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1

Angle-resolved photoemission from oriented thin films of long alkyl molecules: valence band dispersion  

Science.gov (United States)

Angle-resolved photoemission spectra were measured using synchrotron radiation for oriented thin films of hexatriacontane, CH3(CH2)34CH3, and pentatriacontan-18-one, CH3(CH2)16CO(CH2)16CH3, with their long axes perpendicular to the substrate surface. The valence band dispersion was observed along the long molecular axis over the whole Brillouin zone for both compounds. The results demonstrate that the chemical disorder caused by the C=O group in the CH2 chain does not affect the band structure so much and both compounds show valence band dispersion similar to the dispersion in an ideal polyethylene chain. Further, we observed a band which was not observed in our previous work. It determines the top of the valence band at the ? point. Comparison with theoretical results indicates that it corresponds to a band of B1 (B2g at ?) symmetry which consists of only C2p atomic orbitals directed along the carbon chain.

Ueno, Nobuo; Fujimoto, Hitoshi; Sato, Naoki; Seki, Kazuhiko; Inokuchi, Hiroo

1990-01-01

2

Chalcogen doping at anionic site: A scheme towards more dispersive valence band in CuAlO2  

Science.gov (United States)

Using first-principles calculations, we propose to enhance the dispersion of the top of valence band at high-symmetry points by selective introduction of chalcogen (Ch) impurities at oxygen site. As ab-plane hole mobility of CuAlO2 is large enough to support a band-conduction model over a polaronic one at room temperature [M. S. Lee et al. Appl. Phys. Lett. 79, 2029, (2001); J. Tate et al. Phys. Rev. B 80, 165206, (2009)], we examine its electronic and optical properties normal to c-axis. Intrinsic indirectness of energy-gap at ?-point can be effectively removed along with substantial increase in density of states near Fermi level (EF) upon Ch addition. This can be attributed to S 2p-Cu 3d interaction just at or below EF, which should result in significantly improved carrier mobility and conductivity profile for this important p-type TCO.

Mazumder, Nilesh; Sen, Dipayan; Chattopadhyay, Kalyan Kumar

2013-02-01

3

Site-specific intermolecular valence-band dispersion in ?-phase crystalline films of cobalt phthalocyanine studied by angle-resolved photoemission spectroscopy.  

Science.gov (United States)

The valence band structure of ?-phase crystalline films of cobalt phthalocyanine (CoPc) grown on Au(111) is investigated by using angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. The photo-induced change in the ARPES peaks is noticed in shape and energy of the highest occupied molecular orbital (HOMO, C 2p) and HOMO-1 (Co 3d) of CoPc, and is misleading the interpretation of the electronic properties of CoPc films. From the damage-free normal-emission ARPES measurement, the clear valence-band dispersion has been first observed, showing that orbital-specific behaviors are attributable to the interplay of the intermolecular ?-? and ?-d interactions. The HOMO band dispersion of 0.1 eV gives the lower limit of the hole mobility for ?-CoPc of 28.9 cm(2) V(-1) s(-1) at 15 K. The non-dispersive character of the split HOMO-1 bands indicates that the localization of the spin state is a possible origin of the antiferromagnetism. PMID:25494764

Yamane, Hiroyuki; Kosugi, Nobuhiro

2014-12-14

4

Valence holes observed in nanodiamonds dispersed in water.  

Science.gov (United States)

Colloidal dispersion is essential for most nanodiamond applications, but its influence on nanodiamond electronic properties remains unknown. Here we have probed the electronic structure of oxidized detonation nanodiamonds dispersed in water by using soft X-ray absorption and emission spectroscopies at the carbon and oxygen K edges. Upon dispersion in water, the ?* transitions from sp(2)-hybridized carbon disappear, and holes in the valence band are observed. PMID:25597533

Petit, Tristan; Pflüger, Mika; Tolksdorf, Daniel; Xiao, Jie; Aziz, Emad F

2015-02-01

5

Investigation of the XPS valence band structure from Sn chalcogenides  

International Nuclear Information System (INIS)

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

6

Valence band effective Hamiltonians in nitride semiconductors  

Science.gov (United States)

Valence band effective Hamiltonians are useful to determine the electronic states of shallow impurities, quantum wells, quantum wires and quantum dots within the effective mass approximation. Although significant experimental and theoretical work has been performed, basic parameters such as the Rashba Sheka Pikus (RSP) Hamiltonian parameters are still uncertain. In this work, the electronic band structures of AlN, GaN and InN, all in the wurtzite crystal structure, as well as the RSP Hamiltonian parameters are determined by using the QSGW approximation in a FP-LMTO implementation. The corrections offered by this approach beyond the LDA are important to obtain the splittings and effective masses accurately. The present GW implementation, which allows for a real space representation of the self-energy, enables us to interpolate exactly to a fine k-mesh and hence to obtain accurate effective masses. We find the crystal field splitting in GaN (12 meV) in much closer agreement with experiment than previous work and obtain a negative SO coupling for InN. Moreover, we have generalized the method of invariants to crystals with orthorombic symmetry, such as ZnSiN2 ZnGeN2, ZnSnN2 and CdGeN2 and determined the corresponding Hamiltonian parameters.

Punya, Atchara; Schwertfager, Nucharee; Lambrecht, Walter

2012-02-01

7

Valence Band X-Ray Emission Spectra of Compressed Germanium  

International Nuclear Information System (INIS)

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic ?-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic ?-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation

8

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

9

Weak morphology dependent valence band structure of boron nitride  

Science.gov (United States)

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

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

2013-08-01

10

Valence bands spin splitting in GaAs/AlGaAs quantum wells  

Science.gov (United States)

A quantitative comparison between different model calculations of valence band states in GaAs/AlGaAs heterostructures is presented. We demonstrate that a 14-band k.p Hamiltonian using a completely new parameterization based on fits of the tight-binding band structure leads to energy dispersion relations in excellent agreement with experiment, whereas previous parameterizations result in significant deviations. The relevance of the present results to the calculation of spin-related phenomena is discussed.

Nguyen-Quang, T.; Jancu, J.-M.; Voisin, P.

2006-08-01

11

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

12

Luminescence due to radiative transitions between valence band and upper core band in ionic crystals (crossluminescence)  

International Nuclear Information System (INIS)

In some wide band gap halides of heavy alkali and alkaline-earth metals (BaF2, CsCl, RbF etc.) the energy separation between the valence band involving mainly the halogen electronic states and the upper cation core band is smaller than the band gap. Under the excitation generating holes in the core band radiative electronic transitions from the valence band to the core band take place giving rise to a specific kind of intrinsic emission which is proposed to call crossluminescence. Its decay time is about 1 ns and the thermal quenching starts at temperatures well above room temperature. The width of the emission spectrum practically coincides with the width of the valence band deduced from ultraviolet photoelectron spectroscopy and the shape of the spectrum depicts the electronic structure of the valence band. (author)

13

Photon energy dependent valence band response of metallic nanoparticles.  

Science.gov (United States)

We show that the valence band response to photon impact in metallic nanoparticles is highly energy dependent. This is seen as drastic variations of cross sections in valence photoionization of free and initially charge-neutral nanosized metal clusters. The effect is demonstrated in a combined experimental and theoretical study of Rb clusters. The experimental findings are interpreted theoretically using a jellium model and superatom description. The variations are attributed to the changing overlap with the photon energy between the wave functions of diffuse delocalized valence electrons and continuum electrons producing a series of minima in the cross section. PMID:22107629

Jänkälä, K; Tchaplyguine, M; Mikkelä, M-H; Björneholm, O; Huttula, M

2011-10-28

14

Intrinsic valence and conduction bands of Si(111)-1{times}1  

Energy Technology Data Exchange (ETDEWEB)

The valence bands of the unreconstructed H-Si(111)-1{times}1 are investigated using angle-resolved ultraviolet photoelectron spectroscopy. The high quality of the surface and the absence of reconstruction allow us to observe bulk bands comparable to theoretical calculations. The asymmetric dispersion of the valence bands along the {bar {ital M}}{prime}{bar {Gamma}}{bar {ital M}} direction of the surface Brillouin zone confirms the asymmetry observed for the conduction bands. Such an asymmetry, stemming from the fact that the family of (11{bar 2}) planes are not mirror planes in the bulk of Si, provides a supplementary means of disentangling bulk states from surface states. {copyright} {ital 1996 The American Physical Society.}

He, Y.; Bouzidi, S.; Han, B.; Yu, L.; Thiry, P.A.; Caudano, R. [Laboratoire Interdisciplinaire de Spectroscopie Electronique and Laboratoire de Spectroscopie Moleculaire de Surface, Institute for Studies in Interface Sciences, Facultes Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B-5000 Namur (Belgium); Debever, J. [Groupe de Physique des Etats Condenses, Universite de la Mediterranee, 163, avenue de Luminy, Case 901, F-13288 Marseille Cedex 9 (France)

1996-12-01

15

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

16

Valence band photoemission studies of clean metals  

Energy Technology Data Exchange (ETDEWEB)

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..nu.. < 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.

Wehner, P.S.

1978-04-01

17

Measurement of valence band structure in arbitrary dielectric films  

International Nuclear Information System (INIS)

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

18

Instanton contributions to the valence band of the double Sine-Gordon potential  

International Nuclear Information System (INIS)

The energy dispersion relation for the valence band of the double sine-Gordon potential is calculated, approximating the tunneling amplitude by a sum of contributions of multi-instantons and anti-instatons trajectories. The interesting feature of this potential is that they have to deal with two types of instantons, as there are two different potential barriers within one period of the potential. The results with the standard WKB approximation are compared. (Author)

19

Instanton contributions to the valence band of the double sine-Gordon potential  

International Nuclear Information System (INIS)

The energy dispersion relation for the valence band of the double sine-Gordon potential is calculated, approximating the tunneling amplitude by a sum of contributions of instantons and anti-instantons trajectories. The interesting feature of this potential is that it now has to deal with two types of instantons, as there are two different potential barriers within one period of the potential. The results are compared with the standard WKB approximation. (Author)

20

Nonideal anion displacement, band gap variation, and valence band splitting in Cu-In-Se compounds  

International Nuclear Information System (INIS)

Polycrystalline thin films of ternary chalcopyrite CuInSe2 and defect compounds CuIn3Se5 and CuIn5Se8 are prepared in vacuum by three-source coevaporation method. Structural and optical characterizations of the films are done using X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and optical absorbance spectra measurements. With variation in the composition of CuInSe2, a change over from p-type to n-type conductivity is observed (as noted by the hot probe method). The deformation parameters and the anion displacements are calculated from the X-ray diffraction data, and the cation-anion bond lengths are deduced. The dependence of band gap variation on nonideal anion displacement in the ternary compounds and the effect of Se-p-Cu-d repulsion on band gap are studied. The threefold optical structure observed in the fundamental absorption region of the absorption spectra is analysed to extract the valence band splitting parameters. Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation is used to determine the crystal field splittings and spin orbit splittings, and the linear hybridization model is used to calculate the percentage of d-orbital and p-orbital contribution to hybridization in the compounds under consideration

 
 
 
 
21

Valence-band of cubic semiconductors: Clifford algebra approach II  

Energy Technology Data Exchange (ETDEWEB)

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 Cl{sub 4,1}. 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.

Dargys, A, E-mail: dargys@pfi.l [Semiconductor Physics Institute, A. Gostauto 11, LT-01108 Vilnius (Lithuania)

2010-07-15

22

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.

23

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

24

Inter-Valence-Subband/Conduction-Band-Transport IR Detectors  

Science.gov (United States)

Infrared (IR) detectors characterized by a combination of (1) high-quantum-efficiency photoexcitation of inter-valence-subband transitions of charge carriers and (2) high-mobility conduction- band transport of the thus-excited charge carriers have been proposed in an effort to develop focal-plane arrays of such devices for infrared imaging. Like many prior quantum-well infrared photodetectors (QWIPs), the proposed devices would be made from semiconductor heterostructures. In order to obtain the combination of characteristics mentioned above, the proposed devices would be designed and fabricated in novel InAs/GaSb superlattice configurations that would exploit a phenomenon known in the semiconductor art as type-II broken-gap band offset.

Ting, David; Gunapala, Sarath; Bandara, Sumith

2004-01-01

25

The vibronic optical bands of mixed valence dimeric clusters  

International Nuclear Information System (INIS)

The review of vibronic optical bands of mixed valence (MV) cluster is given. The MV dimeric clusters consist of two metal ions Me+n and Me+(n+1) in different oxidation states. The metal ions are exchange coupled. The 'extra' electron migration (double exchange) results in the series of exchange-resonance multiplets. The pairs of exchange-resonance states possessing the same total spin and different parities are coupled by the pseudo-Jahn-Teller vibronic interaction. The band shapes of allowed (?S=0) transitions are observed. The effective spin-dependent dipole moment operator for spin-forbidden transition is constructed. It is shown that the spin-allowed transitions bands have the ?-polarization (along the cluster axis). In the framework of the semiclassical approach the vibronic band-shape theory is developed for both cases of all allowed and forbidden transitions. The detected peculiarities of the band-shapes for these transitions closely connected with the key parameters of MV cluster, i.e. pseudo-Jahn-Teller coupling parameter, Heisenberg-type and double exchange parameters. For the solid coordination MV compounds vibronic coupling with the crystal lattice vibrations (optic and acoustic) is taken into account. IN the case under consideration the expressions for vibronic reduction factors for exchange and tunnel parameters are deduced. The interaction of each centre with crystal lattice vibrations lead to the addition coupling between electr to the addition coupling between electronic shells. (author)

26

Valence level bands and Fermi surface of decagonal Al-Cu-Co  

Energy Technology Data Exchange (ETDEWEB)

Valence electronic states in quasicrystals seem to defy the concept of Bloch states because of the absence of translational periodicity. However, this may not necessarily preclude the existence of delocalized electronic states in quasicrystals, since critical electronic states which fall off with a power law may exist, and evidence for delocalized states has been found in quasicrystals in the region of the deeper valence bands. For an analysis of the transport properties of quasicrystals an investigation of the region near the Fermi level is important, however, Here we examine the s-p-derived electronic structure of decagonal Al-Cu-Co in this region using angle-resolved photoemission. These states are accessible in Al-Cu-Co because, unlike in Al-Ni-Co, the d bands are well removed and do not interfere. We find that the electronic structure can be well represented by parabolic dispersing bands in a region of about 1 eV below the Fermi level. The data are analyzed within a model that uses a subset of specific reciprocal lattice vectors, and that provides a consistent description of the photoemission intensity distribution and the dispersion of the observed state signatures.

Theis, Wolfgang [Fachbereich Physik, FU Berlin (Germany); Dil, Jan Hugo; Horn, Karsten [Fritz Haber Institute of the MPG, Berlin (Germany); Shukla, Ajay; Kim, JeongWon [UGC-DAE-CSR, Indore (India); Koh, Hoon; Rotenberg, Eli [ALS, Lawrence Berkeley Lab. (United States); Gille, Peter [Dept. Geosciences, LMU Munich (Germany)

2009-07-01

27

The Role of Fermi Resonance in Formation of Valence Band of Water Raman Scattering  

Directory of Open Access Journals (Sweden)

Full Text Available The role of Fermi resonance in formation of valence band of water Raman scattering was investigated. Simultaneous measurement of characteristics of bending and valence bands of water in D2O solutions, KBr, and KCl and using genetic algorithms in conjunction with variation methods allowed increasing accuracy of estimation of Fermi resonance coupling constant and of Fermi resonance contribution into formation of water Raman valence band.

Sergey A. Burikov

2008-06-01

28

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

29

Electronic Structure and Valence Band Spectra of Bi4Ti3O12  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

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

1995-01-01

30

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2011-04-15

31

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)

32

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

Science.gov (United States)

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.

Wang, Guan-Yu; Zhang, He-Ming; Gao, Xiang; Wang, Bin; Zhou, Chun-Yu

2012-05-01

33

Rare earth 4f hybridization with the GaN valence band  

International Nuclear Information System (INIS)

The placement of the Gd, Er and Yb 4f states within the GaN valence band has been explored by both experiment and theory. The 4d–4f photoemission resonances for various rare-earth(RE)-doped GaN thin films (RE = Gd, Er, Yb) provide an accurate depiction of the occupied 4f state placement within the GaN. The resonant photoemission show that the major Er and Gd RE 4f weight is at about 5–6 eV below the valence band maximum, similar to the 4f weights in the valence band of many other RE-doped semiconductors. For Yb, there is a very little resonant enhancement of the valence band of Yb-doped GaN, consistent with a large 4f14-? occupancy. The placement of the RE 4f levels is in qualitative agreement with theoretical expectations. (paper)

34

Interband electronic Raman scattering of polarized light in cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The cross-sections are calculated for the Raman scattering processes with polarized light in cubic semiconductors where the valence band top is fourfold degenerate ?8. The exciton effect is taken into account. (author). 17 refs

35

Valence-band satellite in the ferromagnetic nickel: LDA+DMFT study with exact diagonalization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The valence-band spectrum of the ferromagnetic nickel is calculated using the LDA+DMFT method. The auxiliary impurity model emerging in the course of the calculations is discretized and solved with the exact diagonalization, or, more precisely, with the Lanczos method. Particular emphasis is given to spin dependence of the valence-band satellite that is observed around 6 eV below the Fermi level. The calculated satellite is strongly spin polarized in accord with experimental...

Kolorenc, Jindrich; Poteryaev, Alexander I.; Lichtenstein, Alexander I.

2012-01-01

36

Unveiling the valence band nature of GaMnAs anisotropic carriers by inter-band tunneling spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

The discovery of highly anisotropic properties of magnetic tunnel junctions based on GaMnAs has led to new concepts for spintronics devices, such as magnetic memories based on anisotropy relaxation in nanostructures, or magnetic switches relying on electrical control of the magnetic anisotropy. However, despite the success of mean-field theories of hole-induced ferromagnetism, a direct evidence of the existence of Bloch states is still missing, and the nature of carriers remains controversial (impurity-band vs. valence-band models). Here, based on inter-band tunneling spectroscopy of p-GaMnAs/n-GaAs Zener-Esaki diodes, we directly evidence the valence band nature of GaMnAs carriers and, importantly, reveal the spin-split valence bands dependence of their cubic and uniaxial anisotropies (which can be tuned under applied bias or magnetic field). In particular, the Fermi energy is found well below the top of the valence band, and the energy dependence of Bloch states anisotropies shows some specific features predicted by a k.p modelling of the spin-split valence bands.

Giraud, Romain; Granada, Mara; Briones, Edgar; Gennser, Ulf; Lemaitre, Aristide; Faini, Giancarlo [CNRS/LPN, Route de Nozay, 91460 Marcoussis (France)

2011-07-01

37

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

CERN Document Server

In this work we present the electronic band structure for (001)--CdTe interfaces with some other II--VI zinc blende semiconductors. We assume ideal interfaces. We use tight binding Hamiltonians with an orthogonal basis (s p^3 s^*). We make use of the well--known Surface Green's Function Matching method to calculate the interface band structure. In our calculation the dominion of the interface is constituted by four atomic layers. We consider here anion--anion interfaces only. We have included the non common either anion or cation (CdTe/ZnSe), common cation (CdTe/CdSe), and common anion (CdTe/ZnTe) cases. We have aligned the top of the the valence band at the whole interface dominion as the boundary condition. The overall conclusion is that the interface is a very rich space where changes in the band structure with respect to the bulk do occur. This is true not only at interfaces with no common atoms but also at the ones with either common cation or anion atoms irrespective to the fact that the common atomic l...

Olguin, D

1996-01-01

38

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

39

Electronic Structure of Molybdenum Disulfide and Related Disulfide Species: Hr-Xps Hydrogen Adsorption, and Valence Band Analysis.  

Science.gov (United States)

Experimental valence bands were obtained using high resolution electron spectroscopy for chemical analysis (HR-ESCA). Theoretical valence bands were calculated using solid state Extended Huckel theory. Comparison of 2-D MoS_2 theoretical valence bands with the experimental HR-ESCA valence band of polycrystalline MoS_2<=d to parametrization of the S3s, S3p and Mo4d atomic ionization potentials, H _{rm ii}, Slater-type coefficients, c_{rm i}, and exponents, zeta_{rm i}. The S3s and S3p parameters found for MoS_2 were also used to obtain NbS_2 and RuS_2 theoretical valence bands. Curvature and energetic positions of theoretical energy dispersion curves approximated those of experimental curves obtained by ultraviolet photoelectron spectroscopy, UPS. A direct gap (K to K) was calculated for 2-D MoS_2 while an indirect gap (Gamma to K) was calculated for 3-D MoS_2. The indirect gap in 3-D MoS_2 is shown to arise from antibonding interactions of crystal orbitals across the Van der Waals gap at Gamma. Electronic structure studies of 1-D (MoS _2)_5 systems led to the conclusion that addition of edges results in the formation of unoccupied edge states at and above the Fermi level. HR-ESCA valence band comparisons of single crystal and polycrystalline MoS_2 show higher intensity on the low binding energy side of the highest occupied band in the sample containing a higher number of edge sites, polycrystalline MoS_2. Theoretical binding enthalpies for hydrogen adsorption on molybdenum and sulfur sites on various molybdenum disulfide systems were calculated. Of the sulfur sites, fully-coordinated basal plane sites were favored over 1-coordinate and 2 -coordinate edge sites. Of the molybdenum sites, 2-, 3 -, 4-, and 5-coordinate edge sites were favored in that order over fully-coordinated basal plane sites. Angle resolved HR-ESCA of single crystal MoS _2 showed the existence of forward focusing at a polar angle of ca. 49^circ and at azimuthal angles separated by 60^ circ. Forward focusing of second atomic layer molybdenum photoelectrons by first atomic layer sulfurs is shown to occur at the following azimuthal angles: 0 ^circ, 120^ circ and 240^circ (arbitrary designation of 0^circ). Forward focusing of fifth atomic layer molybdenum photoelectrons by fourth atomic layer sulfurs is shown to occur at the following azimuthal angles: 60^circ , 180^circ and 300 ^circ. (Abstract shortened by UMI.).

Richards-Babb, Michelle

1993-01-01

40

Theory of two-photon absorption by exciton states in cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The coefficient of the absorption of two polarized photons is calculated for direct band gap semiconductors with degenerate valence bands. Wannier-Mott exciton states are included in both the intermediate and final states. Numerical calculations are performed for ZnSe and are compared with Sondergeld's experimental and theoretical results. (author). 11 refs, 2 tabs

 
 
 
 
41

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

42

Valence band hybridization in N-rich GaN1-xAsx alloys  

Energy Technology Data Exchange (ETDEWEB)

We have used photo-modulated transmission and optical absorption spectroscopies to measure the composition dependence of interband optical transitions in N-rich GaN{sub 1-x}As{sub x} alloys with x up to 0.06. The direct bandgap gradually decreases as x increases. In the dilute x limit, the observed band gap approaches 2.8 eV; this limiting value is attributed to a transition between the As localized level, which has been previously observed in As-doped GaN at 0.6 eV above the valence band maximum in As-doped GaN, and the conduction band minimum. The structure of the valence band of GaN{sub 1-x}As{sub x} is explained by the hybridization of the localized As states with the extended valence band states of GaN matrix. The hybridization is directly confirmed by soft x-ray emission experiments. To describe the electronic structure of the GaN{sub 1-x}As{sub x} alloys in the entire composition range a linear interpolation is used to combine the effects of valence band hybridization in N-rich alloys with conduction band anticrossing in As-rich alloys.

Wu, J.; Walukiewicz, W.; Yu, K.M.; Denlinger, J.D.; Shan, W.; Ager III, J.W.; Kimura, A.; Tang, H.F.; Kuech, T.F.

2004-05-04

43

Dissipation of core-hole momentum by phonons in soft-x-ray radiation processes from valence band to core level of wide-gap insulators  

Science.gov (United States)

The role of phonons in the soft-x-ray radiation process from a valence band to a core level in an insulator is studied theoretically. A three-band system composed of a dispersionless core band, a conduction band, and a valence band, with wide energy gaps between them, is taken as a typical example. Phonons with a finite dispersion are assumed to couple weakly only with a hole in the core band (core hole). Using this model, we calculate the resonant second-order optical process composed of an excitation of an electron from the core band to the conduction band by an incident x ray, and a subsequent transition from the valence band to the core band by radiating another x ray. Without the phonons, the momentum of the core hole is expected to be well defined by the resonance condition of the incident x ray. However, this momentum is dissipated by the phonons. If the radiation occurs completely after this dissipation, we obtain a so-called luminescence, which is independent of the incident x ray. In this case, the spectral shape fully reflects the density of states (DOS) of the valence band. However, if the radiation occurs long before this dissipation effect, we obtain a resonant Raman scattering that depends on the incident x ray. The spectral shape of this Raman scattering has a sharp peak, quite different from the DOS. The relative intensity between these two components is determined by the phonon dispersion, the lifetime of the core hole, and the core-hole-phonon coupling constant. From this theoretical framework, we have concluded that there are various cases, i.e., Raman-dominant cases and luminescence-dominant cases, as well as intermediate cases, in good agreement with various experimental observations. The B 1s2p transitions of cubic BN are concluded to correspond to a luminescence-dominant case.

Minami, Tatsuya; Nasu, Keiichiro

1998-05-01

44

Effect of valence band splitting on the biexciton binding energy in wurtzite-type semiconductors  

International Nuclear Information System (INIS)

A lower bound for the biexciton binding energy in wurtzite-type semiconductors is calculated using a 64x64 matrix Hamiltonian obtained by the kxp method which takes into account the interband coupling-induced nonparabolicity of the two upper valence bands A and B as well as their different anisotropies and symmetries. A properly symmetric variational ansatz for the ground state vector of the GAMMA1 biexciton (with AB and BB biexciton states admixed) yields an analytic expression for the binding energy. Its numerical evaluation for CdS as a function of the nonparabolicity parameters U and W shows a sensitive dependence on U, but only a weak dependence on W. It turns out that a strong valence band coupling (U = - 2(h/2?)2/2m0 as proposed earlier) would result in a marked increase of biexciton binding energy due to the strong nonparabolicity of the A valence band in this case. (author)

45

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

46

Conformation effects in the XPS valence band spectra of aliphatic polyesters  

Energy Technology Data Exchange (ETDEWEB)

XPS valence band spectra are reported for seven aliphatic polyesters. By comparison of the spectra with that of PMMA a qualitative understanding of the spectral features is achieved. By comparing valence band spectra of semi-crystalline solid and polymer melt samples, chain conformation effects are identified for five of the polyesters. For four of these the effects may be attributed to changes in methylenic hyperconjugation interactions on melting but the fifth contains (CH{sub 2}){sub n} segments that are too short for methylenic hyperconjugation to occur. Only one of the polyesters showed a significant conformation effect in the C 1s spectrum.

Beamson, G. [National Centre for Electron Spectroscopy and Surface Analysis, CCLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD (United Kingdom)], E-mail: g.beamson@dl.ac.uk

2007-02-15

47

Observation of valence band electron emission from n-type silicon field emitter arrays  

Science.gov (United States)

Electron emission from the valence band of n-type Si field emitter arrays is reported. High electrostatic field at the surface of Si was achieved by reducing the radius of the emitter tip. Using oxidation sharpening, 1 ?m aperture polycrystalline Si gate, n-type Si field emitter arrays with small tip radius (˜10 nm) were fabricated. Three distinct emission regions were observed: conduction band emission at low gate voltages, saturated current emission from the conduction band at intermediate voltages, and valence band plus conduction band emission at high gate voltages. Emission currents at low and high voltages obey the Fowler-Nordheim theory. The ratio of the slopes of the corresponding Fowler-Nordheim fits for these two regions is 1.495 which is in close agreement with the theoretical value of 1.445.

Ding, Meng; Kim, Han; Akinwande, Akintunde I.

1999-08-01

48

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

International Nuclear Information System (INIS)

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.

49

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

50

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

Energy Technology Data Exchange (ETDEWEB)

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 x{sub c}=(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 ?{sub 9} symmetry of b{sub ?{sub 9}}=0.85eV, and propose a possible bowing for the crystal field energy of b{sub cf}=?0.12eV. A comparison of the light extraction efficiency perpendicular and parallel to the c axis of Al{sub x}Ga{sub 1-x}N/Al{sub y}Ga{sub 1-y}N quantum well structures is discussed for different compositions.

Neuschl, B., E-mail: benjamin.neuschl@uni-ulm.de; Helbing, J.; Knab, M.; Lauer, H.; Madel, M.; Thonke, K. [Institute of Quantum Matter / Semiconductor Physics Group, University of Ulm, Albert-Einstein-Allee 45, 89069 Ulm (Germany); Meisch, T.; Forghani, K.; Scholz, F. [Institute of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, 89069 Ulm (Germany); Feneberg, M. [Institut für Experimentelle Physik, Otto-von-Guericke-Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg (Germany)

2014-09-21

51

Valence band variation in Si (110) nanowire induced by a covered insulator  

International Nuclear Information System (INIS)

In this work, we investigate strain effects induced by the deposition of gate dielectrics on the valence band structures in Si (110) nanowire via the simulation of strain distribution and the calculation of a generalized 6×6k·p strained valence band. The nanowire is surrounded by the gate dielectric. Our simulation indicates that the strain of the amorphous SiO2 insulator is negligible without considering temperature factors. On the other hand, the thermal residual strain in a nanowire with amorphous SiO2 insulator which has negligible lattice misfit strain pushes the valence subbands upwards by chemical vapour deposition and downwards by thermal oxidation treatment. In contrast with the strain of the amorphous SiO2 insulator, the strain of the HfO2 gate insulator in Si (110) nanowire pushes the valence subbands upwards remarkably. The thermal residual strain by HfO2 insulator contributes to the up-shifting tendency. Our simulation results for valence band shifting and warping in Si nanowires can provide useful guidance for further nanowire device design. (classical areas of phenomenology)

52

Electronic structure of the diamond (111) 1 x 1 surface: Valence-band structure, band bending, and band gap states  

International Nuclear Information System (INIS)

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

53

Theory of valence-band and core-level photoemission from plutonium dioxide  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The correlated-band theory implemented as a combination of the local-density approximation with the dynamical mean-field theory is applied to PuO2. An insulating electronic structure, consistent with the experimental valence-band photoemission spectra, is obtained. The calculations yield a nonmagnetic ground state that is characterized by a noninteger filling of the plutonium 5f shell. The noninteger filling as well as the satellites appearing in the 4f core-level photoemiss...

