bluuebunny/crossref_metadata_2025_split huggingface dataset:
Raman Scattering in ε‐GaSe and ZnIn2S4 Single Crystals under Pressure
Authors: Vinogradov E. A. , Zhizhin G. N. , Melnik N. N. , Subbotin S. I. , Panfilov V. V. , Allakhverdiev K. R. , Baeaev S. S. , Zhitar V. F. 
Published: July 2007

AbstractThe influence of hydrostatic pressure on the Raman scattering spectra in ε‐GaSe (up to 8.5 kbar) and ZnIn2S4 (up to 10.9 kbar) is investigated at 295 K. Parameters γi – (1/vi) (dvi/dp) are obtained for both the single crystals; for e‐GaSe values of the mode‐Gruneisen parameters are also obtained. The set of the parameters γi(Γi) shows that these two crystals are layered ones with ionic‐covalent bonds inside the layers; the degree of ionicity in ZnIn2S4 is higher than in ε‐GaSe. “Rigid‐layer” (interlayer) vibrations determined for ZnIn2S4 and ε‐GaSe using γi‐parameters show that ZnIn2S4 is not a one‐packet polytype.
The Density of States at the Band Tail of a Disordered System with Correlations
Authors: Dúc Nguyẽn Manh 
Published: July 2007

AbstractEdwards' formulation of the electronic states in a disordered system is extended for a generalized probability distribution in which the correlations between the positions of scatterers of different kinds are included. Using Feynman's path integral it is shown that the density of states may be defined in amorphous materials where the short‐range order may play an important role. The evaluation is considered for pure and binary liquid semiconductors and for a heavily doped and compensated semiconductor to give the asymptotic form of the density of states for ε < 0.
Perturbation Expansion for the Asymmetric Anderson Hamiltonian
Authors: Hobvtić B. , Zlatić V. 
Published: July 2007

AbstractAsymmetric single orbital Anderson model is studied by the rapidly convergent perturbation method of Yosida and Yamada. The change in the density of states, specific heat, and transport coefficients due to the second order self energy corrections is evaluated for a dilute alloy with nearly half‐filled virtual bound state.
The Effect of Hydrostatic Pressure and Temperature on the EPB Spectrum of the Eu2+ Ion in CdF2b40 as a Function of the Nearest‐Neighbour Distances
Authors: Rewaj T. , Krupski M. 
Published: July 2007

AbstractThe effect of temperature and hydrostatic stress on the b spin‐Hamiltonian parameter for Eu2+ in CdP2 single crystal is investigated. The measurements are carried out in X‐band in the temperature range from 77 to 447 K and the pressure range up to 5000 at. The following results are obtained: (∂b/∂T)P = 1 at = −220 × 10−8cm−1K−1 and (∂b/∂p)T = 300K = +1.66 × l0−8 cm−1 at−1. It is deduced, on the basis of these results, that about 70% of the temperature changes in b(T) can be related to the thermal expansion of the crystal i.e. to the changes in metal‐ligand distance.
Reflectance and Thermoreflectance of GeTe
Authors: Baleva M. 
Published: July 2007

AbstractThe reflectance spectra of p‐type GeTe crystals are measured in the energy range 0.4 to 3.0 eV at liquid nitrogen and room temperature. The principal optical constants are obtained by inverting the spectra using the Kramers‐Kronig technique. The thermoreflectance spectra at room and liquid nitrogen temperature are measured over the energy range from 0.8 to 3.0 eV. The results are quantitatively interpreted. The relatively large and sharp changes in the thermoreflectance spectra near 1 eV are explained with the thermal width of the Fermi function, and with δ‐type transitions at 1.36 eV.
Luminescence of RbI above Liquid Nitrogen Temperature
Authors: Mitsushima Y. , Itoh N. 
Published: July 2007

AbstractLuminescence spectra of RbI induced by bombardments with pulsed Ne+ and electron beams and X‐rays are measured above liquid nitrogen temperature. The luminescence bands at 3.8 and 2.4 eV are studied in detail. The former shows the usual quenching character, while the latter shows following characters. (1) The intensity of the 2.4 eV band obtained by pulsed Ne+ beams at liquid nitrogen temperature increases superlinearly as the current density increases. (2) The change of the luminescence after an electron pulse bombardment around room temperature shows an exponential decay superposed by a growth followed by another exponential decay. The time constants which characterize the luminescence change after an electron pulse bombardment are obtained between 260 and 300 K and are found to follow the Arrhenius equation with an activation energy of about 0.39 eV. To explain these experimental results, it is proposed that the 2.4 eV luminescence band is caused by the radiative recombination of a complex center formed by the interaction between the mobile VK center and a quasi‐stable electron trapped center, the concentration of which increases linearly with the radiation dose.
Kinetics of Flash‐Stimulated Changes of F‐Centre Absorption in γ‐Irradiated KCl
Authors: Werner B. , Stradowski C. , Sugier H. 
Published: July 2007

