bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic ordering in crystals
Authors: Ionova G. V. , Makarov E. F. , Pachev O. M. , Ionov S. P. 
Published: July 2007

AbstractThe charge and orbital ordering are studied in crystals with doubly degenerate or quasi‐degenerate orbitals. A model Hamiltonian is used including the intercentre electron transfer, the intracentre and intercentre Coulomb, and exchange interactions. To obtain the eigenvalues of the Hamiltonian the Zubarev double time Green function technique is applied. In the disordered state a mixing of bands takes place owing to electron—hole correlations. The conditions for the occurrence of orbital and charge ordering are obtained.
Icosahedral microclusters a possible structural unit in amorphous metals
Authors: Briant C. L. , Burton J. J. 
Published: July 2007

AbstractA model for amorphous metals is proposed. The model is based on the 13 atom icosahedron which is the minimum energy structure for a 13 atom cluster. An amorphous metal is considered as containing a high density of these structural units. The material surrounding the icosahedra is partially ordered. The spaces between the ordered regions may be disordered. This model successfully accounts for the experimental interference functions in amorphous materials. It is explained, why, physically, such a structure should be expected to form. It is also shown that structures such as that proposed are observed when, in a computer experiment, a liquid is quenched very rapidly into the solid phase.
Non‐linear optical properties of the band charge carrier gas
Authors: Fomin V. M. , Pokatilov E. P. 
Published: July 2007

AbstractA theory of non‐linear optical properties of a band charge carrier gas in semiconductors due to the influnece of a strong electromagnetic radiation on the probability of the carrier scattering by phonons or ionized impurities leading to intra‐band transitions is developed. A general expression for the polarization created by the carrier response in a laser field in the presence of a magnetic field and other electromagnetic fields (in particular, of a probe wave) is obtained at arbitrary temperatures using the path‐integral version of the density matrix method. The polarization spectrum is investigated. A self‐consistent set of wave equations is obtained to describe the propagation of electromagnetic waves, both, of those ones whose frequencies are determined by external sources and of those ones which are generated in the non‐linear medium under consideration. The explicit analytical expressions for a generalized dielectric permeability tensor and a generalized generation tensor are obtained for all principal scattering mechanisms and an arbitrary laser field intensity. The macroscopic (observable) manifestations of the polarization non‐linearity — the effects of the dependence of the absorption coefficient an the refractive index the intensity — are studied. The generation of harmonics and mixed frequencies is described; the influence of the generation on the laser wave propagation is investigated and found to be significant.
Excitons in gallium sulphide
Authors: Belenkii G. L. , Godzhaev M. O. 
Published: July 2007

AbstractStrong evidences for existence of indirect excitons in GaS are obtained as a result of an analysis of differential absorption and photoluminescence spectra registered in the 4.2 to 100 K temperature range in polarized light. The location of the exciton band in gallium sulphide Eexc = (2594 ± 2) meV and the energy of phonons which take part in indirect transitions are determined. Lines which are interpreted as a result of non‐phonon transitions are observed in the absorption and emission spectra. Lines which arise from the decomposition of bound excitons are found in emission spectra as well. It is shown that polarization pecularities of GaS absorption and luminescence spectra are explained in terms of the band scheme of gallium selenide which is structurally analogous to GaS. The possible nature of the anisotropy of excitons in GaS is discussed.
Superconductivity originating from quasi‐orbital electrons II. The superconducting ground state of quasi‐orbital conduction electrons
Authors: Krüger E. 
Published: July 2007

AbstractIt is shown that the electrons of a certain narrow half‐filled band couple to Cooper pairs below a transition temperature Tc if these electrons behave similar to orbital electrons. This transition is nearly the same as that described by the BCS theory. However, the coupling of the electrons to Cooper pairs is constrained in a new manner by quasi‐classical constraining forces. A comparison of band structure calculations with superconducting properties of metals suggests that quasiclassical constraining forces must exist in any superconducting state. As a result, only those metals become superconducting which have a narrow half‐filled band of a certain symmetry type in their band structure.
Electron states associated with the core region of the 60° dislocation in silicon and germanium
Authors: Marklund S. 
Published: July 2007