Kolorenc, Jindrich; Kozub, Agnieszka L.; Shick, Alexander B.

2014-01-01

54

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

55

Theoretical analysis of optical gain in quantum well lasers including valence band-mixing effect  

International Nuclear Information System (INIS)

The linear optical gain in the AlGaAs/GaAs quantum well lasers is studied theoretically, taking into account the valence-band mixing effect. Our approach is based on the multiband effective-mass theory (k p method) and the density-matrix formalism. In order to obtain the valence band's structure we employ the 4x4 Luttinger-Kohn Hamiltonian, neglecting the coupling to the split-off band. The spectral dependence of the linear optical gain is calculated using the density-matrix method with interband relaxation. Finally, we analyse the spatial distribution of the optical gain in the quantum well region for the photon energy corresponding to the peak value of the linear gain. (author)

56

Energy spectrum of second valence band in PbSnTe solid solutions  

International Nuclear Information System (INIS)

PbTe, PbSnTe solid solutions have a complex structure of valence band. Energy spectrum of the second valence band was calculated, effective masses and g factor were determined in the paper. P model was used in calculations. Electron spectrum in this model is constructed using metal spectrum of praphase -a hypothetic crystal with a simple cubic lattice and half-filled P bands, appeared from orbitales. It is shown that in Pbsub(1-x)Snsub(x)Te solid solutions energy gap and effective mass depend slightly on composition. P model parameters calculated in the approximation of a virtual crystal for composition x=0.2 give effective masses m2*=0.2 m0, m2*=0.04 m0

57

Electronic structure of alkane chains. Complete one-dimensional band structures of the valence states  

Science.gov (United States)

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

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

1994-03-01

58

Comparative Study of Auger-Free Luminescence and Valence-Band Photoemission in Wide-Gap Materials  

Science.gov (United States)

The line shape of Auger-free luminescence (AFL), caused by radiative recombination of holes in the outermost-core band with valence electrons, has been studied for CsCl, CsBr and BaF2 in comparison to photoelectron spectra of the valence bands. It is confirmed that the main part of AFL locates on the low-energy side of the valence-band region estimated from the maximum of the outermost-core band in the photoelectron spectra. Furthermore, its low-energy tail extends by approximately 0.5 1.0 eV below the minimum of the valence band. These results clearly indicate that the core hole induces considerable lattice distortion around itself, which is kept well during the radiative transition. The line shape of AFL is favorably explained in terms of an energy-band picture in which the lattice relaxation is taken into account in principle.

Itoh, Minoru; Kamada, Masao

2001-11-01

59

Application of factor analysis to XPS valence band of superparamagnetic iron oxide nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

X-Ray photoelectron spectra of nano-sized superparamagnetic iron oxide nanoparticles were examined with the aim to discriminate the different degree of iron oxidation. Careful analysis of the valence band regions reveals the presence of both Fe{sub 3}O{sub 4} and Fe{sub 2}O{sub 3}. The application of factor analysis enabled us to extract the relative molar concentrations of these oxides in the nanoparticles. This is of particular interest in improving the magnetic properties of iron oxide nanoparticles whose superparamagnetic character can be optimized to obtain better contrast in images from nuclear magnetic resonance. As a result, the factor analysis allows tuning the nanoparticle synthesis conditions in order to obtain the optimal magnetic properties for imaging. Results obtained by the XPS valence band analysis were compared to the transmission electron microscopy, X-ray diffraction and Raman measurements.

Minati, L., E-mail: luminati@fbk.eu [FBK, Via Sommarive 18, Povo 38123 Trento (Italy); Micheli, V. [FBK, Via Sommarive 18, Povo 38123 Trento (Italy); Rossi, B. [FBK, Via Sommarive 18, Povo 38123 Trento (Italy); Dept. Physics University of Trento, Via Sommarive 14, Povo 38123 Trento (Italy); Migliaresi, C.; Dalbosco, L. [Dept. Material Engineering and Industrial Technologies University of Trento, via Mesiano 77, 38100 Trento (Italy); Bao, G.; Hou, S. [Dept. of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 30332-0535 (United States); Speranza, G. [FBK, Via Sommarive 18, Povo 38123 Trento (Italy)

2011-10-01

60

Valence band structure of i-ZnMgEr quasicrystal: photoemission study  

International Nuclear Information System (INIS)

The results of photoemission study of icosahedral single-grain ZnMgEr quasicrystals are presented. Synchrotron radiation photoemission measurements were performed on in situ cleaved samples at 10-10 m-bar pressure and low, 140-150 K, temperature. The valence band photoemission spectra measured reveal a simple-metal type valence band of i-ZnMgEr with a distinct Fermi edge cutoff and a spectral feature at 0.7 eV below ?F. Analysis of the PE spectra shows that the spectral feature observed corresponds to the van Hove singularities in the density of states, which are due to intersections of the Fermi surface with the 222100 and 311111 Bragg planes

 
 
 
 
61

Accounting for many-body correlation effects in the calculation of the valence band photoelectron emission spectra of ferromagnets  

Energy Technology Data Exchange (ETDEWEB)

The influence of dynamical correlation effects on the valence band photoelectron emission of ferromagnetic Fe, Co and Ni has been investigated. Angle-resolved as well as angle-integrated valence band photoelectron emission spectra were calculated on the basis of the one-particle Green's function, which was obtained by using the fully relativistic Korringa-Kohn-Rostoker method. The correlation effects have been included in terms of the electronic self-energy which was calculated self-consistently within Dynamical Mean-Field Theory (DMFT). In addition a theoretical approach to calculate high-energy angle-resolved valence band photoelectron emission spectra is presented.

Minar, J. [Department Chemie und Biochemie, Physikalische Chemie, Universitaet Muenchen, Butenandtstr. 5-13, D-81377 Munich (Germany)]. E-mail: jan.minar@cup.uni-muenchen.de; Chadov, S. [Department Chemie und Biochemie, Physikalische Chemie, Universitaet Muenchen, Butenandtstr. 5-13, D-81377 Munich (Germany); Ebert, H. [Department Chemie und Biochemie, Physikalische Chemie, Universitaet Muenchen, Butenandtstr. 5-13, D-81377 Munich (Germany); Chioncel, L. [Institut fuer Theoretische Physik - Computational Physics, Technische Universitaet Graz, A-8010 Graz (Austria); Lichtenstein, A. [Institut fuer theoretische Physik, Universitaet Hamburg, 20355 Hamburg (Germany); De Nadai, C. [European Synchrotron Radiation Facility, BoIte Postale 220, 38043 Grenoble Cedex (France); Brookes, N.B. [European Synchrotron Radiation Facility, BoIte Postale 220, 38043 Grenoble Cedex (France)

2005-07-21

62

Accounting for many-body correlation effects in the calculation of the valence band photoelectron emission spectra of ferromagnets  

International Nuclear Information System (INIS)

The influence of dynamical correlation effects on the valence band photoelectron emission of ferromagnetic Fe, Co and Ni has been investigated. Angle-resolved as well as angle-integrated valence band photoelectron emission spectra were calculated on the basis of the one-particle Green's function, which was obtained by using the fully relativistic Korringa-Kohn-Rostoker method. The correlation effects have been included in terms of the electronic self-energy which was calculated self-consistently within Dynamical Mean-Field Theory (DMFT). In addition a theoretical approach to calculate high-energy angle-resolved valence band photoelectron emission spectra is presented

63

Valence band study of LaNiO3-? thin films  

International Nuclear Information System (INIS)

The resonant photoemission spectroscopy was used to study the surface electronic structure under La 4d?4f and Ni 3p?3d photo-excitation of thin LaNiO3-? 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?4f threshold is accompanied by resonance of the N4,5O2,3O2,3 and N4,5O2,3V 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?3d threshold indicate the loss of nickel species at the LaNiO3-? film surface after heat treatment.

64

Study of the structure of the valence band edge in bismuth-antimony telluride films  

International Nuclear Information System (INIS)

The behaviour of kinetic coefficients in Bi2-xSbxTe3 films with high hole concentrations is studied. The temperature dependences of the coefficients of heat conduction, Hall coefficient and thermal e.m.f. of films with different hole concentrations are given. The effect of the heavy hole zone on the transfer phenomenon is studied. The obtained results explicitly reveals the two-band pattern of the Bi2-xSexTe3 valence band. The effective mass of the state density in the second subband is estimated

65

Valence band offsets at Cu(In,Ga)Se{sub 2}/Zn(O,S) interfaces  

Energy Technology Data Exchange (ETDEWEB)

The energy band alignment at interfaces between Cu-chalcopyrites and Zn(O,S) buffer layers, which are important for thin-film solar cells, are considered. Valence band offsets derived from X-ray photoelectron spectroscopy for Cu(In,Ga)Se{sub 2} absorber layers with CdS and Zn(O,S) compounds are compared to theoretical predictions. It is shown that the valence band offsets at Cu(In,Ga)Se{sub 2}/Zn(O,S) interfaces approximately follow the theoretical prediction and vary significantly from sample to sample. The integral sulfide content of chemical bath deposited Zn(O,S) is reproducibly found to be 50-70%, fortuitously resulting in a conduction band offset suitable for solar cell applications with Cu(In,Ga)Se{sub 2} absorber materials. The observed variation in offset can neither be explained by variation of the Cu content in the Cu(In,Ga)Se{sub 2} near the interface nor by local variation of the chemical composition. Fermi level pinning induced by high defect concentrations is a possible origin of the variation of band offset. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Adler, Tobias; Klein, Andreas [Surface Science Division, Institute of Materials Science, Technische Universitaet Darmstadt, Petersenstrasse 32, 64287, Darmstadt (Germany); Botros, Miriam [Surface Science Division, Institute of Materials Science, Technische Universitaet Darmstadt, Petersenstrasse 32, 64287, Darmstadt (Germany); Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung Baden-Wuerttemberg (ZSW), Industriestrasse 6, 70565, Stuttgart (Germany); Witte, Wolfram; Hariskos, Dimitrios; Menner, Richard; Powalla, Michael [Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung Baden-Wuerttemberg (ZSW), Industriestrasse 6, 70565, Stuttgart (Germany)

2014-09-15

66

Valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces  

International Nuclear Information System (INIS)

The energy band alignment at interfaces between Cu-chalcopyrites and Zn(O,S) buffer layers, which are important for thin-film solar cells, are considered. Valence band offsets derived from X-ray photoelectron spectroscopy for Cu(In,Ga)Se2 absorber layers with CdS and Zn(O,S) compounds are compared to theoretical predictions. It is shown that the valence band offsets at Cu(In,Ga)Se2/Zn(O,S) interfaces approximately follow the theoretical prediction and vary significantly from sample to sample. The integral sulfide content of chemical bath deposited Zn(O,S) is reproducibly found to be 50-70%, fortuitously resulting in a conduction band offset suitable for solar cell applications with Cu(In,Ga)Se2 absorber materials. The observed variation in offset can neither be explained by variation of the Cu content in the Cu(In,Ga)Se2 near the interface nor by local variation of the chemical composition. Fermi level pinning induced by high defect concentrations is a possible origin of the variation of band offset. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

67

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

International Nuclear Information System (INIS)

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

68

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

69

The role of valence-band excitation in laser ablation of KCl  

Science.gov (United States)

We present recent measurements of excited-atom and ion emission from KCl surfaces illuminated by vacuum-ultraviolet synchrotron radiation (h-nu = 8-28 eV) and ultraviolet laser light (h-nu = 4 eV). At low intensities characteristic of the synchrotron experiments, excited atoms are desorbed by simple valence-band excitation process involving the metallization of the KCl surface. At the higher intensities typical of laser desorption and ablation, we observe a strong decrease in K emission as a function of the number of laser shots, but an essentially constant yield of Cl. K(+) and Cl(-) emission at high intensities show similar behavior. The energetics of these desorption phenomena can be treated in a bond-orbital model which shows that creation of a single valence hole is sufficient to excite an ion to an anti-bonding state.

Haglund, Richard F., Jr.; Tang, Kai; Bunton, Patrick H.; Wang, Ling-Jun

1991-01-01

70

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

71

High-energy photoemission in silver: resolving d and sp contributions in valence band spectra  

International Nuclear Information System (INIS)

We present high-resolution valence band and core level spectra of silver for photoelectron kinetic energies up to 8 keV. At these kinetic energies we estimate a surface contribution of less than 3%. Taking advantage of the favourable sp/d relative cross-sections, a comparison with the calculated density of states is presented. We observe an increasing photoemission intensity when approaching the Fermi level, which we assign to a free-electron-like character in the 5p-band, whereas the principal s-like contribution is located at the bottom of the d-band. The difference between measured and calculated values of the sp/d cross-section ratio is discussed

72

Conduction and valence band edge properties of hexagonal InN characterized by optical measurements  

International Nuclear Information System (INIS)

The optical properties of hexagonal InN crystal are investigated by reflectance, absorption and photoluminescence method. With use of the theoretical formula of these spectra, both of conduction and valence band properties are analysed. As a result, the average effective electron and hole mass values at the band edge are found to be about 0.046(±0.01)m0 and 0.2(±0.05)m 0, respectively. The photoluminescence emitted from the cleaved edge was strongly polarized. The agreement of the peak energies of ? and ? polarized light means that A, B, and C band edges are closely located in energy space. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

73

Electronic structure of the valence band of GdS x and Gd 3S 4  

Science.gov (United States)

Electronic structure of clean surfaces of single crystals of magnetic compounds of sulfides of gadolinium GdS x ( x = 0.8-1.25) with NaCl and Th 3P 4 structures has been investigated by ultra-violet and X-ray photoelectron, Auger and electron energy loss spectroscopy. Clean surfaces of GdS x were obtained by cleavage in situ in ultra-high vacuum and by ion bombardment followed by annealing in vacuum. Photoelectron spectra of the valence band of GdS x family have the similar structure with a maxima at 9 eV and 5 eV related to a photoemission from the 4f-states and valence band formed by 3p-states of S anions respectively and features in the region 0-2 eV associated with 6s-5d conduction band of GdS x. An analysis of electron energy loss spectra has shown that the main features in loss spectra at high beam energy are associated with the 4d-4f giant resonance, 5p-5d resonance and plasmon excitation. At low incident energy a prominent multiplet structure is observed and interpreted in terms of the dipole-forbidden 4f ? 4f electron transitions.

Grazhulis, V. A.; Bozhko, S. I.; Bolotin, I. L.; Bulanov, O. R.; Ionov, A. M.

1996-09-01

74

Determination of a natural valence-band offset - The case of HgTe and CdTe  

Science.gov (United States)

A method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structures of semiconductors is proposed. The HgTe-CdTe system is used as an example in which it is found that the valence-band maximum of HgTe lies 0.35 + or - 0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the X-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication on the heterojunction band lineup and the electronic structures of semiconductor alloys, are discussed.

Shih, C. K.; Spicer, W. E.

1987-01-01

75

The Synthesis of NiO/TiO2 Heterostructures and Their Valence Band Offset Determination  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In this work, a heterojunction based on p-type NiO/n-type TiO2 nanostructures has been prepared on the fluorine doped tin oxide (FTO) glass substrate by hydrothermal method. Scanning electron microscopy (SEM) and X-Ray diffraction techniques were used for the morphological and crystalline arrays characterization. The X-ray photoelectron spectroscopy was employed to determine the valence-band offset (VBO) of the NiO/TiO2 heterojunction prepared on FTO glass substrate. The core levels of Ni 2p ...

Ibupoto, Z. H.; Abbasi, M. A.; Liu, X.; Alsalhi, M. S.; Willander, M.

2014-01-01

76

Accuracy of alloy partial densities of states as determined by valence-band photoelectron diffraction  

International Nuclear Information System (INIS)

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

77

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

Olguin, D.; Baquero, R.

1996-01-01

78

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

International Nuclear Information System (INIS)

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

79

Partial cross sections and density of states effects in the valence band photoemission from solid nitrogen  

International Nuclear Information System (INIS)

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

80

Core level and valence band investigation of WO{sub 3} thin films with synchrotron radiation  

Energy Technology Data Exchange (ETDEWEB)

In this work, the electronic properties of the surface of WO{sub 3} films with thickness of 150 nm, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 deg. C and 500 deg. C (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 eV. The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO{sub 3} component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements.

Ottaviano, L.; Bussolotti, F.; Lozzi, L.; Passacantando, M.; La Rosa, S.; Santucci, S

2003-07-22

 
 
 
 
81

Core level and valence band investigation of WO3 thin films with synchrotron radiation  

International Nuclear Information System (INIS)

In this work, the electronic properties of the surface of WO3 films with thickness of 150 nm, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 deg. C and 500 deg. C (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 eV. The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO3 component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements

82

Study of the 4f and valence band density of states in rare-earth metals  

International Nuclear Information System (INIS)

The 4f and valence states of all metallic rare earths have been studied using x-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat spectroscopy (BIS) for the unoccupied part. It is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations. The intensities of the 4f final-state multiplets are well described in terms of the coefficients of fractional parentage. There is a symmetry between the fsup(n) XPS and fsup(14-n) BIS spectra. The observed energies of the 4f excitations correspond to transitions to completely screened final states. These values enable one to predict the elements which are liable to interconfiguration fluctuation when their 4f levels are shifted to Esub(F) by the chemical environment or by compression. The large Coulomb correlation energies, U which prevent the formation of 4f bands in these elements, are directly obtained from the spectra observed and are found to be in good agreement with recent calculations. The linewidths and singularity indices of the XPS and BIS 4f lines are determined and discussed. (author)

83

Experimental determination of conduction and valence bands of semiconductor nanoparticles using Kelvin probe force microscopy  

Energy Technology Data Exchange (ETDEWEB)

The ability to determine a semiconductor's band edge positions is important for the design of new photocatalyst materials. In this paper, we introduced an experimental method based on Kelvin probe force microscopy to determine the conduction and valence band edge energies of semiconductor nanomaterials, which has rarely been demonstrated. We tested the method on six semiconductor nanoparticles ({alpha}-Fe{sub 2}O{sub 3}, CeO{sub 2}, Al{sub 2}O{sub 3}, CuO, TiO{sub 2}, and ZnO) with known electronic structures. The experimentally determined band edge positions for {alpha}-Fe{sub 2}O{sub 3}, Al{sub 2}O{sub 3}, and CuO well matched the literature values with no statistical difference. Except CeO{sub 2}, all other metal oxides had a consistent upward bias in the experimental measurements of band edge positions because of the shielding effect of the adsorbed surface water layer. This experimental approach may outstand as a unique alternative way of probing the band edge energy positions of semiconductor materials to complement the current computational methods, which often find limitations in new synthetic or complex materials. Ultimately, this work provides scientific foundation for developing experimental tools to probe nanoscale electronic properties of photocatalytic materials, which will drive breakthroughs in the design of novel photocatalytic systems and advance the fundamental understanding of material properties.

Zhang Wen, E-mail: wzhang81@njit.edu [New Jersey Institute of Technology, Department of Civil and Environmental Engineering (United States); Chen Yongsheng, E-mail: yongsheng.chen@ce.gatech.edu [Georgia Institute of Technology, School of Civil and Environmental Engineering (United States)

2013-01-15

84

Experimental determination of conduction and valence bands of semiconductor nanoparticles using Kelvin probe force microscopy  

International Nuclear Information System (INIS)

The ability to determine a semiconductor’s band edge positions is important for the design of new photocatalyst materials. In this paper, we introduced an experimental method based on Kelvin probe force microscopy to determine the conduction and valence band edge energies of semiconductor nanomaterials, which has rarely been demonstrated. We tested the method on six semiconductor nanoparticles (?-Fe2O3, CeO2, Al2O3, CuO, TiO2, and ZnO) with known electronic structures. The experimentally determined band edge positions for ?-Fe2O3, Al2O3, and CuO well matched the literature values with no statistical difference. Except CeO2, all other metal oxides had a consistent upward bias in the experimental measurements of band edge positions because of the shielding effect of the adsorbed surface water layer. This experimental approach may outstand as a unique alternative way of probing the band edge energy positions of semiconductor materials to complement the current computational methods, which often find limitations in new synthetic or complex materials. Ultimately, this work provides scientific foundation for developing experimental tools to probe nanoscale electronic properties of photocatalytic materials, which will drive breakthroughs in the design of novel photocatalytic systems and advance the fundamental understanding of material properties.rstanding of material properties.

85

Dispersion Characteristics of a Cylindrical Electromagnetic Band Gap Structure  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

Boutayeb, Halim; Mahdjoubi, Kouroch

2006-01-01

86

Valence-band x-ray photoemission from Ir(001) with high angular resolution and high-energy resolution  

Science.gov (United States)

High angular resolution and high-energy-resolution x-ray photoelectron spectroscopy (XPS) measurements of the Ir valence bands are presented. Valence-band XPS spectra are reported for Ir(001) as a function of the polar angle in the (100) and (110) mirror planes, with an angular resolution of +/-1.7° and an energy resolution of 0.41 eV full width at half maximum. Although some relative intensity variations are observed, the same spectral peaks appear at all angles with only very small (tungsten, that will not yield to advances in electron spectrometer instrumentation alone.

Klebanoff, L. E.; van Campen, D. G.

1992-10-01

87

Effects of second valence band and resonant states on transport in undoped Pbsub(1-x)Snsub(x)Te  

International Nuclear Information System (INIS)

The temperature dependence of the Hall coefficient of n- and p-type Pbsub(1-x)Snsub(x)Te samples (x = 0.2) is analysed taking into account the temperature dependent effective masses and nonparabolicity. It is shown that the energetic separation between the edges of conduction and second valence band, respectively, is ?2 = 0.34 eV as in PbTe. From the analysis of p-type samples the existence of resonant states, which are degenerated with the upper valence band, is deduced. The concentration of these states is of the same order as the hole concentration. (author)

88

Valence band offset of ?-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy.  

Science.gov (United States)

A sample of the ?-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the ?-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the ?-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the ?-Ga2O3/GaN structure is 1.40?±?0.08 eV. PMID:23046910

Wei, Wei; Qin, Zhixin; Fan, Shunfei; Li, Zhiwei; Shi, Kai; Zhu, Qinsheng; Zhang, Guoyi

2012-01-01

89

Valence band offset of ?-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A sample of the ?-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the ?-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the ?-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the ?-Ga2O3/GaN structure is 1.40?±?0.08 eV.

Wei, Wei; Qin, Zhixin; Fan, Shunfei; Li, Zhiwei; Shi, Kai; Zhu, Qinsheng; Zhang, Guoyi

2012-01-01

90

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

91

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2006-05-15

92

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.)

93

X-ray photoelectron valence band spectra from semiconductors Bi2Te3 and Sb2Te3  

International Nuclear Information System (INIS)

Angular resolved X-ray photoelectron spectra (ARXPS) are measured for valence bands of the layer semiconductors Bi2Te3 and Sb2Te3. Starting from simple models taking into consideration the atomic construction, photoionization cross sections and the crystal structure the spectra can be interpreted. Important conclusions are drawn on the electronic structure of these semiconducting compounds. (author)

94

The Crossover from Impurity to Valence Band in Diluted Magnetic Semiconductors: The Role of the Coulomb Attraction by Acceptor  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The crossover between an impurity band (IB) and a valence band (VB) regime as a function of the magnetic impurity concentration in models for diluted magnetic semiconductors (DMS) is studied systematically by taking into consideration the Coulomb attraction between the carriers and the magnetic impurities. The density of states and the ferromagnetic transition temperature of a Spin-Fermion model applied to DMS are evaluated using Dynamical Mean-Field Theory (DMFT) and Monte ...

Popescu, F.; S?en, C.; Dagotto, E.; Moreo, A.

2007-01-01

95

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)

96

All-electron Exact Exchange Treatment of Semiconductors: Effect of Core-valence Interaction on Band-gap and $d$-band Position  

CERN Document Server

Exact exchange (EXX) Kohn-Sham calculations within an all-electron full-potential method are performed on a range of semiconductors and insulators (Ge, GaAs, CdS, Si, ZnS, C, BN, Ne, Ar, Kr and Xe). We find that the band-gaps are not as close to experiment as those obtained from previous pseudopotential EXX calculations. Full-potential band-gaps are also not significantly better for $sp$ semiconductors than for insulators, as had been found for pseudopotentials. The locations of $d$-band states, determined using the full-potential EXX method, are in excellent agreement with experiment, irrespective of whether these states are core, semi-core or valence. We conclude that the inclusion of the core-valence interaction is necessary for accurate determination of EXX Kohn-Sham band structures, indicating a possible deficiency in pseudopotential calculations.

Sharma, S; Ambrosch-Draxl, C

2005-01-01

97

Direct determination of the band offset in atomic layer deposited ZnO/hydrogenated amorphous silicon heterojunctions from X-ray photoelectron spectroscopy valence band spectra  

Energy Technology Data Exchange (ETDEWEB)

The chemical composition and band alignment at the heterointerface between atomic layer deposition-grown zinc oxide (ZnO) and hydrogenated amorphous silicon (a-Si:H) is investigated using monochromatized X-ray photoelectron spectroscopy. A new approach for obtaining the valence band offset ?E{sub V} is developed, which consists in fitting the valence band (VB) spectrum obtained for a-Si:H with a thin ZnO overlayer as the sum of experimentally obtained VB spectra of a bulk a-Si:H film and a thick ZnO film. This approach allows obtaining ?E{sub V}?=?2.71?±?0.15?eV with a minimum of assumptions, and also yields information on the change in band bending of both substrate and ZnO film. The band offset results are compared to values obtained using the usual approach of comparing valence band edge-to-core level energy differences, ?E{sub B,CL} ???E{sub B,VB}. Furthermore, a theoretical value for the VB offset is calculated from the concept of charge neutrality level line-up, using literature data for the charge neutrality levels and the experimentally determined ZnO/a-Si:H interface dipole. The thus obtained value of ?E{sub V}{sup CNL}?=?2.65?±?0.3?eV agrees well with the experimental ?E{sub V}.

Korte, L., E-mail: korte@helmholtz-berlin.de; Rößler, R.; Pettenkofer, C. [Helmholtz-Zentrum Berlin, Institute for Silicon Photovoltaics, Kekuléstrasse 5, 12489 Berlin (Germany)

2014-05-28

98

Direct determination of the band offset in atomic layer deposited ZnO/hydrogenated amorphous silicon heterojunctions from X-ray photoelectron spectroscopy valence band spectra  

International Nuclear Information System (INIS)

The chemical composition and band alignment at the heterointerface between atomic layer deposition-grown zinc oxide (ZnO) and hydrogenated amorphous silicon (a-Si:H) is investigated using monochromatized X-ray photoelectron spectroscopy. A new approach for obtaining the valence band offset ?EV is developed, which consists in fitting the valence band (VB) spectrum obtained for a-Si:H with a thin ZnO overlayer as the sum of experimentally obtained VB spectra of a bulk a-Si:H film and a thick ZnO film. This approach allows obtaining ?EV?=?2.71?±?0.15?eV with a minimum of assumptions, and also yields information on the change in band bending of both substrate and ZnO film. The band offset results are compared to values obtained using the usual approach of comparing valence band edge-to-core level energy differences, ?EB,CL ???EB,VB. Furthermore, a theoretical value for the VB offset is calculated from the concept of charge neutrality level line-up, using literature data for the charge neutrality levels and the experimentally determined ZnO/a-Si:H interface dipole. The thus obtained value of ?EVCNL?=?2.65?±?0.3?eV agrees well with the experimental ?EV.

99

Valence band offset and Schottky barrier at amorphous boron and boron carbide interfaces with silicon and copper  

International Nuclear Information System (INIS)

In order to understand the fundamental charge transport in a-B:H and a-BX:H (X = C, N, P) compound heterostructure devices, X-ray photoelectron spectroscopy has been utilized to determine the valence band offset and Schottky barrier present at amorphous boron compound interfaces formed with (1 0 0) Si and polished poly-crystalline Cu substrates. For interfaces formed by plasma enhanced chemical vapor deposition of a-B4–5C:H on (1 0 0) Si, relatively small valence band offsets of 0.2 ± 0.2 eV were determined. For a-B:H/Cu interfaces, a more significant Schottky barrier of 0.8 ± 0.16 eV was measured. These results are in contrast to those observed for a-BN:H and BP where more significant band discontinuities (>1–2 eV) were observed for interfaces with Si and Cu.

100

X-ray photoemission spectroscopy determination of the InN/yttria stabilized cubic-zirconia valence band offset  

International Nuclear Information System (INIS)

The valence band offset of wurtzite InN(0001)/yttria stabilized cubic-zirconia (YSZ)(111) heterojunctions is determined by x-ray photoemission spectroscopy to be 1.19±0.17 eV giving a conduction band offset of 3.06±0.20 eV. Consequently, a type-I heterojunction forms between InN and YSZ in the straddling arrangement. The low lattice mismatch and high band offsets suggest potential for use of YSZ as a gate dielectric in high-frequency InN-based electronic devices

 
 
 
 
101

Valence one-electron and shake-up ionization bands of fluorene, carbazole and dibenzofuran  

International Nuclear Information System (INIS)

Highlights: • The photoelectron spectra of the title compounds are assigned in details. • Shake-up lines are found to severely contaminate both ?- and ?-ionization bands. • ?-ionization onsets are subject to severe vibronic coupling complications. • We compare the results of OVGF, ADC(3) and TDDFT calculations. - Abstract: A comprehensive study of the He (I) ultra-violet photoelectron spectra of fluorene, carbazole and dibenzofuran is presented with the aid of one-particle Green’s Function calculations employing the outer-valence Green’s Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with Dunning’s correlation consistent basis sets of double and triple zeta quality (cc-pVDZ, cc-pVTZ). Extrapolations of the ADC(3) results for the outermost one-electron ?-ionization energies to the cc-pVTZ basis set enable theoretical insights into He (I) measurements within ?0.15 eV accuracy, up to the ?-ionization onset. The lower ionization energy of carbazole is the combined result of mesomeric and electronic relaxation effects. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital picture of ionization at the ADC(3) level. Comparison is made with calculations of the lowest doublet–doublet excitation energies of the radical cation of fluorene, by means of time-dependent density functional theory (TDDFT)

102

Valence one-electron and shake-up ionization bands of fluorene, carbazole and dibenzofuran  

Energy Technology Data Exchange (ETDEWEB)

Highlights: • The photoelectron spectra of the title compounds are assigned in details. • Shake-up lines are found to severely contaminate both ?- and ?-ionization bands. • ?-ionization onsets are subject to severe vibronic coupling complications. • We compare the results of OVGF, ADC(3) and TDDFT calculations. - Abstract: A comprehensive study of the He (I) ultra-violet photoelectron spectra of fluorene, carbazole and dibenzofuran is presented with the aid of one-particle Green’s Function calculations employing the outer-valence Green’s Function (OVGF) approach and the third-order algebraic diagrammatic construction [ADC(3)] scheme, along with Dunning’s correlation consistent basis sets of double and triple zeta quality (cc-pVDZ, cc-pVTZ). Extrapolations of the ADC(3) results for the outermost one-electron ?-ionization energies to the cc-pVTZ basis set enable theoretical insights into He (I) measurements within ?0.15 eV accuracy, up to the ?-ionization onset. The lower ionization energy of carbazole is the combined result of mesomeric and electronic relaxation effects. OVGF/cc-pVDZ or OVGF/cc-pVTZ pole strengths smaller than 0.85 systematically corroborate a breakdown of the orbital picture of ionization at the ADC(3) level. Comparison is made with calculations of the lowest doublet–doublet excitation energies of the radical cation of fluorene, by means of time-dependent density functional theory (TDDFT)

Reza Shojaei, S.H.; Morini, Filippo; Deleuze, Michael S., E-mail: michael.deleuze@uhasselt.be

2013-05-16

103

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.