AbstractThe decay of F−‐centres, produced by flashing the γ‐irradiated KCl crystals is studied in the microsecond regime at room temperature. The kinetics of the decay of F−‐ and recovery of F‐centre absorption bands are compared and a scheme of reactions in the system investigated is proposed. The decay of flash produced F− ‐centres leads initially mainly to the recovery of the F‐band, probably via tunnelling of an electron to an anion vacancy on relatively long distances. The excess energy, released after tunnelling, allows F‐centres subsequently to diffuse and form aggregates. Another possibility is the migration of F‐centres in electronically excited 2p state, leading also to the formation of aggregate centres.
Short‐Range Effects in Germanium‐Silicon
Authors: Martín‐Rodero A. , Vergés J. A. , Tejedor C. 
Published: July 2007

AbstractIn order to analyze simple approximations used in the study of Ge‐Si alloys, a self‐consistent calculation of the Ge‐Si perfect crystal is presented. The description in terms of localized functions allows to discuss a few‐neighbours approach. Short‐range effects are dominant for valence bands, but this does not apply for conduction electrons.
Studies of the Fine Structure of the Direct Gap of As2Se3 Single Crystals by Electroreflectance
Authors: Althaus H. L. , Weiser G. 
Published: July 2007

AbstractElectroreflectance spectra of As2Se3 single crystals in the energy range between 2 and 2.6 eV are presented for polarization of light parallel and perpendicular to the layer plane. Comparison of the spectra with band structure calculations and bond models reveal a very complicated structure of the energy bands near the direct gap. The direct gap close to 2.20 eV is located near Γ. The spinorbit splitting of the uppermost valence bands of 40 meV is similar to the splitting in trigonal selenium. Optical selection rules and the dependence of the spectra on the orientation of the electric field indicate that the uppermost filled bands arise mainly from the lone pair p‐states of the Se atoms.
Molecular Field Theory and Landau‐Ginzburg Theory of Phase Transitions in Disordered Ferromagnets
Authors: Fähnle M. 
Published: July 2007

AbstractThe equivalence is demonstrated of molecular field theory and Landau‐Ginzburg theory for the description of the ferromagnetic phase transition. In analogy to the original (inhomogeneous) molecular field theory of disordered materials and its homogeneous version (S = Sz in the expression for the molecular field) an inhomogeneous and a homogeneous version of the Landau‐Ginzburg theory are derived. Within the framework of the local versions of these theories the influence of statistically random structure fluctuations on the critical temperature, Tc, is investigated. It is shown that Tc is either unchanged (homogeneous theory) or increased (inhomogeneous theory) in comparison to a corresponding homogeneous system, if correlations between site disorder and bond disorder are neglected. A possible way is described to decide which version of the theory is more appropriate for the description of the phase transition in disordered materials.
On the Interaction between a Vacancy and a Dislocation in Metals
Authors: Masuda K. 
Published: July 2007

AbstractThe interaction ΔEvd between a vacancy and a (screw or an edge) dislocation in metals is investigated using a tight‐binding‐type electronic theory. The repulsive core‐core interaction energies at short distances are simulated by the Born‐Mayer potential. It is shown that there are systematic differences in the ΔEvd between the present results and those obtained by using a pair potential method. Furthermore, it is found that the atomic configuration around a vacancy in the dislocation core region is essentially different from that in the unperturbed lattice.
Electronic Raman Scattering of Polarized Light on Donor Levels in Semiconductors with Direct Band Gaps
Authors: Ba An Nguyen , Van Hieu Nguyen , Toan Thang Nguyen , Ai Viet Nguyen 
Published: July 2007

AbstractExplicite approximate analytical expressions are derived for the differential cross sections of the electronic Raman scattering of circularly polarized light on donor levels with the electronic transition 1s → 2s in semiconductors with zincblende structure and direct band gaps. The photon energies are assumed to be nearly equal to the band gap and therefore the intermediate states in the valence bands give dominant contributions. The real symmetry properties of the degenerate valence bands are taken into account. The absolute values of the cross sections are calculated for GaAs.
Many‐Particle Effects at the E1‐Transition in Silicon
Authors: Jungk G. 
Published: July 2007