AbstractUsing the tight‐binding method the localized states connected with the 60° dislocation in silicon and germanium are calculated. Periodic arrays of large unit cells each containing two 60° dislocations with Burgers vectors of opposite sign are considered. As a consequence of the broken bonds a one‐dimensional band occurs in the direction of the dislocation line for both silicon and germanium. The dilatation region of the dislocation core is also found to cause a second dislocation band to appear below the conduction band edge.
Band structure calculation of ZnSb
Authors: Yamada Y. 
Published: July 2007

AbstractAccurate band structures of ZnSb are obtained by the use of the empirical‐pseudopotential method. The top of the valence band is located at k = (0.93π/a, 0, 0) on the line Γ‐∑−X with symmetry ∑4 and the bottom of the conduction band is located at k = (0.47π/a, 0, 0) on the line Γ−∑−X with symmetry ∑1. The indirect energy gap is 0.60 eV. The interband transition gaps are assigned. A single ellipsoidal band model is presented for the explanation of the experiments of the magnetoresistance in p‐type materials.
Investigation of the photoelectric and photoluminescent properties of crystalline quartz and vitreous silica in the fundamental absorption region. A model for electronic structure and migration of energy in SiO2
Authors: Trukhin A. N. 
Published: July 2007

AbstractElectronic structure and energy migration in SiO2 are studied by the experimental method. The spectral dependences of the related quantum yield of external photoemission, of photoconductivity, and of energy transfer to the luminescence centres are investigated in crystalline quartz and vitreous silica under irradiation by photons with energy from the fundamental absorption region. The direct allowed transition energy gap is found to be equal to 12 eV. The energy gap minimal value is estimated to be 9.5 eV. The fundamental absorption bands of SiO2 at 10.4 and 11.6 eV are shown to be due to excitons, capable of migrating in the „hot”︁ non‐thermalized state and transferring energy to the luminescence centres. Electron‐hole pairs transfer energy in thermalized state with lattice.
Phase diagrams of an Ising magnet with random exchange bonds
Authors: Medvedev M. V. 
Published: July 2007

AbstractIn the molecular field approximation it is shown that there are a random ferromagnetic state, a spin‐glass one and a random antiferromagnetic one, in an Ising magnet with random nearest‐neighbour exchange bonds of different signs. The analytical expressions for the transition temperatures from these states into the paramagnetic ones are obtained. The high‐temperature susceptibility of the system and the susceptibility of the spin‐glass state are investigated as well.
Kerr effect and birefringence from paraelectric CN− defects in nine alkali halides
Authors: Diaz‐Gongora A. , Lüty F. 
Published: July 2007

AbstractSubstitutional CN− defects in the sodium‐, potassium‐ and rubidium‐chlorides, ‐bromides, and iodides are investigated at low temperatures (2 to 8 K) under electric fields E of different symmetry for their field‐induced birefringence Δn(E) (Kerr‐effect). In all nine crystal systems, a para‐electric Kerr‐effect is found with a pronounced anisotropy giving evidence for 〈111〉 CN− symmetry in the six potassium‐ and rubidium‐halides, and 〈100〉 symmetry in the three sodium‐halides. Under increase of the CN− concentration, anisotropic birefringence effects appear (without field) under cooling of the crystals, indicating low temperature ordering of interacting CN− dipole clusters.
The origin of the anomalous TO‐doublet in cubic CuCl studied by light scattering
Authors: Fukushi K. , Nippus M. , Claus R. 
Published: July 2007

AbstractLight scattering experiments are performed concerning polaritons associated with the anomalous TO‐doublet in CuCl. A quantitative description of the dispersion behaviour, recorded at liquid N2 temperature, may be obtained by taking into account coupling of the fundamental zone centered TO‐phonon with second order modes due to two van Hove singularities. This description is in good agreement with recent calculations of the two‐phonon density of states in CuCl.
The infrared absorption spectrum of n‐type GaAs
Authors: Jensen B. 
Published: July 2007