104

Analysis of high-temperature thermoelectric properties of p-type CoSb3 within a two-valence-band and two-conduction-band model  

Science.gov (United States)

Experimental data on the thermoelectric properties of p-type CoSb3 reported by Caillat et al. [J. Appl. Phys. 80, 4442 (1996)] have been analyzed, assuming not only a pair of the first valence (v1) and the first conduction (c1) bands but also the second valence (v2) and the second conduction (c2) bands. By taking into account the excitation of carriers into the v2 and the c2 bands, the behavior of the Hall coefficient as well as that of the Seebeck coefficient at high temperatures is well explained. By taking into account the nonparabolicity of the v1 band, the temperature dependence of mobility is well explained with assuming scattering due to acoustic phonons, nonpolar and polar optical phonons, and ionized impurities. Furthermore, various material parameters of CoSb3, such as the band-gap energy, effective masses, and deformation potentials, have been deduced from fitting the calculation to the experimental data on the temperature dependences of the Hall coefficient, the mobility, and the Seebeck coefficient. Among them, the band-gap energy and the effective mass of the v1 band have been corrected from the original values estimated by Caillat et al. In addition, it is shown that the experimental data on the hole-concentration dependences of both the room-temperature Seebeck coefficient and the cyclotron mass are well reproduced by the theoretical calculation using the deduced values for the nonparabolic v1 band.

Kajikawa, Y.

2014-05-01

105

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

106

Valence band and catalytic activity of Au nanoparticles in Fe2O3/SiO2/Si(100) environment  

International Nuclear Information System (INIS)

A 10-nm-thick gold film was evaporated onto a SiO2/Si(100) substrate and was implanted by Ar+ ions at 40 keV and 1015 at/cm2 dose creating island like Au nanoparticles. An 80-nm-thick gold film was also deposited the onto same substrate and considered as reference. Fe2O3 film was deposited onto the gold nanoparticles and the gold/oxide interface was modeled. The valence band and the structure were measured by means of photoelectron spectroscopy (XPS) and by transmission electron microscopy (TEM), respectively. The catalytic activity was detected by CO oxidation, which was higher after the deposition of Fe2O3 layer onto Au nanoparticles than that on a continuous Au film. This observation was correlated to the nanosize and the redistributed valence band density of states of gold in the Au/Fe2O3 interface

107

Valence band structure and density of states effective mass model of biaxial tensile strained silicon based on k · p theory  

International Nuclear Information System (INIS)

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

108

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

109

Observation and theoretical description of the pure Fano-effect in the valence-band photo-emission of ferromagnets  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The pure Fano-effect in angle-integrated valence-band photo-emission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photo-electrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano-effect observed before for paramagnetic transition metals. While the theoretical photo-curre...

Minar, J.; Ebert, H.; Nadai, C.; Brookes, N. B.; Venturini, F.; Ghiringhelli, G.; Chioncel, L.; Katsnelson, M. I.; Lichtenstein, A. I.

2005-01-01

110

Surface alloying in the Sn/Ni(111) system studied by synchrotron radiation photoelectron valence band spectroscopy and ab-initio density of states calculations  

International Nuclear Information System (INIS)

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

111

Polarization-dependent ARPES measurement for valence band of anatase TiO2  

Science.gov (United States)

Angle-resolved photoelectron spectroscopy utilizing linearly polarized synchrotron radiation was conducted to examine the electronic structure of an anatase TiO2(001) thin film fabricated on LaAlO3(100) by pulsed laser deposition. Polarization-dependent measurements by changing the incidence angle of the light were performed to identify the orbital symmetry. Energy-momentum dispersion curves along the [001] direction are mapped and resolved into four components: O 2p-derived P? non-bonding, O 2p-Ti 3d ? bonding, and two ? bonding states. The intensities of the ? bonding states are obviously different between both polarizations while the intensities of the peaks due to P? and ? are almost equivalent. Two different ? states with polarization dependence were attributed to Ti t1u? orbital. The result implies that the distortion of TiO6 octahedra resolves degeneracy of T1u bands and/or breaking T2g symmetry.

Emori, Masato; Sakino, Akiko; Ozawa, Kenichi; Sakama, Hiroshi

2014-06-01

112

Valence band and core-level analysis of highly luminescent ZnO nanocrystals for designing ultrafast optical sensors  

Science.gov (United States)

The detailed surface analysis such as survey scan, core-level, and valence band spectra of highly luminescent ZnO:Na nanocrystals were studied using the x-ray photoelectron spectroscopy to establish the performed presence of Na+ ions. The observed increase in band gap from 3.30 (bulk) to 4.16 eV (nano), is attributed to the quantum confinement of the motion of electron and holes in all three directions. The photoluminescence and decay measurements have complemented and supported our study to design an efficient and ultrafast responsive optical sensing device.

Joshi, Amish G.; Sahai, Sonal; Gandhi, Namita; Krishna, Y. G. Radha; Haranath, D.

2010-03-01

113

Intra-valence-band mixing in strain-compensated SiGe quantum wells  

CERN Document Server

We explore the midinfrared absorption of strain-compensated p-Si0.2Ge0.8/Si quantum wells for various well thicknesses and temperatures. Owing to the large band offset due to the large bi-axial strain contrast between the wells and barriers, the intersubband transitions energies from the ground state to the excited heavy hole (hh), light hole (lh) and split-off hole (so) states are resolved to ~0.5 eV. When hh2 is within ~30 meV of lh1 or so1 a partial transfer of the hh1-hh2 oscillator strength to the hh1-lh1 or hh1-so1 transitions is observed, which is otherwise forbidden for light polarized perpendicular to the plane of the wells. This is a clear sign of mixing between the hh and lh or so-states. A large temperature induced broadening of hh2 peak is observed for narrow wells indicating a non-parabolic dispersion of the hh2 states due to the mixing with the lh/so continuum. We found that the 6-band k.p theory gives a quantitative account of the observations. A possible role of many-body effects in the tempe...

Tsujino, S; Falub, C; Fromherz, T; Diehl, L; Sigg, H; Grützmacher, D

2005-01-01

114

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

International Nuclear Information System (INIS)

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

115

Valence band engineering for remarkable enhancement of surface emission in AlGaN deep-ultraviolet light emitting diodes  

Energy Technology Data Exchange (ETDEWEB)

Valence band engineering for remarkable enhancement of surface emission in AlGaN deep-ultraviolet (UV) light emitting diodes (LEDs) has been theoretically investigated by the calculation using the 6 x 6 k.p Hamiltonian. It has already been pointed out that the topmost valence band in deep-UV AlGaN material has a Z-like character and this causes the c-axis polarized emission unfavourable for light extraction from c-plane based LEDs. We have found, in this study, that this unfavourable polarization can be switched into favourable in-plane polarization by decreasing quantum well (QW) width and/or introducing in-plane compressive strain, which cause band-crossing and change the character of the topmost band. It is also shown that such polarization changes occur in the QWs on semipolar substrates although that does not take place in those on nonpolar substrates. Furthermore, we have derived the condition of the polarization switch as an approximate analytical expression which could be useful for device structural design. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Atsushi Yamaguchi, A. [Research Laboratory for Integrated Technological Systems, Kanazawa Institute of Technology, Tokyo (Japan)

2008-07-01

116

Evidence in the XPS valence band of the fold structure at the surface of polyethylene lamellae  

Science.gov (United States)

For the purpose of identifying surface conformations, the valence XPS spectra of mats of polyethylene single crystals as prepared from the solution and after annealing are reported and compared with the spectrum of a pressed pellet of hexatriacontane. The relative intensity of the two "C?C peaks" in the inner-valence region exhibit significant variations that are related to the chain folds occuring at the surface of the lamellae, on the one hand, and to planar zig-zag segments for the annealed and pellet samples, on the other hand.

Delhalle, J.; Riga, J.; Denis, J. P.; Deleuze, M.; Dosière, M.

1993-07-01

117

Valence-band mixing induced by sp-d exchange interaction in CdMnTe quantum wires  

International Nuclear Information System (INIS)

Anisotropic magneto-optical effects have been studied theoretically by using multi-band effective mass method. We focus on a model of an ideal CdMnTe quantum wire surrounded by infinite potential barriers. The mixing of heavy- and light-hole bands is caused by both the one-dimensional quantum confinement and the sp-d exchange interaction. As a result of the valence-band mixing characteristic to the one-dimensional diluted magnetic semiconductor, the Zeeman splitting in the magnetic field parallel to the wire direction becomes smaller than that in the perpendicular magnetic field. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

118

Valence-band mixing induced by sp-d exchange interaction in CdMnTe quantum wires  

Energy Technology Data Exchange (ETDEWEB)

Anisotropic magneto-optical effects have been studied theoretically by using multi-band effective mass method. We focus on a model of an ideal CdMnTe quantum wire surrounded by infinite potential barriers. The mixing of heavy- and light-hole bands is caused by both the one-dimensional quantum confinement and the sp-d exchange interaction. As a result of the valence-band mixing characteristic to the one-dimensional diluted magnetic semiconductor, the Zeeman splitting in the magnetic field parallel to the wire direction becomes smaller than that in the perpendicular magnetic field. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Harada, Y.; Kita, T.; Wada, O. [Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai 1-1, Nada, 657-8501, Kobe (Japan); Marsal, L.; Mariette, H. [Laboratoire de Spectrometrie Physique, Universite J. Fourier, Grenoble I, CNRS (UMR 5588), Boite Postal 87, 38402 Saint Martin d' Heres Cedex (France); Ando, H. [Department of Physics, Faculty of Science and Engineering, Konan University, Okamoto 8-9-1, Higashi-Nada, 658-8501, Kobe (Japan)

2006-03-15

119

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

120

Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures  

Science.gov (United States)

The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti50Ni50 and Ti50Ni25Cu25 alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti50Ni50 - x Cu x system ( x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.

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

2012-08-01

 
 
 
 
121

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

International Nuclear Information System (INIS)

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 Mn2NiGa appear at the phase transition. •HAXPES of C1b 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

122

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

123

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

124

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

125

Theoretical study of the satellite bands in the valence shell XPS spectra of N2, CO, O2 and NO molecules  

International Nuclear Information System (INIS)

The energies for the ionized states of N2, CO, O2 and NO are obtained by using an ab initio full valence configuration interaction (FVCI) method with a minimal basis set. The intensities of the corresponding valence shell MgK? X-ray photoelectorn spectroscopy (MgK?-XPS) spectra are estimated with plane wave approximation and the satellite bands in the observed spectra are analyzed. The present FVCI results give a quite reasonable interpretation of the bands, both with regard to the positions and the relative intensities. New assignments are proposed for some bands of O2 and NO. (orig.)

126

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

127

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)

128

Valence bands of oriented finite linear chain molecular solids as model compounds of polyethylene studied by angle-resolved photoemission  

Science.gov (United States)

Angle-resolved photoemission spectra were measured using synchrotron radiation of two kinds of oriented model compounds of polyethylene with their molecular axes perpendicular to the substrate surface, i.e. evaporated films of hexatriacontane CH 3(CH 2) 34CH 3 and Langmuir-Blodgett films of Cd arachidate (CH 3(CH 2) 17COO) 2Cd. Both films show similar photoelectron energy distribution curves determined by the long-alkyl chain. The intramolecular energy-band dispersion of polyethylene was determined from the photon-energy dependence of the normal-emission spectra. This is the first direct observation of an energy-band dispersion in organic solids. The upper bands formed by C 2p and H 1s electrons extend from 8.8 to 15.5 eV below the vacuum level, and the deeper-lying bands originating from C 2s electrons lie from 17.5 to 24.7 eV. The band structure obtained is compared to results from XPS and ESR studies. Furthermore, the experimentally determined band structure is discussed in detail in view of theoretical calculations for polyethylene. Ab initio and extended Hückel calculations give a good description of the experimental results.

Seki, Kazuhiko; Ueno, Nobuo; Karlsson, Ulf O.; Engelhardt, Rainer; Koch, Ernst-Eckhard

1986-06-01

129

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.)

130

Thermoelectric Enhancement in PbTe with K or Na codoping from tuning the interaction of the light- and heavy-hole valence bands  

Energy Technology Data Exchange (ETDEWEB)

The effect of K and K-Na substitution for Pb atoms in the rocksalt lattice of PbTe was investigated to test a hypothesis for development of resonant states in the valence band that may enhance the thermoelectric power. We combined high-temperature Hall-effect, electrical conductivity, and thermal conductivity measurements to show that K-Na codoping do not form resonance states but can control the energy difference of the maxima of the two primary valence subbands in PbTe. This leads to an enhanced interband interaction with rising temperature and a significant rise in the thermoelectric figure of merit of p -type PbTe. The experimental data can be explained by a combination of a single- and two-band models for the valence band of PbTe depending on hole density that varies in the range of 1–15×10{sup 19} ?cm{sup ?3} .

Androulakis, John; Todorov, Iliya; Chung, Duck_Young; Ballikaya, Sedat; Wang, Guoyu Y; Uher, Ctirad; Kanatzidis, Mercouri G.

2010-01-01

131

Strain-dependent intersubband absorption in the valence band of SiGe quantum wells  

Science.gov (United States)

We present intersubband absorption measurements performed on p-type Si1 - xGex/Si(0 0 1) quantum wells (x = 0.35) in the presence of an external strain. We find that the in-plane polarized absorption band is rather insensitive to the strain. For the incident light polarized along the growth direction, the absorption strength increases under the tensile strain. The latter observation is well accounted for by a six-band k · p model and is explained by redistribution of hole population between the heavy- and the light-hole subbands caused by the strain-induced energy shifts of the bands.

Yakimov, A. I.; Kirienko, V. V.; Armbrister, V. A.; Bloshkin, A. A.

2014-04-01

132

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)

133

The observation of a surface resonance state in the valence-band structure of the perovskite La sub 0 sub . sub 6 sub 5 Ba sub 0 sub . sub 3 sub 5 MnO sub 3  

CERN Document Server

The valence-band electronic structure of a crystalline thin film of La sub 0 sub . sub 6 sub 5 Ba sub 0 sub . sub 3 sub 5 MnO sub 3 has been investigated using angle-resolved photoemission. The observed valence-band structure has a substantial surface contribution and is strongly influenced by the surface conditions. Surface contamination and disorder attenuate the emission from a Mn-3d-derived state at a binding energy of 3.3 eV. This state was found to be localized at the surface with dispersion only in the plane of the surface and exhibited a strong intensity dependence on emission angle. On the basis of a simple LMTO-ASA bulk band calculation, it is clear that this state does not fall within a bulk band gap. We have therefore identified this Mn-3d-derived state at 3.3 eV below E sub F as a surface resonance. (author)

Waldfried, C; Liou, S H; Sabiryanov, R; Jaswal, S S; Dowben, P A

1997-01-01

134

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

135

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

136

Observation and theoretical description of the pure Fano-effect in the valence-band photo-emission of ferromagnets  

CERN Document Server

The pure Fano-effect in angle-integrated valence-band photo-emission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photo-electrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano-effect observed before for paramagnetic transition metals. While the theoretical photo-current and spin difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin density approximation (LSDA). Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory (DMFT).

Minar, J; De Nadai, C; Brookes, N B; Venturini, F; Ghiringhelli, G; Chioncel, L; Katsnelson, M I; Lichtenstein, A I

2005-01-01

137

Experimental Observation and Theoretical Description of the Pure Fano Effect in the Valence-Band Photoemission of Ferromagnets  

International Nuclear Information System (INIS)

The pure Fano effect in angle-integrated valence-band photoemission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photoelectrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano effect observed before for paramagnetic transition metals. While the theoretical photocurrent and spin-difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co, only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin-density approximation. Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory

138

Experimental observation and theoretical description of the pure Fano effect in the valence-band photoemission of ferromagnets.  

Science.gov (United States)

The pure Fano effect in angle-integrated valence-band photoemission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photoelectrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano effect observed before for paramagnetic transition metals. While the theoretical photocurrent and spin-difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co, only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin-density approximation. Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory. PMID:16241823

Minár, J; Ebert, H; De Nadaï, C; Brookes, N B; Venturini, F; Ghiringhelli, G; Chioncel, L; Katsnelson, M I; Lichtenstein, A I

2005-10-14

139

Valence-band photoemission in UO2(111) near the 5d resonant photon energy  

International Nuclear Information System (INIS)

Pronounced Fano-like resonant effects are observed in the photoemission spectra of the localized U 5f and OXp ndash: UXf,6d: bonding bands of UO2 when photon energies near the U 5d/sub 3/2,5/2/ thresholds (105 and 97.1 eV, respectively) are employed. The constant-final-state spectra are well described by calculated transition energies and oscillator strengths for the 5d/sup 10/5f2?/sup h//sup ?/5d95f3 process; also, the decay rates of the intermediate-state multiplets are in qualitative agreement with the observed width of the spectral features. Distinctly different constant-initial-state (CIS) spectra are obtained at the intensity maximum of the U 5f photopeak and at the minimum between the 5f peak and the O 2p bonding band. Calculated decay rates indicate that this results from variations in the branching ratio for decay of intense intermediate-state multiplets into 2F/sub 5/2/ and 2F/sub 7/2/ final states. Resonant effects in the OXp ndash: UXf,6d: bonding band show the bonding participation of U orbitals; moreover, the higher intensity of the CIS spectrum obtained near the top of the bonding band agrees with band-structure calculations showing most of the U 5f character in this region

140

Valence-band x-ray photoelectron spectroscopic studies of vanadium phosphates and the formation of oxide-free phosphate films on metallic vanadium  

Science.gov (United States)

The valence-band and core-level x-ray photoelectron spectroscopy (XPS) of vanadium phosphates are reported, and the valence-band spectra interpreted by various calculation models. The spectral interpretation of the vanadium phosphates is used to show that an oxide-free vanadium phosphate film corresponding to VO(H2PO4)2 can be prepared on vanadium metal. Three types of vanadium phosphate were investigated to provide an understanding of the XPS data for vanadium phosphates, and to demonstrate the differences between these phosphates and vanadium oxides. The three phosphates formed, VO(H2PO4)2, VOHPO4.0.5H2O, and VOPO4.2H2O were prepared by reaction of vanadium with phosphoric acid under a variety of conditions. The article focuses upon the valence-band region which shows significant differences between different types of vanadium phosphates as well as clear differences between the phosphates on the one hand and oxides on the other hand. The valence-band spectra are effectively interpreted by the multiple scattered-wave X? calculations and band structure calculations.

Asunskis, Daniel J.; Sherwood, Peter M. A.

2003-07-01

 
 
 
 
141

Valence-band x-ray photoelectron spectroscopic studies of vanadium phosphates and the formation of oxide-free phosphate films on metallic vanadium  

International Nuclear Information System (INIS)

The valence-band and core-level x-ray photoelectron spectroscopy (XPS) of vanadium phosphates are reported, and the valence-band spectra interpreted by various calculation models. The spectral interpretation of the vanadium phosphates is used to show that an oxide-free vanadium phosphate film corresponding to VO(H2PO4)2 can be prepared on vanadium metal. Three types of vanadium phosphate were investigated to provide an understanding of the XPS data for vanadium phosphates, and to demonstrate the differences between these phosphates and vanadium oxides. The three phosphates formed, VO(H2PO4)2, VOHPO4.0.5H2O, and VOPO4·2H2O were prepared by reaction of vanadium with phosphoric acid under a variety of conditions. The article focuses upon the valence-band region which shows significant differences between different types of vanadium phosphates as well as clear differences between the phosphates on the one hand and oxides on the other hand. The valence-band spectra are effectively interpreted by the multiple scattered-wave X? calculations and band structure calculations

142

CuInS2-CdS heterojunction valence band offset measured with near-UV constant final state yield spectroscopy  

Science.gov (United States)

The valence band offset of the heterojunction between CuInS2 (CIS) and chemical bath deposited CdS has been determined both by means of combined x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) and by near-UV constant final state (CFS) yield spectroscopy. The use of the latter shows that this spectroscopic method is indeed suitable for the investigation of chalcopyrite thin films. The higher information depth due to the low excitation energies (7.5-4.0 eV) used in CFS makes it possible to obtain a signal from the valence band of the CIS substrate even after a relatively thick (˜5 nm) CdS layer has been deposited. The number of samples needed to determine the band offset is reduced from three to one and the effects of chemical changes on the CIS surface can be accounted for because the valence band edges of both materials are measured directly. The valence band offsets with the XPS/UPS and CFS methods were determined to be 1.25±0.20 and 1.45±0.20 eV, respectively.

Johnson, B.; Korte, L.; Lußky, T.; Klaer, J.; Lauermann, I.

2009-10-01

143

Valence band discontinuity at the GaN/SiC(0001) heterojunction studied in-situ by synchrotron-radiation photoelectron spectroscopy  

International Nuclear Information System (INIS)

Full text: The valence band discontinuity and the interface formation of the n-type hexagonal GaN/SiC(0001) heterointerface have been studied by means of angle-resolved photoelectron spectroscopy using synchrotron radiation. Gallium nitride thin films were grown on SiC(0001) substrates by molecular beam epitaxy using either nitrogen plasma or ammonia gas as nitrogen source. The interface properties were investigated in-situ by a combination of core level and valence band spectroscopy. The interface formation and the valence band offset of the epitaxial GaN films grown by the two methods have been compared. The GaN/SiC interface shows SiN formation and the measured valence band discontinuity is 1.05 ± 0.05 eV, both for the ammonia and nitrogen plasma grown films. This value is in good agreement with the theoretical prediction and indicates that the band alignment of the GaN/SiC heterojunction is of the staggered type

144

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

Energy Technology Data Exchange (ETDEWEB)

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.

Brandow, B.H.

1985-01-01

145

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

146

Ab initio electronic band structure study of the valence bands of II-VI C(2 × 2) reconstructed surfaces  

Science.gov (United States)

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

Rubio-Ponce, A.; Olguín, D.

2015-01-01

147

Valence and conduction band offsets at low-k a-SiOxCy:H/a-SiCxNy:H interfaces  

International Nuclear Information System (INIS)

In order to understand the fundamental electrical leakage and reliability failure mechanisms in nano-electronic low-k dielectric/metal interconnect structures, we have utilized x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy to determine the valence and conduction band offsets present at interfaces between non-porous and porous low-k a-SiOxCy:H interlayer dielectrics and a-SiCxNy:H metal capping layers. The valence band offset for such interfaces was determined to be 2.7?±?0.2?eV and weakly dependent on the a-SiOC:H porosity. The corresponding conduction band offset was determined to be 2.1?±?0.2?eV. The large band offsets indicate that intra metal layer leakage is likely dominated by defects and trap states in the a-SiOC:H and a-SiCN:H dielectrics.

148

Exciton binding energies and the valence-band offset in mixed type-I--type-II strained-layer superlattices  

International Nuclear Information System (INIS)

Strained CdTe-Cd1-xZnxTe superlattices are of mixed type: electrons and heavy holes are confined to the CdTe layers (type I) while light holes are confined to the Cd1-xZnxTe layers (type II). In this paper we calculate the exciton binding energy (EBE) as a function of superlattice period for both type-I (spatially direct) and type-II (spatially indirect) excitons. For the heavy-hole (type-I) exciton the binding energy is larger than the bulk value, and varies only slowly with the period down to small periods, where the exciton acquires a three-dimensional character and our calculation breaks down. For the light-hole (type-II) exciton the binding energy at large period is much smaller, due to the spatial separation of electron and hole. As the period decreases, the binding energy increases steadily to reach its bulk value for vanishingly small period. Given the EBE's, we can fit the already published data on the exciton transition energies with a single adjustable parameter, the ''average valence-band offset'' (averaged over the heavy and light holes). This is the algebraic sum of the chemical-offset and the hydrostatic-strain contribution, and is found to be (2±4)% of the difference in band gap between the barrier and well. This value lies in the range predicted theoretically

149

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

150

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. PMID:25146151

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

2014-01-01

151

Valence-band orbital character of CdO: A synchrotron-radiation photoelectron spectroscopy and density functional theory study  

Science.gov (United States)

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

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

2014-04-01

152

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 st...

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

2006-01-01

153

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

154

Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: A combined experimental and theoretical approach  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise deter...

Mourad, Daniel; Richters, Jan-peter; Ge?rard, Lionel; Andre?, Re?gis; Bleuse, Joe?l; Mariette, Henri

2012-01-01

155

Band gap and dispersion engineering of photonic crystal devices  

Science.gov (United States)

Photonic crystals (PhCs) have been of great interest in a variety of fields in the past decade due to their great capability for manipulating photons in a manner similar to how electrons are controlled in a semiconductor material. In particular, PhCs are expected to revolutionize such fields as optical signal processing and optical communication by allowing the development of novel optical devices for high-density photonic integrated circuits (PICs). The development of PhC devices will be greatly accelerated by systematic designs. In this dissertation, I developed several procedures to systematically engineer the dispersion properties of PhCs. Using these procedures, I presented a variety of novel applications intended for use in future high-density PICs. These were achieved through efficient implementations of the finite-difference time-domain (FDTD) method and the plane wave method (PWM). Specifically, by combining these efficient electromagnetic tools with the direct binary search (DBS) method or simulated annealing (SA), I developed very efficient synthesis processes and used them to optimize absolute photonic band gaps (PBGs) of PhC structures and a beam steering device based on a PhC with PBG(s). I also presented another novel PhC device working in PBG, namely a PhC ring drop filter. On the other hand, I utilized the FDTD method and the PWM to shape dispersion surfaces and/or contours of PhC structures for manipulating light propagation. In particular, I engineered PhCs with square- and circle-shaped equi-frequency contours (EFCs) and presented several applications using these two unique PhCs. These applications include optical beam routing, coupling and splitting a wide beam into multiple narrow self-guiding beams, a unidirectional emitter, and an in-plane lens coupler. I also explored negative refraction and left-handed behavior in PhCs and presented a flat lens using a PhC exhibiting negative refraction and left-handed behavior.

Chen, Caihua

156

Valence band structure of (Bisub(1-x)Sbsub(x))2Te3 single crystals  

International Nuclear Information System (INIS)

For the first time the anisotropy of reflectivity of polarized infrared light of p-(Bisub(1-x)Sbsub(x))2Te3 single crystals (0.5 -1. In addition to the excitation of free carriers in the reflection spectra of the mixed crystals also interband excitations are observed. For the mixed crystals the optical results are consistently described in terms of a six-valley band model and scattering on acoustical phonons together with the data for the dc conductivity, the Hall coefficients, and the thermopower. The main axis masses msub(ii), the ellipsoid tilt angle theta, the effective density of states mass msub(d)sup(tot), the cyclotron mass msub(Bparallelc)sup((c)), and the susceptibility masses msub(perpendicular, parallelc)sup((CHI)) are given in dependence on composition, also the Fermi level Esub(F), the carrier density p, the mobility usub(perpendicularc), the high frequency dielectric constant epsilon'sub(infinity), and the relaxation time tau. The interband influence is interpreted by a simple band model and the energy gap Esub(g) calculated. (author)

157

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)

158

Electronic structures and valence band XPS spectra of BeO and SiC calculated by X? cluster method  

International Nuclear Information System (INIS)

The DV-X? cluster method has been applied for calculations of the electronic structures and for analysis of valence band XPS spectra of BeO, ?-SiC (Wurutzite type) and ?-SiC (Zinc-blende type). Clusters studied are [Be4O4] for BeO, and [Si4C4] and [Si5C4]sup(0.75+) for ?-SiC and ?-SiC, respectively. The calculation for BeO has yielded the electronic level structure characteristic of an insulating material. For ?-SiC and ?-SiC the level structures can be related well with their semiconducting behavior. The calculated XPS spectrum of ?-SiC is very similler to that of ?-SiC and is in good agreement with the observed one. However, the effective charge on Si atom in ?-SiC obtained is about twice that in ?-SiC (?-SiC : +1.56, ?-SiC : +0.75). The marked difference indicates that ?-SiC is a material more ionic than ?-SiC. (author)

159

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

International Nuclear Information System (INIS)

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 ?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 ?-band system (? 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 ?-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-de radical cations, by means of time-dependent density functional theory (TDDFT)

160

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

 
 
 
 
161

Theoretical study of influencing factors on the dispersion of bulk band-gap edges and the surface states in topological insulators Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

The dispersion of the band-gap edge states in bulk topological insulators Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} 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.

Rusinov, I. P., E-mail: rusinovip@gmail.com; Nechaev, I. A. [Tomsk State University (Russian Federation); Chulkov, E. V. [Donostia International Physics Center (DIPC) (Spain)

2013-06-15

162

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

Science.gov (United States)

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.