AbstractThe ellipsometric determination of the dielectric function of Si around the controversial 3.4 eV optical structure yields at least three critical points at 3.34, 3.41, and 3.58 eV with types M0, M1 and M0, respectively. The exact lineshape fit of the energy‐derivative spectra shows that the M1‐type singularity is modified by many‐particle effects, which can be accounted for by an admixture of M2‐type lineshape.
Infrared and Raman Spectra of Non‐Stoichiometric Cerium Hydrides
Authors: Fujimori A. , Ishii M. , Tsuda N. 
Published: July 2007

AbstractInfrared reflectivity and Raman spectra of CeHx; (2 < x < 3) are reported. Some of phonon features in the spectra shift with the hydrogen concentration. The spectra are analyzed in terms of lattice vibrations of supercells which simulate non‐stoichiometric CeHx. Force constants thus obtained show that the hydrogen atoms at octahedral sites are more weakly bound than those at tetrahedral sites. Anomalous changes in the force constants are suggested to account for the observed shifts.
Local Density of States for an Arbitrary Array of Muffin‐Tin Potentials
Authors: d'Albuquerque J. , Castro E. , Muniz B. Bechara , Jacobs R. L. 
Published: July 2007

AbstractNew formulae are presented for the local density of states in both the spherically symmetric regions and the interstitial region of an arbitrary array of muffin‐tin potentials. No assumption is made about periodicity, there need be no underlying lattice, the muffin‐tins may be of arbitrary sizes (provided that they do not overlap) and the spherically symmetric potentials are arbitrary. The formula at a point in the spheres is site‐diagonal and reduces in spherical average to that of Chen and Fromhold. A similar formula is obtained for points in part of the interstitial region. The formulae when integrated over all space reproduce the result for the total density of states due to Lloyd.
Itinerant and Localized d‐Electrons in Transition Metals from Positron Annihilation Data
Authors: Johnson O. 
Published: July 2007

AbstractThe positron annihilation data for non‐transition metals are treated as a combination of annihilation by conduction electrons, ion‐core electrons, and d‐electrons localized on the ion core. It is possible to account for the high positron annihilation rate of transition metals and the gradation across the transition series by the further partition of the d‐electrons into localized and itinerant d‐electrons. All d‐electrons are itinerant for Sc, Ti, and V, and only a fraction of a d‐electron is itinerant in Fe, Co, and Ni; there are two and five localized d‐electrons for Cr and Mn, respectively.
Infrared Spectrum and Structure of Hydrogenated Amorphous Silicon
Authors: Shanks H. , Fang C. J. , Ley L. , Cardona M. , Demond F. J. , Kalbitzer S. 
Published: July 2007

AbstractThe ir absorption spectrum of hydrogenated amorphous silicon is discussed in the context of structural models for this material. It is shown that the integrated strength of the bond stretching bands in hydrogenated amorphous silicon cannot be used to determine the hydrogen concentration because the local effective charge for ir absorption is a function of hydrogen concentration and sample preparation. The effective charge for the wagging—rocking—rolling vibrations at 640 cm−1, however, is independent of hydrogen concentration and sample preparation. Hence the integrated strength of this band can be used to measure the hydrogen concentration provided the proportionality constant is determined empirically. Changes in the ir absorption bands as a function of isochronal annealing temperature show that two different types of bonds contribute to the bond stretching band at 2100 cm−1. The bond bending bands at 890 and 840 cm−1 are associated with one of the bonds producing the 2100 cm−1 stretching band. The relative strengths of the 890 and 840 cm−1 bands to the 2100 cm−1 band depend strongly on the film preparation method: the glow discharge produced films show much stronger bending bands than those produced by rf sputtering. A structural model for hydrogenated amorphous silicon is presented in which a Maxwell‐Garnett‐type local field correction, which is dependent on the dipole location within a microvoid and the shape of the microvoid, is used to explain the data.
A Full Single‐Quantum Radiative Recombination of Biexcitons Due to Exciton—Biexciton Interaction
Authors: Petrashku K. G. , Russu S. S. , Khadzhi P. I. 
Published: July 2007

