bluuebunny/crossref_metadata_2025 huggingface dataset:
The one‐ and two‐photon absorption due to surface state excitons
Authors: Bobrysheva A. I. , Beryl S. I. , Zyukov V. T. , Moskalenko S. A. , Pokatylov E. P.
Published: July 2007

The oscillator strengths are calculated for one‐ and two‐photon absorption by surface state Wannier excitons. The model of two surface bands is used, and excitons and electron‐hole pairs act as intermediate states. The cases of direct allowed and forbidden dipole transitions are investigated. Numerical calculations of the ratios of two‐ and one‐photon oscillator strengths are performed for (100) GaAs.
Induced polarization charge density around an electron in a polar semiconductor
Authors: Peeters F. M. , Devreese J. T.
Published: July 2007

A calculation is presented of the polarization charge density induced by an electron in a polar semiconductor subjected to a magnetic field. The result, which is derived for the anisotropic Feynman polaron model studied earlier, is expected to be applicable for arbitrary values of the electron—phonon coupling strength (α), temperature (T), and magnetic field strength (ℋ︁).
Approximate wave functions for the F+‐centers in LiNbO3 and LiTaO3
Authors: Ho Fat Duen
Published: July 2007

The improved hydrogen‐like wave function is used as a trial wave function of the 1s‐state to calculate the energy levels of F+‐centers in lithium niobate and lithium tantalate. The point‐ion model is employed. The Schrödinger equation is solved variationally. The transition energies agree within 6.4% and 13.4% with the observed peaks in the absorption spectra for LiNbO3 and LiTaO3, respectively. In comparison with previous work along this line the results obtained here are much better in the case of LiTaO3, and are rather compatible in the case of LiNbO3. Furthermore, in order to save computer time various groups of ions out of the “lattice” of 810 ions are examinated to find the minimum numbers of ions needed to guarantee an essential physics of the systems. The advantages of using this more accurate one‐parameter wave function are discussed.
Penn‐like model calculations application to hydrogenated amorphous silicon films prepared by glow discharge
Authors: Ravindra N. M. , Ance C. , Ferraton J. P. , Donnadieu A. , Robin S.
Published: July 2007

Recent model calculations are studied of Ance et al. concerning the valence electron density and hydrogen concentration in hydrogenated amorphous silicon films prepared by glow discharge decomposition of silane. This study is essentially necessitated by the fact that the model of Ance et al. yields values of Penn gap which are fairly large when compared to energies corresponding to peaks in the spectra of the imaginary part of the dielectric constant. Also a method is proposed to evaluate the density of the films. The densities hence determined are shown to be in accord with available experimental data. In the passing, results of the electronic polarizability of these films are also reported.
Study of energy band parameters in p‐type Ge0.9Si0.1 alloys
Authors: Takeda K. , Maeda Y. , Ohta E. , Sakata M. , Kido G. , Miura N.
Published: July 2007

The electronic properties of Ge0.9Si0.1 are investigated by cyclotron resonance, optical absorption, and galvanomagnetic measurements. The cyclotron effective masses are determined for light and heavy holes and interpreted using the band structure theories of group IV semiconductors including the subband interaction. The temperature dependence of the mobility of holes is obtained both by Hall effect and cyclotron resonance. The optical phonon energy involved in the indirect transition is also obtained by intrinsic absorption spectra.
Intrinsic recombination in dependence on doping concentration and excitation level application to lead chalcogenides
Authors: Mocker M. , Ziep O.
Published: July 2007

On the base of anisotropic and parabolic band structure recombination rates are calculated for the mixed crystals Pb0.78Sn0.22Te and PbS0.1Se0.9 in dependence on non‐equilibrium carrier concentration δn up to 1020 cm−3 in a wide range of temperatures and doping levels. Degeneracy causes different changes of lifetimes or recombination rates for material with different activation energies of the recombination process. Both, weakening or strengthening of the known n2p‐law of the Auger transition rate can occur at beginning degeneracy. At δn > 1019 cm−3 always sublinear behaviour should be observed. For radiative recombination a weakening of the np‐law is obtained generally; in the limit δn > 1019 cm−3 the rate behaves like (δn)4/3.
Study of thermophysical and anharmonic properties of fluorite compounds
Authors: Singh R. K. , Pandey N. K.
Published: July 2007

An extensive study is made of thermophysical and anharmonic properties of fluorite compounds using an interionic potential, which consists of a long‐range Coulomb and three‐body interactions and the short‐range overlap repulsion and van der Waals attraction. The agreement achieved between experimental and the theoretical results on third‐order elastic constants and pressure derivatives of second order elastic constants are generally better than those obtained by others. This potential succeeds in predicting various thermophysical properties, like compressibility and its pressure and temperature derivatives, thermal expansion and Grüneisen parameters of seven crystals of fluorite structure.
Role of the electron scattering processes in the Hubbard model in the insulator‐to‐metal transition
Authors: Kalra M. L.
Published: July 2007

The role of the spin disorder scattering and dynamical processes in the Hubbard model in the insulator‐to‐metal transition is investigated at finite temperatures. An explicit expression for the dc conductivity is obtained by studying the two particle double time retarded Green function by the equation of motion method. Frequency and temperature dependence of the conductivity is discussed in detail. It is found that the system exhibits a continuous insulator‐to‐metal transition with increasing temperature. The conductivity changes by a considerable factor around transition temperature. The results obtained can be applied to explain the electrical properties of transition metal oxides and other compounds which show the continuous insulator‐to‐metal transition.
Absorption, fluorescence, and excitation spectra of thallium‐activated rubidium bromide
Authors: Sharan V. B.
Published: July 2007