>M Zieli?ski,

2013-11-01

163

Relationship between thermoelectric power and electron density of states in valence band measured with XPS for Fe-Cr binary alloys and Fe-Cr-Ni ternary alloys  

International Nuclear Information System (INIS)

In order to clarify the mechanism behind the change in thermoelectric power (TEP) of Fe-Cr binary alloys and Fe-Cr-Ni ternary alloys due to Cr concentration, using these alloys with various Cr concentrations, dependence of TEP on Cr concentration was investigated. According to the Mott-Jones theory, TEP is largely related to electron density of states at the Fermi level. The electron density of states for the alloys in valence band was measured with X-ray photoelectron spectroscopy (XPS), and theoretically calculated TEP from the XPS spectrum was compared with experimentally measured TEP. Electron density of states in valence band of the both alloys changed due to Cr concentration, and the theoretically calculated TEP from XPS spectrum and the experimentally measured TEP similarly changed with Cr concentration. We think that the change in TEP with Cr concentration is due to the change in electron density of states with Cr concentration. (author)

164

Transport properties and valence band feature of high-performance (GeTe)85(AgSbTe2)15 thermoelectric materials  

Science.gov (United States)

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (AgxSbTex/2+1.5)15(GeTe)85 (TAGS) thermoelectric materials with 0.4 ? x ? 1.2. All samples exhibit good thermoelectric figures of merit (zT) which reach 1.5 at 700 K for x = 0.6. Thermoelectric and thermomagnetic transport properties (electrical resistivity, Seebeck, Hall and transverse Nernst-Ettinghausen coefficients) are measured and used to calculate the scattering factor, the Fermi energy, the density-of-states (DOS) effective mass and hole mean free path (mfp). The DOS effective mass is very high due to the large band mass of the primary valence band and the high degeneracy of pockets in the Fermi surface from the second valence band. The highly degenerate Fermi surface increased the total DOS without decreasing mobility, which is more desirable than the high DOS that comes from a single carrier pocket. The high-temperature hole mfp approaches the Ioffe-Regel limit for band-type conduction, which validates our discussion based on band transport and is also important for TAGS alloys having high zT with heavy bands. The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity.

Chen, Y.; Jaworski, C. M.; Gao, Y. B.; Wang, H.; Zhu, T. J.; Snyder, G. J.; Heremans, J. P.; Zhao, X. B.

2014-01-01

165

Microstructure of reorientation bands in niobium alloys with ultra disperse particles of nonmetallic phase  

International Nuclear Information System (INIS)

Crystal lattice reorientation bands at initial stages of plastic flow of high-strength niobium alloys with ultra disperse particles of nonmetallic phase are detected by transmission electron microscopy. Fine structure of these bands is studied. Mechanism of their formation and the role of a high-strength state in deformation rotation mode development are discussed

166

Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy  

International Nuclear Information System (INIS)

The valence band offset (VBO) at the interface CdS/Cu2ZnSnS4 was investigated by x-ray photoelectron spectroscopy (XPS). The VBO was measured by two different procedures: an indirect method involving the measurements of the core levels together with the XPS bulk valence band (VB) spectra and a direct method involving the analysis of XPS VB spectra at the interface. The indirect method resulted in a VBO value of (?1.20 ± 0.14) eV while the direct method returned a similar value of (?1.24 ± 0.06) eV but affected by a lower uncertainty. The conduction band offset (CBO) was calculated from the measured VBO values. These two measured values of the VBO allowed us to calculate the CBO, giving (?0.30 ± 0.14) eV and (?0.34 ± 0.06) eV, respectively. These values show that the CBO has a cliff-like behaviour which could be one of the reasons for the Voc limitation in the CdS/CZTS solar cells. (paper)

167

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

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

2005-01-01

168

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

169

Simultaneous nonlinearity suppression and wide-band dispersion compensation using optical phase conjugation.  

Science.gov (United States)

Optical phase conjugation is demonstrated to enable simultaneous wide-band compensation of the residual dispersion and the fiber nonlinearities in dispersion-managed fiber transmission lines employing slope-compensating fibers. When the dispersion slope of transmission fibers is equalized by slope-compensating fibers, the residual dispersion and the slope of dispersion slope are compensated by middle-span optical phase conjugation. More importantly, fiber nonlinearity may be largely suppressed by arranging the fibers into conjugate pairs about the phase conjugator, where the two fibers of each pair are in scaled translational symmetry. The translational symmetry is responsible for cancelling optical nonlinearities of the two fibers up to the first-order perturbation, then a mirror-symmetric ordering of the fiber pairs about the conjugator linearizes a long transmission line effectively. PMID:19475027

Wei, Haiqing; Plant, David

2004-05-01

170

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

171

Spectroscopy of Neutron-Rich $^{168,170}$Dy: Yrast Band Evolution Close to the $N_{p}N_{n}$ Valence Maximum  

CERN Document Server

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

Söderström, P A; Regan, P H; Algora, A; de Angelis, G; Ashley, S F; Aydin, S; Bazzacco, D; Casperson, R J; Catford, W N; Cederkäll, J; Chapman, R; Corradi, L; Fahlander, C; Farnea, E; Fioretto, E; Freeman, S J; Gadea, A; Gelletly, W; Gottardo, A; Grodner, E; He, C Y; Jones, G A; Keyes, K; Labiche, M; Liang, X; Liu, Z; Lunardi, S; Muarginean, N; Mason, P; Menegazzo, R; Mengoni, D; Montagnoli, G; Napoli, D; Ollier, J; Pietri, S; Podolyák, Z; Pollarolo, G; Recchia, F; ?ahin, E; Scarlassara, F; Silvestri, R; Smith, J F; Spohr, K M; Steer, S J; Stefanini, A M; Szilner, S; Thompson, N J; Tveten, G M; Ur, C A; Valiente-Dobón, J J; Werner, V; Williams, S J; Xu, F R; Zhu, J Y; 10.1103/PhysRevC.81.034310

2010-01-01

172

Energy gap and valence band structure of Cd/sub 1-z/Mn/sub z/Te/sub 1-y/Se/sub y/ alloys  

International Nuclear Information System (INIS)

Measurements of wavelength modulated reflectance are made as a function of temperature on Cd/sub 1-z/Mn/sub z/Te/sub 1-y/Se/sub y/ alloys for both zinc blende and wurtzite fields. The room temperature energy gap E0 is found to show a square law variation with y independent of the change in crystal structure and the relation proposed by Moon et al. for E0 values at a general composiion (z, y) is found to be satisfied. At low temperatures for the Cd/sub 1-z/Mn/sub z/Se alloys, the E0 values show an increase ?E0 due to magnetic effects and the values of ?E0 agree with the theoretical predictions of Alexander et al. Also for Cd/sub 1-z/Mn/sub z/Se, the values of valence band splittings are used to show that the fractional d-like character of the valence band increases linearly with z and has a value of approximately 0.26 at z = 0.2. (author)

173

The impact of health status on dispersal behavior in banded mongooses (Mungos mungo).  

Science.gov (United States)

While disease and injury have obvious impacts on mortality, they can have less understood non-lethal impacts on behavior. These behavioral effects might have a significant consequences for population-level disease dynamics if diseased individuals are more or less likely to disperse. We opportunistically observed dispersal events in banded mongooses (Mungos mungo) that were either healthy or unhealthy due to injury and/or clinical signs of a novel tuberculosis pathogen, Mycobacterium mungi. We found that diseased and/or injured mongooses were significantly less likely to disperse than healthy individuals, suggesting that disease may have an important consequences for dispersal that could in turn affect population-level disease dynamics. PMID:24504905

Fairbanks, Bonnie M; Hawley, Dana M; Alexander, Kathleen A

2014-06-01

174

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

175

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

176

Detection of Fe 3d electronic states in the valence band and magnetic properties of Fe-doped ZnO film  

International Nuclear Information System (INIS)

Fe-doped ZnO film has been grown by laser molecular beam epitaxy (L-MBE) and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), all of which reveal the high quality of the film. No secondary phase was detected. Resonant photoemission spectroscopy (RPES) with photon energies around the Fe 2p–3d absorption edge is performed to detect the electronic structure in the valence band. A strong resonant effect at a photon energy of 710 eV is observed. Fe3+ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV?8 eV. There are no electronic states related to Fe near the Fermi level. Magnetic measurements reveal a typical superparamagnetic property at room temperature. The absence of electronic states related to Fe near the Fermi level and the high quality of the film, with few defects, provide little support to ferromagnetism. (condensed matter: structural, mechanical, and thermal properties)

177

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

178

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes  

International Nuclear Information System (INIS)

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP- (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states

179

Order N photonic band structures for metals and other dispersive materials  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We show, for the first time, how to calculate photonic band structures for metals and other dispersive systems using an efficient Order N scheme. The method is applied to two simple periodic metallic systems where it gives results in close agreement with calculations made with other techniques. Further, the approach demonstrates excellent numerical stablity within the limits we give. Our new method opens the way for efficient calculations on complex structures containing a w...

Arriaga, J.; Ward, A. J.; Pendry, J. B.

1998-01-01

180

Order N photonic band structures for metals and other dispersive materials  

CERN Document Server

We show, for the first time, how to calculate photonic band structures for metals and other dispersive systems using an efficient Order N scheme. The method is applied to two simple periodic metallic systems where it gives results in close agreement with calculations made with other techniques. Further, the approach demonstrates excellent numerical stablity within the limits we give. Our new method opens the way for efficient calculations on complex structures containing a whole new class of material.

Arriaga, J; Pendry, J B

1998-01-01

 
 
 
 
181

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)

182

Excitonic properties related to valence band levels split by spin-orbit interaction in layered oxychalcogenide LaCuOCh(Ch=S,Se)  

International Nuclear Information System (INIS)

The energy splitting of valence band in oxychalcogenides LaCuOCh (Ch=S, Se) was studied through degenerative four-wave mixing (DFWM) signals excited by femtosecond laser pulses and high-resolution absorption spectra at 4 K. The DFWM signals for LaCuOS exhibited a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states were split by 9 meV, while the corresponding splitting in LaCuOSe was 125 meV. The spin-orbit interaction of Ch ion accompanied by the hybridization of Cu 3d orbital causes the splitting of the exciton levels

183

Valence band resonant photoemission spectroscopy on Ce compounds at Ce 3d-4f and 4d-4f thresholds: surface vs. bulk electronic structure  

International Nuclear Information System (INIS)

Electron correlation in 4f orbitals strongly influences the low energy properties of Ce and its inter-metallic compounds. By exploiting the different surface sensitivity of valence band resonant photoemission (VBRP) at Ce 3d-4f and Ce 4d-4f thresholds, a lowering of the 4f hybridization has been recently observed on going from the bulk to the surface layers. We report an extensive VBRP work at both the thresholds (3d and 4d) on three Ce compounds (Ce7Rh3, CeRh2, and CeCo2). In addition to surface effects, we found different resonance behaviours for f0 and f1 configurations. These differences should not be disregarded when extracting the 4f bulk and surface contributions from the photoemission spectra. ((orig.))

184

Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn(2+)-Doped CdSe Quantum Dots.  

Science.gov (United States)

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1Se, the dominant hole character of each excitonic transition can be identified, making use of the fact that the g-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals. PMID:25438717

Fainblat, Rachel; Muckel, Franziska; Barrows, Charles J; Vlaskin, Vladimir A; Gamelin, Daniel R; Bacher, Gerd

2014-12-23

185

Demonstrating Valence Band-Edge Effective Work Function by Aluminum Implantation in High- k/Metal Gate p-MOSFET with Incorporated Fluorine  

Science.gov (United States)

This work demonstrates the valence band-edge effective work function ( ? m ,eff) of a titanium nitride (TiN) gate with a hafnium oxide (HfO2) dielectric using a cost-effective, low-complexity gate-first integration scheme. Aluminum (Al) ion implantation following TiN gate stack formation yielded a ? m ,eff of 5.0 eV without an equivalent oxide thickness penalty. Additionally, the incorporation of fluorine (F) into the HfO2 dielectric by the channel implantation approach further improved ? m ,eff to 5.1 eV. This technique for modulating ? m ,eff has potential for threshold-voltage tuning without any process complexity in high- k/metal gate low-power applications.

Chen, Y. W.; Lai, C. M.; Cheng, L. W.; Hsu, C. H.; Liang, C. W.

2012-07-01

186

Wide-band Simultaneous Observations of Pulsars: Disentangling Dispersion Measure and Profile Variations  

CERN Document Server

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 100000 across our observing band. We use this fact to constrain some of the properties of the ISM along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the 1 implications that it could have for precision pulsar timing projects such as the detection of gravitati...

Hassall, T E; Hessels, J W T; Kramer, M; Alexov, A; Anderson, K; Coenen, T; Karastergiou, A; Keane, E F; Kondratiev, V I; Lazaridis, K; van Leeuwen, J; Noutsos, A; Serylak, M; Sobey, C; Verbiest, J P W; Weltevrede, P; Zagkouris, K; Fender, R; Wijers, R A M J; Bahren, L; Bell, M E; Broderick, J W; Corbel, S; Daw, E J; Dhillon, V S; Eisloffel, J; Falcke, H; Griessmeier, J -M; Jonker, P; Law, C; Markoff, S; Miller-Jones, J C A; Osten, R; Rol, E; Scaife, A M M; Scheers, B; Schellart, P; Spreeuw, H; Swinbank, J; ter Veen, S; Wise, M W; Wijnands, R; Wucknitz, O; Zarka, P; Asgekar, A; Bell, M R; Bentum, M J; Bernardi, G; Best, P; Bonafede, A; Boonstra, A J; Brentjens, M; Brouw, W N; Bruggen, M; Butcher, H R; Ciardi, B; Garrett, M A; Gerbers, M; Gunst, A W; van Haarlem, M P; Heald, G; Hoeft, M; Holties, H; de Jong, A; Koopmans, L V E; Kuniyoshi, M; Kuper, G; Loose, G M; Maat, P; Masters, J; McKean, J P; Meulman, H; Mevius, M; Munk, H; Noordam, J E; Orru, E; Paas, H; Pandey-Pommier, M; Pandey, V N; Pizzo, R; Polatidis, A; Reich, W; Rottgering, H; Sluman, J; Steinmetz, M; Sterks, C G M; Tagger, M; Tang, Y; Tasse, C; Vermeulen, R; van Weeren, R J; Wijnholds, S J; Yatawatta, S

2012-01-01

187

Influence of In0.15Ga0.85As capping layers on the valence and conduction band structure of InAs quantum dots  

International Nuclear Information System (INIS)

We have prepared self-assembled InAs quantum dots (QDs) capped by GaAs and In0.15Ga0.85As, respectively, by molecular beam epitaxy. The In0.15Ga0.85As cap layer shifts the ground state photoluminescence (PL) emission from 1261 nm to 1319 nm, which might be useful for telecommunication purposes. The QDs were embedded into n- or p-type capacitance-voltage (C(V)) structures to investigate the conduction and valance band states, respectively. The red-shift of the interband transitions due to the In 0.15Ga0.85As layer observed in PL is compared to the shift of the corresponding energy levels obtained from C(V) measurements. The shifts of the ground states obtained from C(V) spectroscopy sum up to 42 meV, which is in good agreement with 43 meV observed in PL measurements. A small difference could be caused by a change in the exciton binding energy. From the 42 meV overall red-shift, 83% originate from the conduction and only 17% from the valence band. This is probably due to the smaller effective mass in the conduction band, so that here changes in the confinement potential result in larger changes in the energy levels

188

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

International Nuclear Information System (INIS)

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.

189

Angle-resolved x-ray photoemission from the valence bands of tungsten with high angular resolution and temperature variation  

International Nuclear Information System (INIS)

Angle-resolved valence photoemission spectra have been obtained for W(011) and W(001) using Al K? and Mg K? excitation and with both normal and off-normal emission directions. The angular resolution was approx. 0, and thus the highest to date in this energy regime, and the temperature also was varied from 295 to 1000 K. These spectra exhibit features very strongly dependent on emission direction and temperature that can be more precisely associated with direct-transition emission from different Brillouin-zone points than in prior studies. Data obtained at different temperatures are used to deconvolute direct- and nondirect-transition components. The direct-transition components are found to include residual intensity due to nondirect transitions. The simple direct-transition model applied previously to the analysis of such data is also extended to include the effects of correlated lattice vibrations via a cylindrical broadening in k space. This model is found to describe semiquantitatively the effects of phonon-assisted nondirect transitions on spectra

190

Valence-band-mixing effects on the optical gain of GaAs/AlxGa1-xAs graded single quantum wells  

International Nuclear Information System (INIS)

Full text: In the last few years the semiconductor lasers have been attractive for research because they are both physically very interesting and technologically important. This is especially true for quantum well lasers since, with the current technology of quantum wells, it is possible to control the range, depth, and arrangement of quantum mechanical potential wells, which can be useful to make very good lasers. In the last decade the importance of the quantum well laser has steadily grown until today, where it is preferred for most semiconductor laser applications. This growing popularity is because, in almost every aspect, the quantum well laser is somewhat better than the conventional one with bulk active layers. In this work, we have calculated the optical gain spectra in the unstrained graded GaAs/Al0.2Ga0.8As single quantum well laser as a function of the energy of the radiation and the interface width. The calculation of the electronic structure was done using the parabolic band model. The calculation of the valence band structure was done taking into account the effects of sub-band mixing between the heavy and light holes, as well as by using an effective Hamiltonian approach based on the k-vector · p-vector Luttinger-Kohn method. The optical gain was calculated using the density matrix approach, considering all sub-band transitions in the quantum well. We consider both the transversal electrical and the transversal magnetic light polal and the transversal magnetic light polarization. Our results show that the peak gain is sensitive to the width and the graded profile of the interfaces, and the gain spectrum is blue-shifted as a function of the interface width. (author)

191

Effect of temperature on the valency bands of HDO in water in the liquid and solid states. Effects on the analysis of heavy water using infra-red absorption  

International Nuclear Information System (INIS)

After, a description of the technique used, a qualitative examination is made of the influence of the temperature on the ?OH(3,400 cm-1) and ?OD (2,500 cm-1) valence bands of HDO in the liquid state and then during the passage to the solid state. Quantitative examination with two cells of different thickness makes it possible to define the influence of temperature on the residual absorption of the pure liquid (D2O or H2O and on the valency bands (?OH and ?OD respectively). It is found that a similar change occurs in the two bands but that the changes in the background are very different. During the passage from the liquid to the solid state the shape of the bands varies considerably but little change occurs in the total intensity. It has been possible to express these results in a simple form which is directly applicable to analytical problems. (authors)

192

Valence band electronic structure of C60F18 and C60F36 studied by photoelectron spectroscopy  

International Nuclear Information System (INIS)

Photoemission spectra of solid fluorinated fullerenes C60Fx (x = 18, 36) were measured in comparison with parent C60 using synchrotron radiation (h? = 120 eV) providing the spectra in the density of states mode. Creation of the ion-like bonding of fluorine and disappearance of the ?-states were observed to be a result of fluorination. Disappearance of the upper lying ?-states was revealed to be a reason of the band gap widening upon fluorination

193

Valence-band orbital character of CdO: A synchrotron-radiation photoelectron spectroscopy and density functional theory study  

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

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

2014-01-01

194

Stimulated emission spectra and valence band structure in the Cd3(As/sub x/P/sub 1-x/)2 system  

International Nuclear Information System (INIS)

Recombination radiation in Cd3(As/sub x/P/sub 1-x/)2 solid solutions near cadmium phosphide (x 3(As/sub x/P/sub 1-x/)2 single crystals is detected. It is shown that for Cd3P2 the width of emission spectrum and its position relative to the transmission edge may be adequately described in terms of a direct interband-transition model. The same model is used to calculate the luminescence I/sub cv1/(?) and the optical transmission T/sub cv1/(?) in the presence of a small (i.e. up to 5% of the Brillouin zone size) K1 shift between the extrema of the heavy hole valence band V1 and conduction band C. It is shown that at k1 is not equal to 0 the dependence I/sub cv1/(?) may have two maxima. The structure of stimulated emission spectra and the reference data on luminescence in Cd3(As/sub x/P/sub 1-x/)2 single crystals are interpreted taking into account the calculations carried out in this work. (author)

195

Thermoelectric, thermomagnetic properties and valence band structure of Pbsub(1-x)Snsub(x)Te single crystals  

International Nuclear Information System (INIS)

Single crystals of Pbsub(0.82)Snsub(0.18)Te solid solution of p-type conductivity grown by the vapour phase method were investigated. Experimental data were obtained on concentration dependences of coefficients of thermal emf and the transverse Nerst-Ettinghausen effect in zero and weak magnetic fields. Concentration dependence of the effective mass of state density at the Fermi level was calculated at 90 K. The dependence of current carrier energy on the wave vector was calculated and generalized on the basis of the latter. The experimental data were compared with calculations for different scattering mechanisms taking into account empirically determined dispersion law

196

Experimental realisation of a double-core dispersion-compensating optical fibre for S-band optical communications  

International Nuclear Information System (INIS)

Experimental realisation of a double-core optical fibre having a dispersion of -1750 ps km-1 nm-1 at a wavelength of 1470 nm is reported. This is the highest negative dispersion ever reported in the S-band (1450 nm to 1530 nm). The double-core optical fibre is characterised in terms of geometrical parameters, attenuation measurement, MFD measurement, bend loss measurement, etc. This work will be useful for the S-band optical fibre communication. (fibre optics)

197

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

198

Features of crystal structure, electrophysical properties and spectrum model of valence bands of MoS2 type laminated compounds  

International Nuclear Information System (INIS)

Character of chemical bond metal-sulphur (selenium) and states of valent electrons in MoS2 type laminated compounds are analyzed based on measuring ion radii and electrophysical properties (conductivity, Hall effect, Seebeck effect). It is shown that layers in crystals are formed from trigonal-prismatic polyhedrons formed by contiguous spherical S or Se anions the charge of which are close to -2. Valent electrons of cations which did not transfer on anion p-levels, are collected and they are in an energy band formed by dsub(zsup(2)) (msub(e)=0) states and which is located above these filled levels

199

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

200

High-energy anomaly in the band dispersion of the ruthenate superconductor.  

Science.gov (United States)

We reveal a "high-energy anomaly" (HEA) in the band dispersion of the unconventional ruthenate superconductor Sr2RuO4, by means of high-resolution angle-resolved photoemission spectroscopy (ARPES) with tunable energy and polarization of incident photons. This observation provides another class of correlated materials exhibiting this anomaly beyond high-T(c) cuprates. We demonstrate that two distinct types of band renormalization associated with and without the HEA occur as a natural consequence of the energetics in the bandwidth and the energy scale of the HEA. Our results are well reproduced by a simple analytical form of the self-energy based on the Fermi-liquid theory, indicating that the HEA exists at a characteristic energy scale of the multielectron excitations. We propose that the HEA universally emerges if the systems have such a characteristic energy scale inside of the bandwidth. PMID:23006289

Iwasawa, H; Yoshida, Y; Hase, I; Shimada, K; Namatame, H; Taniguchi, M; Aiura, Y

2012-08-10

 
 
 
 
201

Observation of magnonic band gaps in magnonic crystals with nonreciprocal dispersion relation  

Science.gov (United States)

An effect of metallization of a magnonic crystal surface on band-gap formation in the spectra of a surface spin wave (SSW) is studied both theoretically and experimentally. The structures under consideration are one-dimensional magnonic crystals based on yttrium iron garnet with an array of etched grooves, with and without a metal screen on the top of the corrugated surface. Due to nonreciprocity of propagation of the SSW, the shift of the band gap to higher frequency and from the border of the Brillouin zone in the presence of a conducting overlayer was measured in a transmission line experiment. Results of numerical calculations and model analysis are in agreement with experimental data and give further insight into the origin of the band gap and properties of the nonreciprocal SSW in metalized magnonic crystals. This gives a positive answer to the outstanding question about the possibility of detection of magnonic band gaps in the spectra of spin waves with nonreciprocal dispersion in magnonic crystals, and creates the potential for new applications and improvements of already existing prototype magnonic devices.

Mruczkiewicz, M.; Pavlov, E. S.; Vysotsky, S. L.; Krawczyk, M.; Filimonov, Yu. A.; Nikitov, S. A.

2014-11-01

202

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

203

Band structure peculiarities of three-valence REM monosulphides (CeS, NdS, CdS)  

International Nuclear Information System (INIS)

Complex investigation into peculiarities of a zone structure of trivalent REM monosulfides (CeS, NdS, GdS) was conducted on the basis of comparing data of calculation by the quasiband crystal field method with results of X-ray spectrum investigation. When constructing crystalline potentials for the calculation the following REM atom configurations were used: [Xe]+4fsup(0.9)5ssup(2)5psup(6)5dsup(1.1) for Ce, [Xe]+4fsup(2.75)5ssup(2)5psup(6)5dsup(1.25) for Nd and [Xe]+4fsup(6.5)5ssup(2)5psup(6)5dsup(1.5) for Gd([Xe]-xenon shell). Schemes of energy bands of GdS, NdS and CeS are presented and interpretation of peculiarities obtained for zone structures is given

204

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

205

Band dispersion and temperature dependence of the HOMO and LUMO photoemission features of alkali intercalated C60  

International Nuclear Information System (INIS)

Full text: Here we report on the temperature dependent phonon-broadening of the HOMO and LUMO photoemission features in alkali metal intercalated fullerides, including C60, K6C60, Cs6C60, K4C60 and Cs4C60. Compounds with different alkali metal stoichiometry (different C60 electron filling) show different temperature behaviours that may be related to differences in the electron phonon coupling constant (?). In particular we have estimated a ratio ?(C60+)/ ?(C60-) 1.7 ± 0.3, in agreement with theoretical predictions.We will also show angle resolved photoemission measurements of the band dispersion in bcc K6C60 and fcc K3C60. While the dispersion of the filled LUMO-derived bands near the Fermi level in K3C60, as measured at 30 K, is less than 100 meV along the two main symmetry directions of the (111) surface (i.e. about a factor of 2 less than expected), the band dispersion of both LUMO and HOMO derived bands in K6C60 compares well with calculations. Possible reasons for this difference will be discussed and the band dispersion of these 'bulk' intercalated compounds will be compared with the band dispersion measured for C60 monolayers on Ag surfaces

206

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

207

I-band-like non-dispersive inter-shell interaction induced Raman lines in the D-band region of double-walled carbon nanotubes  

Science.gov (United States)

Non-dispersive, inter-layer interaction induced Raman peaks (I bands)—in the region of the D band—have been observed recently for bilayer graphene, when the two layers were rotated with respect to each other. Here, similar observations for double-walled carbon nanotubes are theoretically predicted. The prediction is based on double resonance theory, involving non-zone-centered phonons, and the effect of disorder is replaced by interaction between the two tubes.

Gyimesi, Bálint; Koltai, János; Zólyomi, Viktor; Kürti, Jen?

2015-02-01

208

I-band-like non-dispersive inter-shell interaction induced Raman lines in the D-band region of double-walled carbon nanotubes  

Science.gov (United States)

Non-dispersive, inter-layer interaction induced Raman peaks (I bands) -- in the region of the D band -- have been observed recently for bilayer graphene, when the two layers were rotated with respect to each other. Here, similar observations for double-walled carbon nanotubes (DWCNTs) are theoretically predicted. The prediction is based on double resonance theory, involving non-zone-centered phonons, and the effect of disorder is replaced by interaction between the two tubes.

Gyimesi, Bálint; Koltai, János; Zólyomi, Viktor; Kürti, Jen?

2014-09-01

209

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 charge transfer to Mott

210

Valence band offset at GaN/{beta}-Si{sub 3}N{sub 4} and {beta}-Si{sub 3}N{sub 4}/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy  

Energy Technology Data Exchange (ETDEWEB)

Ultra thin films of pure {beta}-Si{sub 3}N{sub 4} (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using {beta}-Si{sub 3}N{sub 4} 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/{beta}-Si{sub 3}N{sub 4}/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the {beta}-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 {beta}-Si{sub 3}N{sub 4} and a type-II heterojunction. The conduction band offset was deduced to be {approx} 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/{beta}-Si{sub 3}N{sub 4} interface formation.

Kumar, Mahesh; Roul, Basanta [Materials Research Centre, Indian Institute of Science, Bangalore- 560012 (India); Central Research Laboratory, Bharat Electronics, Bangalore-560013 (India); Bhat, Thirumaleshwara N.; Rajpalke, Mohana K. [Materials Research Centre, Indian Institute of Science, Bangalore- 560012 (India); Kalghatgi, A.T. [Central Research Laboratory, Bharat Electronics, Bangalore-560013 (India); Krupanidhi, S.B., E-mail: sbk@mrc.iisc.ernet.in [Materials Research Centre, Indian Institute of Science, Bangalore- 560012 (India)

2012-05-31

211

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

212

Intersubband absorption of strain-compensated Si1-xGex valence-band quantum wells with 0.7?x?0.85  

International Nuclear Information System (INIS)

Strain-compensated, p-type SiGe quantum wells with a high Ge concentration of up to 85% have been grown on commercially available Si0.5Ge0.5 pseudosubstrates by molecular-beam epitaxy. Structural investigations by transmission electron microscopy and high-resolution x-ray reflection and diffraction showed that at a growth temperature around T=300 deg. C, samples in excellent compliance with the design parameters, comparatively sharp interfaces, and negligible increase of growth-induced surface roughness can be grown. Comparison of polarization-dependent intersubband absorption measurements with simulated intersubband absorption spectra shows that for the quantum wells investigated in this work, the hole eigenstates, their in-plane dispersion, and the polarization-dependent intersubband transition matrix elements are accurately described by a strain-dependent, six-band k·p Luttinger-Kohn Hamiltonian in which only one fitting parameter--the intersubband transition linewidth--is used

213

Features of the core valence luminescence and electron energy band structure of A1-xCsxCaCl3 (A = K,Rb) crystals  

Science.gov (United States)

From luminescence spectroscopy of CsCaCl3, Rb1-xCsxCaCl3 and K1-xCsxCaCl3 crystals, we have found evidence for intrinsic and impurity core-valence luminescence due to the radiative recombination of valence electrons with the holes of intrinsic or impurity 5p Cs+ core states. The structural similarity of core-valence luminescence spectra has been revealed for the A1-xCsxCaCl3 (A = K,Rb) crystals investigated. The electron energy structure of the CsCaCl3 crystal has been calculated using the pseudopotential approach taking into account the gradient corrections for the exchange-correlation energy. The calculated density of the electronic states of CsCaCl3 has been compared with corresponding parameters obtained from the analysis of core-valence luminescence spectra.

