bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron states associated with the core region of the 60° dislocation in silicon and germanium
Authors: Marklund S.
Published: July 2007

Using 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

Accurate 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

Electronic 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

In 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

Substitutional 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

Light 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

The 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

Energy 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

An 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

The 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

For 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

The 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

The 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

In 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

The 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

The 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

Creation 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

On 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

Starting 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

Using 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

A 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

It 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

X‐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

The 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.
Quasi‐Local Acceptor Levels and Electron Mobility in Hg1−xCdxTe
Authors: Mekhtiev M. A. , Seid‐Rzaeva S. M.
Published: July 2007

The scattering problems of conduction electrons by quasi‐local acceptor levels is considered. The relaxation time of electrons and the analytical expression for the ϵg‐dependence (ϵg is the energy gap between light‐carrier bands) of the energy of the quasi‐local acceptor level are determined for a more general form of the acceptor potential taking into account the contributions from three upper bands (the conduction band, and the bands of heavy and light holes) as well as the effects of non‐parabolicity. The calculated results for the ϵg‐dependence of the quasi‐local level energy and electron mobility are in good agreement with experimental data in the range of ϵg > 60 meV.
On the Theory of the Ising Model with a Transverse Field
Authors: Kamieniarz G.
Published: July 2007

The zero‐temperature behaviour of the transverse Ising model is investigated in the paramagnetic region. Two versions of the Green's function technique are used and the results are in excellent agreement with the high density and series expansions results in both, the weak and strong interaction limit.
Hybrid Resonance in a Quantizing Magnetic Field in BiSb
Authors: Stegmann R. , Oelgart G.
Published: July 2007

Hybrid resonances are investigated in a quantizing magnetic field in semiconductive BiSb alloys. The measurements are carried out at 1.6 K at frequencies ranging between 120 and 70 GHz with the magnetic field being parallel to the binary axis. It is found that above a definite measuring frequency hybrid resonances can be detected, the occurrences and the field location of which are in effect determined by the redistribution of charge carriers between the component surfaces of the parted Fermi surface in the quantizing magnetic field. The discussion of the results allows the determination of the spin splitting parameter, which is found to be G2 = 0.9. A quadratic relation between energy and momentum gives good agreement between theory and experiment.
Crystal Field Levels of Pr3+ in PrAl3 Studied by Inelastic Neutron Scattering
Authors: Andreeff A. , Kaun L. P. , Lippold B. , Matz W. , Moreva N. I. , Walther K.
Published: July 2007

The crystal field splitting of the 3H4 ground multiplet of the Pr3+ ion in the intermetallic compound PrAl3 is investigated by inelastic scattering of thermal neutrons. At several temperatures crystal field transitions are measured by TOF and TAS methods for polycrystalline PrAl3. In order to decide between crystal field transitions and phonon scattering, the isostructural, nonmagnetic compound LaAl3 is also measured. The crystal field parameters and the energy level scheme for the ground multiplet splitting in PrAl3 are given.
Surface Phenomena in the Polarization‐Dependent Electroreflectance Spectra of Si
Authors: Tyagai V. A. , Snitko O. V. , Evstigneev A. M. , Krasiko A. N.
Published: July 2007

The polarization anisotropy in the longitudinal surface barrier electroreflectance (ER) of heavily doped Si (110) single crystal is measured as a function of the bias for the 3.4 eV peak (E′0). The ER anisotropy under depletion and weak accumulation conditions is found to be characterized by the usual polarization ratio signature of S = 1.35. Both, in the flatband and in the strong accumulation regions practically no polarization dependence is observed. The data obtained indicate that the surface‐related electrooptic effect contributes essentially to the isotropic behaviour of surface‐barrier ER of Si due to surface quantization (flatband region) and to surface amorphism (strong accumulation). The unusual lineshape of the ER spectra taken in the flatband region is shown to be due to the mixing of the depletion‐ and accumulation‐type spectra in the nondifferential ER measurement conditions.
The Dynamic Dielectric Function in HgSe and HgTe
Authors: Szuszkiewicz W. , Witowski A. M. , Grynberg M.
Published: July 2007

Absorption measurements in the infrared region from 5 to 30 μm for HgTe at 295 K and for HgSe at 10, 80, and 295 K are performed. On the basis of the observed interference phenomena the dependence of the refractive index on the wavelength is obtained. These results are analysed in terms of the dynamic dielectric function. The contribution of the Γ → Γ transitions is calculated in terms of Kane's band structure model. From the comparison of theoretical results and experimental data the value of ϵ′∞, treated as a fitting parameter, is determined. The relation ϵ′∞(T) determined for HgSe is somewhat different from that found for HgTe and differs from the value proposed earlier for HgSe.
Exciton Propagation and Memory Effect in Molecular Crystals at Low Temperatures
Authors: Šubrta V.
Published: July 2007

