach is to decorate the colloidal surface with patches of single-stranded DNA that selectively bind those on other colloids. Here we propose a design principle that exploits many-body effects to control the valency of otherwise isotropic colloids. Using a combination of theory and simulation, we show that we can tune the valency of colloids coated with mobile ssDNA, simply by tuning the nonspecific repulsion between the particles. Our simulations show that the resulting effective interactions lead to low-valency colloids self-assembling in peculiar open structures, very different from those observed in DNACCs with immobile DNA linkers.

Angioletti-Uberti, Stefano; Varilly, Patrick; Mognetti, Bortolo M.; Frenkel, Daan

2014-09-01

399

Valence mixing in YbCuAl: a case study  

International Nuclear Information System (INIS)

Results are presented of a study of the valence state of Yb in the intermetallic compound YbCuAl. Both macroscopic physical properties (magnetic susceptibility, heat capacity, thermal expansion, electric resistivity) and microscopic physical properties (neutron inelastic scattering, nuclear magnetic resonance) are determined. The results are compared with a local Fermi liquid theory. (G.T.H.)

400

The problem of fluctuating valence in f-electron metals  

International Nuclear Information System (INIS)

Besides some general semi-philosophical ideas about the phenomenon of mixed-valence, the question of the chemistry of this phenomenon and heavy-electron metals are discussed, especially why it is that cerium and uranium can be so similar. 11 refs.; 2 figs

 
 
 
 
401

Real-time observation of valence electron motion  

International Nuclear Information System (INIS)

The superposition of quantum states drives motion on the atomic and subatomic scales, with the energy spacing of the states dictating the speed of the motion. In the case of electrons residing in the outer (valence) shells of atoms and molecules which are separated by electronvolt energies, this means that valence electron motion occurs on a subfemtosecond to few-femtosecond timescale (1 fs = 10-15 s). In the absence of complete measurements, the motion can be characterized in terms of a complex quantity, the density matrix. Here we report an attosecond pump-probe measurement of the density matrix of valence electrons in atomic krypton ions. We generate the ions with a controlled few-cycle laser field and then probe them through the spectrally resolved absorption of an attosecond extreme-ultraviolet pulse, which allows us to observe in real time the subfemtosecond motion of valence electrons over a multifemtosecond time span. We are able to completely characterize the quantum mechanical electron motion and determine its degree of coherence in the specimen of the ensemble. Although the present study uses a simple, prototypical open system, attosecond transient absorption spectroscopy should be applicable to molecules and solid-state materials to reveal the elementary electron motions that control physical, chemical and biological properties and processes.

402

Regional versus Global Entanglement in Resonating-Valence-Bond states  

CERN Document Server

We investigate the entanglement properties of resonating valence bond states on two and higher dimensional lattices, which play an important role in the theory of conductivity. We show that these states are genuinely multipartite entangled, while there is only a negligible amount of two-site entanglement. We comment on possible physical implications of our findings.

Chandran, A; Sen, A; Sen, U; Vedral, V; Chandran, Anushya; Kaszlikowski, Dagomir; Sen, Aditi; Sen, Ujjwal; Vedral, Vlatko

2007-01-01

403

Pairing mechanisms in superconductors with valence-skipping dopants  

Science.gov (United States)