Chornodolskyy, Ya; Stryganyuk, G.; Syrotyuk, S.; Voloshinovskii, A.; Rodnyi, P.

2007-11-01

214

Features of the core-valence luminescence and electron energy band structure of A1-xCsxCaCl3 (A = K,Rb) crystals  

International Nuclear Information System (INIS)

From luminescence spectroscopy of CsCaCl3, Rb1-xCsxCaCl3 and K1-xCsxCaCl3 crystals, we have found evidence for intrinsic and impurity core-valence luminescence due to the radiative recombination of valence electrons with the holes of intrinsic or impurity 5p Cs+ core states. The structural similarity of core-valence luminescence spectra has been revealed for the A1-xCsxCaCl3 (A = K,Rb) crystals investigated. The electron energy structure of the CsCaCl3 crystal has been calculated using the pseudopotential approach taking into account the gradient corrections for the exchange-correlation energy. The calculated density of the electronic states of CsCaCl3 has been compared with corresponding parameters obtained from the analysis of core-valence luminescence spectra

215

Design and analysis of a broadband dispersion compensating photonic crystal fiber Raman amplifier operating in S-band.  

Science.gov (United States)

This paper presents an optimized design of a dispersion compensating photonic crystal fiber (PCF) to achieve gain-flattened Raman performances over S-band using a single pump. Genetic algorithm interfaced with an efficient full-vectorial finite element modal solver based on curvilinear edge/nodal elements is used as an optimization tool for an accurate determination of PCF design parameters. The designed PCF shows high negative dispersion coefficient (-264 ps/nm/km to -1410 ps/nm/km) and negative dispersion slope, providing coarse dispersion compensation over the entire S-band. The module comprised of 1.45-km long optimized PCF exhibits +/-0.46 dB gain ripples over 50 nm wide bandwidth and shows a very low double Rayleigh backscattering value (-59.8 dB). The proposed module can compensate for the dispersion accumulated in one span (80-km) of standard single mode fiber with a residual dispersion of +/-700 ps/nm, ensuring its applicability for 10 Gb/s WDM networks. Additionally, the designed PCF remains single mode over the range of operating wavelengths. PMID:19516499

Varshney, Shailendra K; Fujisawa, Takeshi; Saitoh, Kunimasa; Koshiba, Masanori

2006-04-17

216

Valence and conduction band offsets at low-k a-SiO{sub x}C{sub y}:H/a-SiC{sub x}N{sub y}:H interfaces  

Energy Technology Data Exchange (ETDEWEB)

In order to understand the fundamental electrical leakage and reliability failure mechanisms in nano-electronic low-k dielectric/metal interconnect structures, we have utilized x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy to determine the valence and conduction band offsets present at interfaces between non-porous and porous low-k a-SiO{sub x}C{sub y}:H interlayer dielectrics and a-SiC{sub x}N{sub y}:H metal capping layers. The valence band offset for such interfaces was determined to be 2.7?±?0.2?eV and weakly dependent on the a-SiOC:H porosity. The corresponding conduction band offset was determined to be 2.1?±?0.2?eV. The large band offsets indicate that intra metal layer leakage is likely dominated by defects and trap states in the a-SiOC:H and a-SiCN:H dielectrics.

King, Sean W., E-mail: sean.king@intel.com; Brockman, Justin; French, Marc; Jaehnig, Milt; Kuhn, Markus [Logic Technology Development, Intel Corporation, Hillsboro, Oregon 97124 (United States); French, Benjamin [Ocotillo Materials Laboratory, Intel Corporation, Chandler, Arizona 85248 (United States)

2014-09-21

217

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

218

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

219

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

220

Deduction of the chemical state and the electronic structure of Nd2Fe14B compound from X-ray photoelectron spectroscopy core-level and valence-band spectra  

International Nuclear Information System (INIS)

Characterization of chemical state and electronic structure of the technologically important Nd2Fe14B compound is attractive for understanding the physical nature of its excellent magnetic properties. X-ray photoelectron spectroscopy (XPS) study of such rare-earth compound is important and also challenging due to the easy oxidation of surface and small photoelectron cross-sections of rare-earth 4f electrons and B 2p electrons, etc. Here, we reported an investigation based on XPS spectra of Nd2Fe14B compound as a function of Ar ion sputtering time. The chemical state of Fe and that of B in Nd2Fe14B compound can be clearly determined to be 0 and ?3, respectively. The Nd in Nd2Fe14B compound is found to have the chemical state of close to +3 instead of +3 as compared with the Nd in Nd2O3. In addition, by comparing the valence-band spectrum of Nd2Fe14B compound to that of the pure Fe, the contributions from Nd, Fe, and B to the valence-band structure of Nd2Fe14B compound is made more clear. The B 2p states and B 2s states are identified to be at ?11.2 eV and ?24.6 eV, respectively, which is reported for the first time. The contribution from Nd 4f states can be identified both in XPS core-level spectrum and XPS valence-band spectrum. Although Nd 4f states partially hybridize with Fe 3d states, Nd 4f states are mainly localized in Nd2Fe14B compound.

 
 
 
 
221

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

222

Vibronic dynamic problem of the valence tautomeric interconversion [low-spin Co(III)(N-circumflex N)(sq)(cat)] ?[high-spin Co(II)(N-circumflex N)(sq)2]. Magnetic properties and charge-transfer band  

International Nuclear Information System (INIS)

For a single valence tautomeric complex the vibronic problem of pseudo-Jahn-Teller effect is solved. The vibronic (hybrid) wave-functions and energy levels are used for the calculation of the temperature dependence of the molecular magnetic moment and the shape of the optical band in the near infrared range. The magnetic moment is shown to increase gradually from 1.73?B to 4.34?B. The spectrum of the isolated complex in the near infrared range is revealed to represent a band related to the light induced transfer of an electron from the cat ligand to the metal ion. The suggested model is in qualitative agreement with the experimental data

223

Determination of the valence band offsets at HfO2/InN(0001) and InN/In0.3Ga0.7N(0001) heterojunctions using X-ray photoelectron spectroscopy  

International Nuclear Information System (INIS)

The valence band offset (VBO) at a InN/In0.3Ga0.7N(0001) as well as HfO2/InN(0001) heterojunction is investigated by X-ray photoelectron spectroscopy using monochromated AlK? radiation. The InN and In0.3Ga0.7N films were grown using plasma-assisted molecular beam epitaxy, whereas HfO2 layers were deposited by plasma-assisted electron beam evaporation. The VBOs were determined by analysing the core level binding energy and valence band maxima of bulk-like films as well as of In0.3Ga0.7N and InN layers covered with 5 nm thick overlayers of InN and HfO2, respectively. The resulting VBO values are ?0.5 eV for the InN/In0.3Ga0.7N heterojunction and ?0.9 eV in the case of the HfO2/InN heterointerface. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

224

Chemical bonding and charge redistribution - Valence band and core level correlations for the Ni/Si, Pd/Si, and Pt/Si systems  

Science.gov (United States)

Via a systematic study of the correlation between the core and valence level X-ray photoemission spectra, the nature of the chemical bonding and charge redistribution for bulk transition metal silicides has been examined. Particular emphasis is placed on Pt2Si and PtSi. It is observed that the strength of the metal (d)-silicon (p) interaction increases in the order Ni2Si, Pd2Si, Pt2Si. It is also observed that both the metal and silicon core lines shift to higher binding energy as the silicides are formed. The notion of charge redistribution for metallic bonds is invoked to explain these data.

Grunthaner, P. J.; Grunthaner, F. J.; Madhukar, A.

1982-01-01

225

Wide-band and Air Dispersion Effecting the ABCD Algorithm of Phase-Recovery in Long-baseline Interferometry  

CERN Document Server

Long-baseline interferometry detects fringes created by superposition of two beams of light collected by two telescopes pointing into a common direction. The external path difference generated by pointing away from the zenith is commonly compensated by adding a variable optical path length (delay) through air for one beam such that the optical path difference between the beams remains close to zero near the detector. The ABCD formula assigns a (wrapped) phase to the amplitudes A to D of an interference pattern shifted by multiples of 90 degrees in phase. We study the interplay between a wide band pass of the optics and the dispersion of the air in the compensating delay, which leads to small deviations between the ABCD phase and the reduced, monochromatic group-delay representation of the wave packets. In essence, this adds dispersion to the effects that have been discussed for evacuated interferometers (telescopes in space) before [J. Opt. Soc. Am. A 22 (2005) 2774].

Mathar, R J

2006-01-01

226

Properties of bands rolled from nickel- and copper powders containing dispersion oxides  

International Nuclear Information System (INIS)

The effect of various kinds was studied of treatment on the physicomechanical properties of bands rolled from nickel and copper powders containing oxides, 3% each (by mass): SiO2, ?-Al2O3, ZrO2, HfO2. A combination of cold compression with intermediate sintering produced high-density bands of precipitation-strengthened materials, which showed high mechanical properties over a wide range of test temperatures

227

The anomalous dispersion of the disorder-induced and the second-order Raman Bands in Carbon Nanotubes  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english In this work we have studied the dispersion of the disorder-induced (D) and the second-order (G') Raman bands in single wall carbon nanotubes using several laser excitation energies (E laser) in the range 1.5-3.0 eV. An anomalous step-like behavior was observed in the E laser dependence of the G'-ba [...] nd frequency. This result is interpreted as a manifestation of the one-dimensional (1D) behavior of the phonon spectrum in carbon nanotubes.

M. A., Pimenta; E. B., Hanlon; A., Marucci; P., Corio; S. D. M., Brown; S. A., Empedocles; M. G., Bawendi; G., Dresselhaus; M. S., Dresselhaus.

2000-06-01

228

Comparison of the valence band structure peculiarities in the Bi0.84-Sb0.16-Sn0.1 and pure semiconducting Bi1-xSbx alloys  

International Nuclear Information System (INIS)

On the basis of measured low magnetic field galvanomagnetic coefficients both for Sn-doped Bi0.84-Sb0.16-Sn0.1 and pure semiconducting Bi-Sb alloys, kinetic parameters of the investigated materials have been determined. It is shown that hole ellipsoids corresponding to the obtained results are less anisotropic in comparison with L-hole ellipsoids. Therefore peculiarities of the valence band for the Bi0.84-Sb0.16 alloy and other seniconducting alloys with high content of antimony can not be adequately described by involving dominant contribution of the hole valleys localized in T, H and L points of the Brioullin Zone

229

Change of a rate of the conversion E1-transition of the 110mAg isomer (??=1.11+-0.003 keV) in variation of valence band configuration of silver atomic shell  

International Nuclear Information System (INIS)

The conversion spectrum for the E1-transition in 110Ag with an energy ??=1110±30 eV is calculated for nine configurations of the valence band (VB) of the silver atom in the framework of the relativistic variant of the Hartree-Fock-Slater method. It is found that orbits of the 3s-4s range exhibit a very high stability (variation 0 (a0=5.29x10-9 cm). This means that one may expect a signi ficant influence of the chemical environment in the conversion spectrum notwithstanding that the calculation in the scheme of variations of the VB configuration for an isolated atom yields small changes in the decay rate (??/? -3)

230

Influence of In{sub 0.15}Ga{sub 0.85}As capping layers on the valence and conduction band structure of InAs quantum dots  

Energy Technology Data Exchange (ETDEWEB)

We have prepared self-assembled InAs quantum dots (QDs) capped by GaAs and In{sub 0.15}Ga{sub 0.85}As, respectively, by molecular beam epitaxy. The In{sub 0.15}Ga{sub 0.85}As cap layer shifts the ground state photoluminescence (PL) emission from 1261 nm to 1319 nm, which might be useful for telecommunication purposes. The QDs were embedded into n- or p-type capacitance-voltage (C(V)) structures to investigate the conduction and valance band states, respectively. The red-shift of the interband transitions due to the In {sub 0.15}Ga{sub 0.85}As layer observed in PL is compared to the shift of the corresponding energy levels obtained from C(V) measurements. The shifts of the ground states obtained from C(V) spectroscopy sum up to 42 meV, which is in good agreement with 43 meV observed in PL measurements. A small difference could be caused by a change in the exciton binding energy. From the 42 meV overall red-shift, 83% originate from the conduction and only 17% from the valence band. This is probably due to the smaller effective mass in the conduction band, so that here changes in the confinement potential result in larger changes in the energy levels.

Richter, Mirja; Reuter, Dirk; Wieck, Andreas D. [Lehrstuhl fuer Angewandte Festkoerperphysik, Ruhr-Universitaet Bochum, D-44780 Bochum (Germany); Duboz, Jean-Yves [Centre de Recherche sur l' Hetero-Epitaxie et ses Applications, CNRS, Sophia-Antipolis, F- 06560 Valbonne (France)

2007-07-01

231

Levels of valence  

Directory of Open Access Journals (Sweden)

Full Text Available The distinction between the positive and the negative is fundamental in our emotional life. In appraisal theories, in particular in the component process model of emotion (Scherer, 1984, 2010, qualitatively different types of valence are proposed based on appraisals of (unpleasantness, goal obstructiveness/conduciveness, low or high power, self- (incongruence, and moral badness/goodness. This multifaceted conceptualization of valence is highly compatible with the frequent observation of mixed feelings in real life. However, it seems to contradict the one-dimensional conceptualization of valence often encountered in psychological theories, and the notion of valence as a common currency used to explain choice behavior. Here, we propose a framework to integrate the seemingly disparate conceptualizations of multifaceted valence and one-dimensional valence by suggesting that valence should be conceived at different levels, micro and macro. Micro-valences correspond to qualitatively different types of evaluations, potentially resulting in mixed feelings, whereas one-dimensional macro-valence corresponds to an integrative “common currency” to compare alternatives for choices. We propose that conceptualizing levels of valence may focus research attention on the mechanisms that relate valence at one level (micro to valence at another level (macro, leading to new hypotheses and addressing various concerns that have been raised about the valence concept, such as the valence-emotion relation.

VeraShuman

2013-05-01

232

The quasiparticle band gap in the topological insulator Bi2Te3  

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We present a theoretical study of dispersion of states which form the bulk band-gap edges in the three-dimensional topological insulator Bi2Te3. Within density functional theory, we analyze the effect of atomic positions varying within the error range of the available experimental data and approximation chosen for the exchange-correlation functional on the bulk band gap and k-space location of valence- and conduction-band extrema. For each set of the positions with different...

Nechaev, I. A.; Chulkov, E. V.

2013-01-01

233

Influence of dye content on the conduction band edge of titania in the steam-treated dye-dispersing titania electrodes.  

Science.gov (United States)

The titania and dye-dispersing titania electrodes were prepared by a nitric acid-catalyzed sol-gel process. The dye-dispersing titania contains the dye molecules dispersed on the surface of the individual nanosized titania particles. The photo-cyclic voltammetry (Photo-CV) and photoelectric measurements of the dye-dispersing titania electrodes were conducted to clarify the factors changing the conduction band edge of the titania and the open-circuit voltage (Voc ) of the electrodes. The remaining nitrate ions caused a negative shift of conduction band edge of the titania of the dye-dispersing titania. The conduction band edge of the titania was shifted in a negative direction in the electrode containing a greater amount of the dye. These results are due to the adsorption of nitrate ions and the dye-titania complex formation on the titania particle surface. The effect of the dye-titania complex formation on the shift in the titania conduction band edge was greater than that of the adsorption of nitrate ions due to strong interaction between the dye and titania through the carboxylate and quinone-like groups of the dye. The shift in the titania conduction band edge corresponded to the change in the Voc value. PMID:24893823

Setiawan, Rudi Agus; Nishikiori, Hiromasa; Tanaka, Nobuaki; Fujii, Tsuneo

2014-01-01

234

Evidence of valence band perturbations in GaAsN/GaAs(001): A combined variable-angle spectroscopic ellipsometry and modulated photoreflectance investigation  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The contribution of the fundamental gap E_ as well as those of the E_ + Delta(so) and E+ transitions to the dielectric function of GaAs1-xNx alloys near the band edge were determined from variable-angle spectroscopic ellipsometry and modulated photoreflectance spectroscopy analyses. The oscillator strength of the E_ optical transition increases weakly with nitrogen incorporation. The two experimental techniques independently reveal that not only the oscillator strength of th...

Turcotte, S.; Larouche, S.; Beaudry, J. -n; Martinu, L.; Masut, R. A.; Desjardins, P.; Leonelli, R.

2008-01-01

235

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

236

Depolarisation of light scattered by disperse systems of low-dimensional potassium polytitanate nanoparticles in the fundamental absorption band  

Science.gov (United States)

The results of experimental studies of depolarising properties of disperse systems on the basis of potassium polytitanate nanoplatelets and nanoribbons in the visible and near-UV spectral regions are presented. It is shown that in the fundamental absorption band of the nanoparticle material the increase in the depolarisation factor takes place for the radiation scattered perpendicularly to the direction of the probing beam. For nanoribbons a pronounced peak of depolarisation is observed, which is caused by the essential anisotropy of the particles shape and the peculiarities of the behaviour of the material dielectric function. The empirical data are compared with the theoretical results for 'nanodiscs' and 'nanoneedles' with the model dielectric function, corresponding to that obtained from optical constants of the titanium dioxide dielectric function.

Zimnyakov, D. A.; Yuvchenko, S. A.; Pravdin, A. B.; Kochubey, V. I.; Gorokhovsky, A. V.; Tretyachenko, E. V.; Kunitsky, A. I.

2014-07-01

237

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

238

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

239

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

240

Phase Linear, Flat Wide Band, Low-Loss Filters Using New Configuration of Unidirectional Up- and Down-Chirp Dispersive Inter Digital Transducers  

Science.gov (United States)

Dispersive transducers have sharp cut-off and flat wideband frequency characteristics. Also, phase linear and very low-loss characteristics are obtained by combining down- and up-chirp unidirectional dispersive transducers (DUDIST and UUDIDT). We proposed wide band filters combining DUDIDT and UUDIDT using the conventional UDT with the grating thin films. In this case, we could not obtain the low loss results. In this paper, the theoretical results of phase linear, flat wide band and low loss filters using a combinations of the new DUDIDT and UUDIDT are described. DUDIDT and UUDIDT are obtained by using the new configuration of Dispersive IDT. The large unidirectionalities of the DUDIDT and UUDIDT are obtained by changing of the electrode width and thickness, and using the open and short-circuit electrodes as the reflectors.

Satoh, Yusuke; Yamanouchi, Kazuhiko

2010-09-01

 
 
 
 
241

Corrugated flat band as an origin of large thermopower in hole doped PtSb2  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The origin of the recently discovered large thermopower in hole-doped PtSb$_2$ is theoretically analyzed based on a model constructed from first principles band calculation. It is found that the valence band dispersion has an overall flatness combined with some local ups and downs, which gives small Fermi surfaces scattered over the entire Brillouin zone. The Seebeck coefficient is calculated using this model, which gives good agreement with the experiment. We conclude that ...

Kouta Mori; Hidetomo Usui; Hirofumi Sakakibara; Kazuhiko Kuroki

2012-01-01

242

Electronic band structure of beryllium oxide  

International Nuclear Information System (INIS)

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

243

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

244

Intrinsic accuracy in 3-dimensional photoemission band mapping  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Fundamental principles of mapping 3-dimensional quasiparticle dispersions in the valence band using angle-resolved photoemission spectroscopy are discussed. Such mapping is intrinsically limited in accuracy owing to damping of the final states, resulting in equivalent broadening in the surface-perpendicular wavevector. Mechanisms of the intrinsic accuracy are discussed in depth based on a physically transparent picture involving interplay of the final- and initial-state spec...

Strocov, V. N.

2002-01-01

245

One-band tight-binding model parametrization of the high-$T_c$ cuprates, including the effect of $k_z$-dispersion  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We discuss the effects of interlayer hopping and the resulting $k_z$-dispersion in the cuprates within the framework of the one-band tight binding (TB) model Hamiltonian. Specific forms of the dispersion relations in terms of the in-plane hopping parameters $t$, $t'$, $t''$ and $t'''$ and the effective interlayer hopping $t_z$ in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) and the added intracell hopping $t_{bi}$ between the CuO$_2$ bilayers in Bi$_2$Sr...

Markiewicz, R. S.; Sahrakorpi, S.; Lindroos, M.; Lin, Hsin; Bansil, A.

2005-01-01

246

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

International Nuclear Information System (INIS)

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

247

Dispersion of Refractive Index of ?-Ga2O3 Thin Films  

Science.gov (United States)

The dispersion of the refractive index of ?-Ga2O3 thin films obtained by RF-sputtering was investigated. Anomalous dispersion was observed for films annealed in hydrogen; normal dispersion, for films annealed in oxygen or argon. The spectral dependence in the visible region of the refractive index with normal dispersion was determined mainly by transitions from the top of the valence band formed by oxygen 2p-states to the bottom of the conduction band formed by hybridization of oxygen 2p-states and gallium 4s-states. The single-oscillator approximation parameters, dispersion energy, degree of ionic chemical bonding, and coordination number were found for films with normal dispersion.

Bordun, O. M.; Kukharskyy, I. Yo.; Bordun, B. O.; Lushchanets, V. B.

2014-11-01

248

[Research on and correction of interference fringes phenomenon in dispersive hyperspectral imaging spectrometer using back-illuminated CCDs in near-infrared band].  

Science.gov (United States)

Dispersive hyperspectral imaging spectrometer using back-illuminated CCDs will cause interference fringes in near-infrared band, reducing the near-infrared spectral resolution. To solve this problem, we established a multi-beam interference model similar to a Farby-Pérot interferometer, estimated the intensity of distribution of interference fringes from 700 to 900 nm, verified its correctness with measured data, and analyzed the relationship between CCD photosensitive zone thickness and the interference phenomenon. On this base, the authors used the improved flat-field correction algorithm to correct the interference. From 751.83 to 1 010.04 nm wavelength, the correction efficiency can reach 96.6%. The results show that the algorithm can effectively eliminate the interference of dispersive hyperspectral imaging spectrometer in near-infrared band. PMID:25269323

Ma, Liang; Wei, Jun; Huang, Xiao-Xian

2014-07-01

249

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

250

Integrated C-band erbium-doped fiber amplifier and dispersion compensation module with negligible pump power penalty.  

Science.gov (United States)

Conventional dispersion compensation requires erbium-doped fiber amplifiers with high pump power to compensate for excess losses introduced by the dispersion-compensating fiber. We report and demonstrate an optimized single-pump amplifier configuration with a pump power penalty of less than 5% when integrated with a fiber Bragg grating dispersion compensator. The system is capable of compensating 100 km of G.652 single-mode fiber. PMID:19881651

Varghese, Samuel; Blixt, Peter; Darby, David; Nair, Suresh

2009-11-01

251

Electronic band structure of magnetic bilayer graphene superlattices  

Energy Technology Data Exchange (ETDEWEB)

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

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

2014-09-28

252

Electronic band structure of magnetic bilayer graphene superlattices  

International Nuclear Information System (INIS)

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

253

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

254

Ab initio calculation of core-valence-valence Auger spectra in closed shell systems.  

Science.gov (United States)

We propose an ab initio method to evaluate the core-valence-valence Auger spectrum of systems with filled valence bands. The method is based on the Cini-Sawatzky theory, and aims at estimating the parameters by first-principles calculations in the framework of DFT. Photoemission energies and the interaction energy for the two holes in the final state are evaluated by performing DFT simulations for the system with varied population of electronic levels. Transition matrix elements are taken from atomic results. The approach takes into account the non spherical density of states of the emitting atom, spin-orbit interaction in core and valence, and non quadratic terms in the total energy expansion with respect to fractional occupation numbers. It is tested on two benchmark systems, Zn and Cu metals, leading in both cases to L23M45M45 Auger peaks within 2 eV from the experimental ones. Especially problematic is the evaluation of the hole-hole interaction for systems with broad valence bands: our method underestimates its value in Cu, while we obtain excellent results for this quantity in Zn.

Brivio, Gian Paolo; Fratesi, Guido; Italo Trioni, Mario; Ugenti, Simona; Perfetto, Enrico; Cini, Michele

2009-03-01

255

VALENCE STATES OF SULFUR IN POLLUTION SAMPLES BY X-RAY ANALYSIS  

Science.gov (United States)

A flat single crystal spectrometer was configured to measure the valence band x-ray spectra of various forms of sulfur in air pollutants. While most different valence states showed differences in the structure of the K sub beta band, particular emphasis was placed on distinguishi...

256

Optimization of a dual-core dispersion slope compensating fiber for DWDM transmission in the 1480-1610 nm band through G.652 single-mode fibers  

Science.gov (United States)

We report design optimization of a dual-core dispersion slope compensating fiber (DSCF) for broadband dense wavelength division multiplexing (DWDM) transmission separately in the S-, C-, and L-bands through a 1310 nm-optimized conventional single-mode fiber (CSF). Index profile parameters of the DSCF have been adjusted to simultaneously achieve a mode effective area, which match that of the G.652 fiber as closely as possible in order to contain potentially detrimental nonlinear propagation effects like four-wave mixing and cross-phase modulation (XPM). Theoretical figure of merit of the designed dispersion compensating fiber (DCF) is ˜900 ps/(nm-dB) @ 1550 nm, and its estimated bend loss for a single turn bend diameter of 32 mm is negligible.

Pal, Bishnu P.; Pande, Kamna

2002-01-01

257

Size effects on mixed valence CePd{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

To study the size effects on mixed-valence state of CePd{sub 3}, nanoparticles of CePd{sub 3}, sizes ranging from 5.2 to 9.5 nm, were prepared. The mixed valence increased from 3.3 to 3.5 as particle size reduced from bulk to 5.2 nm. This consequence was illustrated by the enhancements of valence fluctuations and 4f electron hybridization with conduction band through size reduction. Another interesting finding in the nanoparticles is that a certain fraction {approx} 25% of the sample becomes trivalent which is evident from the increase of magnetic susceptibility at low temperatures.

Lin, Y H; Ou, M N; Chen, Y Y [Institute of Physics, Academia Sinica, Taipei, Taiwan (China); Wang, C R [Department of Physics, Tunghai University, Taichung, Taiwan (China); Dong, C L, E-mail: linyh@phys.sinica.edu.tw, E-mail: cheny2@phys.sinica.edu.tw [National Synchrotron Radiation Research Center, Hsinchu, Taiwan (China)

2011-01-01

258

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 2 p and Ni 2 p 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

2015-01-01

259

Optical Dispersion Behavior and Band Gap Energy of Relaxor Ferroelectric 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 Single Crystal  

International Nuclear Information System (INIS)

Refractive indices and extinction coefficients of 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 (PMN-0.08PT) single crystal are investigated by variable angle spectroscopic ellipsometry (VASE) at different wavelengths. The parameters relative to the energy band structure are obtained by fitting to the single-oscillator dispersion equation, and the band gap energy is also deduced from the Tauc equation. Similar to most oxygen-octahedra ferroelectrics, PMN-0.08PT has the same dispersion behavior described by the refractive-index dispersion parameters. (condensed matter: electronicstructure, electrical, magnetic, and opticalproperties)

260

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 tha...

Myagkota, S.

2003-01-01

 
 
 
 
261

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)

262

Micro-Valences: Affective valence in “neutral” everyday objects  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Affective valence influences both our cognition and our perception of the world. Indeed, the speed and quality with which we recognize objects in a visual scene can vary dramatically depending on its affective content. However, affective processing of visual objects has been typically studied using only stimuli with strong affective valences (e.g., guns or roses). Here we explore whether affective valence must be strong or obvious to exert an effect on our perception. We conclude that the maj...

MichaelJTarr; SophieLebrecht; MosheBar; LisaFBarrett

2012-01-01

263

Dispersive optical constants and temperature-dependent band gap of cadmium-doped indium selenide thin films  

Science.gov (United States)

Polycrystalline cadmium-doped indium selenide thin films were obtained by the thermal co-evaporation of ?-In2Se3 crystals and Cd onto glass substrates kept at a temperature of 200 °C. The temperature dependence of the optical band gap in the temperature region of 300-450 K and the room temperature refractive index, n(?), of these films have been investigated. The absorption edge shifts to lower energy as temperature increases. The fundamental absorption edge corresponds to a direct energy gap that exhibits a temperature coefficient of -6.14 × 10-4 eV K-1. The room temperature n(?) which was calculated from the transmittance data allowed the identification of the oscillator strength and energy, static and lattice dielectric constants and static refractive index as 20.06 and 3.07 eV, 7.43 and 10.52 and 2.74, respectively.

Qasrawi, A. F.

2005-08-01

264

Dielectric dispersion and energy band gap of Bi1.5-xSmxZn0.92Nb1.5O6.92 solid solution  

Science.gov (United States)

The optical transmittance and reflectance spectra of samarium doped bismuth-zinc-niobium-oxide (BZN) pyrochlore ceramics are investigated in the wavelength range of 200-1050 nm (200-1500 THz). The Sm content in the Bi1.5-xSmxZn0.92Nb1.5O6.92 solid solution significantly alters the optical properties. Therefore, increasing the Sm doping ratio from x=0.10 to x=0.13 decreased the indirect forbidden energy band gap from 3.60 to 3.05 eV. In addition, above 350 THz, increasing the Sm content decreases the dielectric constant values and alters the dielectric dispersion parameters. The dielectric spectra which were evaluated in the frequency range of 200-1500 THz reflected a sharp decrease in the dielectric constant with increasing frequency down to 358 THz. The spectra reflected a resonance peak at this frequency. Such resonance spectrum is promising for technological applications as it is close to the illumination of 870 nm IR lasers that are used in optical communications. The calculated oscillator (Eo) and dispersion (Ed) energies near that critical range (375-425 THz) reflected an increase in both Eo and Ed with increasing Sm content.

Qasrawi, A. F.; Mergen, A.