Absorption, excitation, and fluorescence spectra of polycrystalline RbBr: Tl phosphors are presented at various thallium concentrations. For very low thallium content two absorption bands are obtained peaking at 224 and 260 nm. Additional absorption bands appear around 230, 244 267, and 278 nm with rise of thallium content. An asymmetric fluorescence band in the ultraviolet region is obtained at low thallium concentration which can be resolved into two Gaussian emission bands peaking at 317 and 365 nm. Two additional visible fluorescence bands appear at 440 and 535 nm with rise of thallium content. The excitation spectra for the ultraviolet and visible emission bands are found to be different. Accordingly the ultraviolet emission band is attributed to the characteristic A‐emission in Tl+ monomer entering substitutionally and the visible bands are attributed to dimer centers having D4h site symmetry.
Study of electronic structure of reduced TiO2 and VxTi1−xO2 crystals by ESCA and optical absorption
Authors: Sakata K.
Published: July 2007

An analysis is made of optical absorption spectra and electron spectroscopy for chemical analysis (ESCA) on pure TiO2, reduced TiO2, and VxTi1−xO2. It is found that in the reduced TiO2, the Ti ion takes the variable valence states, Ti4+, Ti3+, and Ti2+, and they form a broad donor band, which overlaps with the conduction band originating from Ti4s levels. 3d–4s mixing results in a large change of the conductivity of TiO2 from insulator to metal depending upon the degree of reduction. In TiO2 doped with V, which behaves like a free ion, the excess 3d electrons of the V ions form impurity level just above the O2p valence band. In the reduced VxTi1−xO2, four different valence states, Ti4+, Ti3+, Ti2+, and V4+, are found and this accounts for the narrow optical gap of this material.
On the elastic anisotropy of ferroelectric triglycine selenate
Authors: Khanna S. K.
Published: July 2007

The numerical values of the elastic constants C11, C22, C33, C15, C35, C55 are determined experimentally by using an acousto‐optical method. Results are reported of the anisotropy of the velocity of 10 MHz ultrasonic waves in the direct vicinity of the phase transition temperature. It is observed that in the ferroelectric phase the velocity rises with a rotation of the sound wave vector by 2 to 4° with respect to the z‐axis in the Y–Z plane. On the other hand these critical anomalies of velocity are found to follow an identical logarithmic behaviour in the paraelectric phase. The results are discussed on the basis of Landau theory but at the same time taking into account the long range dipole–dipole interactions as well as the fluctuations of the order parameter. The correlation parameter is estimated from the temperature dependence of the elastic coefficient in the paraelectric phase (δ = 0.2 × 10−16 cm2).
Generalized pseudoatom model of inhomogeneous systems and its application to a simple‐metal surface
Authors: Streitenberger P.
Published: July 2007

General expressions in real‐space representation are derived for charge density, total energy, and Hellmann‐Feynman forces on an ion. They represent a uniform and systematic extension of the pseudoatom or pair‐potential model to highly inhomogeneous electron–ion systems. Modifications appear in form of non‐central pair potentials and additional one‐body potentials arising from the screening in the inhomogeneous electron gas. The corresponding self‐consistent charge density results from the superposition of overlapping non‐spherical pseudoatoms and an additional charge density that is mainly localized in the defect region. A first application to the simple‐metal surface is given where some general results within the jellium model are obtained.
Reaction field approximation for inhomogeneous ferromagnets
Authors: Wojtczak L. , Balcerzak T.
Published: July 2007

The reaction field approximation method for inhomogeneous magnetic systems is extended. The Curie temperature of inhomogeneous samples is calculated on the basis of the perturbation theory. As a result the shift of the Curie temperature is obtained with respect to the value of the Curie temperature for an effective crystalline structure. A comparison of the results with those following from other methods shows that the accuracy of the presented method is the same as the cluster method and is much better than the molecular field approximation.
Determination of the correlation between the crystal field axis system and the crystallographic axes in chromium‐doped β‐Ga2O3 by EPR
Authors: Gunsser W. , Rohwer K.
Published: July 2007

The correct relationship is found between the crystal field axes and crystallographic axes in β‐Ga2O3 by application of a simplified orientation method. A computer program is set up, which is able to calculate and to plot the angular variation of the resonance field strengths and of the energy levels involved in EPR. In this way, the strange angular dependence of the EPR lines can be explained and the correctness of the stated correlation between the two axis systems can be verified. From the measured resonance fields a set of spin Hamiltonian parameters is calculated.
Exciton–phonon interaction in crystalline and vitreous SiO2
Authors: Godmanis I. T. , Trukhin A. N. , Hübner K.
Published: July 2007

An investigation of the Urbach rule in crystalline SiO2 (α‐quartz), crystalline SiO2Ge, and vitreous SiO2 in the framework of the models of Dow and Redfield and of Toyozawa shows that the fundamental optical absorption edge of SiO2 is determined by strong exciton–phonon interactions. In the crystalline and vitreous SiO2 low‐energy excitons with E = 9.1 eV (α‐quartz) and E = 8.7 eV (glass) interact mainly with longitudinal optical phonons. In the crystalline SiO2Ge alloy the interaction with phonons is realized by excitons localized on germanium impurities. The exciton–phonon interaction in SiO2Ge and vitreous SiO2 is stronger than in crystalline SiO2 due to the higher degree of localization of low‐energy excitons in these disturbed SiO2 forms. The strong exciton–phonon interaction leads to a momentary self‐trapping of the low‐energy excitons in crystalline and non‐crystalline SiO2 and to a corresponding relaxation of the SiO2Ge excitons localized on Ge impurities. Numerical values of the fundamental exciton parameters are deduced and discussed.
Low‐frequency secondary radiation in polar semiconductors at finite temperature
Authors: Comas F. , Giner C. Trallero
Published: July 2007