Recent studies of the degenerate semiconductor, lead telluride, doped with the valence skipping element, thallium, have lead to propositions of negative U superconductivity. In this model, charge 2e fluctuations between the stable +1 and +3 thallium valence states stimulate formation of cooper pairs. Significantly, correlated with superconductivity in this material is a minimum in resistance, similar to that found in metals doped with trace magnetic impurities. This behavior has been attributed to a charge-Kondo effect, where scattering from impurities with valence fluctuations between +1 and +3 charge states mimics the effect of scattering from magnetic impurities, causing a minimum in resistance and an enhancement of the density of states at the Fermi level. In this dissertation, I will describe experiments performed on three different sets of materials designed to illuminate aspects of superconductivity in materials that have been doped with valence-skipping elements. In the first project, I examine the effect of additional indium donors on samples of Pb 1-yTlyTe that would otherwise exhibit superconductivity and charge-Kondo behavior. In this work, I find that, as additional electrons are injected into Pb.99Tl.01Te by increasing the indium counterdopant concentration, the Kondo-like behavior and superconductivity are both suppressed near the same indium concentration. This is interpreted as a suppression of the degeneracy between +1 and +3 valence states by introducing additional electrons, which favor the +1 valence state, resulting in a suppression of charge-Kondo scattering and negative U pairing effects. The second project extends this study to the related material indium-doped tin telluride, where the valence-skipping nature of indium has lead others to propose a similar negative U superconducting state. In contrast to thallium-doped lead telluride, where the host material, PbTe, is nonsuperconducting, SnTe superconducts when doped with high concentrations of vacancies on the tin site. This allows for comparisons between the superconducting state in SnTe without additional dopants, and SnTe that has been doped with the valence-skipping element, indium. By comparing the density of states at the Fermi level and the superconducting critical temperature in Sn.995-xInxTe, grown for this study, and Sn1-deltaTe, collected from the literature, I show that enhanced superconductivity in indium-doped material cannot be explained by changes in the density of states at the Fermi level. Instead, a stronger superconducting pairing strength is inferred, which could arise from negative U pairing effects at the mixed-valence impurities. Additionally, comparing the temperature of a structural phase transition to that of the superconducting transition temperature as a function of indium content rules out softening of phonon modes related to this transition as a source of the Tc enhancement. The third study utilizes the high solubility limit of indium in tin telluride of up to 20%, compared to the limit of 1.4% thallium dopants in lead telluride, to examine the question of how interactions between valence-fluctuating impurities might affect superconductivity and the resistance minimum. In this case, resistivity measurements reveal an anomalously large resistance minimum in Sn988-x InxTe doped with 2.7% 6.1%, where samples are found to superconduct. While a detailed theoretical model with which to compare these data is lacking, some possible explanations relying on interactions between indium sites are proposed.

Erickson, Ann Shaklee

404

Superconductivity and mixed valency in thallium-doped lead telluride  

Science.gov (United States)

Tl-doped PbTe (Pb1-xTl xTe) is a degenerate semiconductor exhibiting superconductivity at a remarkably high Tc given its relatively low carrier concentration of ˜1020 cm-3. For Tl concentrations x beyond a critical value x c˜0.3%, it is observed to superconduct with T c rapidly increasing with x up to a maximum Tc of 1.5 K for the highest Tl doping of 1.5%. In comparison, other similar superconducting semiconductors can achieve comparable Tc values with much higher carrier concentrations. Since thallium is the only impurity known to cause superconductivity in PbTe, there has been considerable discussion as to the role of the Tl impurities in this material. In particular, it has been suggested that the superconductivity can be linked to the presence of a mixed Tl valence fluctuating between Tl + and Tl3+ states. We investigate the superconducting and normal state properties of single crystal Tl-doped PbTe samples for a range of Tl concentrations up to the solubility limit of approximately 1.5%. Estimates of superconducting parameters from heat capacity and upper critical field measurements show that the material is a Type-II, weak-coupled BCS superconductor in the dirty limit. Evidence for the mixed valence state appears as a carrier concentration saturation in the Hall effect measurements, indicating a self-compensating mixture of Tl+ (acceptors) and Tl3+ (donors) beyond a characteristic Tl concentration near xc. Furthermore, we show evidence that an anomalous low-temperature resistivity upturn may be associated with a charge Kondo effect arising; from the quantum valence fluctuations between the degenerate valence states of the Tl impurities. The resistivity anomaly was observed only in superconducting samples where x > xc These observations would support the notion of an alternate pairing mechanism for superconductivity in this material which is linked to the presence of the mixed valence state.

Matsushita, Yana

405

Asymmetry gap in the electronic band structure of bilayer graphene  

CERN Document Server

A tight binding model is used to calculate the band structure of bilayer graphene in the presence of a potential difference between the layers that opens a gap $\\Delta$ between the conduction and valence bands. In particular, a self consistent Hartree approximation is used to describe imperfect screening of an external gate, employed primarily to control the density $n$ of electrons on the bilayer, resulting in a potential difference between the layers and a density dependent gap $\\Delta (n)$. We discuss the influence of a finite asymmetry gap $\\Delta (0)$ at zero excess density, caused by the screening of an additional transverse electric field, on observations of the quantum Hall effect.

