bluuebunny/crossref_metadata_2025 huggingface dataset:
Time‐Dependent Momentum Distribution of Polarons at Arbitrary Temperature and Electric Field
Authors: Brosens F. , Devreese J. T.
Published: July 2007

The charge carriers in a polar semiconductor are assumed to be described by the Fröhlich polaron Hamiltonian, i.e. the interaction with the LO phonons is considered to be the dominant scattering mechanism. In the presence of an electric field, and at sufficiently weak electron–phonon coupling, the dynamics of the charge carriers are studied from the Boltzmann equation. A rigorous solution for the time‐dependent momentum distribution function is found at arbitrary temperature and electric field. This solution consists of a recurrence relation between the expansion coefficients αn (p, t), if the momentum distribution function is expanded in Legendre polynomials: \documentclass{article}\pagestyle{empty}\begin{document}$ f(p,t) = {\textstyle{1 \over 2}}\sum\limits_{n = 0}^\infty {(2n + 1)} {\rm a}_{\rm n} (p,t)P_n (\cos \theta ) $\end{document} where θ is the angle between the electric field and the momentum p under consideration. It is shown that from Boltzmann's equation one of these expansion coefficients is left completely unspecified, and has to be determined from physical boundary conditions.
A Cooperative Mechanism Limiting the Spectral Width of Stress Relaxation Processes in Solids
Authors: Kubát J.
Published: July 2007

A recently developed two‐level cooperative model of stress relaxation in solids is shown to describe correctly the experimental finding F/σ ≈ 0.1, with F denoting the maximum slope of the relaxation curves in a σ (stress)–ln t (time) diagram, and σ the initial effective stress. The basic idea underlying the model is the notion of multiple transitions between the two levels, such transitions being induced by spontaneous events by a similar mechanism as in B‐E statistics. The spectrum of relaxation times, τ, is determined by the size distribution of the clusters, the τ values being inversely proportional to the cluster size. A subdivision of the relaxing system into smaller cooperating regions is one of the consequences of the constancy of the ratio F/σ.
Lattice Dynamics of GaP under Hydrostatic Pressures
Authors: Patel C. , Sherman W. F. , Wilkinson G. R.
Published: July 2007

The “eleven‐parameter” rigid‐ion model is used to describe the lattice dynamics of GaP under variable hydrostatic pressures. The one‐phonon and two‐phonon density of states functions are computed for various pressures and compared with the second order Raman spectrum measured by the authors. The effect of high pressure on the phonon dispersion curves is shown to lead to “softening” in the transverse acoustic modes and this suggests a possible driving mechanism for the phase transformation in this compound.
NMR on Metastable Pt–P and Ni–Pt–P
Authors: Goebbels J. , Khan H. R. , Lüders K. , Raub Ch. J.
Published: July 2007

Knight shift and linewidth of 31P and 195Pt and the electrical resistance as a function of temperature formeta stable crystalline Pt82 P18 and amorphous (Ni0.4 Pt0.6)80 P20 are measured. It seems that the 195Pt Knight shift is mainly determined by the content of P, indicating a filling of the d‐states of Pt. The 31P linewidth δ is frequency dependent in the case of the amorphous sample. The calculated linewidth is smaller by a factor of about four compared to the frequency independent part of δ, which may be due to the indirect dipole–dipole interaction.
Effect of Electric Fields on Luminescence in GaAs
Authors: Yamawaki M. , Hamaguchi C.
Published: July 2007

Photoluminescence spectra near the exciton region are investigated under the application of external electric fields parallel and perpendicular to epitaxial GaAs surfaces. Quenching of the luminescence is observed and explained in terms of impact ionization of excitons. The emission peaks of the upper (UPB) and lower polariton branches (LPB) shift to higher and lower energy sides, respectively, with increasing the electric fields or the excitation intensity. The calculated peak of the LPB emission is located around 1.515 eV and shifts to higher energy side and dominates the UPB emission, which is contradictory to the present observation. New emission peak appears at 1.515 eV under an application of an electric field in pure samples with carrier concentrations less than 6×1013 cm−3.
On the Existence of a Phase Transition for the Fröhlich Polaron
Authors: Peeters F. M. , Devreese J. T.
Published: July 2007