2014-05-01

265

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

266

Electron localization in mixed-valence manganites  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Mixed-valence manganites (A3+0.7B2+0.3)MnO3 exhibit a resistivity peak with giant negative magnetoresistance below the Curie point. Residual resistivity ranges from 5?10-8 to 8?103 m, but the electronic heat capacity when A=Y or La is that of a normal d-band metal; 6 mJ mole-1K-2. Spatial fluctuations in the Coulomb and spin-dependent potentials localize the eg(*) electrons in wave packets that are large on the scale of the Mn-Mn separation. Transport in the magnetically localized state i...

Coey, John Michael David

1995-01-01

267

Similarities of the band structure of In4Se3 and InSe under pressure and peculiarities of the creation of the band gap  

Science.gov (United States)

The existence of the physical elementary energy bands describing the Davydov splitting was demonstrated in the energy spectra of the layered orthorhombic In4Se3 crystal. It was confirmed that the physical elementary energy bands are related to the exactly determined Wyckoff position in the unit cell, where the maximum of the valence electron density is localized. By means of ab initio calculations, the dispersion laws with the low-energy non-parabolicity were obtained for electrons and holes of the In4Se3 crystal. The reasons for such a dispersion law to occur in In4Se3 are discussed as well as its similarity to the dispersion law for holes in the ? -InSe crystal under pressure.

Sznajder, M.; Rushchanskii, K. Z.; Kharkhalis, L. Yu.; Bercha, D. M.

268

From mixed valence to the Kondo lattice regime  

Science.gov (United States)

Many heavy fermion materials are known to cross over from the Kondo lattice regime to the mixed valence regime or vice versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valence regime: a low temperature coherence peak and a high temperature ‘Hubbard band’ peak. These two peaks merge, yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity likewise exhibits an unusual absorption feature (shoulder) in the deep mid-infrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport and of the effective f-level in the optical conductivity are shown to be responsible for the anomalous transport properties. A two-band hybridization-gap model, which neglects incoherent effects due to many-body scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed valence regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn5, Ce2Ni3Si5, CeFeGe3 and YbIr2Si2, (b) pressure dependent resistivity of YbInAu2 and CeCu6, and (c) optical conductivity measurements in YbIr2Si2 yields excellent agreement.

Kumar, Pramod; Vidhyadhiraja, N. S.

2011-12-01

269

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

270

Parameterization of CuIn1-xGaxSe2 (x = 0, 0.5, and 1) energy bands  

International Nuclear Information System (INIS)

Parameterization of the electronic band structure of CuIn1-xGaxSe2 (x = 0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [Ej(k); j = v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the ?-point valence-band maximum Ev1(0). Also the lowest conduction band Ec1(k) is anisotropic and non-parabolic for energies ? 0.05 eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn1-xGaxSe2 need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses mj(k), and also an average energy-dependent approximation of the masses mj(E).

271

Fermi surface and electron dispersion of PbTe doped with resonant Tl impurity from KKR-CPA calculations  

Science.gov (United States)

We present results of a detailed study on the electron dispersions and Fermi surface of lead telluride doped with 2% of thallium, which is a resonant impurity in PbTe. Using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA), Bloch spectral functions (BSFs), which replace the dispersion relations in alloys, are calculated, and BSF intensity maps over the Brillouin zone (alloy Fermi surface cross sections) are presented. It is shown that, close to the valence band edge, Tl does not create an isolated impurity band, but due to its resonant character, strongly disturbs the host electronic bands, leading to the disappearance of sharp and well-defined electronic energy bands. The consequences of this effect on the transport properties are discussed and a qualitative explanation for the improvement in the thermoelectric properties of PbTe:Tl is suggested.

Wiendlocha, Bartlomiej

2013-11-01

272

??????????? ? ??????????(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

273

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

International Nuclear Information System (INIS)

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

274

Micro-Valences: Perceiving Affective Valence in Everyday Objects  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Perceiving the affective valence of objects influences how we think about and react to the world around us. Conversely, the speed and quality with which we visually recognize objects in a visual scene can vary dramatically depending on that scene’s affective content. Although typical visual scenes contain mostly “everyday” objects, the affect perception in visual objects has been studied using somewhat atypical stimuli with strong affective valences (e.g., guns or roses). Here we explor...

Lebrecht, Sophie; Barrett, Lisa Feldman; Tarr, Michael J.; Bar, Moshe

2012-01-01

275

Effects of Optical-density and Phase Dispersion of an Imperfect Band-limited Occulting Mask on the Broadband Performance of a TPF Coronagraph  

Science.gov (United States)

Practical image-plane occulting masks required by high-contrast imaging systems such as the TPF-Coronagraph introduce phase errors into the transmitting beam., or, equivalently, diffracts the residual starlight into the area of the final image plane used for detecting exo-planets. Our group at JPL has recently proposed spatially Profiled metal masks that can be designed to have zero parasitic phase at the center wavelength of the incoming broadband light with small amounts of' 00 and phase dispersions at other wavelengths. Work is currently underway to design. fabricate and characterize such image-plane masks. In order to gain some understanding on the behaviors of these new imperfect band-limited occulting masks and clarify how such masks utilizing different metals or alloys compare with each other, we carried out some modeling and simulations on the contrast performance of the high-contrast imaging testbed (HCIT) at .JPL. In this paper we describe the details of our simulations and present our results.

Sidiek, Erkin; Balasubramanian, Kunjithapatham

2007-01-01

276

Nonlinear hopping induced valence fluctuations  

International Nuclear Information System (INIS)

A model is introduced to describe intermediate valence states: any f-electron hopping process influences the hopping probability for other f-electrons. This nonlinear coupling may result from higher order interaction terms between f-electrons and conduction electrons or from a strong coupling to the lattice and its dynamics. As a function of coupling parameters and temperature several phase transitions are possible characterized by nonintegral valence changes. A possible relation to the ?-? phase transition in cerium is discussed. (orig.)

277

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)

278

A Multidimensional Measure of Work Valences  

Science.gov (United States)

Work valence is derived from expectancy-valence theory and the literature on children's vocational development and is presumed to be a general appraisal of work that emerges during the childhood period. Work valence serves to promote and inhibit the motivation and tasks associated with vocational development. A measure of work valence, composed of…

Porfeli, Erik J.; Lee, Bora; Weigold, Ingrid K.

2012-01-01

279

An experimental and theoretical study of the valence shell photoelectron spectrum of bromochlorofluoromethane  

Energy Technology Data Exchange (ETDEWEB)

The complete valence shell photoelectron spectrum of bromochlorofluoromethane (CHFClBr), covering the binding energy range {approx}10-50 eV, has been recorded using synchrotron radiation and the observed structure has been interpreted using ionization energies and relative spectral intensities computed using the third-order algebraic-diagrammatic-construction (ADC(3)) scheme for the one-particle Green's function and the outer valence Green's function (OVGF) method. The theoretical results demonstrate that the inner valence region of the photoelectron spectrum is dominated by satellite structure. Angle-resolved photoelectron spectra, recorded at selected excitation energies, have enabled the orbital assignments for the outer valence bands to be confirmed. The four outermost photoelectron bands, ascribed to the two pairs of orbitals associated with the nominally chlorine and bromine lone-pairs, exhibit characteristic angular distributions. The photon energy dependent variations in the relative photoelectron band intensities provide additional support for the orbital assignments.

Holland, D M P [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom); 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); Novak, I [Department of Chemistry, National University of Singapore, Singapore 0511 (Singapore); Zaytseva, I L; Trofimov, A B; Gromov, E V [Laboratory of Quantum Chemistry, Irkutsk State University, 664003 Irkutsk (Russian Federation); Schirmer, J, E-mail: david.holland@stfc.ac.u [Theoretische Chemie, Physikalisch-Chemisches Institut, Universitaet Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg (Germany)

2010-07-14

280

Choice of compounds with fast core-valence transitions  

International Nuclear Information System (INIS)

It is now well established that in a number of wide gap ionic crystals a specific intrinsic luminescence can be observed due to radiative hole transitions between the upper core band and valence band. The progress in knowledge about core-valence (CV) transitions is considered in papers. The CV transitions were first observed and interpreted in BaF2 crystals. The intrinsic luminescence due to CV transitions has a short decay time (?1ns), a high thermal stability in all its parameters, and a relatively high yield. Because of these luminescence properties, crystals with radiative CV transitions are promising scintillators. Such scintillators are particularly important in devices with a high counting rate, for instance in emission tomography. The presence of a filled valence band above the core band in which the hole is created presents the possibility of population inversion at high excitation densities. This aspect creates new possibilities for producing optical amplification and generation. This work examines different approaches to the problem of CV transitions. A class of ionic crystals, in which the radiative transitions are most efficient, is identified. It is shown how to obtain the necessary spectral width of emission, and the possibility of controlling the decay time of the emission is discussed

 
 
 
 
281

Valence effects of sorption: laboratory control of valence state  

International Nuclear Information System (INIS)

Estimation of the rates of migration of nuclides from nuclear waste repositories required knowledge of the interaction of these nuclides with the components of the geological formations in the path of the migration. These interactions will be dependent upon the valence state and speciation of the nuclide. If the valence state is not known, then there can be little confidence in use of the data for safety analysis. An electrochemical method of valence state control was developed which makes use of a porous electrode in a flow system containing a column of the adsorbent. By use of this method and solvent extraction analyses of the valence states, a number of reactions of interest to HLW repositories were investigated. These include the reduction of Np(V) and Tc(VII) by crushed basalt and other minerals. For the reduction of Np(V) by basalt, the experiments indicate that sorption on basalt increases with pH and that most of the Np is reduced to Np(IV). The adsorbed Np(IV) is very difficult to remove from the basalt. For the experiments with Tc(VII), the results are considerably more complicated. The results of these experiments are used to assess some of the techniques and methods currently used in safety analyses of proposed HLW repositories. Perhaps the most important consideration is that predictive modeling of valence change reactions, such as the reduction of Np(V) and Tc(VII), must be used with considerable caution, and the occurrence of such reactions should be verified as best as possible with experiments using valence state control and analyses. 13 references, 3 figures, 1 table

282

Parameterization of CuIn{sub 1-x}Ga{sub x}Se{sub 2} (x = 0, 0.5, and 1) energy bands  

Energy Technology Data Exchange (ETDEWEB)

Parameterization of the electronic band structure of CuIn{sub 1-x}Ga{sub x}Se{sub 2} (x = 0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [E{sub j}(k); j = v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the {Gamma}-point valence-band maximum E{sub v1}(0). Also the lowest conduction band E{sub c1}(k) is anisotropic and non-parabolic for energies {approx} 0.05 eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn{sub 1-x}Ga{sub x}Se{sub 2} need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses m{sub j}(k), and also an average energy-dependent approximation of the masses m{sub j}(E).

Chen Rongzhen, E-mail: rche@kth.se; Persson, Clas

2011-08-31

283

Magnetic excitations in intermediate valence semiconductors with singlet ground state  

International Nuclear Information System (INIS)

The explanation of the origin inelastic peaks in magnetic neutron scattering spectra of the mixed-valent semiconductor SmB6 is proposed. It is shown that the excitonic theory of intermediate valence state not only gives the value of the peak frequency but also explains the unusual angular dependence of intensity of inelastic magnetic scattering and describes the dispersion of magnetic excitations in good agreement with experiment

284

Bi2Te3 band structure reconstruction under hydrostatic compression  

International Nuclear Information System (INIS)

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

285

Core-valence luminescence in scintillators  

International Nuclear Information System (INIS)

The state of the art of core-valence luminescence (CVL) in a number of halides (BaF2, CsCl, LiBaF3, etc.) is analysed. The following items are discussed: common properties of CVL, a suppression of slow emission component in BaF2, an effect of secondary excitation density on intensity and decay profile of the emission, a neutron-gamma discrimination, spectrum simulations, impurity-associated CVL, an origin of slow- and long-wavelength CVL components, a lattice relaxation, vibrations of lattice environment and phonon broadening of the emission bands. Experimental results on impurity-associated CVL of Rb1-xCsxBr (x<0.2) are presented and discussed. It is shown that the impurity-associated luminescence gives us a convenient way for the improvement of light output and variation of the emission wavelength of the scintillators

286

Band structure of ABC-trilayer graphene superlattice  

Science.gov (United States)

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

Uddin, Salah; Chan, K. S.

2014-11-01

287

Understanding intermediate-band solar cells  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. To make this possible, these cells incorporate an energy band that is partially filled with electrons within the forbidden bandgap of a semiconductor. Photons with insufficient energy to pump electrons from the valence band to the conduction band can use this intermediate band as a stepping stone to generate an electron-hole pair. Nanostructured materials and certain...

Luque Lo?pez, Antonio; Marti? Vega, Antonio; Stanley, Colin

2012-01-01

288

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

Science.gov (United States)

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

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

2015-01-01

289

XANES Pb LIII spectra of mixed-valence compound: Minium, Pb3O4  

CERN Document Server

Mixed-valence compound Pb3O4 (minium) has been studied using X-ray absorption near-edge structure (XANES) spectroscopy and DFT calculations. In spite of presence of two valence states of lead [Pb(II) and Pb(IV)], the XANES spectrum of studied system is corresponding to apparently unified, an intermediate valence state of Pb. On the other hand, the 207Pb NMR spectra definitely show two different spectral bands corresponding to different Pb2+ and Pb4+ ions in Pb3O4 crystal structure. The explanation of this contradiction is related to the basics of XANES and NMR spectroscopy.

Gabuda, S P; Erenburg, S B; Bausk, N V

2003-01-01

290

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

291

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

292

Photoemission from valence bands of transition metal-phthalocyanines  

Energy Technology Data Exchange (ETDEWEB)

Research highlights: {yields} The HOMO mainly comes from the carbon atoms of Pc rings and the central metal atoms almost have no contribution on the highest occupied molecular orbital (HOMO: a{sub 1u}) distribution of CoPc as well as NiPc. {yields} Influence by central metal atom on the photoemission intensities from the HOMO of two single molecule systems is negligible for the major. {yields} The modification of the distribution for {pi}-orbital upon adsorption as well as the scattering effects of the central metal on the photoemission intensities are negligible for the major. - Abstract: Angular dependencies of ultraviolet photoelectron spectrum of transition metal-phthalocyanines (TM-Pcs), NiPc and CoPc, have been studied by using multiple-scattering theory to explore the electronic structure of the organometallic complexes influenced by central metal atom. The calculated angular distributions of photoelectrons for the highest occupied molecular orbital (HOMO: a{sub 1u}) from the two single systems are nearly the same and represent well the experimental results obtained for the well-ordered monolayer on the highly oriented pyrolytic graphite substrate. The central metal atoms almost have no contribution on the HOMO distribution, which mainly comes from the carbon atoms of Pc ring. Moreover, the modification of the distribution for {pi} orbital upon adsorption as well as the scattering effects of the central metal on the photoemission intensities are negligible for the major.

Shang, Ming-Hui, E-mail: smh2875@graduate.chiba-u.jp [Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522 (Japan); Nagaosa, Mayumi [Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522 (Japan); Nagamatsu, Shin-ichi [University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585 (Japan); Hosoumi, Shunsuke [Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522 (Japan); Kera, Satoshi, E-mail: kera@faculty.chiba-u.jp [Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522 (Japan); Institute for Molecular Science, Nishigonaka 38, Myodaiji, Okazaki 444-8585 (Japan); Fujikawa, Takashi; Ueno, Nobuo [Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522 (Japan)

2011-04-15

293

Photoemission from valence bands of transition metal-phthalocyanines  

International Nuclear Information System (INIS)

Research highlights: ? The HOMO mainly comes from the carbon atoms of Pc rings and the central metal atoms almost have no contribution on the highest occupied molecular orbital (HOMO: a1u) distribution of CoPc as well as NiPc. ? Influence by central metal atom on the photoemission intensities from the HOMO of two single molecule systems is negligible for the major. ? The modification of the distribution for ?-orbital upon adsorption as well as the scattering effects of the central metal on the photoemission intensities are negligible for the major. - Abstract: Angular dependencies of ultraviolet photoelectron spectrum of transition metal-phthalocyanines (TM-Pcs), NiPc and CoPc, have been studied by using multiple-scattering theory to explore the electronic structure of the organometallic complexes influenced by central metal atom. The calculated angular distributions of photoelectrons for the highest occupied molecular orbital (HOMO: a1u) from the two single systems are nearly the same and represent well the experimental results obtained for the well-ordered monolayer on the highly oriented pyrolytic graphite substrate. The central metal atoms almost have no contribution on the HOMO distribution, which mainly comes from the carbon atoms of Pc ring. Moreover, the modification of the distribution for ? orbital upon adsorption as well as the scattering effects of the central metal on the photoemission intensities are negligible for the majorities are negligible for the major.

294

Band structure and optical electron spectra of (TrMA)CoCl{sub 3}.2H{sub 2}O crystal  

Energy Technology Data Exchange (ETDEWEB)

DFT calculations of the band structure, density of states and optical spectra of trimethylammonium cobalt chloride dihydrate [(CH{sub 3}){sub 3}NH]CoCl{sub 3}.2H{sub 2}O single crystal have been performed for the first time within a framework of the density functional theory using the CASTEP code (Cambridge serial total energy package). The valence band energy dispersion E(k) was found to be flat, although the bottom of the conduction band reveals noticeable dispersion along the lines [000]-[0012] and [-12012]-[-1200]-[-12120]. The calculations performed in the generalized gradient spin approximation (GGS) predict nonzero magnetic moment for the TrMA-CoCl{sub 3} crystal. The optical spectra calculated agree qualitatively with the experimental absorption spectra of the crystal in the range of Co ions excitation, 1.50-2.75eV.

Andriyevsky, B. [Technical University of Koszalin, 2 Sniadeckich Str., PL-75-453 Koszalin (Poland)]. E-mail: bandri@tu.koszalin.pl; Kapustianyk, V. [Scientific-Technical and Educational Center of Low Temperature Studies, Lviv I. Franko National University, Dragomanova Str., 50, UA-79005 Lviv (Ukraine); Ciepluch-Trojanek, W. [Technical University of Koszalin, 2 Sniadeckich Str., PL-75-453 Koszalin (Poland); Batiuk, A. [Scientific-Technical and Educational Center of Low Temperature Studies, Lviv I. Franko National University, Dragomanova Str., 50, UA-79005 Lviv (Ukraine)

2005-10-01

295

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)

296

An Intracranial EEG Study of the Neural Dynamics of Musical Valence Processing.  

Science.gov (United States)

The processing of valence is known to recruit the amygdala, orbitofrontal cortex, and relevant sensory areas. However, how these regions interact remains unclear. We recorded cortical electrical activity from 7 epileptic patients implanted with depth electrodes for presurgical evaluation while they listened to positively and negatively valenced musical chords. Time-frequency analysis suggested a specific role of the orbitofrontal cortex in the processing of positively valenced stimuli while, most importantly, Granger causality analysis revealed that the amygdala tends to drive both the orbitofrontal cortex and the auditory cortex in theta and alpha frequency bands, during the processing of valenced stimuli. Results from the current study show the amygdala to be a critical hub in the emotion processing network: specifically one that influences not only the higher order areas involved in the evaluation of a stimulus's emotional value but also the sensory cortical areas involved in the processing of its low-level acoustic features. PMID:24904066

Omigie, Diana; Dellacherie, Delphine; Hasboun, Dominique; George, Nathalie; Clement, Sylvain; Baulac, Michel; Adam, Claude; Samson, Severine

2014-06-01

297

Effects of rare-earth substitution in ?: mixed-valence and magnetic properties  

Science.gov (United States)

The lattice parameters and magnetic properties of 0953-8984/11/8/015/img9 (R = Ho, Er) have been investigated. The deviation of the lattice parameters from Vegard's law indicates that the Ce valence in both systems fluctuates with the concentration of the dopant R. This fluctuation is directly relevant to the degree of localization of Ce 4f electrons, and is independent of the atomic size of the dopant element. The observed metamagnetic transition of 0953-8984/11/8/015/img10 has been analysed from the description in the band picture. The Ce valence fluctuation from the mixed-valence state to the localized state increases the moments of the Ce and Fe ions, and the Curie temperature. The changes in the mixed-valence state also affect the properties of the Ce-Fe coupling.

Tang, C. C.; Li, Y. X.; Du, J.; Wu, G. H.; Zhan, W. S.

1999-03-01

298

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

299

A macroscopic model for electron transport in silicon using analytic descriptions for both the electron bands and the phonon dispersion relations  

Science.gov (United States)

We present a macroscopic model for electron transport in silicon, in which the state variables are moments of the electron distribution functions. The evolution equations are obtained from the Boltzmann transport equations and are closed by means of the maximum entropy principle (MEP). Analytic approximations are used for the electron and phonon dispersion relations. All the main scattering mechanisms electrons undergo in silicon are taken into account.

Mascali, Giovanni; Romano, Vittorio

2013-10-01

300

Attosecond time-resolved photoelectron dispersion and photoemission time delays.  

Science.gov (United States)

We compute spectrograms and relative time delays for laser-assisted photoemission by single attosecond extreme ultraviolet pulses from valence band (VB) and 2p core levels (CLs) of a Mg(0001) surface within a quantum-mechanical model. Comparing the time-dependent dispersion of photoelectron (PE) wave packets for VB and CL emission, we find striking differences in their dependence on the (i) electron mean free path (MFP) in the solid, (ii) screening of the streaking laser field, and (iii) chirp of the attosecond pulse. The relative photoemission delay between VB and 2p PEs is shown to be sensitive to the electron MFP and screening of the streaking laser field inside the solid. Our model is able to reproduce a recent attosecond-streaking experiment [S. Neppl et al., Phys. Rev. Lett. 109, 087401 (2012)], which reveals no relative streaking time delay between VB and 2p PEs. PMID:24484011

Liao, Q; Thumm, U

2014-01-17

 
 
 
 
301

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

Science.gov (United States)

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

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

2004-12-01

302

Topological Crystalline Kondo Insulator in Mixed Valence Ytterbium Borides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The electronic structures of two mixed valence insulators YbB$_6$ and YbB$_12$ are studied by using the local density approximation (LDA) supplemented with the Gutzwiller method and dynamic mean field theory (DMFT). YbB$_6$ is found to be a moderately correlated Z$_2$ topological insulator, similar to SmB$_6$ but having much larger bulk band gap. Notably, YbB$_12$ is revealed to be in a new novel quantum state, strongly correlated topological crystalline Kondo insulator, whi...

Weng, Hongming; Zhao, Jianzhou; Wang, Zhijun; Fang, Zhong; Dai, Xi

2013-01-01

303

Electronic polaron effects in mixed valence and heavy fermion materials  

International Nuclear Information System (INIS)

It is shown that the Coulomb interaction between line-integral electrons and broad band electrons has a profound influence on the dynamical properties of an line-integral hole in mixed valence and heavy fermion materials. At zero temperature the dynamics of the screening process contains an infrared divergence. The broadening of this divergence by temperature causes the motion of the line-integral electrons to crossover from wave propagation to diffusion. This mechanisms explains the observed dual nature of the line-integral electrons, namely that at low temperatures they behave like a Fermi liquid, while at high temperatures they evolve into a localized magnetic moments

304

Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure  

Science.gov (United States)

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduction states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as a model system for mixed valent 4f-electron materials. Using high-resolution x-ray-absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength.

Jarrige, I.; Yamaoka, H.; Rueff, J.-P.; Lin, J.-F.; Taguchi, M.; Hiraoka, N.; Ishii, H.; Tsuei, K. D.; Imura, K.; Matsumura, T.; Ochiai, A.; Suzuki, H. S.; Kotani, A.

2013-03-01

305

DERIVATION AND SIGNIFICANCE OF VALENCE MOLECULAR CONNECTIVITY  

Science.gov (United States)

The physical basis for valence molecular connectivity was studied. The sigma sup upsilon and sigma values are cardinal numbers describing the electronic structure of atoms in their valence states. The value (sigma sup upsilon) + (sigma) describes the volume of a bonding atom whil...

306

Valence-Bond Theory and Chemical Structure.  

Science.gov (United States)

Discussed is the importance of valence bond theory on the quantum-mechanical theory of chemical structure and the nature of the chemical bond. Described briefly are early VB theory, development of VB theory, modern versions, solid-state applications, models, treatment in textbooks, and flaws in criticisms of valence bond theory. (KR)

Klein, Douglas J.; Trinajstic, Nenad

1990-01-01

307

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

308

Electronic band structures for zinc-blende and wurtzite CdS  

International Nuclear Information System (INIS)

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

309

Indirect-direct band gap transition through electric tuning in bilayer MoS2  

International Nuclear Information System (INIS)

We investigate the electronic properties of bilayer MoS2 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

310

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

311

Verb Valencies and Croatian as a Foreign and Second Language  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The article deals with verb valency and the description and list of verb valencies in Croatian as the first and as the second and foreign language. Since the importance of researching verb valency was observed in linguistics several decades ago, it has been researched in many languages, and many verb valency dictionaries have been written. In Croatia, the valency structure of the Croatian verb system has been the subject of several projects, resulting in the publishing of valency lexicons and...

Marica ?ilaš-Mikuli?; Sanda Lucija Udier; Milvia Guleši?-Machata

2011-01-01

312

Understanding the valency of rare earths from first-principles theory  

Science.gov (United States)

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. Their magnetic properties are determined by the occupancy of the strongly localized 4f electronic shells, while the outer s-d electrons determine the bonding and other electronic properties. Most of the rare-earth atoms are divalent, but generally become trivalent in the metallic state. In some materials, the energy difference between these valence states is small and, by changing some external parameter (such as pressure), a transition from one to the other occurs. But the mechanism underlying this transition and the reason for the differing valence states are not well understood. Here we report first-principles electronic-structure calculations that enable us to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions. We find that there are two types of f electrons: localized core-like f electrons that determine the valency, and delocalized band-like f electrons that are formed through hybridization with the s-d bands and which participate in bonding. The latter are found only in the trivalent systems; if their number exceeds a certain threshold, it becomes energetically favourable for these electrons to localize, causing a transition to a divalent ground state.

Strange, P.; Svane, A.; Temmerman, W. M.; Szotek, Z.; Winter, H.

1999-06-01

313

Modified sf model for mixed-valence Eu chalcogenides  

International Nuclear Information System (INIS)

EuO exhibits at room temperature a pressure-induced metal-insulator transition of first order into a mixed-valence phase (psub(c) approximately equal to 300 kbar), which is followed by a structural NaCl-CsCl transition at about 400 kbar. For EuS and EuSe only the structural transitions are observed. A modified sf model is proposed for describing the mixed valency in the ferromagnetic semiconductors EuO, EuS and EuSe. It turns out that the temperature-dependent shift of the conduction band edge, occurring below Tsub(C) due to the sf exchange interaction, drives the metal-insulator transition point to substantially lower pressure values. As a consequence of the dynamic interplay between the red shift of the band edge and the 'dilution' of the Heisenberg magnet, caused by f electron fluctuations, the electronic phase transitions change from first to second order with decreasing temperature. At T = O the following critical regions are found. EuO, 200-260 kbar; EuS, 130-160 kbar; EuSe, 125-150 kbar. For EuS the electronic collapse happens in the low-temperature region clearly before the NaCl-CsCl transition and should therefore be observable as in EuO. (author)

314

A First-Principles Investigation of Core Valence Luminescence in Barium Fluoride  

Science.gov (United States)

Rodnyi (P. Rodnyi, Sov. Phys.- Solid State 34, 1078(1992)) showed how the core valence transition (CVL) in a binary compound is determined by the various band parameters associated with the band gap of the material and its valence and core band structures. The most crucial parameter for this process is shown to be ?, which is approximately equal to the ratio of the energy gap between the top of the core band and bottom of the conduction band, and the band gap. We have studied the variation of this band parameter among the alkaline-earth fluorides. Using the band structures from the augmented spherical wave method, the values of ? are found to be 2.79, 2.11 and 1.74 for CaF_2, SrF2 and BaF2 respectively. They are in reasonable agreement with the empirical values reported by Rodnyi^1. The value of ? for BaF2 lies within the limits of 1.34 and 2.00 which is favorable for CVL. A comparison of the band parameters from the ASW method and a cluster approach using the scattered wave X-? method will also be made.

Mishra, K. C.; Johnson, K. H.; Osterloh, I.; Schmidt, P. C.

1997-03-01

315

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

316

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

317

Magnetic Properties and Eu Valence in EuCu2(SixGe1-x)2  

International Nuclear Information System (INIS)

We performed measurements of magnetic susceptibility ( ? ) and LIII - edge X-ray absorption spectroscopy (XAS) in EuCu2(SixGe1-x)2. For 0.70 ? x ? 0.80, with decreasing temperature, the ? deviates from Curie-Weiss (CW) law with a Eu2+ state and exhibits almost temperature-independent behaviour at lower temperatures. The behaviour, which is roughly similar to that accompanied by the valence transition in EuNi2(Si1-xGex)2, cannot be interpreted only in terms of the valence change. The hybridization between 4f electrons and a conduction band should also be taken into consideration. (author)

318

Towards a unified understanding of the valence transition in rare-earth systems under pressure  

Science.gov (United States)

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduction states. I will present a unified picture of pressure-induced valence transition using lanthanum monochalcogenides as model system for 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, I will show that the pressure-induced valence transition in Sm and Yb monochalcogenides is driven by the promotion of a 4f electron into the lowest unoccupied (LU) 5dt2g band. This band holds the key to explaining the diverse transitions of the monochalcogenides, offering a complete picture of the f electron delocalization mechanism. I demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.

Jarrige, Ignace; Yamaoka, Hitoshi; Rueff, Jean-Pascal; Lin, Jung-Fu; Taguchi, Munetaka; Hiraoka, Nozomu; Ishii, Hirofumi; Tsuei, Ku-Ding; Imura, Keiichiro; Matsumura, Takeshi; Ochiai, Akira; Suzuki, Hiroyuki; Kotani, Akio

2013-03-01

319

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

320

Action versus valence in decision making  

Science.gov (United States)

The selection of actions, and the vigor with which they are executed, are influenced by the affective valence of predicted outcomes. This interaction between action and valence significantly influences appropriate and inappropriate choices and is implicated in the expression of psychiatric and neurological abnormalities, including impulsivity and addiction. We review a series of recent human behavioral, neuroimaging, and pharmacological studies whose key design feature is an orthogonal manipulation of action and valence. These studies find that the interaction between the two is subject to the critical influence of dopamine. They also challenge existing views that neural representations in the striatum focus on valence, showing instead a dominance of the anticipation of action. PMID:24581556

Guitart-Masip, Marc; Duzel, Emrah; Dolan, Ray; Dayan, Peter

2014-01-01

 
 
 
 
321

Symmetry of valence states of Heusler compounds explored by linear dichroism in hard-x-ray photoelectron spectroscopy.  