A new method of theoretical low‐temperature calculation of the memory functions (MF) governing the exciton propagation in a model of a three‐dimensional perfect crystal with one molecule per elementary cell is presented. This method is suitable for all MF irrespective of distance between sites involved. The influence of initial condition on the exciton propagation is also tested.
Vibronic Spectra of Molecular Systems
Authors: Zgierski M. Z. , Henneker W. H.
Published: July 2007

A model for investigation of the vibronic spectra of molecular crystals and polymers is presented. The model assumes that Frenkel excitons and intramolecular phonons (vibrational excitons) move independently and permits a study of two interacting Frenkel exciton bands in a crystalline system containing one molecule per unit cell. In particular the hypo‐ and hyperchromic effects on intensity distributions among different vibronic bands are investigated. A possible generalization of the model to crystalline systems of more than one molecule per unit cell is indicated.
A Kinetic Model of Triplet Exciton Dynamics in Organic Crystals with Discrete Trapping Levels
Authors: Zboiński Z.
Published: July 2007

A simple kinetic model is developed to provide expressions for interpreting data from delayed fluorescence and phosphorescence experiments. The expressions for steady state, transient, and chopped light excitations are obtained for a system containing several triplet trapping levels which are well separated in energy. The interrelation of the experimental data obtained for the three modes of excitation enables the evaluation of depths, triplet lifetimes, and other parameters of the trapping levels.
The Motional Effect in Nuclear Double‐Resonance
Authors: Whitaker M. A. B.
Published: July 2007

The behaviour of a nuclear double‐resonance system is discussed for the case where the atoms are in motion. In contrast to the motional narrowing found in conventional nuclear magnetic resonance it is predicted that the line should become broad and shallow.
EPR Investigations of ADP:Cr3+ above and below TC
Authors: Diemer H. K. , Bijvank E. J. , den Hartog H. W.
Published: July 2007

Cr3+ impurities in ADP crystals have been investigated employing the EPR technique. A detailed study has been made at temperatures higher than the Curie temperature; it is suggested that the Cr3+ impurities are located at an NH4+ site which is accompanied by two neighboring proton vacancies. This is concluded from the directions of the principal axes with respect to the crystal axes. In the paraelectric phase there is only one type of Cr3+ center but below TC there are two different centers probably due to different arrangements of the H+ vacancies.
Phenomenological Study of Vacancies in Aluminium within Density‐Functional and Second‐Order Perturbation Formalisms
Authors: Perrot F.
Published: July 2007

The vacancy‐formation energy in aluminium is calculated up to second order in electron–ion interaction within the density‐functional formalism, where the explicit gradient‐expansion of the kinetic energy is used. The ground state for the defect corresponds to an uniform positive background with a spherical hole and an electron density profile represented by an analytical (Gaussian) form which minimizes the total energy. An entirely analytical expression is obtained for zero‐and first‐order energies. The density induced by the perturbative ionic potential (superposition of pseudopotentials minus positive background) is calculated using a linear‐response function which, in the defect, takes into account the charge depletion around the vacancy. The following conclusions are drawn: the perturbation series is convergent, the vacancy‐formation energy is very sensitive to the choice of the (local) pseudopotential.
Chemisorption on a Model Transition Metal. Coverage Dependence of the Adsorption Behaviour
Authors: Masuda K.
Published: July 2007

The adsorption both of a single atom and a monolayer of atoms on the (001) surface of the model transition metal is investigated using the Green's function formalism and the phase shift technique. The electronic structure of the surface is obtained by the Kalkstein‐Soven method. For comparison, both the single‐ and two‐peaked models of the surface density of states (DOS) are used. The adatom charge, heat of adsorption, and the change in the DOS due to chemisorption are calculated within the Newns‐Anderson model and are compared with the available experimental results as well as with those of the previous chemisorption calculations. It is shown that the two‐peaked substrate DOS model can qualitatively account for the strong coverage dependence of the photoemission spectra observed in the system such as H/W(100). The present theory is also extended to the chemisorption system with general coverages.
Lattice Dynamics, Specific Heat, and Bulk Modulus of Scandium
Authors: Rao R. Ramji , Ramanand A.
Published: July 2007