Secondary radiation in polar direct–gap semiconductors is considered for finite, but not too large, temperatures. For sufficiently pure and perfect samples, photoexcited electrons (holes) relax via phonon scattering. Assuming an infinite recombination time, the differential cross‐section for photon emission is calculated in the low‐frequency region of the spectra. Rather intense secondary radiation is predicted for high temperatures. The general shape of emission and excitation spectra is discussed in detail.
Investigation of asymmetrical grain boundaries and their coincidence site lattices
Authors: An Nguyen , Herrmann R. , Worm G.
Published: July 2007

The subject of this paper is a method for the examination of asymmetrical grain boundaries with randomly directed axes of rotation. In so far as the axis of rotation lies in a plane of symmetry of the crystal the geometrical relation between symmetrical and asymmetrical grain boundaries is shown. The CSL Bravais cells corresponding to asymmetrical grain boundaries and the facet angles for the considered grain boundaries are calculated.
Decay behaviour of high‐excitation luminescence in GaP:N. I. Model for the decay of two coexisting phases
Authors: Weinert H.
Published: July 2007

The time behaviour of the radiative recombination of nitrogen‐doped GaP is investigated at pulsed intense optical excitations and low temperatures (T = 2 and 27 K). The decay behaviour of the luminescence intensities corresponding to the electron–hole liquid (EHL) and the coexisting lowdensity phase (LDP) is found for all nitrogen concentrations (nN = 2 × 1017 to 2.9 × 1018 cm−3) to be nonexponential. It is interpreted as a consequence of interactions between both coexisting phases. A model is introduced where the EHL is fed by the LDP continuously. It describes the observed decay of the luminescence very well, even for different nitrogen concentrations and different excitation intensities.
Theory of the verwey transition in the mixed valence compound Eu3S4
Authors: Ihle D. , Lorenz B.
Published: July 2007

The Verwey transition in Eu3S4 at TV = 186 K is explained on the basis of a model which takes into account the Coulomb interaction of the extra 4f electrons at the Eu2+ ions and the formation of small polarons due to the interaction with phonons. This model is treated in the Hartree and cluster variation approximations. At TV a first‐order Verwey transition is obtained. The magnitude of the observed entropy change at TV is explained. Above TV the polarons are found to be considerably short‐range ordered.
The electrical resistivity of short‐range ordered alloys and its temperature dependence
Authors: Richter J. , Schubert G.
Published: July 2007

The electrical resistivity R of short‐range ordered binary alloys with b.c.c. structure is calculated using the force‐force correlation function scheme in the weak‐scattering limit. For the description of short‐range order for temperatures above the critical temperature the theory of Clapp and Moss is applied yielding the Fourier transform of the Cowley short‐range order parameters in dependence on the interatomic potential, the concentration and the temperature. In AB ordering alloys the short‐range order causes a decrease of the resistivity R in comparison with the totally stochastic alloy and a positive temperature coefficient (1/R) dR/dT. In clustering alloys the atomic short‐range order increases the resistivity and yields negative (1/R) dR/dT. This qualitative relation between R and (1/R) dR/dT is in agreement with Mooij's rule. Furthermore the short‐range ordering causes characteristic deviations from the Nordheim behaviour.
On the role of the soliton mechanism in the electron transfer along the quasi‐one‐dimensional molecular structures
Authors: Satarić M. , Žakula R.
Published: July 2007

An expression for the probability of electron transfer from a donor molecule (D) to an acceptor molecule (A) along a quasi‐linear molecular chain (biopolymer), to which D and A are attached is derived. Investigated is the case with an indirect interaction caused by modification of Coulomb interaction due to electron transition from D to A and vice versa via a periodical structure, in addition to the direct, resonant interaction of D and A. The indirect interaction may lead to the electron capture by a soliton wave. The one‐way transfer of an electron caused by relaxation phenomena for the dynamical system in contact with a thermostat (T = 0 K) is discussed.
Raman scattering in GeS single crystals at low temperatures
Authors: Vorlíček V. , Gregora I. , Chvostová D.
Published: July 2007

The Raman spectra of GeS single crystals are investigated in the temperature interval 20 to 300 K. The possible origin of new features observed at low temperatures is discussed. A new assignment of the off‐diagonal B1g and B3g modes is proposed.
Optical properties of K2SeO4 in the vicinity of phase transitions
Authors: Pačesová S. , Březina B. , Jastrabík L.
Published: July 2007