Science.gov (United States)

This study reports on the linear dichroism in angular-resolved photoemission from the valence band of the Heusler compounds NiTi0.9Sc0.1Sn and NiMnSb. High-resolution photoelectron spectroscopy was performed with an excitation energy of h? = 7.938??keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. The valence band spectra exhibit the typical structure expected from first-principles calculations of the electronic structure of these compounds. Noticeable linear dichroism is found in the valence band of both materials, and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry-dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation. PMID:21838382

Ouardi, Siham; Fecher, Gerhard H; Kozina, Xeniya; Stryganyuk, Gregory; Balke, Benjamin; Felser, Claudia; Ikenaga, Eiji; Sugiyama, Takeharu; Kawamura, Naomi; Suzuki, Motohiro; Kobayashi, Keisuke

2011-07-15

322

Valence resonance states of N2/-/  

Science.gov (United States)

Resonant scattering observed in the transmission of electrons between 9 and 11 eV in collision with N2 has been attributed to the attachment of the scattered electron to an excited valence state. Resonance valence excited states of N2(-) are calculated that could support this hypothesis. Although calculated and observed resonance energies are in close agreement, the calculations do not reflect the observed complex angular behavior of the scattered electron.

Krauss, M.; Neumann, D.

1973-01-01

323

Valence and valence-core interactions in transition-metal diatomic molecules  

Science.gov (United States)

In this study, the correlation between open-shell valence and closed-shell valence-core electrons is shown to be significant, with regard to an adequate description of the bonding in transition metal diatomic molecules. The SCMEH-MO (self-consistent modified extended Huckel molecular orbital) method is well suited for partitioning these two influences, in terms of their independent and collective effects on the electrons within the occupied mo-s. Two test cases, Cr2 and Mo2 are presented to demonstrate the nature and magnitude of these valence/valence-core interactions.

Boudreaux, Edward A.; Baxter, Eric

324

The band structures of superlattices (Ge2)1/(GaAs)m(110) with m=1-20  

International Nuclear Information System (INIS)

The electronic band structures of superlattices (Ge2)1/(GaAs)m(110) with m=1-20 were calculated by using the renormalization technique in a tight-binding frame. The results show that the valence band top and the conduction band bottom of the superlattices (Ge2)1/(GaAs)m(110) are not at the same point in the Brillouin zone and the electronic states at the bottom of the conduction band display distinct 2-dimensional character. For the monolayer superlattice (Ge2)1/(GaAs)1 both the indirect and direct forbidden bands are very small. As the number of the GaAs layers increases the electronic states at the bottom of the conduction bands transit from 3-dimensional character to 2-dimensional character gradually and the degrees of the transitions are different for each point in the Brillouin zone. The transverse energy dispersion of the lowest conduction band remains unchanged when the number of GaAs layers m increases beyond 10. (author). 9 refs, 8 figs, 4 tabs

325

Physical properties and valence state of cerium in the filled skutterudite CePt?Ge??.  

Science.gov (United States)

Electronic, magnetic, and transport properties of the filled platinum-germanium skutterudite CePt?Ge?? are investigated. High resolution x-ray absorption spectroscopy measurements at the cerium L(III) edge demonstrate that CePt?Ge?? in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal a broad maximum at Tmax D 65-80 K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient ? = 105 mJ mol?¹ K?², deduced from specific heat, indicates moderately enhanced band masses for CePt?Ge??. We discuss these findings and conclude that CePt?Ge?? represents a system at the border between intermediate valence (IV) and Kondo lattice behavior. In addition, the lattice specific heat and the thermal conductivity are discussed with respect to the vibrational dynamics of Ce in the [Pt?Ge??] framework. PMID:22056917

Gumeniuk, R; Kvashnina, K O; Schnelle, W; Nicklas, M; Borrmann, H; Rosner, H; Skourski, Y; Tsirlin, A A; Leithe-Jasper, A; Grin, Yu

2011-11-23

326

Optically detecting spin-split bands in semiconductors in magnetic fields  

Science.gov (United States)

We report on combined theoretical and experimental studies of spin-split bands in semiconductors in magnetic fields. We have studied a wide range of systems including: 1) electron and valence band splitting in dilute magnetically doped semiconductors (DMS) systems like InMnAs, 2) electron and valence band splitting in strained InSb/AlInSb heterostructures and 3) valence band splitting in GaAs. The systems have been studied with a variety of experimental techniques including: i) ultra-high magnetic field cyclotron resonance ii) magnetoabsorption and iii) optically pumped NMR (OPNMR). Calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the quantum confinement potential as well as pseudomorphic strain at the interfaces and sp-d coupling between magnetic impurities and conduction band electrons and valence band holes. Optical properties are calculated within the golden rule approximation and compared with experiments. Detailed comparison to experiment allows one to accurately determine conduction and valence band parameters including effective masses and g-factors. Results for InMnAs show shifts in the cyclotron resonance peaks with Mn doping. For InSb, we find a sensitive dependence of the elecronic structure on the strain at the pseudomorphic interfaces. For GaAs, we show that OPNMR allows us to spin-resolve the valence bands and that structure in the OPNMR signal is dominated by the weaker light hole to conduction band Landau level transitions.

Pan, X.; Sun, Y.; Saha, D.; Sanders, G. D.; Santos, M. B.; Doezema, R. E.; Hayes, S.; Khodaparast, G.; Munekata, H.; Matsuda, Y. H.; Kono, J.; Stanton, C. J.

2012-10-01

327

Valence space electron momentum spectroscopy of diborane  

International Nuclear Information System (INIS)

A non-classical mechanism of binding in diborane (B2 H6) is derived quantum-mechanically (B3LYP/6-311++G**) using a dual-space analysis. High-resolution binding-energy spectra of diborane, generated using an outer-valence Green's-function and density-functional theory with a statistical average of model orbital potentials (SAOP), agree satisfactorily with experiment. Electron-correlation energies of diborane produce orbital-based variations in ionization energy in the valence space, but with negligible impact on the shape of only ag symmetry orbitals as indicated in momentum space. The present work indicates quantitatively that (a) the pair of three-centre banana-shaped B-Hb-B bonds are more accurately described as one diamond-shaped bond with B-Hb-B-Hb, (b) all bonds in diborane are electron-deficient including the four equivalent B-Ht bonds, (c) there is no pure B?B bond but contributions from all valence orbitals form an unconventional electron-deficient B-B bond, and (d) only two innermost valence orbitals - 2ag and 2b1u - are sp2-hybridized and no evidence indicates other valence orbitals of diborane to be hybridized

328

Coherent potential approximation treatment of the Sm valence transition in SmS induced by alloying  

International Nuclear Information System (INIS)

The Sm valence transition, similar to the pressure-induced transition in pure SmS, has been observed in a large number of cases by substituting a trivalent rare earth, B, for Sm, thereby forming isostructural alloys with a lattice constant smaller than that of semiconducting SmS. Such substitutions are expected to exert a 'chemical' pressure which simulates the external pressure. However, divalent substitutions (Yb, Eu and Ca), although having a favourable 'size' factor, do not induce any valence transition. Accordingly, band structure effects, essentially due to the relative position of the conduction bands of SmS and BS, should also be taken into consideration. In order to clarify the role which the lattice constant and the electronic structure play in the valence transition, these alloys have been studied using the coherent potential approximation (CPA) including both the crystal field effect and the Coulomb interaction between localised and itinerant states as driving mechanisms. For Sm, the 'homogeneous' picture is considered, each Sm site having the average valence; then the Smsub(1-x)Bsub(x)S system is reduced to a binary alloy. Charge transfer between the f states of Sm and the alloy conduction band (i.e. Sm valence change) is determined self-consistently. The calculations are consistent with the experimental behaviour and allow us to understand qualitatively the variation of the critical concentration xsub(c) for the transition as a function of the differencetransition as a function of the difference ?a between the lattice parameters of SmS and BS. (author)

329

Band Structure and Effective Masses of ZnMgO  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

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

2013-01-01

330

Molybdenum Valence in Basaltic Silicate Melts  

Science.gov (United States)

The moderately siderophile element molybdenum has been used as an indicator in planetary differentiation processes, and is particularly relevant to core formation [for example, 1-6]. However, models that apply experimental data to an equilibrium differentiation scenario infer the oxidation state of molybdenum from solubility data or from multivariable coefficients from metal-silicate partitioning data [1,3,7]. Partitioning behavior of molybdenum, a multivalent element with a transition near the J02 of interest for core formation (IW-2) will be sensitive to changes in JO2 of the system and silicate melt structure. In a silicate melt, Mo can occur in either 4+ or 6+ valence state, and Mo6+ can be either octahedrally or tetrahedrally coordinated. Here we present first XANES measurements of Mo valence in basaltic run products at a range of P, T, and JO2 and further quantify the valence transition of Mo.

Danielson, L. R.; Righter, K.; Newville, M.; Sutton, S.; Pando, K.

2010-01-01

331

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

332

Intermediate valence of plutonium chalcogenides as determined by photoemission  

Science.gov (United States)

There has been considerable experimental evidence, like magnetism, electrical conductivity, specific heat, lattice constant, elasticity and compressibility, showing that the plutonium monochalcogenides are intermediate valent and thus were the first actinide compounds where this aspect could be experimentally verified. Also, theoretical calculations confirm the experimental findings. Recently, photoemission has been performed on PuSe layers and PuTe single crystals and the findings have been interpreted as a 5f localization and not an intermediate valent configuration where 5f and 6d states hybridize. In this paper, it will be shown that also these photoemission data give direct evidence of intermediate valence. However, in addition, a narrow 5f band is observed at the Fermi level, causing the observed strongly enhanced Pauli paramagnetism.

Wachter, P.

2003-09-01

333

Role of many body shake-up in core-valence-valence electron emission from single wall carbon nanotubes.  

Science.gov (United States)

Auger core-valence-valence transitions from single wall Carbon nanotubes are studied using a tight-binding calculational scheme with nearest neighbor overlap, hopping interactions, and a double-zeta basis set. The resulting Hamiltonian approximates the unperturbed pi and sigma bands of the nanomaterials coupled with the free electron states outside the solid and the core-hole. As a first step, the Fermi's golden rule is applied to determine the so called one-electron spectrum of emitted electrons from different tubes, in which either the neutralizing or the ejected electrons, in the initial state, lie within nearest neighboring atomic sites to the core-hole. Many-body corrections are effectively modeled using a broadening function, which accounts for dynamic screening effects involving the initial and final states. Particular attention is paid to the asymmetric component of the broadening function, responsible for the shake-up of pi electrons. Finally, the Cini-Sawatzky distortion function is used to describe the final state effect of the hole-hole interaction. A quantitative estimation of the interplay of shake-up processes is proposed by adjusting the asymmetric parameters of the broadening function to reproduce measurements of Auger electrons ejected from bundles of single wall Carbon nanotubes. PMID:22400315

Sindona, A; Pisarra, M; Maletta, S; Commisso, M; Riccardi, P; Bonanno, A; Barone, P; Falcone, G

2011-10-01

334

Observation of phonon anomalies in the intermediate-valence compound CePd3  

International Nuclear Information System (INIS)

We report measurements of the phonon dispersion relation in intermediate-valence CePd3. The longitudinal-acoustic modes show a distinct softening above 100 K and a temperature-dependent broadening and splitting at higher temperatures. Especially in the direction the longitudinal modes differ from those of other Cu3Au structures. A good fit of the room-temperature dispersion curves is given by a 16-parameter Born--von Karman model with a breathing force as an additional degree of freedom

335

Thulium valence systematics in some chalcogenides  

International Nuclear Information System (INIS)

A relation is established connecting the lattice parameter (a0) and the valence of Tm (vm) derived from the susceptibility (?) of several diluted and concentrated Tm chalcogenide alloys. For Yb1-xTmxSe, values of vm predicted using this relation and the measured a0 are found to agree fairly well with those derived from ?. For Yb0.9Tm0.1S0.8Se0.2 the normally used Vegard's law would give v?2 whereas this relation gives an intermediate valence of Tm in agreement with the experiment. The resistivity of these crystals is presented. (orig.)

336

Valence states of actinides in synthetic monazites  

International Nuclear Information System (INIS)

The valence states of various actinides doped into the lanthanide orthophosphates (LnPO4) have been investigated. Actinide-doped LnPO4 single crystals were grown by means of a flux technique and the actinide valence states were determined by optical absorption spectrophotometry. Radiation damage effects were also studied in these systems. The lanthanide orthophosphates are found in nature in the form of the mineral monazite and synthetic analogs of this substance represent a promising primary containment medium for the isolation of high-level nuclear wastes

337

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2006-03-15

338

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

339

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

Science.gov (United States)

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.

Vistovskyy, V. V.; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Myagkota, O. S.; Gloskovskii, A.; Gektin, A. V.; Vasil'ev, A. N.; Rodnyi, P. A.; Voloshinovskii, A. S.

2013-11-01

340

Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

The influence of the BaF{sub 2} 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? ? E{sub g}) 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.

Vistovskyy, V. V., E-mail: vistvv@gmail.com; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S. [Ivan Franko National University of Lviv, 8 Kyryla i Mefodiya, 79005 Lviv (Ukraine); Myagkota, O. S. [Lviv Polytechnic National University, 12S. Bandera, 79013 Lviv (Ukraine); Gloskovskii, A. [Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg (Germany); Gektin, A. V. [Institute for Scintillation Materials, NAS of Ukraine 60 Lenina Ave, 61001 Kharkiv (Ukraine); Vasil' ev, A. N. [Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Rodnyi, P. A. [Saint-Petersburg State Polytechnical University, 29, Polytekhnicheskaya, 195251 Saint-Petersburg (Russian Federation)

2013-11-21

 
 
 
 
341

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

342

Mixed-valence states of polynuclear iron complexes  

Energy Technology Data Exchange (ETDEWEB)

The valence-delocalization in mixed-valence binuclear ferrocene derivatives is investigated by using several physicochemical results. It is demonstrated that the mixed-valence state is controlled by the environment in the solid state. The mechanism of the valence-delocalization in 1',1'''-diethylbiferrocenium triiodide is discussed. The difference between the valence-delocalization accompanied by the symmetry change of the counter anion and the valence-delocalization without connection with the symmetry of the counter anion is discussed. The results are compared with those of oxo-centered trinuclear iron carboxylates. (orig.).

Nakashima, Satoru (Dept. of Chemistry, Faculty of Science, Hiroshima Univ. (Japan))

1993-04-01

343

Dispersion Basics  

Science.gov (United States)

A webcast presentation by Dr. Timothy Spangler (Director of the COMET Program and a former air quality consultant). This 25-minute lecture provides an overview of the basics of dispersion, the effects of different atmospheric conditions on dispersion, and how dispersion is commonly modeled after an accidental release of a hazardous material.

COMET

2002-11-12

344

The band energy structure of RbKSO4 crystals  

Directory of Open Access Journals (Sweden)

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

O.V.Bovgyra

2007-01-01

345

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?)

346

The chemical response of main-group extended solids to formal mixed valency: the case of LixBC.  

Science.gov (United States)

The introduction of mixed valency into extended main-group solids is discussed using the example of hole-doped LiBC, where a combination of experimental measurements and density functional theory calculations is used to understand the observed electronic properties in terms of deviation from the expected rigid-band electronic structure behaviour. PMID:17827126

Fogg, A M; Darling, G R; Claridge, J B; Meldrum, J; Rosseinsky, M J

2008-01-13

347

A study of the valence shell photoionisation dynamics of pyrimidine and pyrazine  

Energy Technology Data Exchange (ETDEWEB)

Graphical abstract: The complete valence shell photoelectron spectra of pyrimidine and pyrazine have been recorded with synchrotron radiation and interpreted with the aid of vertical ionisation energies and relative spectral intensities calculated using time-dependent density functional theory. Highlights: Black-Right-Pointing-Pointer Valence shell photoelectron spectra of pyrimidine and pyrazine have been recorded. Black-Right-Pointing-Pointer Many-body effects are important. Black-Right-Pointing-Pointer Photoionisation dynamics are affected by shape resonances. Black-Right-Pointing-Pointer Theoretical predictions for single-hole ionic states are satisfactory. - Abstract: The complete valence shell photoelectron spectra of pyrimidine and pyrazine have been recorded with synchrotron radiation and the observed structure has been interpreted with the aid of vertical ionisation energies and relative spectral intensities calculated using time-dependent density functional theory. The theoretical predictions for the single-hole ionic states due to outer valence shell ionisation agree satisfactorily with the experimental results. Ionisation from the inner valence orbitals is strongly influenced by many-body effects and the intensity associated with a particular orbital is spread amongst numerous satellites. Photoelectron angular distributions and partial cross sections have been determined both experimentally and theoretically, and demonstrate that shape resonances affect the valence shell photoionisation dynamics. In addition to shape resonances occurring a few eV above the ionisation threshold, the calculations indicate that many of the orbitals are influenced by shape resonant processes at much higher energies. Some of these higher energy resonances have been confirmed through a comparison between the relevant theoretical and experimental photoelectron asymmetry parameters. The spectral behaviour of asymmetry parameters associated with {pi}-orbitals has been shown to differ from that of asymmetry parameters associated with {sigma}-orbitals. These differences provide a means of distinguishing between the two types of orbitals even in heavily congested regions of the photoelectron spectrum suffering from band overlap.

Holland, D.M.P., E-mail: david.holland@stfc.ac.uk [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom); 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); Stener, M.; Decleva, P. [Dipartimento di Scienze Chimiche, Universita di Trieste, Via L. Giorgieri, I-34127 Trieste (Italy); Consorzio Interuniversitario Nazionale per la Scienze e Tecnologia dei Materiali, INSTM, Unita' di Trieste, Via L. Giorgieri, I-34127 Trieste (Italy); CNR-IOM, Area Science Park - Basovizza, Strada Statale 14 km 163,5, I-34149 Trieste (Italy)

2011-11-18

348

Electronic band structure of the layered dichalcogenide MoTe{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

The transition metal dichalcogenide {beta}-MoTe{sub 2} 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 {beta} 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 {beta}-MoTe{sub 2} 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 {beta}-MoTe{sub 2}. Fermi level crossings are studied by Fermi-surface maps. The experimental findings will be compared with tight-binding band structure calculations.

Heimburger, Robert; Zandt, Thorsten; Janowitz, Christoph; Manzke, Recardo [Humboldt-Universitaet zu Berlin, Institut fuer Physik, Newtonstrasse 21, 12489 Berlin (Germany)

2007-07-01

349

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

350

Band Structure of AlSb Nanoclusters  

Directory of Open Access Journals (Sweden)

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

Haider I. Isa

2012-08-01

351

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

352

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

353

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

354

Neutron inelastic scattering from mixed valence materials  

International Nuclear Information System (INIS)

Paramagnetic scattering from mixed valence materials is reviewed. Some of the early measurements identified a broad quasielastic spectral distribution (e.g. a Lorentzian centered on zero energy with large, practically Q-independent widths (half-width approx.5-30 MeV) which remain finite as T ? OK. More recent measurements using high energy neutrons on several mixed valance systems reveal inelastic peaks superposed on the broad quasielastic spectrum at low temperatures. These inelastic peaks progressively melt away with increasing temperature, accompanied possibly by some softening, and disappear almost completely around the temperature of the maximum in the susceptibility. Several possible mechanisms could semi-qualitatively account for the observed spectral response, a deeper insight into whose origin would aid in understanding the mixed valence phenomena

355

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.

2005-12-01

356

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

357

Robustness of Quantum Criticality of Valence Fluctuations  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 ro...

Watanabe, Shinji; Miyake, Kazumasa

2013-01-01

358

Valence Bond Solids for Quantum Computation  

CERN Document Server

Cluster states are entangled multipartite states which enable to do universal quantum computation with local measurements only. We show that these states have a very simple interpretation in terms of valence bond solids, which allows to understand their entanglement properties in a transparent way. This allows to bridge the gap between the differences of the measurement-based proposals for quantum computing, and we will discuss several features and possible extensions.

Verstraete, F

2003-01-01

359

Correlated Topological Insulators with Mixed Valence  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We propose the local density approximation (LDA)+Gutzwiller method incorporating Green's function scheme to study the topological physics of correlated materials from the first-principles. Applying this method to typical mixed valence materials SmB6, we found its non-trivial Z2 topology, indicating that SmB6 is a strongly correlated topological insulator (TI). The unique feature of this compound is that its surface states contain three Dirac cones in contrast to most known TIs.

Lu, Feng; Zhao, Jianzhou; Weng, Hongming; Fang, Zhong; Dai, Xi

2012-01-01

360

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

 
 
 
 
361

Valence fluctuations between two magnetic configurations  

International Nuclear Information System (INIS)

The subject of this work is the study of a microscopic model which describes TmSe through its most important feature, i.e.: the valence fluctuations between two magnetic configurations. Chapter I is a general review of the most important physical properties of rare-earth systems with intermediate valence (I.V.) and a general description of experimental results and theoretical models on Tm compounds. In Chapter II the Hamiltonian model is discussed and the loss of rotational invariance is also analyzed. Chapter III is devoted to the study of non-stoichiometric Tsub(x)Se compounds. It is shown that these compounds can be considered as a mixture of TmSe (I.V. system) and Tm3+0.87Se. Chapter IV is devoted to the calculation of spin-and charge susceptibilities. The results obtained permit to explain the essential features of the neutron scattering spectrum in TmSe. In Chapter V, an exactly solvable periodic Hamiltonian is presented. From the experimental results, some fundamental features are deduced to describe TmSe as an intermediate valence system whose two accessible ionic configurations are magnetic (degenerated fundamental state). (M.E.L)

362

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

363

Multidimensional x-ray spectroscopy of valence and core excitations in cysteine  

Science.gov (United States)

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 {k}_I =-{k} _1+{k} _2+{k} _3 and {k}_II ={k} _1-{k} _2+{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.; Zhang, Yu; Healion, Daniel; Mukamel, Shaul

2013-04-01

364

New materials for intermediate band photovoltaic cells. A theoretical and experimental approach  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Density functional theory calculations of certain transition-metal doped semiconductors show a partially occupied relatively narrow band located between valence band and conduction band. These novel systems, containing the metallic band, are called intermediate-band materials. They have enhanced optoelectronic properties which allow an increase in solar energy conversion efficiency of conventional solar cells. We previously proposed III-V, chalcopyrite and sulfide derived compounds show...

Wahno?n Benarroch, Perla; Palacios Clemente, Pablo; Aguilera Bonet, Irene; Semino?vski Pe?rez, Yohanna; Conesa, Jose Carlos; Lucena, Raquel

2010-01-01

365

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

366

Formation of an unconventional Ag valence state in Ag2NiO2  

CERN Document Server

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 specific heat coefficient, gamma, is likely due to strong spin fluctuations that stem from nearly complete compensation of the ferro- (metallic double exchange and the 90 degree superexchange) and antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path) interactions.

Johannes, M D; Mazin, I I; Khomskii, D I

2006-01-01

367

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

368

Valence transition behavior of the doped Falicov-Kimball model at nonzero temperatures  

CERN Document Server

The extrapolation of small-cluster exact-diagonalization calculations is used to study the influence of doping on valence transitions in the spinless Falicov-Kimball model at nonzero temperatures. Two types of doping are examined, and namely, the substitution of rare-earth ions by non-magnetic ions that introduce (i) one or (ii) none additional electron (per non-magnetic ion) into the conduction band. It is found that the first type of substitution increases the average $f$-state occupancy of rare-earth ions, whereas the second type of substitution has the opposite effect. The results obtained are used to describe valence transition behavior of samarium in the hexaboride solid solutions $Sm_{1-x}M_xB_6$ ($M=Y^{3+},La^{3+}, Sr^{2+},Yb^{2+}$) and a very good agreement of theoretical and experimental results is found.

Farkasovsky, P

2005-01-01

369

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

370

Actinides, the narrowwest bands  

Energy Technology Data Exchange (ETDEWEB)

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.

Smith, J.L.; Riseborough, P.S.

1984-01-01

371

Probe for spin- and valence-selective X-ray absorption fine structure spectroscopy: EuLgamma4 emission.  

Science.gov (United States)

EuLgamma(4) emission spectra of EuS, EuF(3), EuCl(3), and Eu(2)O(3) were measured using a multicrystal, multidetector spectrometer. Splitting of the EuLgamma(4) band was observed in the EuS spectra and attributed to exchange interactions between Eu5p and Eu4f. It was shown that because of this splitting, the EuLgamma(4) emission could be used for performing spin-selective measurements. This emission exhibited a large chemical shift (approximately 5 eV), depending on the valence state of a compound. The applicability of this emission to valence-selective X-ray absorption fine structure spectroscopy was demonstrated through partial fluorescence yield measurements performed on a 1:1 mixture of EuS and Eu(2)O(3) and on a valence-fluctuating compound Eu(3)Pd(20)Ge(6). PMID:19170607

Hayashi, Hisashi; Kawamura, Naomi; Mizumaki, Masaichiro; Takabatake, Toshiro

2009-02-15

372

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

373

Constraints to the flat band potential of hematite photo-electrodes.  

Science.gov (United States)

We revisit the fundamental constraints that apply to flat band potential values at semiconductor photo-electrodes. On the physical scale, the Fermi level energy of a non-degenerate semiconductor at the flat band condition, EF(FB), is constrained to a position between the conduction band, EC, and the valence band, EV,: |EC| band potentials appear to lie outside these fundamental boundaries. In order to assess the validity of any determined flat band potential, the boundaries set by the conduction band and the valence band must be computed on both scales a priori, where possible. This is accomplished with the aid of an analytical reconstruction of the semiconductor|electrolyte interface in question. To illustrate this approach, we provide a case study based on synthetic hematite, ?-Fe2O3. The analysis of this particular semiconductor is motivated by the large variance in the flat band potential values reported in the literature. PMID:24968087

Hankin, A; Alexander, J C; Kelsall, G H

2014-08-14

374

Interconsistent band structure of narrow-gap Hg1-xCdxTe alloys obtained with consideration of far-band influence  

Science.gov (United States)

Faraday rotation, absorption spectra, and the intrinsic carrier concentration temperature dependencies were investigated in order to determine the interconsistent system of values of the narrow-gap Hg1-xCdxTe band parameters. The undoped and doped with indium or copper crystals with x eV cm have been obtained with a high degree of certainty. The parameters of the second approach of the Kane model were found too: (gamma) 1 equals 4.0; (gamma) equals 0.7; (gamma) s equals 0.5. A new method of determination of the heavy hole effective mass have been elaborated and have been used successively in the heavy hole energy range (epsilon) eV. A new form of the light carrier dispersion relation and more exact relation for the temperature dependence of (epsilon) g have been obtained too. The heavy hole Hall factor at T equals 77 K, the heavy hole mobility at T equals 295 K, and the constant of valence band deformation potential E1 equals 12.5 eV have been determined. It has been found that doping of the material with indium, causes the appearance of donor states in the conduction band about 0.1 eV above the bottom of the band. The band structure of Hg1-xCdxTe (MCT) is studied for a long time, and its peculiarities are well known. The light carriers spectrum is believed to be described in framework of the 3-band Kane's model, and the parabolic approximation for heavy hole subband is thought to be sufficient as a rule. The influence of far bands is taken into account rather seldom. But at the same time, the values of band parameters obtained by different authors are not in a good agreement often. In result, it was impossible to agree with an acceptable accuracy the measured and the calculated values of some physical characteristics of MCT by using the known values of band parameters. In order to achieve perfect conformity between the theory and the experiment, another approach must be used. It is necessary that the complex experiment be carried out with a high accuracy of not only the measuring, but of the mathematical models of studied effects and the calculations too. All the main sources of systematic error should be revealed and removed, and the values of band parameters should be determined over again. Such a study was carried out, and its results are reported in this paper.

Bogoboyashchiy, V. V.

1997-08-01

375

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

Science.gov (United States)

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

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

2002-11-01

376

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

Energy Technology Data Exchange (ETDEWEB)

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.

Powis, I. [School of Chemistry, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Trofimov, A.B. [Laboratory of Quantum Chemistry, Irkutsk State University, 664003 Irkutsk (Russian Federation); A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk (Russian Federation); Bodzuk, I.L. [Laboratory of Quantum Chemistry, Irkutsk State University, 664003 Irkutsk (Russian Federation); Holland, D.M.P., E-mail: david.holland@stfc.ac.uk [Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD (United Kingdom); 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)

2013-03-29

377

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

378

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

379

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

380

Band discontinuities at epitaxial SrTiO3/Si(001) heterojunctions  

International Nuclear Information System (INIS)

We have used photoemission methods to directly measure the valence and conduction band offsets at SrTiO3/Si(001) interfaces, as prepared by molecular-beam epitaxy. Within experimental error, the measured values are the same for growth on n- and p-Si, with the entire band discontinuity occurring at the valence band edge. In addition, band bending is much larger at the p-Si heterojunction than at the n-type heterojunction. Previously published threshold voltage behavior for these interfaces can now be understood in light of the present results. (c) 2000 American Institute of Physics

 
 
 
 
381

Interband interaction between Ge states and surface resonance band of Pb on Ge(001)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We investigate the valence band structure of Pb on Ge(001) by Angle-Resolved Photoelectron Spectroscopy. Three Ge bands, G1, G2, and G3, were observed on Ge(001) 2x1 clean surface. In addition to these three bands, a forth band (R band) is found in the 2 ML of Pb coverage. The R band continues to appear even when the surface superstructure changed. The position of the R band does not depend on Pb coverage. These results indicate that the R band derives from Ge subsurface sta...

Sakata, Tomohiro; Takeda, Sakura N.; Kitagawa, Kousuke; Kumeda, Haruka; Kokui, Kazuki; Daimon, Hiroshi

2014-01-01

382

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu1-xNix)4Ga  

International Nuclear Information System (INIS)

The compounds Ce(Cu1-xNix)4Ga crystallize in the hexagonal CaCu5-type structure for the whole doping range 0?x?1. The border compounds CeCu4Ga and CeNi4Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu1-xNix)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 4f1 states for up to x=0.5. - Research highlights: ? Ni substitution for Cu in CeCu4Ga 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

383

Attosecond band-gap dynamics in silicon  

Science.gov (United States)

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.