The phonon dispersion curves for scandium along the symmetry directions are obtained using the nearest‐neighbour central‐force model for h.c.p. metals proposed by Srinivasan and Ramji Rao. The theoretical curves are in good agreement with experiment, except in the case of two transverse optic branches. The calculated lattice heat capacity is in good agreement with the Cvl‐ values estimated from the experimental Cp‐data of scandium. The theoretical third‐order elastic (T.O.E.) constants of scandium are used to determine its Anderson‐Grüneisen (A.G.) parameter and the temperature dependence of the bulk modulus of scandium is calculated using Anderson's theory. The calculated change in Bs from 300 to 0 K is in accord with that estimated from the measured elastic constants of scandium.
Higher Anharmonicities in the Improved Unsymmetrized Self‐Consistent Field Approximation for a Crystal. II. Comparison of Calculations with Experiment
Authors: Zubov V. I.
Published: July 2007

By the method considered in the preceding papers the thermodynamic properties of strongly anharmonic crystals of Ar, Kr, and Xe are calculated in extensive temperature ranges. Various pair potentials are used and the three‐body interactions are taken into account. The thermal stability of strongly anharmonic crystals is discussed. The calculated thermodynamic properties of Ar, Kr, and Xe are compared with both the experimental data and Monte Carlo computations.
Study of the Ultraslow Critical Dynamics of the Order Parameter of the Ferroelectric AgNa(NO2)2 by the Spin Relaxation Times T1, T1ϱ, T2 of the 23Na Nucleus
Authors: Grossmann J. , Müller D. , Petersson J.
Published: July 2007

Measurements are made of the temperature dependences of the spin relaxation times T1, T1ϱ, and T2 of the 23Na nucleus in the uniaxial ferroelectric AgNa(NO2)2. Although none of these quantities shows an anomalous behaviour near Tc their temperature dependences can be ascribed to the dynamics of the electric dipoles which cause the ferroelectricity.
The Influence of Surface Layers on Photoreflectance in the Exciton Region of CdS
Authors: Risch L.
Published: July 2007

Changes of exciton reflectance spectra by an intense light beam (photoreflectance) are used to investigate electric fields on the free, unprepared crystal surface, originating from surface states. There are two types of photoreflectance spectra in CdS at 105 K, which are associated to different crystal properties A and B. The electric field increases in type A crystals and decreases in the case of type B crystals. This mechanism produces the main contribution to photoreflectance; the screening of the Coulomb interaction of the excitons by free electrons is negligible.
Quasi‐Molecular Model Interpretation of the Resonances in the Fundamental Absorption Region of Valence Semiconductors
Authors: Shatalov V. M.
Published: July 2007

An interpretation of the two main maxima in the valence semiconductor optical spectra and an explanation of the shape of the dielectric constant curve are suggested using the quasi‐molecular model of these crystals suggested by Tolpygo. According to this model the resonances in the continuous spectrum are due to the transitions to the metastable Frenkel exciton states. The interband transitions give rise only to the background component of the spectrum. Estimates of the parameters of the theory are given and experimental sequences of the model are discussed.
Photoemission Studies of 2p Core Levels of Pure and Heavily Doped Silicon
Authors: Eberhardt W. , Kalkoffen G. , Kunz C. , Aspnes D. , Cardona M.
Published: July 2007

The energy distribution curves of the 2p core levels of pure and heavily doped Si are investigated using highly monochromatized (≈︁ 0.1 eV) synchrotron radiation (105 to 140 eV). Shifts and broadenings of the spectra with doping and with the energy of the exciting photons are observed and interpreted in terms of the electrostatic band bending at the semiconductor surface and, for pure Si, of surface core shifts. The “absorption” spectrum of the 2p → conduction band transitions is studied for the same materials with the partial yield technique. By combining the absorption spectra and the core level EDC's a value of EB = (0.18 ± 0.2) eV for the core exciton binding energy is determined. Small differences in the 2p → conduction edge both with doping and with the energy of the exciting photons are observed and discussed.
Hopping conduction in the presence of an electromagnetic wave
Authors: Zvyagin I. P.
Published: July 2007

Kinetic equations describing the electronic transitions between localized states are generalized to take account of strong electromagnetic waves. The solution is analyzed and the amplitude and frequency dependences of the dc phonon–photon‐assisted hopping conductivity are obtained.
Hall Coefficient of Dilute Aluminum Alloys. I. Calculation by Four‐OPW Approximation
Authors: Yonemitsu K. , Takano K. , Matsuda T.
Published: July 2007