The temperature dependence of the absorption edge of K2SeO4 is measured in the range 300 to 4.2 K. In this temperature range the dependence of the absorption coefficient on the energy of photons and temperature is described by Urbach's rule. The magnitudes of the band gap obtained are E∥ = 6.06 and E⟂ = 6.14 eV for light polarized parallel and perpendicular to the a‐axis. The temperature gradient of the band gap (∂E/∂T)∥, ⟂ reveals a jump at the paraelectric‐incommensurate phase transition T2 = 129.5 K for both polarizations of incident light in agreement with the second‐order nature of the phase transition. In the paraelectric phase, phonons with eigenfrequencies ω∥ = 218 cm−1 and ω⟂ = 181 cm−1 are interacting with the optical transitions. A switch‐over to the optical phonon of frequency ω∥,⟂ = 92 cm−1 is observed at T2 together with the change of the electron‐optical‐phonon coupling constants indicating the increase of the ionic character of the bonds in the incommensurate phase between the K+ ions and the (SeO4)2− tetrahedral groups.
On the two‐site spinless Falicov‐Kimball model with hybridization
Authors: Zieliński J. , Kowalska J.
Published: July 2007

The exactly solvable two‐site extended Falicov‐Kimball model is considered. It is shown that the increase of hopping energy lowers the tendency to an abrupt change of the average occupation number of f‐electrons. Some thermodynamic quantities are evaluated. The results are used for the alloy analogy coherent potential approximation treatment of the periodic model.
Far ultraviolet photoelectric study of thin SnSe evaporated films
Authors: Bennouna A. , Tessier P. Y. , Priol M. , Dang Tran Q. , Robin S.
Published: July 2007

Energy distribution curves of photoemitted electrons are obtained for SnSe thin polycrystalline films in the 10 to 40 eV energy range. The method of preparation of the sample under ultrahigh vacuum conditions is described and the synthesis of the initial material. The quality of the sample is checked by Auger analysis which reveals a weak reactivity of the material. The EDC's show structures appearing at 0.9, 2.1 to 2.7, 3.9, 7.4 eV below the top of the valence band. Also the Sn 4d levels are investigated. The 4d(5/2) is found at 24.3 eV and the 4d(3/2) at 25.3 eV below the top of the valence band. Structures in the EDC's are compared with those of existing similar works and are interpreted by means of theoretical bands scheme.
Screening of electrostatic fields in crystalline semiconductors by electrons hopping over defects
Authors: Poklonski N. A. , Stelmakh V. F.
Published: July 2007

The expression for the electrostatic field screening length is obtained and it is shown that the effective concentration of electrons hopping via defects is included both into the screening length and electrical conductivity. The quotient of the diffusion coefficient to the drift mobility for hopping electrons is found.
Self‐consistent embedded‐cluster calculations of the electronic structure of alkaline earth fluorides in the hartree‐fock‐slater approximation
Authors: Amaral N. C. , Maffeo B. , Guenzburger Diana
Published: July 2007

Molecular orbital calculations are performed for clusters representing the CaF2, SrF2, and BaF2 ionic crystals. The discrete variational method is employed, with the Xα approximation for the exchange interaction; a detailed investigation of different models for embedding the clusters in the solids led to a realistic description of the effect of the neighbour ions in the infinite crystal. The results obtained are used to interpret optical and photoelectron data reported in the literature. In the case of CaF2, comparisons are made with existing band structure calculations.
Picosecond reflectivity measurements on glow discharge a‐Si: H
Authors: Bergner H. , Brückner V. , Dietrich D. , Kerstan F. , Nowick W.
Published: July 2007

The temporal behaviour of photoinduced charge carriers in amorphous hydrogenated silicon films is studied by means of time‐resolved reflectivity measurements using a Nd:YAG laser. The investigated a‐Si:H films, deposited by the glow discharge technique under systematic variation of the silane—hydrogen pressure, show different structures and defects. The temporal behaviour of the non‐equilibrium charge carriers of all samples is determined by a two‐step relaxation process. In accordance with the model the photoexcited carriers relax from the conduction band to an intermediate state in the mobility gap and than to the ground state. The characteristic times of these relaxation processes are 30 to 85 ps and 0.75 to 2.5 ns, respectively. The influence of the structure of the a‐Si:H films on the relaxation times is discussed.
Model for optical absorption in transition metals containing d impurities
Authors: Parlebas J. C.
Published: July 2007

The effect of a ‘d’ impurity is investigated on the contribution to the optical absorption in a transition metal from electronic intra‐ and interband transitions. Within a tight‐binding approximation a previous calculation of the imaginary part ϵ2(ω) of the dielectric constant in simple models is extended to realistic multiband dilute alloys. The possibility is shown of exploring critical points in nickel host band structure, for example, by wave‐vector non‐conserving transitions induced by impurities, in relation with recent experimental data.
Amorphous problem in exafs data analysis
Authors: Ageev A. L. , Babanov Yu. A. , Vasin V. V. , Ershov N. V. , Serikov A. V.
Published: July 2007

It is shown that the radial atomic distribution function for disordered systems can, in principle, be determined from the oscillating extended fine structure of X‐ray absorption spectra using the regular algorithms of solution of incorrect problems. The radial atomic distribution function for Fe80B20, obtained from experimental EXAFS data and corresponding to the results of diffraction studies, is presented.
Polarized light absorption of the magnetic semiconductor CdCr2Se4
Authors: Edelman I. S. , Dustmuradov G. , Kononov V. P.
Published: July 2007

The absorption of polarized light in CdCr2Se4 thin films in the spectral region 10000 to 22000 cm−1 at different temperatures is investigated. Through the whole spectral region the absorption intensity differs for right‐handed and left‐handed circularly polarized light or for parallel and perpendicular linearly polarized light. The character of the temperature and polarization dependence of the absorption intensity is different for different parts of the spectrum. The observed absorption is considered as a sum of the interband transitions and the transitions between localized states.
Orthoferrite Dynamical Properties in the Vicinity of the Tricritical Point
Authors: Tarasenko S. V. , Medvedovskaya O. G.
Published: July 2007