McCann, E

2006-01-01

406

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

Energy Technology Data Exchange (ETDEWEB)

Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap-transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor.

Yastrubchak, O.; Gluba, L.; ?uk, J. [Institute of Physics, UMCS, Pl. Marii Curie-Sk?odowskiej 1, 20-031 Lublin (Poland); Sadowski, J. [Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa (Poland); MAX-Lab, Lund University, 22100 Lund (Sweden); Krzy?anowska, H. [Institute of Physics, UMCS, Pl. Marii Curie-Sk?odowskiej 1, 20-031 Lublin (Poland); Department of Physics and Astronomy, Vanderbilt University, 6506 Stevenson Center, Nashville, Tennessee 37325 (United States); Domagala, J. Z.; Andrearczyk, T.; Wosinski, T. [Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa (Poland)

2013-08-07

407

Spectroscopic studies of oxo-centered, carboxylate-bridged, trinuclear mixed-valence iron (???, ???, ??) complexes with aromatic hydroxycarboxylic acids  

Science.gov (United States)

New type of oxo-centered, carboxylate-bridged, trinuclear, mixed-valence iron complexes of the general formula [Fe3O(OOCR)3(OOCR*)3L3] (where R = C13H27 or C15H31 and R* = C6H4(OH), (R'); C6H5CH(OH), (R?) or (C6H5)2C(OH), (R) and L = Methanol) were synthesized by the reaction of [Fe3O(OOCCH3)6(H2O)3] with straight chain carboxylic acids and aromatic hydroxycarboxylic acids. These were characterized by elemental analyses, spectral (electronic, infrared, Mössbauer, FAB mass and powder XRD) studies, conductance and magnetic susceptibility measurements. Infrared spectra suggested bidentate and bridging mode of coordination of both the carboxylate and hydroxycarboxylate anions along with Fe3O vibrations in the complexes. Mössbauer parameters indicated the presence of high-spin Fe(??) (S = 2) and high-spin Fe(???) (S = 5/2) centers in the complexes, confirming the valence-localized type of species. An intervalence-transfer band observed at 13,690-13,850 cm-1 range in the room-temperature electronic spectra of the complexes also suggested the complexes containing iron in mixed-valence state. Trinuclear nature of the complexes was confirmed by their FAB mass spectra. Magnetic moment values displayed octahedral geometry around each iron in the complexes and a net anti-ferromagnetic exchange coupling via ?-oxo atom related to mixed-valence pairs. A plausible structure for these complexes has been established on the basis of spectra and magnetic moment data.

Singh, Atresh Kumar; Singh, Alok Kumar

2013-08-01

408

Rotational bands terminating at maximal spin in the Rh-Pd region  

International Nuclear Information System (INIS)

The interplay between collective and non-collective degrees of freedom is well accounted for in configurations evolving from rotational behaviour to terminating sates. These states are characterized by spins built from the angular momentum of the valence holes and valence particles and by oblate (? = +60 degree) or prolate (? = -120 degree) shape. Results obtained with the multidetector EUROGAM2 are presented. The cases of 102Pd, which is the first nucleus in which terminating bands built on valence space and core excited configurations have been observed, and 102Rh, which is the first odd-odd nucleus between the Z = 28 and Z = 50 shell closures in which terminating bands have been found, are discussed. Comments are made on lifetimes and transitions feeding the terminations

409

Secondary seed dispersal and its role in landscape organization  

Science.gov (United States)

Mathematical models of banded vegetation patterns predict rapid upslope migration of vegetated patches not realized in field experiments, a key point of disagreement between theory and observation. It is shown that the disagreement between model results and field observations can arise from how seed migration dynamics is incorporated. Two approaches to biomass movement are used to test the hypothesis that secondary seed dispersal in overland flow inhibits band migration. The first is based on a coupling between down-slope water transport and seed advection. The second uses a heavy-tailed kernel-based representation of seed transport in which an anisotropic dispersal kernel combines the effect of isotropic primary and downslope secondary seed dispersal. This approach ensures that conclusions about secondary dispersal are independent of diffusive representations of biomass movement. The analysis demonstrates that secondary seed dispersal can retard upward movement of banded vegetation irrespective of the precise representation of biomass movement as long as the anisotropic effects are accounted for.