The Hall coefficients of dilute aluminum alloys are computed by 4‐OPW approximation. The Hall coefficient is found to be very sensitive to the scattering potential. By slight modification of the potential, the calculated results are fitted to the experimental values. Formulae to calculate the mean relaxation time for various parts of the Fermi surface from the measured values of the Hall coefficient and electrical resistivity are deduced.
Phonon Attenuation in n‐Type Semiconductors with ZB Structure in Presence of Internal Strains and Magnetic Field
Authors: Singh Mahiradhwaj
Published: July 2007

Expressions for the attenuation coefficient of phonons by donor electrons are obtained in presence of internal strains for n‐type semiconductors with ZB structure for the situations when besides the usual spin‐orbit splitting of the six degenerate states into a quartet and a lower lying doublet, the quartet further splits into two doublets due to internal strains present in the crystal. Similarily the expression for ultrasonic attenuation is also obtained in presence of a magnetic field for the most general case when the degeneracy of both the doublets is removed. These expressions are used in explaining the magnetic dependence of ultrasonic attenuation in Ge doped n‐type InSb. Good agreement between theory and experiment is observed.
Susceptibility Calculations on Spin‐Doped Crystals
Authors: Fakioǧlu S.
Published: July 2007

The susceptibilities of some transition elements, Fe3+ and Gd3+, in various crystals are calculated using the known crystal field parameters. These parameters are used in crystal field theory with the correlation of effective spin Hamiltonian. Two different methods, namely the density matrix formalism and van Vleck's momentum expansion, are utilized to calculate the susceptibilities. It is possible to see the effect of host crystals, and the temperature dependence of susceptibilities of such salts.
X‐Ray K‐Absorption Structure of Zirconium Metal and Its Compounds
Authors: Agarwal B. K. , Johri R. K.
Published: July 2007

The K‐absorption spectra of zirconium are studied in the following substances: Zr, ZrO2, ZrC, ZrBr4, Zr(SO4)2 · 4 H2O, Zr(NO3)4 · 5 H2O, ZrOCl2 · 8 H2O, and K2Zr(C2O4)3. The fine structure within the main edge is discussed on the basis of band structure and is in general accordance with the overlap of d‐ and sp‐bands in transition metals. The observed extended fine structure is analysed in each case to give information on the bond length.
Effect of Pressure on Hole Effective Masses in GaAs
Authors: Alekseeva Z. M. , Vyatkin A. P. , Karavaev G. F. , Krivorotov N. P.
Published: July 2007

The effect of pressure on light and heavy hole masses in GaAs is studied. A theoretical calculation of the dependence of the hole masses on pressure is carried out taking into account the influence of the lowest conduction band levels Γ and Γ. The experimental values are obtained from the change of the tunnel diode characteristics with pressure. The agreement of the calculated values with the experimental data is good for the light holes.
Thermoreflectance of PbTe
Authors: Baleva M.
Published: July 2007

Two groups of p‐type PbTe grown by direct synthesis and Bridgman methods are investigated. The thermoreflectance spectra at room and liquid nitrogen temperature are measured over the energy range from 1.0 to 4.6 eV. The spectra are found to be different for both the groups of samples. Quantitatively the results obtained are interpreted on the basis of theoretical and experimental information. The spectral positions are determined of the energy transitions, the values of dEg/dT for the different critical points and the dependence of dν/dT on the growth method and the temperature. The difference in the spectra for both the groups of crystals is explained.
Boundary Effects on the Transition into Excitonic Phase
Authors: Lozovik Yu. E. , Nishanov V. N. , Kaminskaya E. P.
Published: July 2007

The boundary influence on the transitions of semiconductors or semimetals into the excitonic phase is considered. Proximity effects lead to a decreasing of the dielectric gap in the excitonic phase near the boundary with vacuum or an insulator. In the case when two contacting materials have different critical transition temperatures (T and T) in the excitonic phase, the sample with the low critical temperature becomes insulator for temperature T > T > T. On taking into account the hybridization and interband transition the Ginzburg‐Landau equation for the effective order parameter describing the semimetal–“excitonic insulator”; phase transition is derived. The equation describing the proximity effects in the transition semiconductor‐“excitonic insulator” is equivalent to the equation for Wannier‐Mott excitons near the boundary.
Reaction Rate Constants of Triplet Excitons in Naphthalene Single Crystals
Authors: Asami H. , Nakayama T. , Chong T. , Itoh N.
Published: July 2007