The peculiarities are investigated of the behaviour characteristic of the components of the high‐frequency magnetic susceptibility tensor of the electronic spin subsystem considering and not considering the nuclear spin subsystem and also the amplification coefficient of the nuclear magnetic resonance and the shape of the curves of the resonance absorption of the electromagnetic energy in the vicinity of the NMR frequency.
Influence of Interaction Range on Computed Peierls Stress and Dislocation Displacement in B.C.C. Lithium
Authors: Beauchamp P. , Prevost J.
Published: July 2007

The simple form of interaction between [111] rows deduced from pseudo‐potential theory is used to compute the core structure, the Peierls stress and the glide plane of a a/2 [111] screw dislocation at 0 K in b.c.c. lithium. Particular attention is given to the influence of the cut‐off radius of the row interaction. It is found that the unstressed core structure and the stresses producing a movement of the dislocation are only weakly dependent on the cut‐off radius. The glide plane is either {112} twinning or {112} antitwinning depending on the range of row interaction so that one cannot forecast what it could be if interactions are taken into account completely.
The Magneto‐Optical Effect Under a Strong Electric Field in Semiconductors
Authors: Stramska H.
Published: July 2007

A magneto‐optical effect theory based on the nonlinear Boltzmann equation is derived in the presence of a strong dc electric field. The addition of a phenomenological energy relaxation term to the Boltzmann equation provides analytical expressions for the magnetoconductivity in the limit of a quadratic deviation from Ohm's law. The current density is derived for a strongly degenerate electron gas, with an energy dependence of the momentum relaxation time. The Faraday rotation and ellipticity in a strong dc electric field are calculated.
Point Defect Aggregates in γ‐Irradiated LiF Single Crystals
Authors: Frugoli P. A. , Pimentel C. A.
Published: July 2007

Diffuse X‐ray scattering near the Bragg reflection and Bragg line profile analyses are made in γ‐irradiated LiF single crystals using a double crystal X‐ray diffractometer. A preferential alteration in the profile parameters is noted. Clusters with mean parameter sizes from hundreds to thousands of angstroms are observed but each sample presents a set of average size values. The nature of the clusters is found to depend on the γ‐dose: vacancies at low dose (≈︁ 10 Mrd) and interstitials at high dose (≈︁ 50 Mrd). Some process of coalescence at 50 Mrd seems to occur.
On the Influence of Tunneling States on the Transition Temperature of Metallic Glassy Superconductors
Authors: Drechsler S. L. , Vujičič G. M.
Published: July 2007

Estimates for the transition temperature Tc of amorphous superconductors taking into account both electron–phonon and electron–two‐level system (TLS) interactions are obtained. Using experimental data for metallic glasses like PdZr it is found, that the electron–TLS interaction gives quite an observable effect in the Tc‐increase.
Bistability of the Electron‐Hole Plasma Density in a Strong Electromagnetic Field
Authors: Harbich Th. , Mahler G.
Published: July 2007

Differing from the usual treatment of an electron‐hole plasma in quasi‐thermal equilibrium, where the particle density is kept constant, a plasma is considered with a fixed chemical potential. Such a situation is expected to be realistic for semiconductors with a direct gap in the presence of a strong monochromatic electromagnetic field. The appearance of a bistability in the plasma density is predicted for photon energies in the vicinity of the gap.
Investigation of the Electronic Structure of TihX and Ti1−Y VYHX by Positron Annihilation
Authors: Dębowska E.
Published: July 2007

The annihilation parameter changes as function of x and y are studied for TiHx and Ti1−yVyHx systems. The interpretation of the results is based upon s‐ and d‐like electron number changes. The conclusions, being in agreement with the band structure calculation results, confirm the uselessness of the rigid band approximation. The Jahn‐Teller effect, suggested by other authors as the γ → δ transition mechanism, finds its justification in the light of the present results, too. Especially interesting seems to be the way in which s‐ and d‐bands are occupied for different x in the cubic phase.
Hyperfine Field Investigations in Some Ru and Rh Based Heusler Alloys
Authors: Patil V. S. , Pillay R. G. , Tandon P. N. , Devare H. G.
Published: July 2007

The hyperfine fields at 57Fe and at 119Sn doped as a dilute impurity replacing Ge, in the new Heusler alloys Ru2FeGe and Rh2FeGe, are reported. The Mössbauer spectra at 119Sn indicate preferential RGe (R = Ru, Rh) disorder like in other Ru and Rh based Heusler alloys. The hyperfine field value is consistent with recent predictions based on the Blandin and Campbell model.
Optical Orientation and Polarized Luminescence in Silicon
Authors: Efanov A. V. , Entin M. V.
Published: July 2007

The polarization properties of the recombinative photoluminescence of free electrons excited to the conduction band of a cubic multivalley semiconductor by linearly and circularly polarized light are studied theoretically. The electron transitions from the valence band and impurities to the valleys of the conduction band are considered. The phonon‐assisted and phononless indirect transitions are taken into account. Unlike the case of semiconductors with direct interband transitions polarized light is emitted not only by hot electrons but also by cold ones, and by excitons.
Theory of Free–Carrier Absorption in n‐Type Lead Chalcogenides Due to Electron‐Electron Scattering
Authors: van Huong Nguyen
Published: July 2007