Katul, G. G.; Thompson, S.

2008-12-01

410

Birefringence and band structure of CdP2 crystals  

Science.gov (United States)

The spatial dispersion in CdP2 crystals was investigated. The dispersion is positive (nk||?>nk||?) at ?>?0 and negative (nk||?Kramers-Kronig analyses. All features were interpreted as optical transitions on the basis of both theoretical calculations of band structure.

Beril, S. I.; Stamov, I. G.; Syrbu, N. N.; Zalamai, V. V.

2013-08-01

411

Impurity band in B-doped diamond: an 11B NMR study  

Science.gov (United States)

We report an 11B NMR study of heavily boron-doped diamond. By comparing 11B spectra with those from 13C, we find that substitutional boron dopants are limited to 0.26 at.% in an isolated form. This observation reveals that, above the metal-insulator transition, boron dopants are incorporated into carbon sites mostly in the form of boron aggregates and suggests that, in addition to the holes in the valence band, an impurity band formed by boron aggregates plays an important role in the superconductivity in the heavily B-doped regime. A strong disorder effect deduced from Knight shift and nuclear spin-lattice relaxation rate measurements is attributed to the overlap between the intrinsic valence band and a boron impurity band.

Baek, S.-H.; Curro, N. J.; Ekimov, E. A.; Sidorov, V. A.; Bauer, E. D.; Thompson, J. D.

2009-06-01

412

Identification of the 0.95 eV luminescence band in n-type GaAs:Si  

International Nuclear Information System (INIS)

The luminescence band at 0.95 eV has been identified as originating from the transition within (SiGaVGaSiGa) complexes by comparing cathodoluminescence and positron annihilation spectra. The upper and lower energy levels of the molecule-like defect complexes are suggested to lie at 22 meV below the conduction band and at about 0.5 eV above the valence band, respectively

413

Thermodynamics of zinc insertion in CuGaS2:Ti, used as a modulator agent in an intermediate-band photovoltaic material  

Digital Repository Infrastructure Vision for European Research (DRIVER)

An intermediate-bandphotovoltaicmaterial, which has an isolated metallic band located between the top of the valence band and bottom of the conduction band of some semiconductors, has been proposed as third generation solar cell to be used in photovoltaic applications. Density functional theory calculations of Zn in CuGaS2:Ti have previously shown that, the intermediate-band position can be modulated in proportion of Zn insertion in such a way that increasing Zn concentration can lead to aban...

Semino?vski Pe?rez, Yohanna; Palacios Clemente, Pablo; Conesa, Jose Carlos; Wahno?n Benarroch, Perla

2011-01-01

414

Features of the band structure for semiconducting iron, ruthenium, and osmium monosilicides  

International Nuclear Information System (INIS)

The pseudopotential method has been used to optimize the crystal lattice and calculate the energy band spectra for iron, ruthenium and, osmium monosilicides. It is found that all these compounds are indirect-gap semiconductors with band gaps of 0.17, 0.22, and 0.50 eV (FeSi, RuSi, and OsSi, respectively). A distinctive feature of their band structure is the 'loop of extrema' both in the valence and conduction bands near the center of the cubic Brillouin zone.

415

Mapping of valence energy losses via energy-filtered annular dark-field scanning transmission electron microscopy  

International Nuclear Information System (INIS)

The advent of electron monochromators has opened new perspectives on electron energy-loss spectroscopy at low energy losses, including phenomena such as surface plasmon resonances or electron transitions from the valence to the conduction band. In this paper, we report first results making use of the combination of an energy filter and a post-filter annular dark-field detector. This instrumental design allows us to obtain energy-filtered (i.e. inelastic) annular dark-field images in scanning transmission electron microscopy of the 2-dimensional semiconductor band-gap distribution of a GaN/Al45Ga55N structure and of surface plasmon resonances of silver nanoprisms. In comparison to other approaches, the technique is less prone to inelastic delocalization and relativistic artefacts. The mixed contribution of elastic and inelastic contrast is discussed.