AbstractThe single‐quantum annihilation process of the biexciton as a whole at the inelastic collision of the biexciton with an exciton is studied. Analytic expressions for the spectral and temperature dependence of the transition probability are obtained for allowed interband optical transition for various temperatures of the exciton and biexciton subsystems. It is shown that the single‐photon radiative recombination process of the biexciton as a whole in the presence of the exciton is characterized by a transition probability different from zero in the crystal with the symmetry centre in the dipole approximation, while the biexciton radiative recombination itself is impossible.
On the Core Structure of the Glide‐Set 90° and 30° Partial Dislocations in Silicon
Authors: Marklund S. 
Published: July 2007

AbstractBy considering valence force potentials the cores of the glide‐set 30° and 90° partial dislocations in silicon are investigated. It is shown that the 30° partial probably does not contain dangling bonds, while on the other hand dangling bonds at the 90° partial are more likely to exist as pairing of bonds across its core is opposed by a strong shear field. The implications of the results on the electron states of the partials are briefly discussed.
Ultraviolet Photoelectron Spectroscopy (UPS) and LEED Studies of BaTiO3 (001) and SrTiO3 (100) Surfaces
Authors: Courths R. 
Published: July 2007

AbstractUPS studies (21.2 and 40.8 eV) are performed on BaTiO3 (001) and SrTiO3 (100) surfaces. In both the compounds there is an enhanced emission at the top of the valence band for hv = 21.2 eV, which cannot be explained with the existing theoretical bulk densities of states. From the results of angle‐dependent UPS measurements it is concluded that the top of the valence band emission is due to an oxygen surface state. This interpretation is supported by band structure calculations for thin films of BaTiO3 of Marathe et al. A band gap state at 0.9 eV (BaTiO3) and 1.2 eV (SrTiO3) below the bottom of the conduction band can be detected on oxygen deficient surfaces and is interpreted as arising from Ti3+ species. In UHV annealed surfaces are 1 × 1 ordered. They show a 2 × 2 or two mutually perpendicular 2 × 1 LEED patterns in case of a higher concentration of Ti3+—O‐vacancy defects due to defect ordering.
Photoelectron and Optical Spectra Derived from Self‐Consistent Charge MO and Band Calculations for Bulk and Surface of BaTiO3
Authors: Marathe V. R. , Lauer S. , Trautwein A. 
Published: July 2007

AbstractBoth MO and band structure calculations are performed for the cubic phase of BaTiO3 based on a self‐consistent charge procedure following the semi‐empirical Cusachs scheme. The photoelectron spectra derived from the MO calculations are considerably cluster dependent. The experimental XPS valence band (VB) spectrum is well represented by the band structure calculations for the bulk. Comparing band structure calculations for three hypothetical thin films of BaTiO3 with experimental UPS VB results from the preceding paper of Courths it is concluded (i) that the surface of BaTiO3 consists of a TiO2 layer, and (ii) that the experimental UPS VB spectra may be interpreted in terms of contributions from the surface TiO2 layer plus those from the inner BaO and TiO2 layers. Calculated optical transitions in the range 0 to 15 eV are in rough agreement with experimental reflectivity spectra of Bäuerle et al. and Cardona. The calculations show that selfconsistency has significant effects on ionization and Madelung potentials and therefore on the electronic structure of BaTiO3.
Influence of Exciton Gas and Electron—Hole Plasma on Exciton Energy Levels
Authors: Röpke G. , Seifert T. , Stolz H. , Zimmermann R. 
Published: July 2007

AbstractCorrections to the energy levels of ground state excitons embedded in a gas of excitons, electrons, and holes are obtained within the framework of the Green's function technique. Contributions of the interaction with free carriers and excitons are considered in the first Born approximation, and plasmon effects are taken into account. Numerical values are given for the exciton energy shift linear in the densities at different temperatures and different electron—hole mass ratios.
The Dissociation Energy of the Surface Biexciton
Authors: Bobrysheva A. I. , Beryl S. I. , Moskalenko S. A. , Pokatylov E. P. 
Published: July 2007

AbstractThe effective Hamiltonian of two electrons and two holes located near the boundary of the polar crystal and the vacuum, interacting via the Coulomb law and through exchange by virtual surface longitudinal optical phonons is derived. On ground of the approximate form of this Hamiltonian the binding energies Wm(σ) of the surface biexciton are estimated in the whole range of mass ratio 0 ≦ σ = me/mh ≦ 1 (me and mh is the effective mass of the electron and of the hole, respectively). The surface biexciton is stable at all σ and the dissociation energy is greater than that of bulk biexciton.
New Model Exhibiting Metamagnetic Transitions in Absence of Magnetic Field
Authors: Onyszkiewicz Z. 
Published: July 2007