The theory of the infrared absorption by free carriers in lead chalcogenides of n‐type due to electron‐electron (e–e) interaction is discussed within the model of multivalley, nonspherical, and parabolic conduction band. Only the e‐e scattering of electrons of different valleys contributes to the absorption. A formula for the absorption coefficient for an arbitrary anisotropy of the valleys is derived.
Analyse du spectre d'absorption infrarouge de BaF2: Y3+
Authors: Jeandel G. , Warin A. , Morlot G.
Published: July 2007

Le formalisme des fonctions de Green permet de déterminer les modes résonnants induits par la présence d'un cation de substitution dans le réseau du fluorure de baryum. Cette théorie nous a permis d'interpréter la différence observée entre nos spectres d'absorption infrarouge de BaF2:Y3+ et ceux de BaF2 activé par des ions de terres rares.
On the Kinetics of a Paramagnetic Oxygen Centre in CaF2
Authors: Fuchs H. J. , Karthe W. , Börner S.
Published: July 2007

In calcium fluoride crystals, doped with oxygen, centres of the VK type are generated at T = 77 K by X‐irradiation. The VK type centres recombine at T ≈ 150 K and paramagnetic oxygen fluorine‐vacancy centres (O–FV)− originate in the same concentration due to hole transfer whereas X‐irradiation at T = 300 K generates the (O–FV)− centres directly. The generation kinetics and optical and thermal bleaching of the centres are investigated. The activation energy of thermal bleaching EA ≈ 0.6 V indicates a fluorine‐vacancy mechanism. The oxygen diffusion in undoped CaF2 is studied by means of the (O–FV)− centres. The activation energy of the diffusion amounts to E = (1.8 ± 0.2) eV, the diffusion constant D0 depends on the crystal lattice defects.
Surface‐Ripple Brillouin Scattering from a Glass–Metallic Liquid Interface
Authors: Albuquerque E. L.
Published: July 2007

A theoretical calculation is presented for the Brillouin lineshapes of a glass‐metallic liquid interface. The theory takes into account only the surface‐ripple mechanism for the cross section since it is the dominant mechanism for opaque materials. In the Brillouin spectrum the contribution is shown from the surface acoustic wave of the Stoneley type. A comparison between the theoretical results and the experimental spectrum is also done and they are in a good agreement.
Density Functional Approach for Metals. The Matheiss Prescription
Authors: Ghosh A. K. , Chakraborty S. , Manna A.
Published: July 2007

Charge densities of free atoms modify to a great extent when they form metallic systems. This modification is examined in this paper in the light of Löwdin's α‐expansion formalism as adopted by Matheiss in his muffin‐tin potential calculation. Wigner‐Seitz radius and cohesive energy of copper are calculated through the density functional approach by using this modified charge density. Charge densities at the cell boundaries of 3d transition metals are also calculated to test the applicability of the method to all metals. The results are quite encouraging, the simplicity of the method notwithstanding.
Thermopower of Bismuth in Low Longitudinal Magnetic Fields
Authors: Vašk P. , Svoboda P.
Published: July 2007

The thermopower of a Bi single crystal is measured at temperatures below 8 K and in longitudinal magnetic fields up to 0.014 T. The change of the thermopower α(B) due to the magnetic field isfound to be negative below about 5K and positive above. This behaviour can be understood on the basis of the standard theory of the phonon‐drag thermopower and the temperature dependence of partial thermopowers can be estimated.
Hall Coefficient and Magnetoresistance of Semiconductors with Anisotropic Energy Dispersion
Authors: Tomchuk P. M. , Shenderovskii V. A. , Gorley P. N. , Gavaleshko N. P.
Published: July 2007

Hall coefficient and transverse magnetoresistance of narrow‐gap semiconductors are investigated theoretically in a wide temperature region. It is shown that in a weak magnetic field the transport coefficients exhibit non‐monotonic temperature dependence and in arbitrary classical magnetic fields the transverse magnetoresistance may grow anomalously. These regularities are due to inelastic scattering of carriers by optical phonons and anisotropy of the temperature dependences of the components of the effective mass tensor. The contributions which different scattering mechanisms (those by acoustic phonons, impurity ions, and interelectron scattering) give to the transport coefficients are evaluated. In particular, the contributions of impurity and interelectron scattering processes are shown to be negligibly small in lead chalcogenides.
Direct Determination of Indium Antimonide Energy Band Parameters from Diamagnetic Exciton Spectra
Authors: Kanskaya L. M. , Kokhanovskii S. I. , Seysyan K. P. , Efros Al. L.
Published: July 2007

Diamagnetic exciton spectra of InSb crystals derived from transmission measurements carried out on thin unstressed samples at 1.8 K and in magnetic fields up to 80 kOe are used for a direct determination of parameters of the c‐ and v‐bands. Detailed identification and precise binding energy calculations permit one to derive the cyclotron frequencies of the electron and the holes, and the spin splittings. The m(ϵc) and g(ϵc) relationships thus obtained exhibit good agreement with Kane's model up to energies close to ϵc ϵg. Light holes also reveal a strong nonparabolicity characterized in the linear‐in‐k4 approximation by the coefficient p1h = 0.47 meV−1. An analysis of the splitting of two light‐hole long‐wavelength transitions including the diamagnetic exciton binding energies and the data on the c‐band obtained permits one to calculate the v‐band parameter set.
Two Types of F Centres in Alkaline Earth Fluoro‐Halides. I. Superhyperfine Interactions and Electronic Structure
Authors: Bauer R. U. , Niklas J. R. , Spaeth J.‐M.
Published: July 2007