The ground state energy and its first and second derivative with respect to the electron‐phonon coupling are calculated for the polaron theories of Feynman, Lépine and Matz, Mańka, Pekar, and the Gaussian approximation. It is suggested that the phase transition, from the mobile to the localized polaron state, found by some authors is a feature of their approximation rather than an intrinsic property of the Fröhlich Hamiltonian.
Estimation of Force Constants and Phonon Frequencies of Ternary Semiconducting Compounds with Chalcopyrite Structure from Specific Heat Data
Authors: Deus P. , Frei V. , Bilek O. , Cordts W.
Published: July 2007

The force constants of a Keating model are calculated for CuInS2 and AgInTe2 from spectral moments by means of the traces of powers of the dynamical matrix. The spectral moments are derived from measurements of specific heat given in the literature. By diagonalizing the dynamical matrix composed with the calculated force constants the phonon frequencies in the centre of Brillouin zone are obtained. The comparison with published data from measurements on CuInS2 shows reasonable agreement.
Positron Annihilation in Mn2+‐Doped KCl Single Crystals
Authors: Chinnusamy M. , Nagarajan T.
Published: July 2007

The angular distribution of annihilation photons (ADAP) is measured in pure KCl single crystals and in that doped with Mn2+ impurities to concentrations of 60 and 250 ppm. In unannealed, impurity‐doped crystals, the intensity of the narrow component increases with the increase of the impurity concentration. The narrowing of the ADAP curves is explained by assuming trapping of positrons at impurity induced positive ion vacancies. On annealing, for 60 ppm impurity concentration, the intensity of the narrow component is greater than that for the unannealed sample of the same concentration, indicating dissolution of impurity–vacancy complexes, at elevated temperatures. For 250 ppm impurity concentration, comparing the results of the unannealed and annealed samples no great difference is found between the intensities of the narrow components indicating, that the dissolution of the impurity–vacancy complexes is not appreciable at higher concentrations, on heating.
Phonon Scattering by Paraelectrical Centers in Crystals I. General
Authors: Akhmadullin I. Sh. , Saburova R. V.
Published: July 2007

A resonance phonon scattering by a system of randomly distributed electric dipoles over the crystal interacting with each other and with the lattice (strongly) is considered. A distribution function of random molecular fields f(Δ) is found with allowance for both, electric dipole‐dipole and dipole‐phonon interactions. A general expression for the exact one‐phonon propagator is found for the system under study. It is shown that the phonon self‐energy part M possesses critical temperature features inherent in the distribution function f(Δ).
Embedded Cluster Approach to the Density of Tail States for Disordered s‐Scatterers
Authors: Blaudeck P.
Published: July 2007

The density of states in the band tail is calculated by multiple scattering in a disordered cluster of s‐scatterers. A backscattering technique is used for embedding this cluster in a surrounding medium. It is shown that the tail results from states which are related to bound states in a potential box. There exists a characteristic cluster size which is most suitable for taking the configurational average of the density of states.
Nonlocal Pseudopotential Calculations for Two Isoelectronic Series: Ge‐GaAs‐ZnSe and α‐Sn‐InSb‐CdTe
Authors: Humphreys T. P. , Srivastava G. P.
Published: July 2007

Using the empirical local and an energy dependent semiempirical nonlocal pseudopotential schemes, the band structures for two isoelectronic series, viz. Ge‐GaAs‐ZnSe and α‐Sn‐InSb‐CdTe are calculated. The local pseudopotential atomic form factors are determined analytically and nonlocal corrections are incorporated within an energy dependent linearly screened model. Spinorbit interactions are also included in the nonlocal calculations. The band structure and charge density calculations are found to be in very good agreement with recent experimental and theoretical results. The effect of nonlocality is assessed and charge localization and bonding trends are studied within each series.
Analysis of the Grüneisen Parameter and Its Volume Dependence in Alkali Halide Crystals
Authors: Shanker J. , Singh K.
Published: July 2007

An analysis of the Grüneisen parameter γ and its volume derivative for the alkali halide crystals is performed using interionic potential parameters derived from low‐temperature data for interionic distance and bulk modulus. Existing theories of the Grüneisen parameter are used to obtain numerical values of γ and d ln γ/d ln V for sixteen alkali halides. Values of first‐ and second‐order pressure derivatives of bulk modulus derived from interionic potentials are also reported and compared with experimental data.
Force Model Dependence and Asymptotic Behaviour of the Static Lattice Green Function for H.C.P. Crystals
Authors: Khanna R.
Published: July 2007