Schultze, Martin; Ramasesha, Krupa; Pemmaraju, C. D.; Sato, S. A.; Whitmore, D.; Gandman, A.; Prell, James S.; Borja, L. J.; Prendergast, D.; Yabana, K.; Neumark, Daniel M.; Leone, Stephen R.

2014-12-01

384

Bulk band gaps in divalent hexaborides  

International Nuclear Information System (INIS)

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

385

Properties of terminating bands in nuclei  

International Nuclear Information System (INIS)

General properties of terminating bands are briefly reviewed and exemplified on the observed high-spin properties of 158Er and 156Er. The very similar features of the positive parity high-spin spectra of the N=88 isotones of Dy, Er and Yb are pointed out and discussed. The possibility of estimate B(E2)-values in terminating bands from measured feeding times is explored. The spin contribution from different orbitals is calculated in terminating bands as well as more collective bands. One aim is to get some idea of the interaction between different configurations and to this end we also consider a single-j shell model. The limits of terminating bands in the yrast region when the number of valence nucleons increases is discussed. The usefulness of these results when planning future experimental studies is considered. (orig.)

386

Collective magnetic excitations in mixed-valence Sm0.83Y0.17S  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The magnetic spectral response of black-phase mixed-valence Sm0.83Y0.17S has been measured by inelastic neutron scattering on a single crystal. Two magnetic peaks are observed in the energy range of the Sm2+ spin-orbit transition (25-40 meV). Both of them exhibit significant dispersion along the three main symmetry directions, reminiscent of the spin-orbit exciton branch found in pure divalent SmS. The results can be reproduced by a simple phenomenological model accounting f...

Alekseev, P. A.; Mignot, J. -m; Ochiai, A.; Nefeodova, E. V.; Sadikov, I. P.; Clementyev, E. S.; Lazukov, V. N.; Braden, M.; Nemkovski, K. S.

2001-01-01

387

An Ag3PO4/nitridized Sr2Nb2O7 composite photocatalyst with adjustable band structures for efficient elimination of gaseous organic pollutants under visible light irradiation  

Science.gov (United States)

A new Ag3PO4/nitridized Sr2Nb2O7 (N: 0-6.18 wt%) heterojunction was designed to eliminate gaseous pollutants under visible light irradiation. The phase compositions, optical properties, and morphologies of the heterojunction photocatalysts were systematically investigated via powder X-ray diffraction, UV-visible absorption spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. Calculations of the electronic structure indicated that the top of the valance band of Sr2Nb2O7 could be raised by nitrogen doping. Therefore, the electronic structure of the Ag3PO4/nitridized Sr2Nb2O7 composite photocatalysts could be continually changed by controlling the amount of nitrogen in nitridized Sr2Nb2O7. Photocatalytic degradation of isopropyl alcohol (IPA) was carried out to test the photocatalytic activity of the heterojunction. The highest activity (CO2 evolution rate, 10.32 ppm h-1) was observed over the Ag3PO4/nitridized Sr2Nb2O7 heterojunction prepared by nitridation of Sr2Nb2O7 (SNO) at 1023 K. The CO2 evolution rate over the heterojunction was about 40 times higher than that over pure Ag3PO4 (CO2 evolution rate, 0.26 ppm h-1) under visible light irradiation. An investigation of the energy-band structure via valence band X-ray photoelectron spectroscopy indicated that the conduction band (CB) and valence band (VB) of Ag3PO4 are both more positive than those of nitridized Sr2Nb2O7, which facilitates the separation and transfer of photogenerated electrons and holes between the two photocatalysts. By continually adjusting the electronic structures, an optimal band gap for the nitridized Sr2Nb2O7 of 2.15 eV was obtained, and the potential of the valance band was +1.88 eV.

Guo, Jianjun; Zhou, Han; Ouyang, Shuxin; Kako, Tetsuya; Ye, Jinhua

2014-06-01

388

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

389

Neutron scattering on intermediate valence systems  

International Nuclear Information System (INIS)

The temperature dependence of the magnetic relaxation line widths (quasielastic (QE) line widths) and the crystal field excitations of some novel intermediate valent systems were measured by means of inelastic neutron scattering. Some striking new features of the alloys YbBe13, YbPd and Yb3Pd4 appoint them to belong to a new type of intermediate valent systems. YbAl3 shows several inelastic lines, but no QE-line was measurable. The heavy fermion systems CeCu6 and URu2Si2 show a strongly temperature dependent QE-line width, in course of which CeCu6 tends towards a nonvanishing residual value for T->0, which matches with theoretical predictions. For the first time valence instabilities of Pr (PrPd) and Sm (Smsub(0.51)Ysub(0.49)Al2) could be prooved by neutron scattering. In addition to these experiments, in YbCu2Si2 and TmTe the shifts in valency, caused by external hydrostatic pressure, were studied with a pressure cell for neutron scattering, especially designed and constructed for these special purposes. The corresponding observed changes of the QE-line widths coincide with the expected values, both in sign and absolute value. (orig.)

390

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

391

Energy band alignment of PbTe/CdTe(111) interface determined by ultraviolet photoelectron spectra using synchrotron radiation  

International Nuclear Information System (INIS)

The energy band structure with type-I alignment at the PbTe/CdTe(111) heterojunction interface is determined by the ultraviolet photoelectron spectrum using synchrotron radiation. The valence band and conduction band offsets are obtained to be 0.09±0.12 and 1.19±0.12 eV, respectively. These results are in agreement with theoretically predicted ones. The accurate determination of the valence band and conduction band offsets is useful for the fundamental understanding of the mid-infrared light emission from the PbTe/CdTe heterostructures and its application in devices. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

392

Relationship Between Iron Valence States of Serpentine in CM Chondrites and Their Aqueous Alteration Degrees  

Science.gov (United States)

The 0.6-0.7 micron absorption band observed for C-type asteroids is caused by the presence of Fe(3+) in phyllosilicates . Because Fe-bearing phyllosilicates, especially serpentine, are the most dominant product of aqueous alteration in the most abundant carbonaceous chondrites, CM chondrites, it is important to understand the crystal chemistry of serpentine in CM chondrites to better understand spectral features of C-type asteroids. CM chondrites show variable degrees of aqueous alteration, which should be related to iron valences in serpentine. It is predicted that the Fe(3+)/Sum of (Fe) ratios of serpentine in CM chondrites decrease as alteration proceeds by Si and Fe(3+) substitutions from end-member cronstedtite to serpentine, which should be apparent in the absorption intensity of the 0.6-0.7 micron band from C-type asteroids. In fact, the JAXA Hayabusa 2 target (C-type asteroid: 1993 JU3) exhibits heterogeneous spectral features (0.7 micron absorption band disappears by rotation). From these points of view, we have analyzed iron valences of matrix serpentine in several CM chondrites which span the entire observed range of aqueous alteration using Synchrotron Radiation X-ray Absorption Near-Edge Structure (SR-XANES). In this abstract we discuss the relationship between obtained Fe(3+)/Sum of (Fe) ratios and alteration degrees by adding new data to our previous studies

Mikouchi, T.; Zolensky, M.; Satake, W.; Le, L.

2012-01-01

393

Strongly correlated impurity band superconductivity in diamond: X-ray spectroscopic evidence  

Directory of Open Access Journals (Sweden)

Full Text Available In a recent X-ray absorption study in boron doped diamond, Nakamura et al. have seen a well isolated narrow boron impurity band in non-superconducting samples and an additional narrow band at the chemical potential in a superconducting sample. We interpret the beautiful spectra as evidence for upper Hubbard band of a Mott insulating impurity band and an additional metallic 'mid-gap band' of a conducting 'self-doped' Mott insulator. This supports the basic framework of a recent theory of the present author of strongly correlated impurity band superconductivity (impurity band resonating valence bond, IBRVB theory in a template of a wide-gap insulator, with no direct involvement of valence band states.

G. Baskaran

2006-01-01

394

A study of core and valence levels in ?-PbO2 by hard X-ray photoemission  

International Nuclear Information System (INIS)

The core and valence levels of ?-PbO2 have been studied using hard X-ray photoemission spectroscopy (h? = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy

395

Valence states and electronic structures of Co and Mn substituted spin gapless semiconductor PbPdO2  

International Nuclear Information System (INIS)

Electronic structures of Pb(Pd0.9T0.1)O2 (T?=?Mn, Co) spin gapless semiconductors have been investigated by employing soft X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). The valence states of Co and Mn ions are found to be mixed-valent (?2.7) and tetravalent, respectively. The measured valence-band PES and O 1s XAS spectra show that both PbPdO2 and PbPd0.9Co0.1O2 are small-gap semiconductors. This finding is supported by the calculated band structures, obtained in the density functional theory with the modified Becke-Johnson potential (mBJ) scheme. This work also shows evidence for the existence of the phase separation in Mn-substituted PbPd0.9Mn0.1O2

396

Abnormal physics of group-II telluride system: valence contribution of d electrons  

International Nuclear Information System (INIS)

The physical trend of group-II tellurides is unexpected and contrary to the conventional wisdom. The present first-principles calculations give fundamental insights into the extent to which group-II telluride compounds present special properties upon mixing the d valence character. Our results provide explanations for the unexpected experimental observations based on the abnormal binding ordering of metal d electrons and their strong perturbation to the band edge states. The insights into the binary tellurides are useful for the study and control of the structural and chemical perturbation in their ternary alloys and heterostructures. (atomic and molecular physics)

397

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

398

The atomic environment and valence state of Ce implanted in Y2O3  

International Nuclear Information System (INIS)

Ce ions were implanted in Y2O3. Via x-ray absorption spectroscopy, Ce was found to occupy the Y site, and to be surrounded by oxygen atoms. However, contrary to what one would expect on the basis of this substitution, the oxidation state was not purely trivalent. Thanks to the Anderson impurity model, it was possible to interpret the x-ray absorption features near the edge. Ce is found in an intermediate charge state resulting from the interaction of the f state with the valence band of Y2O3. (author)

399

Valence photoelectron spectroscopy of Gd silicides  

International Nuclear Information System (INIS)

Gd3Si5, GdSi, and Gd5Si3 were investigated with photoemission spectroscopy in the photon-energy range 40.8--149 eV by exploiting the energy dependence of the photoemission cross sections and the valence resonance at the crossing of the Gd 4d-4f threshold. The modification of the spectra versus photon energy, along with their stoichiometry dependence, show the relevance of covalent mixed Gd 5d--Si 3sp states in the formation of the chemical bond. In the region close to the Fermi level an increase of the d contribution is observed. These points are discussed in connection with the existing models of the silicide bond

400

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

 
 
 
 
401

Auger-free luminescence due to interatomic transitions of valence electrons into core holes in BaF 2  

Science.gov (United States)

The nature of BaF 2 emission is investigated by measuring excitation spectra using synchrotron radiation for the energy range up to 40 eV. Intense emission bands, appearing at 5.6 and 6.4 eV in the Auger-transparent region, are ascribed to a radiative decay of valence electrons into Ba 2+ 5 p core holes. This luminescence process of a new type is discussed.

Itoh, Minoru; Hashimoto, Satoshi; Sakuragi, Shiro; Kubota, Shinzou

1988-02-01

402

Angular- and light-polarization-dependent valence UV photoelectron spectra of the hexatriacontane ( n-C 36H 74) crystal  

Science.gov (United States)

Valence-band angle-resolved photoelectron spectra of oriented polycrystalline films of hexatriacontane ( n-C 36H 74) were measured by He II (40.8 cV) light. Significant dependence of the spectra on photoemissive angle and light-incidence angle was observed. The advantages and problems of angle-resolved UV photoelectron spectroscopy to the study of electronic structures of organic crystals are discussed.

Seki, Kazuluko; Inokuchi, Hiroo

1982-06-01

403

Teaching Valence Shell Electron Pair Repulsion (VSEPR) Theory  

Science.gov (United States)

This "Science Note" looks at the way that the shapes of simple molecules can be explained in terms of the number of electron pairs in the valence shell of the central atom. This theory is formally known as valence shell electron pair repulsion (VSEPR) theory. The article explains the preferred shape of chlorine trifluoride (ClF3),…

Talbot, Christopher; Neo, Choo Tong

2013-01-01

404

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

Science.gov (United States)

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

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

2002-10-01

405

Electronic band structure of lithium, sodium and potassium fluorides  

International Nuclear Information System (INIS)

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

406

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

407

Infrared spectroscopy of electronic bands in bilayer graphene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare it with tight binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A strong gate voltage dependence of these structures and a sig...

Kuzmenko, Alexey; Heumen, Erik; Marel, Dirk; Lerch, P.; Blake, P.; Novoselov, K. S.; Geim, A. K.

2008-01-01

408

The surface band structure of W(112)  

International Nuclear Information System (INIS)

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

409

Band alignment of SnS/Zn(O,S) heterojunctions in SnS thin film solar cells  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Band alignment is critical to the performance of heterojunction thin film solar cells. In this letter, we report band alignment studies of SnS/Zn(O,S) heterojunctions with various compositions of Zn(O,S). Valence band offsets (VBOs) are measured by femtosecond laser pump/probe ultraviolet photoelectron spectroscopy (fs-UPS) from which conduction band offsets (CBOs) are calculated by combining with band gaps obtained by optical transmission/reflection measurements. The SnS/Zn(O,S) heterojuncti...

Sun, Leizhi; Haight, Richard; Sinsermsuksakul, Prasert; Bok Kim, Sang; Park, Helen Hejin; Gordon, Roy Gerald

2013-01-01

410

Substrate-controlled band positions in CH?NH?PbI? 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

411

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

412

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

Energy Technology Data Exchange (ETDEWEB)

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.

Potts, A.W. [Department of Physics, King’s College, Strand, London WC2R 2LS (United Kingdom); Holland, D.M.P., E-mail: david.holland@stfc.ac.uk [Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD (United Kingdom); Powis, I. [School of Chemistry, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Karlsson, L. [Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala (Sweden); Trofimov, A.B. [Laboratory of Quantum Chemistry, Irkutsk State University, 664003 Irkutsk (Russian Federation); A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk (Russian Federation); Bodzuk, I.L. [Laboratory of Quantum Chemistry, Irkutsk State University, 664003 Irkutsk (Russian Federation)

2013-03-29

413

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

Science.gov (United States)

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

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

2004-01-01

414

Photoinduced mixed valency in zinc porphyrin dimer of triruthenium cluster dyads.  

Science.gov (United States)

The preparation, electrochemistry, and spectroscopic characterization of three new species, (ZnTPPpy)Ru3O(OAc)6(CO)-pz-Ru3O(OAc)6(CO)L, where ZnTPPpy = zinc(II) 5-(4-pyridyl)-10,15,20-triphenylporphyin, L = pyridyl ligand, and pz = pyrazine, are reported. These porphyrin-coordinated Ru3O–BL–Ru3O (BL = bridging ligand) dyads are capable of undergoing intramolecular electron transfer from the photoexcited Zn porphyrin to Ru3O donor–bridge–acceptor dimer systems. Seven reversible redox processes are observed in the cyclic voltammograms of the newly synthesized dyads, showing no significant electrochemical interaction between the redox active porphyrin and the pyrazine-bridged ruthenium dimer of Ru3O trimers. From the electrochemical behavior of the dyads, large comproportionation constants (Kc = 6.0 × 10(7) for L = dmap) were calculated from the reduction potentials of the Ru(III)Ru(III)Ru(II) clusters, indicating a stable mixed-valence state. Electronic absorption spectra of the singly reduced mixed-valence species show two intervalence charge transfer (IVCT) bands assigned within the Brunschwig–Creutz–Sutin semiclassical three-state model as metal-to-bridge and metal-to-metal in character. The progression from most to least delocalized mixed-valence dimer ions, as determined by the divergence of the IVCT bands and in agreement with electrochemical data, follows the order of L = 4-dimethylaminopyridine (dmap) > pyridine (py) > 4-cyanopyridine (cpy). These systems show dynamic coalescence of the infrared spectra in the ?(CO) region of the singly reduced state. This sets the time scale of electron exchange at <10 ps. The electron transfer from the S1 excited state of the coordinated porphyrin to the dimer is predicted to be thermodynamically favorable, with ?GFET(0) ranging from ?0.54 eV for L = dmap to ?0.62 eV for L = cpy. Observation of IVCT band growth under continual photolysis (?exc = 568 nm) confirms a phototriggered intramolecular electron transfer process resulting in a strongly coupled singly reduced mixed-valence species. PMID:25260187

Henderson, Jane; Kubiak, Clifford P

2014-10-20

415

Photoemission investigation of the ZnSe/CdTe heterojunction band discontinuity  

International Nuclear Information System (INIS)

Synchrotron radiation soft x-ray photoemission spectroscopy and reflection high-energy electron diffraction were used to investigate the structural and electronic properties at the ZnSe/CdTe(100) heterojunction interface. ZnSe overlayers were sequentially grown in steps on p-type CdTe(100) single crystals at 200 degree C. In situ photoemission measurements were acquired after each growth in order to observe changes in the valence band electronic structure as well as changes in the Cd 4d, Zn 3d, and Te 4d core lines. The results were used to correlate the interfacial chemistry with the electronic structure and to directly determine the ZnSe/CdTe heterojunction valence band discontinuity and the consequent heterojunction band diagram. Results of these measurements reveal that the valence band offset is ?Ev=0.20 eV. copyright 1995 American Institute of Physics

416

Theoretical studies of inner-valence-shell photoionization cross sections in N2 and CO  

International Nuclear Information System (INIS)

Theoretical studies in the intensity-borrowing sudden approximation are reported of inner-valence-shell photoionization cross sections in N2 and CO. The required ionic-state energies and spectroscopic amplitudes are obtained from appropriate Green's-function and configuration-interaction calculations, and previously devised Stieltjes-Tchebycheff moment-theory techniques are employed in determinations of corresponding continuum dipole transition moments in the static-exchange approximation. Comparisons are made of the Green's-function calculations in the two-particle-hole Tamm-Dancoff approximation with wavefunction results obtained from single-excitation and polarization configuration-interaction calculations. Detailed descriptions are given of the calculated spectroscopic intensity distributions and of the hole-particle configurational compositions of the corresponding inner-valence-shell ionic states, and comparisons are made with previously reported wavefunction studies in N+2 and CO+. Spectroscopic assignments are suggested on basis of the present calculations for the strong features observed recently in higher-resolution inner-valence-shell photoelectron spectra. The corresponding calculated partial-channel photoionization cross sections for the designated C2 ?+sub(u), F 2?+sub(g), G 2?+sub(g), and (2sigma-1sub(g)) 2?+/sup>sub(g)) 2?+sub(g) bands in N2 and C 2?+, D 2PI, F 2?+, G 2?+, and (3sigma-1) 2?+ bands in CO are found to be in good quantitative accord with dipole (e,2e), (e,e + ion), and synchrotron-radiation studies. (orig.)

417

Flat band states: disorder and nonlinearity  

CERN Document Server

We reveal the critical behaviour of Anderson localized modes near intersecting flat and dispersive bands in the quasi-one-dimensional diamond ladder with weak diagonal disorder $W$. The localization length $\\xi$ of the flat band states scales with disorder as $\\xi \\sim W^{-\\gamma}$, with $\\gamma = 1.30 \\pm 0.01$, in contrast to the dispersive bands with $\\gamma =2$. A small fraction of dispersive modes mixed with the flat band states is responsible for the unusual scaling. Anderson localization is therefore controlled by two different length scales. Nonlinearity couples these critical modes, resulting in qualitatively different wave spreading regimes, from enhanced expansion to resonant tunneling and self-trapping.

Leykam, Daniel; Bahat-Treidel, Omri; Desyatnikov, Anton S

2013-01-01

418

Broadband dispersion-compensating photonic crystal fiber.  

Science.gov (United States)

We present a modified dual-core photonic crystal fiber, based on pure silica, with special grapefruit holes in the inner cladding. The fiber has large, broadband negative dispersion, and the dispersion value varies linearly from -380 t o-420 ps/(nm km) in the C band. To decrease the fabrication difficulty, large air holes are adopted. Furthermore, the chromatic dispersion of the fiber is not sensitive to the structure parameters. So the proposed fiber structure can greatly facilitate fiber drawing and can be used for broadband dispersion compensation. PMID:16969392

Yang, Sigang; Zhang, Yejin; He, Lina; Xie, Shizhong

2006-10-01

419

Measurement of the spectral momentum distribution of valence electrons in amorphous carbon by (e,2e) spectroscopy  

International Nuclear Information System (INIS)

The spectral momentum density of the valence band of arc evaporated amorphous carbon has been measured by (e,2e) spectroscopy with significantly improved energy resolution relative to earlier studies. The valence band has been studied over a range of momenta from 0 to 1.6 a.u. with a resolution of 0.15 a.u. and over a range of binding energies from 9 eV above to 35 eV below the Fermi energy with a resolution of 1.5 eV. As seen in earlier studies, two major peaks are observed in the spectral momentum density which previously have been associated with ?- and ?-bands in graphite. A third feature in the spectra, a weak shoulder approximately 4 eV below the Fermi energy, is observed for the first time. A heuristic model is introduced based on the assumption that the spectral momentum density of evaporated amorphous carbon is an angular average of the spectral momentum density of graphite. The behavior of the strongest feature in the experimental spectra is described well by this model, but the other two features, which are in the energy range of the graphitic ?-, ?-2-, and ?3- bands, are poorly represented by the model. It is suggested that the poor agreement is due to re hybridization of these graphitic bands. 29 refs., 9 figs

420

A study of the inner-valence ionization region in HCl and DCl  

Energy Technology Data Exchange (ETDEWEB)

An in-depth photoionization study of the inner-valence electrons in HCl and DCl has been performed using synchrotron radiation. A series of photoelectron spectra of HCl were obtained at a resolution of 23 meV over the binding energy range 25-30.5 eV at various excitation energies and at two different electron collection angles relative to the plane of polarization of the undulator radiation. In addition, photoelectron spectra of DCl were recorded at two different excitation energies. These spectra were compared directly with the threshold photoelectron spectra of HCl and DCl that were recorded previously under similar resolution conditions ({approx}30 meV). This comparative study reveals new information on the nature of the numerous band systems observed in this binding energy region. In addition, we present the experimental confirmation of the theoretical prediction given by Andersson et al (2001 Phys. Rev. A 65 012705) that a vibrational progression showing interference structure would appear in the main inner-valence ionization band in the photoelectron spectrum of DCl at a resolution of 10 meV.

Burmeister, F [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Andersson, L M [Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala (Sweden); Oehrwall, G [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Richter, T [TU-Berlin, Institut fuer Atomare Physik, Hardenbergstrasse 36, D-10623 Berlin (Germany); Zimmermann, P [TU-Berlin, Institut fuer Atomare Physik, Hardenbergstrasse 36, D-10623 Berlin (Germany); Godehusen, K [BESSY GmbH, Albert-Einstein-Strasse 15, D-12489 Berlin (Germany); Martins, M [Institut fuer Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg (Germany); Karlsson, H O [Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala (Sweden); Sorensen, S L [SLF, Institute of Physics, University of Lund, Box 118, S-221 00 Lund (Sweden); Bjoerneholm, O [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Feifel, R [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Wiesner, K [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Goscinski, O [Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala (Sweden); Karlsson, L [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Svensson, S [Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala (Sweden); Yencha, A J [Department of Chemistry, State University of New York at Albany, Albany, NY 12222 (United States)

2004-03-28

 
 
 
 
421

A study of the inner-valence ionization region in HCl and DCl  

International Nuclear Information System (INIS)

An in-depth photoionization study of the inner-valence electrons in HCl and DCl has been performed using synchrotron radiation. A series of photoelectron spectra of HCl were obtained at a resolution of 23 meV over the binding energy range 25-30.5 eV at various excitation energies and at two different electron collection angles relative to the plane of polarization of the undulator radiation. In addition, photoelectron spectra of DCl were recorded at two different excitation energies. These spectra were compared directly with the threshold photoelectron spectra of HCl and DCl that were recorded previously under similar resolution conditions (?30 meV). This comparative study reveals new information on the nature of the numerous band systems observed in this binding energy region. In addition, we present the experimental confirmation of the theoretical prediction given by Andersson et al (2001 Phys. Rev. A 65 012705) that a vibrational progression showing interference structure would appear in the main inner-valence ionization band in the photoelectron spectrum of DCl at a resolution of 10 meV

422

Band Together!  

Science.gov (United States)

After nearly a decade as band director at St. James High School in St. James, Missouri, Derek Limback knows that the key to building a successful program is putting the program itself above everything else. Limback strives to augment not only his students' musical prowess, but also their leadership skills. Key to his philosophy is instilling a…

Olson, Cathy Applefeld

2011-01-01

423

Band alignment of InN/6H-SiC heterojunction determined by x-ray photoelectron spectroscopy  

Science.gov (United States)

The valence band offset (VBO) of InN/6H-SiC heterojunction has been directly measured by x-ray photoelectron spectroscopy. The VBO is determined to be -0.10 ± 0.23 eV and the conduction band offset is deduced to be -2.47 ± 0.23 eV, indicating that the heterojunction has a type-II band alignment. The accurate determination of the valence and conduction band offsets is important for applications and analysis of InN/6H-SiC optoelectronic devices.

Jing, Qiang; Wu, Guoguang; Zhang, Yuantao; Gao, Fubin; Cai, Xupu; Zhao, Yang; Li, Wancheng; Du, Guotong

2014-08-01

424

Valence fluctuation driven quantum phase transition  

Science.gov (United States)

In recent years quantum critical phenomenon have acquired a great interest in the condensed matter community. Many rare earth intermetallic compounds, which are also heavy fermions can be tuned easily to quantum critical point by application of external perturbations like magnetic field and pressure. YbRh2Si2 and CeCu2Si2 are a few examples. The periodic Anderson model (PAM) is a paradigm for studying these kind of systems. We investigate the extended periodic anderson model (EPAM), which includes Coulomb interaction of conduction and localised electrons using local moment approach (LMA) within dynamical mean field theory (DMFT) with the objective of developing an understanding of quantum phase transitions due to valence fluctuations. We show that tuning c-f interaction and on-site energy of localised electron (which can be achieved by varying external perturbation like pressure) leads to some exotic phenomena like vanishing of Fermi liquid scale. We study transport properties near quantum critical point and highlight the anomalies due to the proximity of QCP.

Kumar, Pramod; Vidhyadhiraja, N. S.

2012-02-01

425

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

426

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.

427

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

International Nuclear Information System (INIS)

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

428

Plasmon bands in multilayer graphene  

Science.gov (United States)

High-energy collective electronic excitations (plasmons) in freestanding multilayer graphene are studied by momentum-resolved electron energy-loss spectroscopy (EELS). For normal incidence, only the high-energy plasmon band is excited and we measure a blueshift of the ? -plasmon dispersion with increasing thickness. The observed transition between two-dimensional and three-dimensional behavior is explained using a layered-electron-gas (LEG) model. We propose a method to measure all individual plasmon bands by tilting the sample with respect to the electron beam. As a proof of concept, EELS experiments for three-layer graphene are compared with predictions from the LEG model.

Wachsmuth, P.; Hambach, R.; Benner, G.; Kaiser, U.

2014-12-01

429

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

430

Effect of valence on the electromigration in silver  

International Nuclear Information System (INIS)

It is shown that the apparent effective valence ZB** of a solute deduced from experiments differs from the true effective valence ZB* defined in the atomic models by a corrective term due to the 'vacancy flow effect'. The experimental results suggest that this corrective term is very important and that it is negative for transition elements; this hypothesis is confirmed for the case of iron in a copper matrix. For the elements to the right of silver in the periodic table, where the correction can be neglected, the effective valence of the solute varies linearly with z (z + 1), z being the difference between the valency of the solute and the solvent; in contrast, the further the solute is from the solvent in the periodic table the more nearly the electronic structure of the ion at the saddle point resembles that of the ion at the equilibrium position. (author)

431

Spatial Representation of Odorant Valence in an Insect Brain  

Directory of Open Access Journals (Sweden)

Full Text Available Brains have to decide whether and how to respond to detected stimuli based on complex sensory input. The vinegar fly Drosophila melanogaster evaluates food sources based on olfactory cues. Here, we performed a behavioral screen using the vinegar fly and established the innate valence of 110 odorants. Our analysis of neuronal activation patterns evoked by attractive and aversive odorants suggests that even though the identity of odorants is coded by the set of activated receptors, the main representation of odorant valence is formed at the output level of the antennal lobe. The topographic clustering within the antennal lobe of valence-specific output neurons resembles a corresponding domain in the olfactory bulb of mice. The basal anatomical structure of the olfactory circuit between insects and vertebrates is known to be similar; our study suggests that the representation of odorant valence is as well.

Markus Knaden

2012-04-01

432

Some formal results for the valence bond basis  

CERN Document Server

In a system with an even number of SU(2) spins, there is an overcomplete set of states--consisting of all possible pairings of the spins into valence bonds--that spans the S=0 Hilbert subspace. Operator expectation values in this basis are related to the properties of the closed loops that are formed by the overlap of valence bond states. We construct a generating function for spin correlation functions of arbitrary order and show that all nonvanishing contributions arise from configurations that are topologically irreducible. We derive explicit formulas for the correlation functions at second, fourth, and sixth order. We then extend the valence bond basis to include triplet bonds and discuss how to compute properties that are related to operators acting outside the singlet sector. These results are relevant to analytical calculations and to numerical valence bond simulations using quantum Monte Carlo, variational wavefunctions, or exact diagonalization.

Beach, K S D

2006-01-01

433

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

434

Inelastic collisions of positrons with one-valence-electron targets  

Science.gov (United States)

The total elastic and positronium formation cross sections of the inelastic collisions between positrons and various one-valence-electron atoms, (namely hydrogen, lithium, sodium, potassium and rubidium), and one-valence-electron ions, (namely hydrogen-like, lithium-like and alkaline-earth positive ions) are determined using an elaborate modified coupled-static approximation. Special attention is devoted to the behavior of the Ps cross sections at the energy regions lying above the Ps formation thresholds.

Abdel-Raouf, Mohamed Assad

1990-01-01

435