In the alkaline earth fluoro‐halides BaFCl, SrFCl, and SrFBr two types of F centres with different symmetry are produced, where the electron occupies either an F− vacancy (F(F−) centres) or a Cl− or Br− vacancy (F(Cl−, Br−) centres), respectively. With electron nuclear double resonance the superhyperfine and quadrupole interactions with up to nine shells of neighbour nuclei are determined. The spin density distribution is rather anisotropic. For F(Cl−) centres it can be explained with an 1s‐type centre wave function if the mutual overlap of the lattice ions is taken into account. For the F(F−) centres a small admixture of a 2p‐ or a 3d‐type orbital to the 1s function is required.
Variational Calculations for Ground‐State Energies of Donor–Acceptor Pairs
Authors: Englisch R.
Published: July 2007

For the calculation of ground‐state energies of donor‐acceptor (DA) pairs (and also of two related three‐body problems) variational ansatzes are tested including the correlation between electron and hole. A refinement of a paper by Kartheuser et al. is presented for small distances of the DA pairs. Further, another method to solve the integrals occurring is shown which avoids numerical integration.
The Influence of the Structure of Different Arsenic Chalcogenide Glasses on Refractive Index and Dispersion
Authors: Burckhardt W. , Feltz A.
Published: July 2007

The refractive index dispersion of different arsenic chalcogenide glasses is analyzed by the use of a two‐term Sellmeier dispersion formula. In a region from 3 μm up to the beginning of the absorption edge the wavelength dependence of the refractive index can be described by a single oscillator model. At wavelengths λj, determined by λ1j, = const, the molar refraction of the different glasses is a linear function of neff λ. In representations RM = RM(neff) and RM = RM(λ) the influence of the structure of the different glasses is clearly illustrated. For the glass AsSe3 a special short‐range order is indicated.
Towards a Molecular Theory of the Solid–Liquid Phase Boundary. Structural Theory and Effective Lattice Models
Authors: Klupsch Th.
Published: July 2007

From the molecular, structural theory, an unrestricted quadratic Ising model is derived the subcritical region of which enables a semi‐quantitative description of the f.c.c. crystalline phase in coexistence with its melt including the interface between them for simple one‐component systems with small melting heat at thermal equilibrium, especially for Lennard‐Jones systems near the triple point. The Ising Hamiltonian is determined by the free excess energy and not by the internal energy; except of the melting heat, it involves contributions from the entropy difference connected with the higher degree of disorder of liquids compared with the crystalline state. Therefore, a remarkably large Jackson‐Temkin factor of about three results, but it does not uniquely determine the interface structure because the effective interaction is not restricted to next‐neighbouring lattice sites. A new interpretation of the wetting condition in terms of the effective potential is presented.
Determination of Stresses in SiSiO2 and Mechanically Affected Si by EPR
Authors: Gehlhoff W. , Segsa K. H.
Published: July 2007

The influence of stresses generated by roughening or oxidation of silicon samples on the EPR spectra of Fe0 in silicon is investigated. Starting from previous calculations it is possible to explain the changes in the angular dependence of the spectra, and to determine the stress components in the SiO2 film and in the Si surface affected by roughening. The stress values determined by the EPR analysis are compared with the results obtained by other methods.
On an Anomalous Behaviour of Spin‐Wave Stiffness Constant in Amorphous Ferromagnets
Authors: Kaneyoshi T.
Published: July 2007

For the anomalous behaviour of the spin‐wave stiffness constant observed in some amorphous ferromagnets, changing with the applied field, a model is suggested and discussed.
Renormalized Atom Model (RAM) Applied to the Heavy Rare‐Earth Sesquioxides in Cubic Crystal Structure
Authors: López‐Aguilar F. , Costa‐Quintana J.
Published: July 2007

The electronic structure of the b.c.c. heavy rare‐earth sesquioxides is analyzed through the singlesite solutions within the renormalized atom model (RAM) and using the relativistic version. The Coulomb correlation energy is calculated in order to determine the unoccupied 4f levels.
Precise Positron Lifetime Measurements in Indium
Authors: Puff W. , Mascher P. , Kindl P. , Sormann H.
Published: July 2007

The positron lifetime is measured in polycrystalline indium in the temperature range between 80 and 420 K. The data show a pronounced prevacancy effect which is fairly well approximated by a straigth line. The results suggest transient dilatation trapping rather than metastable self‐trapping. The occurrence of this effect has some influence on the values deduced for the monovacancy formation enthalpy. In the temperature range below 180 K detrapping of positrons cannot be ruled out.
An Approximate Method for the Free Energy
Authors: Ferreira J. R. Faleiro , Silva N. P.
Published: July 2007

A variational approximate free energy expression is derived and used to calculate the critical temperature of the anisotropic Ising model. The results are better consistent than others variationally obtained.
Diffusion‐Induced Quadrupole Relaxation of 27Al Nuclei in Dilute AlTi, AlCr, AlMn, and AlCu Alloys at High Temperatures
Authors: Bottyán L. , Beke D. L. , Tompa K.
Published: July 2007

The temperature dependence of the laboratory frame spin‐lattice relaxation time of 27Al nuclei is measured in 5N Al and in dilute AlTi, AlCr, AlMn, and AlCu alloys at 5.7 and 9.7 MHz resonance frequencies. The relaxation in pure aluminium is found to be purely due to the conduction electrons. An excess T1‐relaxation contribution is detected in all Al–3d alloys investigated above 670 K. The excess relaxation rate is proportional to the impurity content and the temperature dependence of the excess contribution is of Arrhenius‐type with an activation energy of (1.3 ± 0.3) eV for all of the investigated alloys. The relaxation contribution is found to be quadrupolar in origin and is caused by the relative diffusional jumps of solute atoms and Al atoms relatively far from the impurity.
Intraband Absorption Due to Alloy Scattering in Lead Chalcogenide Semiconductors
Authors: Sierański K. , Szatkowski J.
Published: July 2007