The elastic Green function (GF) and static lattice GF for different force constant models are compared in zinc and magnesium. The lattice GF and its approach to the asymptotic value is found to be highly model dependent when the force models differ in the long wavelength limit. The results support the view that the lattice GF is mainly determined by the long wavelength behaviour of the dynamical matrix and emphasize the need for better force models.
Form Effects in the Description of the Orientation Distribution Function (ODF) of Texturized Materials by Model Components
Authors: Matthies S.
Published: July 2007

Describing the ODF of texturized materials by model components quantitative statements about the intensities of the several components can be made. However, depending of the form of the model components and of the chosen χ2‐function drastic deviations from the true intensities can appear, which are connected with the metric of the G‐space. Lorentz‐like standard functions are described and existing fit‐strategies are analysed.
Electron Spin Resonance Study of HD‐Type BrCl− Defects in KCl Doped with Divalent Cation Impurities
Authors: Van Puymbroeck W. , Schrijvers N. , Bouwen A. , Schoemaker D.
Published: July 2007

HD‐type BrCl− defects produced by X‐irradiation at low temperatures in KCl containing both Br− and Sr2+ or Ba2+ impurities are studied by electron spin resonance. In KCl:Sr2+ two HD‐type BrCl− defects, both with their molecular axes tilted in a {100} plane, are observed. The models that are proposed consist of a BrCl− molecule ion, on an anion site, next to a Sr2+ and an associated positive ion vacancy which are nearest neighbors of each other. In one center the bromine is on a more interstitial site and in the other on a more substitutional site. The two HD‐type BrCl− defects observed in KCl:Ba2+ are tilted in a {110} plane. The models consist of a BrCl− on an anion site in the immediate vicinity of a Ba2+ configuration. Again the bromine is either more substitutionally or more interstitially situated. A so‐called HDD<100>‐type BrCl− defect, involving a dimer of impurityvacancy complexes, is also observed.
A Modified Local Density Approximation. Electron Density in Inversion Layers
Authors: Paasch G. , Übensee H.
Published: July 2007

In the local density approximation (LDA) the density of a many‐electron system is expressed as a function of the spatially varying potential. Here a modified LDA is derived applicable for a potential with a high step at some plane which is e.g. a model for the band edges in an inversion layer at the semiconductor—insulator interface. The local density of states shows oscillations and decreases to zero at the interface as a consequence of quantum mechanical reflection. Simple expressions for the density are obtained for the strongly degenerate and for the non‐degenerate system. The comparison with exact results for a model system (triangular potential) shows that the modified LDA yields good results for a wide range of parameters corresponding to real inversion layers.
Tight Binding Quasiparticle Motion in a Poisson cOrrelated Discrete Stochastic Potential. The Density of States
Authors: Behn U.
Published: July 2007

The average one‐particle Green function of a tight binding quasiparticle moving in a discrete stochastic time dependent potential which jumps between two states is calculated in single site approximation under the assumption that the number of jumps is governed by a Poisson distribution. The density of states is discussed over the whole range of parameters and shows a noise induced transition from monomodal (jump frequency α ≫ jump amplitude U) to a bimodal (α ≪ U) feature. The theory contains the stochastic theory of ESR line shapes in the limit of vanishing bandwidth and the coherent potential approximation for binary alloys in the static limit (α = 0).
Calculation of Local Densities of States, Local Charges, and Adsorption Energy on Large Semiconductor Substrate with Single Adsorbate
Authors: Engler C. , Lorenz W.
Published: July 2007

Quantumchemical calculations of local densities of states, local charges, and adsorption energy on large semiconductor substrate with a single adsorbate are reported for GaP with ideal or 1 × 1 reconstructed (110) surface. The dependences of the named quantities on atomar adsorbate diagonal and hopping matrix elements are discussed.
Neutron Scattering Studies on Strontium Barium Niobate
Authors: Prokert F.
Published: July 2007

The dispersion of low‐energetic phonons is measured on Sr0.45Ba0.55Nb2O6 single crystals by inelastic neutron scattering at room temperature for the [ξ, 0,0], [ξ, ξ, 0], and [0, 0, ξ] directions. The elastic constants at this temperature are evaluated from the initial slopes of the acoustic branches. From careful measurements at low energy transfer a sequence of weak dispersionless excitations, separated by about 0.5 meV, is found. The appearance of these low‐lying flat branches is discussed in connection with the results of quasi‐elastic scattering on these samples.
Optical and Electrical Properties of Hydrogenated Amorphous Silicon Carbide
Authors: Dutta R. , Banerjee P. K. , Mitra S. S.
Published: July 2007