AbstractA new model is discussed, defined through a Hamiltonian with anisotropic fourth order terms in spins. It turns out that such a term when added to the ferromagnetic Heisenberg coupling, stabilizes the antiferromagnetic phase at low temperatures. With increasing temperature the meta‐magnetic transition to the ferromagnetic state occurs even when the external magnetic field is absent.
The Mobility of Electrons and Holes in HgTe in the Temperature Range of Intrinsic Conduction
Authors: Dziuba Z. , Wróbel J. 
Published: July 2007

AbstractHgTe samples containing no acceptors are studied. The results of the investigation of Hall coefficient and conductivity versus magnetic field up to 20 kG in the temperature range 4 up to 100 K are presented. Concentration and mobility of electrons and holes in the intrinsic range of the conductivity are calculated. Density state effective mass of holes mh = 0.3m0 is determined. It is shown that acoustic and optical phonon scattering are most important modes limiting the hole mobility.
On the Nonstationary Processes in Exciton Systems
Authors: Hadžiselimović E. A. , Marinković M. M. , Tošić B. S. 
Published: July 2007

AbstractIn consequence of the nonconservation of excitons, the excitation number operator as well as the exciton pair creation and annihilation operators are expressed as functions of time and of the corresponding stationary operators. The nonstationary pair correlation processes and the nonstationary excitation density fluctuation processes are analysed using the nonstationary operators. It is found that the nonstationary pair correlations are characterized by a specific kind of dissipation that is linearly proportional with time. In contradistinction to stationary processes, the corresponding nonstationary ones, lead to elementary excitations having a double energy: E (nonstationary) = 2E (stationary). It is shown that in contrast to exciton density fluctuations, pair correlations have a tendency of localization related to a lattice point. It is also shown that the exciton non‐statinoary pair correlations are in many respect similar to the density fluctuations that take place in the crystal due to a near slow neutron passage. Finally, a general method for the evaluation of the correlation function, applicable even in the case when the Green's function has a n‐th order pole, is developed in the Appendix.
CNDO/2 Calculations of α‐Quartz Electronic Properties of the Perfect Cluster and EPR Parameters of the O−; Vacancy
Authors: Haberlandt H. , Ritschl F. 
Published: July 2007

AbstractThe CNDO/2 method is applied is applied to a cluster cut out from a perfect α‐quartz crystal. The result is compared with that of an extended Hückel theory (EHT) calculation of the same cluster. The dominant character of valence band states in terms of atomic orbitals and the width of the energy gap are discussed. The formal charges of central cluster atoms are given. Furthermore EPR parameters of a relaxed O−; vacancy in α‐quartz are calculated on the basis of CNDO‐RHF and ‐ UHF approximations. The results are compared with those of an EHT calculation of the same model and with the experimental result of the E′1 centre in SiO2.
Theory of the Anomalous Temperature Dependence of C12 in NaCl‐Like Structure Solids Direct Observation of Phonon—Lattice Interactions
Authors: Shrivastava U. C. 
Published: July 2007

AbstractThe basic quantum theory is presented for the temperature dependence of the elastic constants of solids. The phonon—lattice interaction is invoked to explain the anomalous temperature dependence of C12 in NaCl‐like structure alkali halides. The calculated values of dC12/dT for lithium and sodium halides are negative and dC12/dT for potassium and rubidium halides (except KF) are positive (showing an anomalous temperature dependence of C12). These results are in excellent agreement with available experimental data.
Modulational Instability of an Electromagnetic Wave in Magnetized Plasmas
Authors: Guha S. , Namjoshi P. 
Published: July 2007

AbstractA modulational instability is investigated in a piezoelectric semiconductor plasma when the amplitude modulation of an electromagnetic wave by an acoustic wave is considered. Analytical expressions for threshold electric field amplitude as well as the growth rate of the unstable mode are obtained. A numerical analysis is made for n‐InSb crystals.
Cascade Capture of Non‐Equilibrium Carriers
Authors: Bhattacharya D. P. 
Published: July 2007

AbstractThe low‐temperature galvanomagnetic effects in non‐polar semiconductors are investigated under the condition that the non‐equilibrium carriers are captured by shallow attractive traps. The calculations are carried out assuming spherical constant energy surfaces in the momentum space. Numerical calculations are made for n‐Ge at 4.2 K. The possibility of using the results for the determination of some of the capture parameters is discussed.