The temperature dependence of the decay of the delayed fluorescence in a naphthalene crystal doped with β‐chloronaphthalene is measured. The results are analyzed in terms of the reaction kinetics comprising a term for the collision of a free exciton with an exciton trapped by β‐chloronaphthalene (X‐trap). The reaction rate constant between the free and trapped excitons is obtained and found to be larger than the reaction rate constants of the triplet exciton with another triplet exciton, a radical, and a guest anthracene molecule in naphthalene crystals. Assuming that the reaction rate constant between the free and trapped excitons is the value at the diffusion limit, the transition probabilities from the neighbouring pair of an exciton and an energy acceptor to the excited state of the energy acceptor are obtained and compared with the results of the analysis using Dexter's theory.
Pressure Dependence of the Electron—Phonon Coupling in Lanthanum
Authors: Glocker R. , Fritsche L.
Published: July 2007

On the basis of self‐consistent band calculations for f.c.c. lanthanum the first moment of the Eliashberg function and its dependence on hydrostatic pressure is evaluated. The formula of Allen and Dynes is used to connect it to the critical temperature of superconductivity, Tc, and the mean phonon frequency Ω. On adjusting Ω for each pressure p such that Allen and Dynes' value of Tc coincides with the respective experimental transition temperature, it is concluded that Ω is lowered with increasing pressure up to 35 kbar reflecting a phonon softening under pressure. Following an ensuing pressure interval of relatively little variation of Ω(p) there is a marked upward bending beyond 53 kbar suggesting restored lattice stability after a passage through (possibly isostructural) phase transitions at 35 and 53 kbar. The results substantiate identical conclusions of Wittig and collaborators drawn from pressure experiments on superconducting lanthanum.
Electron Spin Resonance of Mobile Electrons in AgCl
Authors: Eachus R. S. , Graves R. E. , Olm M. T.
Published: July 2007

Divalent cationic impurities such as Cd2+ and Pb2+ and low levels of Br− ions extend the photo‐electron lifetime in silver chloride. The free carrier is observed by conventional ESR in the doped solid, and a free‐electron g‐value of 1.8775 ± 0.0005 is obtained. A careful correlation of kinetic data obtained by ESR and the ac microwave photoconductivity technique leads to a reasonable mechanism to explain the photoinduced decomposition of the doped material.
Binding of an Exciton to a Neutral Acceptor
Authors: Stébé B. , Munschy G.
Published: July 2007

The binding energy of the exciton—neutral acceptor complex is calculated by means of an atomic‐like variational wave function, suitable for not too large electron‐to‐hole mass ratios σ = m/m. The results obtained with a 46‐term trial function do not disagree with previous deductions stating that there is a σ‐region of instability. Further, the empiric “Haynes rule” is not well verified in the limited region (σ ≦ 0.431), where the stability can be achieved by means of our trial function. The computed energies are in good agreement with the experimental data relative to GaAs.
A Note on the Polarization Dependence of the Two‐Photon Absorption Coefficient in Crystals. Extension to Circularly Polarized Light
Authors: Doni E. , Girlanda R. , Parravicini G. Pastori
Published: July 2007

The two‐photon absorption coefficient in crystals is discussed for the case, where one or both exciting beams are circularly polarized. The procedure previously used for linearly polarized light is extended to include circularly polarized photons, and also in this case the absorption coefficient is factorized into dynamical factors containing matrix elements of the momentum operator, and into geometrical factors which depend on the polarization directions of the linearly polarized beams, and on the propagation vectors of the circularly polarized beams. Detailed tables of the geometrical factors are given in the case of cubic crystals. For crystals with tetragonal, trigonal, and hexagonal symmetry, the geometrical factors are given in the case of light propagating along the anisotropy axis. The dynamical selection rules are also reported. It is indicated how the two‐photon spectroscopy, using circularly polarized photons too, is able to lead to a complete experimental determination of the symmetry properties of the excited states in crystals.
X‐Ray Emission Spectrum of Metallic Lithium
Authors: Mgbenu E. N. , Maduemezia A.
Published: July 2007

Using improved wave functions, the X‐ray K‐emission spectrum of metallic lithium is computed in the one‐electron band approximation. An adaptation of the multi‐Lagrangian formula is employed in the interpolation required in the density of states evaluation. The shape of the transition probability curve and the gradient of the intensity spectrum at the Fermi energy are investigated in detail. The conclusion is that self‐consistency in the wave functions has a dramatic effect on the X‐ray transition probability. Nozières and de Dominicis' singularity is applied at the threshold to obtain agreement between the experimental and computed spectra. It is found that the value of the cut‐off energy ξ0 required for correspondence between the two spectra in the threshold region, is of the order of the Fermi energy. This confirms predictions obtained analytically.
Non‐Linear Ultrasound Absorption in an Electromagnetic Wave Field under Quantum Size Effect Conditions
Authors: Arakelyan M. M.
Published: July 2007