Hydrogenated amorphous silicon carbide thin films are prepared by RF sputtering. The amorphous structure of the hydrogenated films and their hydrogen content are ascertained by Raman and infrared spectroscopy. The principal reststrahlen band does not shift in frequency but appears to sharpen with the increase in hydrogen concentration. The hydrogen content is estimated by the integrated band intensities of silicon‐hydrogen and carbon‐hydrogen and carbon‐hydrogen modes in the infrared absorption. The hydrogen content has a dominant effect on the optical pseudo‐gap and the dc conductivity of the amorphous films. A blue shift is observed in the absorption edge with increasing hydrogen content. The dc conductivity of the films is studied in the temperature range of 120 to 400 K. High temperature data yield an activation energy gap which increases significantly with hydrogen content, while the low temperature conductivity is explained by hopping between localized states.
Comments on the Behaviour of the Absorption Edge of Cd1−xMnxTe for x > 0.4
Authors: Grancharova E. I. , Lascaray J. P. , Diouri J. , Allègre J.
Published: July 2007

A pinning of the absorption edge of Cd1–xMnxTe occurs for x > 0.4. Some authors have attributed this behaviour to a transition within the 3d5 localized levels of the Mn2+ ions. The possibility is analysed to observe such a transition in CdMnTe. A comparison with observations on spectra of various substitutional impurities of the 3d series on II—VI crystals leads to the determination of an upper limit for the crystal field parameter Dq of the Mn atom in CdMnTe crystals and consequently to a lower limit for the energy difference between the ground and the first excited states in the d‐shell of Mn. From a temperature and composition dependence recall of absorption and luminescence measurements previously performed, it is deduced that transitions between 3d5 levels of Mn2+ do not explain the anomalous behaviour of the absorption edge of CdMnTe for x > 0.4 and an alternative possible explanation is given.
The model of metamagnetic phase transition in Mn3GaC
Authors: Cofta H. , Pawlicki P.
Published: July 2007

A new model of metamagnetic phase transition with temperature in Mn3GaC is proposed. The magnetoelastic Hamiltonian takes into account two independent strains (trigonal and isotropic ones) as functions of temperature, calculated similarly to the microscopic theory of magnetostriction. Necessary conditions for the transition are established within the molecular field approximation and the fit with experiment is discussed.
Optical properties of AsO−2‐impurities in alkali halide crystals
Authors: Lohmeier F. J. , Fischer F.
Published: July 2007

The optical properties of AsO−2‐ions dissolved in several alkali halide crystals are investigated. In optical absorption four bands are found, which belong to the long‐wavelength excitation of the impurity centre. In emission just one band is observed. Radiationless transitions between excited singlet states and the lowest lying B1‐triplet state are induced by a strong spin—orbit coupling. The 3B1‐state has a lifetime around 10−4 s, depending on the size of the anion of the host lattice. Above a temperature between 60 and 80 K (depending on the host lattice) radiationless transitions compete in the depopulation of the 3B1‐state. At the temperature of 4.2 K the long‐wavelength bands show a fine structure, that corresponds to the excitation of normal vibrations of the molecule. According to the strong electron–phonon‐coupling, no zero phonon lines are observed.
Decay of excitons into long‐lived F, H and α, I pairs in KCl
Authors: Lushchik Ch. , Kolk J. , Lushchik A. , Lushchik N. , Taiirov M. , Vasilchenko E.
Published: July 2007

The spectra of the creation of long‐lived Frenkel defects (F, H and α, I pairs) by 7.5 to 9.0 eV photons and the thermal annealing of defects created at exciton decay or at the recombination of electrons with holes are measured in KCl crystals by the luminescence methods at 4.2 K. Pairs of defects with various distances between F and H centers are formed at the decay of Γ (1/2) and Γ (3/2) excitons with n = 1, 2. The generation efficiency of long‐lived α, I pairs is especially high at the tunneling recharge of F, H pairs created via the decay of Γ (3/2) excitons.
Integral representation of the incoherent field in disordered materials IV. Relation to multiple scattering theory and inclusion of pair correlations
Authors: Lenk R.
Published: July 2007