AbstractThe optical conductivity of free electrons in polar semiconducting compounds has recently been calculated by use of a generalized Boltzmann equation derived from the equation of motion of the quantum density matrix. This reduces to the quasi‐classical Boltzmann transport equation in the low frequency limit. In this paper, the optical conductivity is calculated for GaAs as a function of carrier concentration in terms of a frequency dependent relaxation time which reduces to the usual relaxation time in the limit of low frequencies and an elastic scattering mechanism. The relation of the frequency dependent relaxation time to the electron cross section is discussed. The optical constants and the frequency dependent relaxation time are given for GaAs at 298 K over the spectral region from 45 to 3.5 × 103 cm−1 for carrier concentrations from 3.4 × 1015 to 8.7 × 1018 cm−3. The real and imaginary parts of the complex refractive index and the reflectivity are calculated from the complex dielectric constant in terms of the frequency dependent relaxation times. The frequency at the minima of the reflectivity curve and at the plasma edge, where the real part of the electric constant vanishes, are correlated with carrier concentration. Comparison is made with recent experimental results.
Optical properties of amorphous and polycrystalline BeO thin films from electron energy loss measurements
Authors: Gründler R. , Breuer K. , Tews W. 
Published: July 2007

AbstractEnergy loss measurements on amorphous and polycrystalline BeO thin films are performed applying 25 keV electrons. From the energy‐dependent loss function Im (−1/ε) the dielectric function and its first energy derivative, the reflectance, ηeff, ε2E2 up to 44 eV are computed using the Kramers‐Kronig‐analysis and well‐known formulae. The results obtained on polycrystalline BeO are discussed with respect to band structure information. The corresponding spectra of amorphous BeO are compared with those of the polycrystalline material.
Thermodynamics of an electron—hole system in semiconductors
Authors: Combescot M. 
Published: July 2007

AbstractAn investigation is made of the thermodynamics of an electron–hole system in semiconductors. It is shown that the electron–hole liquid, which exists even if it is not the stablest state, can evaporate a gas more complex than for germanium. Possible phase diagrams are given according to the relative values of the binding energies of the excitons, biexcitons, and electron–hole plasma. The possibility of more than one critical point is also considered.
Molecular orbital approach of antiferromagnetic coupling in linear chains AMCl3 with M  V, Mn, and Ni and A  Cs or N(CH3)4
Authors: Charlot M.‐F. , Girerd J.‐J. , Kahn O. 
Published: July 2007

AbstractThe antiferromagnetic contribution JAF to the magnetic exchange interaction parameter J defined by the HDVV spin Hamiltonian is calculated in linear chains AMCl3 (M = V, Mn, and Ni; A = Cs or N(CH3)4) using a model described in a previous paper. Each of the N coupled transition ions possesses n unpaired electrons and its ground term is an orbital singlet. JAF is then expressed by JAF = — where Sμμ is the overlap integral between two magnetic orbitals of the same symmetry μ centred on nearest‐neighbour transition ions and Δμ is the width of the molecular orbital band built from these magnetic orbitals. Δμ is obtained by an extended Hückel calculation with charge iteration and Madelung correction for the potential. Sμμ is calculated by applying the Wolfsberg‐Helmholz approximation to the magnetic orbitals. The calculated values of JAF are compared to the experimental J values. They have the good order of magnitude and their relative order agree with the experimental data.
Multiple scattering in condensed materials I. The embedding field and the effective medium
Authors: Lenk K. 
Published: July 2007

AbstractFor the approximate treatment of the elastic multiple scattering of waves within a system composed of close‐packed muffin‐tin scatterers a new method is developed which is applicable to arbitrarily strong scattering and higher partial waves. The embedding of each individual scatterer is performed by a fit of the wave field within the muffin‐tin spheres to a coherent wave propagating outside the scatterers. Thereby the local surroundings of such points outside are assumed to be isotropic. All mathematical problems involved are solved analytically with the result of a closed dispersion relation for the wave number of the medium. Its solution is discussed for weak scattering and for long wavelengths.
Quantum defect theory of excitons solid neon
Authors: Resta R. 
Published: July 2007