Non‐linear effects in dimensionally quantized films are considered at amplification of hypersound waves in an electromagnetic wave field with frequency Ω under the condition ql ≫ (q'quasimomentum of a phonon, l free‐path length of an electron). Strong resonance singularities in the sound absorption are observed under the condition qvov ∼ ω (v = 1, 2, 3, …, V0v Fermi velocity of the electron of the vth subband of the film, ω sound wave frequency). The momentum distribution function of electrons is distored. Under certain conditions the absorption is reversed to amplification.
Ground State Energy of Bound Polarons
Authors: Lépine Y. , Badder G. , Matz D.
Published: July 2007

The properties of the ground state of the bound large polaron within the Fock approximation are investigated by using a new model spectrum which is known to describe better the polaron at low external fields. The results are compared with those of Matsuura who used the same approximation, obtained from a path integral formulation, with the usual Gaussian and Coulombic trial spectra, Both formulations are variational. It is found that the new spectrum gives better results (lower variational energies) than those of Matsuura for intermediate or large values of electron–phonon coupling and small values of the electrostatic coupling of the electron to the defect. This is related to that fact that the polaron seems to prefer a cylindrical symmetry to a spherical one in the absence of external fields.
Binding Energies of Positron to F and F′ Colour Centres in Alkali Halides
Authors: Lam C. S. , Varshni Y. P.
Published: July 2007

The binding energy of a positron to a F centre is calculated for twenty alkali halides, and that to a F′ centre for nine alkali halides.
Optical Investigation of Atomic Hydrogen Centres in Alkali Chlorides Doped with I− or Br−
Authors: Lohse F. , Reuter G. , Spaeth J. M.
Published: July 2007

In alkali chlorides doped with I− or Br− atomic hydrogen can be trapped in an interstitial configuration where one of the four Cl− neighbours is replaced by I− or Br−. These H‐centres in “mixed configuration” have several absorption bands in the near UV region. The optical bands are due to charge transfer transitions which is shown by magnetooptical experiments as well as by a simple theoretical model.
Exact Derivation of a Generalized Master Equation for the Motion of Excitons
Authors: Kühne R. , Reineker P.
Published: July 2007

Starting from the Nakajima‐Zwanzig equation, by an exact evaluation of its kernel, a generalized master equation, describing the coherent motion of excitons is derived. The time dependence of the kernel is discussed in detail. From this coupled set of integro‐differential equations a set of second order differential equations is obtained. In the continuum approximation, this set results in a partial differential equation in space and time variables, which only for large times assumes the form of a wave equation. Explicit solutions of the various equations of motion are presented and discussed.
Residual Resistivity of Disordered Systems
Authors: Richter J. , Schiller W.
Published: July 2007

By using the method of force‐force correlation functions the residual resistivity is calculated for disordered systems in second order of the fluctuating quantities. Going in this order beyond the single‐site character of usual CPA calculations diagonal disorder as well as off‐diagonal disorder, fluctuations of atomic distances, and coupling between the several disorder fluctuations are taken into account. In the weak scattering limit the distance fluctuations yield no direct contributions to the resistivity. In general, the off‐diagonal disorder gives rise to an important term in the residual resistivity and may cause a deviation from Nordheim's rule for binary disordered crystalline alloys.
Dispersion of Dielectric Axes in Monoclinic and Triclinic Crystals Related to Lattice Dynamics
Authors: Merten L. , Claus R.
Published: July 2007

It is shown that the infrared orientational frequency dispersion of dielectric axes in monoclinic and triclinic crystals is determined by those polartion wave vector directions in which purely transverse modes exist. Simultaneously the wave vector directions of purely longitudinal phonons are obtained.
On the Dielectric Function of the Hubbard Model
Authors: Gasser W.
Published: July 2007

The dielectric function of the Hubbard model is discussed in the strong correlation limit. It is shown that there are in general two zero points of the real part of the dielectric function outside the absorption region which correspond to different types of charge density oscillations. In the zero frequency limit an expression for the dielectric function is derived which allows the explanation of the screening of the potential of a donor ion in a narrow energy band system.