Free carrier absorption allowed by the chemical disorder is calculated for lead chalcogenide mixed semiconductors. It is found that the absorption due to the intervalley transitions may be greater than that due to the intravalley transitions.
Two‐photon interband absorption of light in semiconductors in external parallel electric and magnetic fields
Authors: Katana P. K.
Published: July 2007

The constant of the two‐photon interband absorption of light (TPA) in a semiconductor placed in external, parallel quantizing electric and magnetic fields is calculated. It is shown that the TPA constant near each Landau level must have an oscillatory behaviour due to the presence of discrete Stark levels, that is, at high electric field strengths the “Stark ladder” must be observed. The following particular cases are considered: the one‐photon absorption (OPA), the two‐photon electroabsorption (TPEA), and the two‐photon magneto absorption (TPMA).
The polariton soliton formation due to the effect of the dipole momentum saturation
Authors: Shibarshina G. D. , Moskalenko S. A. , Khadzhi P. I.
Published: July 2007

The energy flux density of the incident light upon the crystal surface and the form of the wave packet envelope with frequencies lying in the exciton range of the spectrum, are determined. They must be of such a nature that after the penetration into the semi‐infinite crystal, the laser pulse can travel at once as a solitary wave without any transient stage. In the case considered the effect of the dipole momentum saturation (DMS) for the transition from the ground state of the crystal to that of the exciton is the main nonlinearity. The Maxwell‐Fresnel boundary conditions are used on the interface between the vacuum and the crystal.
On incommensurate phases in the magnetic phase diagram of the hubbard model
Authors: Schumacher W.
Published: July 2007

The existence of “heat magnetization” in the antiferromagnetic phase of the Hubbard model is discussed in connection with the occurrence of incommensurate magnetic phases. A magnetic phase diagram is derived for temperatures T > 0 in the framework of the Hartree‐Fock approximation. In a great region of this phase diagram incommensurate phases exist. For certain parameters there occurs a phase transition from the incommensurate to the antiferromagnetic phase if the temperature is increased.
Isovalent‐impurity clusters in koster‐slater approximation
Authors: Goede O. , Hennig D.
Published: July 2007

The binding energies of an electron (hole) bound to an m‐atomic isovalent‐impurity cluster in crystals with f.c.c. or h.c.p. cation (anion) sublattices are calculated in a Koster‐Slater one‐band one‐site approximation using Green's function technique. The conduction (valence) band of the host crystal is described by an one‐band tight‐binding Hamiltonian. The binding energies εm, μ are obtained for the ground and excited states of the various types μ of the clusters with respect to the number of nearest‐neighbour steps between the impurities as functions of J/T, where J is the Koster‐Slater parameter and T the effective conduction (valence) band width. The calculated energies using a Hubbard density of states (model H) or a Bethe‐lattice approach for the Green's functions (model B) are discussed and compared with experimental results for CdS: Te and GaP:N.
Local heine‐abarenkov model potential for III–V and II–VI covalent compounds
Authors: Soma T. , Kagaya H.‐Matsuo
Published: July 2007

A local Heine‐Abarenkov model potential is proposed for zincblende‐type crystals. The potential parameters are determined by satisfying the zero pressure condition and the first zero of the empirical pseudopotential interpolated from band calculations. Two sets of parameters are presented for thirteen tetrahedral compounds such as AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, and CdTe.
Optical Detrapping in Molecular Crystals. I. Vibrational Structure of Detrapping Transitions
Authors: Petelenz P.
Published: July 2007

A simple model of optical detrapping in molecular crystals is proposed. It includes the coupling of electronic and vibrational degrees of freedom, due to the displacement of the equilibrium position in intramolecular vibrations upon ionization or electronic excitation of the molecule. The coupling is shown to shift the apparent distribution of trap depths towards greater values. Possible relevance of the model to the interpretation of experimental data is discussed and new experiments are suggested.
Image Forces at the Metal Electrode‐Electrolyte Solution Interface: Account of Spatial Dispersion of the Solution Permittivity
Authors: Rubinshtein A. I.
Published: July 2007

The effect of spatial dispersion of the permittivity of an electrolyte solution on the character of image forces W(a) near a metal electrode is investigated. The effect of penetration of an electric field into the metal is taken into account. The asymptotic expressions obtained for W(a) show that in highly dilute electrolyte solutions the charge in addition to usual attraction encounters an effective repulsion in two distance regions from the metal, so that the image of W(a) has an essentially non–monotonic form. For a more concentrated solution, the image of W(a) is modified and may take a form corresponding to a monotonic repulsion of the charge from the interface.
The Theory of Dynamical X‐Ray Diffraction on a Superlattice
Authors: Khapachev Yu. P.
Published: July 2007

A general theory is given of dynamical X‐ray diffraction on a superlattice. The approach is based on an analysis of the solution of the Takagi equation system. Diffraction for two types of super‐lattices: T « ∧ and T ≈ ∧. (T period of superlattice, ∧ extinction length) is considered. Angle position of satellites and their widths are essentially different from analogous results of the kinemetical theory.