AbstractThe close analogies between Rydberg states in rare‐gas atoms and excitons in rare‐gas solids are discussed. A quantum defect classification of exciton states is proposed and a new value of the “experimental” gap is found. A simple semi‐empirical theory is also established, whose only ingredients are the exciton effective mass equation and the free‐atom Rydberg quantum defects. The present theory compares extremely well to the observed spectra in solid neon for all the excitons, including the first.
The depolarization interaction of adsorbed non‐polar molecules
Authors: Antoniewicz P. R. 
Published: July 2007

AbstractThe depolarizing interaction between two non‐polar molecules, on a metal surface, is investigated. It is found that the classical depolarizing field between two dipole moments is sufficient to describe the interaction for spherically symmetric molecules or atoms. However the effective polarizability should be used instead of the free atom polarizability. For an anisotropic molecule there is a correction to the classical result of up to about twenty percent at a monolayer coverage.
Stopping power of solids in planar channelling
Authors: Pathak Anand P. 
Published: July 2007

AbstractIn order to find the effects of extended defects (like dislocations) on the energy loss rate of charged particles in channelling one needs to know the position dependence of the stopping power in the channels. The variations of the stopping power Se for MeV α‐particles with position in (111) and (100) planar channels of gold, silver, and copper single crystals are calculated. The effective electron density responsible for stopping in planar channelling is calculated by taking suitable planar average of density due to different shells of individual atoms in the plane. The results are compared with available experimental data for gold. The energy dependence of stopping power for well channelled α‐particles in Au(111) planar channels in the energy range 1 to 5 MeV is found to follow an empirical relation Se∼E−3/4. The dependence of energy loss on the oscillation amplitude in Au(111) channels is also calculated and compared with available experimental data and good agreement is found.
Charge Distribution and Ionicity of the Chemical Bonds in TiC, VC, and TiN
Authors: Mokhracheva L. P. , Tskhai V. A. , Geld P. V. 
Published: July 2007

AbstractThe space charge distribution of the valence electrons in TiC, VC, and TiN is calculated by using the crystal wave functions obtained previously in terms of the tight‐binding method. Along the bond directions MeX, MeMe, and XX, the comparison of the calculated charge density with that obtained by superposition of the atomic densities is made, which shows that there is an increasing electron transfer from the intermediate region of the MeMe bonds to the bond direction MeX in TiCVCTiN. Charge changing in the atomic volumes of the nonmetal atoms points out that the carbon atom in TiC has not an additional charge, whereas in VC and TiN there is a charge transfer into nonmetal atomic sphere of approximately 0.1 and 0.4 electrons, respectively.
Inadequacy of the Direct Interband Transition Scheme Applied to Valence Semiconductor Fundamental Absorption Spectra
Authors: Shatalov V. M. , Telezhkin V. A. , Tolpygo K. B. 
Published: July 2007

AbstractThe appearance of areas with a nearly constant interband difference EG(k) near the L‐ and K‐points of the Brillouin zone, corresponding to the absorption peaks E1 and E2, is considered within the model of an electron in a periodic field. Large values of the reduced masses μ‖ result in relatively deep levels of the Wannier‐Mot texciton (≈ 0.1 eV for L and ≈ 1 eV for K). To obtain absorption frequencies in agreement with experiment one has to redetermine the periodic potential Wg, as the interband distance at the points mentioned above must exceed the energy of the corresponding absorption peak by the value of the exciton binding energy.
Statistical Mechanics of Impurity States. II. Applications
Authors: Foglio M. E. 
Published: July 2007

AbstractCreation and annihilation operators for impurities introduced in a previous paper are applied 1) to describe some known models of the metal–insulator and other phase transitions, 2) to discuss the problem of mixed‐valence using the thermodynamic perturbation method. The results derived are extensions of those previously obtained and coincide with them in the proper limit. The calculation of the equation of state (i.e. the pressure) is discussed in relation to this treatment.
Bound Polarons in YAG Crystals
Authors: Bernhardt Hj. 
Published: July 2007

AbstractOn the basis of investigations of different coloration centres in pure and doped YAG crystals, most of them are already published, the applicability is proved of the generalized conception of a bound polaron to all hitherto known coloration centres occurring after X‐ray irradiation. It is shown that the optical properties described can be explained by means of the model of a linear harmonic oscillator.
Theoretical Investigation of the Anisotropy of Beryllium by Polarized K‐Emission, APS, and Compton Profiles
Authors: Rennert P. , Dörre Th. , Gläser U. 
Published: July 2007

AbstractStarting from a pseudopotential calculation for metallic beryllium the K‐emission spectra of different polarizations, the Compton profiles for several directions, and the APS are calculated. Besides the shape of the emission spectra also the absolute values of the intensity are highly anisotropic due to the existence of localized charges along the hexagonal axis of Be. The calculated spectra agree well with a recently published measurement. The Compton profiles for three directions are calculated in momentum approximation. The comparison of the anisotropies obtained with those of several measurements and a LCAO calculation shows a good correlation.
Cu+ Luminescent Centre in LiCl Single Crystals
Authors: Pedrini C. 
Published: July 2007

AbstractUsing the cluster approximation, a detailed semi‐empirical molecular orbital calculation is reported for computing the energy levels of Cu+ luminescent centres in NaCl and LiCl. Excited states energies are obtained on the basis of 3d94s configuration of Cu+, including spin–orbit interaction. Excitation and emission bands of LiCl:Cu+ are reported and their assignments are discussed. The emission process is studied by measuring lifetimes of the emitting level at various temperatures. The energy level scheme provides a satisfactory explanation of the emission process, with the presence of active metastable levels, in agreement with the decay time data.
Widths of 5d Energy Bands in the Heavy Rare‐Earth Metals
Authors: Eagles D. M. 
Published: July 2007

AbstractA model in which the partial densities of states associated with individual 5d levels are spread over regions of widths of only about 1 eV in the heavy rare‐earth metals is used to discuss recent results on X‐ray emission and electronic specific heats in these metals, and on crystal fields in alloys. Evidence for and against narrow 5d bands from results on saturation magnetic moments, neutron‐diffraction‐determined 4f moments, magnetic hyperfine fields, electric field gradients at nuclei, magnetic anisotropy, and X‐ray and magneto‐optical absorption in the heavy rare‐earth metals and alloys are reviewed. There is more evidence for than there is against the proposition that the bands associated with partially‐occupied 5d states are narrow.
The Frequency Dependent Dielectric Function of Gapless Semiconductors in a Magnetic Field
Authors: Gelmont B. L. , Ivanov‐Omskii V. I. , Ukraintsev E. N. 
Published: July 2007

AbstractIt is shown that in gapless semiconductors in a magnetic field the transmission of electromagnetic waves is possible at frequencies less than the plasma frequency even in the Voigt geometry. This is due to the combined resonance between two sets of Landau levels.
ESR Study of a Cr5+ Centre in Ferroelectric Ammonium Sulphate
Authors: Manjunath C. V. , Srinivasan R. 
Published: July 2007

AbstractX‐band electron spin resonance (ESR) studies of (CrO4)2‐ doped, X‐irradiated single crystals of ferroelectric ammonium sulphate ((NH4)2SO4, TC = 223 K) at 300 and 208 K are reported. The paramagnetic centre responsible for the ESR spectrum is identified to be Cr5+. Superhyperfine interaction of the unpaired electron with two equivalent protons is observed. The spin‐Hamiltonian parameters which are nearly axial at 300 K, with g‖ < g⟂ indicating a d orbital ground state, acquired rhombic character below TC indicating a distortion of the sulphate tetrahedron. An increase in the value of the proton superhyperfine constant in the ferroelectric phase is indicative of stronger hydrogen bonding.
The Critical Voltage Effect in Transmission Electron Microscopy. X. Experimental Observatisons in the Presence of Non‐Systematic Reflections
Authors: David M. , van Tendeloo G. , van Landuyt J. , Gevers R. 
Published: July 2007

AbstractThe nature of the 2nd order critical voltage effect in the presence of non‐systematic reflections is discussed by means of a dispersion surface analysis, and illustrated by some typical experiments. It is shown that a voltage variation is to a certain extent equivalent to tilting around the basic row. This entails a possible extension of the use of the effect in e.g. structure factor determination. A particular convergent beam technique is proposed for optimal observation of the behaviour of the intensity with varying voltage and orientation.