The theoretical scheme developed in Parts I to III is related to equations which exactly describe the multiple scattering problem. This procedure exhibits that the hitherto investigated simple variant is restricted to uncorrelated scatterers and that it rests on the same approximation idea as a previously developed medium approach to the coherent field. The neglect of fluctuations which is characteristic for a medium theory can be seen explicitly. The basic integral representation is generalized by the inclusion of pair correlations. The generalized representation is determined uniquely and forms the starting point for a generalized wave theory of diffusion.
Structure factors and compressibilities of liquid metals
Authors: Bretonnet J. L. , Gasser J. G. , Bath A. , Kleim R.
Published: July 2007

A shouldered hard sphere interatomic potential is used to evaluate, within the mean spherical approximation, the experimental structure factors of 13 liquid metals. For 10 metals, a positive shoulder of the potential is more appropriate than a square well potential. A parabolic well potential is also tried and in many cases a repulsive tail is obtained. The isothermal compressibility and the electrical resistivity for some metals are calculated. A good agreement with experimental values is achieved, especially when an electronic contribution is introduced for the compressibility.
On thermodynamics of spontaneous metamagnetism
Authors: Tuszyński J. A. , Cofta H.
Published: July 2007

A general phenomenology of the Heisenberg spontaneous metamagnet is developed on thermodynamical basis. Analysis of the power expanded free energy of an ambigious magnetoelastic spin system reveals the crucial role of temperature terms in inducing the spontaneous metamagnetic transition. The temperature term results from the influence of the lattice vibrations on the exchange parameter. The neglecting of such a vibration effect is identified as the source o serious discrepancies involved in all existing exchange‐inversion theories.
Phonon anomalies in copper
Authors: Weiss L. , Eschrig H.
Published: July 2007

Phonon anomalies in copper are investigated. With the shape of the Fermi surface obtained from self‐consistent band structure calculations various kinds of anomalies are predicted in the (110)‐plane of the wave‐vector space. Some of them are investigated experimentally and the measured anomalies confirm the theoretical predictions in position and shape. Especially, an anomaly corresponding to the touching from inside of a Fermi surface with its shifted copy predicted by Kaganov et al. is observed for the first time. The experimental results are compared with earlier measurements of other authors.
Cluster model for interstitial hydrogen bonding in nickel
Authors: Skála L. , Künne L. , Fritsche H.‐G. , Müller H.
Published: July 2007

The CNDO, EHT, and SW‐Xαmethods are used to investigate tetrahedral and octahedral Ni clusters with and without the interstitial hydrogen in the center. Optimized distances between atoms, one‐electron energy levels, and the bonding mechanism of hydrogen in the nickel clusters are given and discussed in detail.
Discovery of low‐temperature metastable state in amorphous bismuth—hydrogen systems
Authors: Belevtsev B. I. , Komnik Yu. F. , Odnokozov V. I. , Fomin A. V.
Published: July 2007

Electric and superconducting properties of Bi films condensed in the H2 atmosphere (pH2 = = 5 × 10−6 to 2 × 10−5 Torr) onto a liquid‐helium‐cooled substrate are studied. The dissolved hydrogen is found to increase essentially the resistivity Q, to suppress Tc and raise the amorphous—crystalline transition temperature Tac. If pH2 exceeds some critical values, the films prepared are in the metastable phase, a‐Bi(H), highly saturated with hydrogen. At about 5 K this phase transforms into ordinary, also metastable, amorphous Bi, which results in a sharp decrease of the normal resistance (down to two orders of magnitude) and a Tc rise (up to 2 K). In a number of cases this transition proceeds as a heating‐induced jump from the resistive to the superconducting state. The properties of pure and hydrogen‐saturated Bi films are thoroughly compared. The results obtained are discussed on the base of the up‐to‐date knowledge.
A new theoretical approach to the low‐temperature specific heat of disordered solids
Authors: Hüttner B. , Pompe W.
Published: July 2007

Starting from a perturbation theory for the free energy of disordered solids, the anomalous temperature dependence of the specific heat can be calculated. The model under consideration is based on the two common features of a large group of disordered solids, firstly, the distorted SRO, and, secondly, the cluster‐like arrangement of strongly distorted atoms which are imbedded in a crystallike matrix. The theory yields the increase of the specific heat proportional to T for very low temperatures and the anomalous high value of the T3‐term. The calculated values coincide very well with the experimentally determined temperature dependence for disordered covalent solids.
Variable reconstruction of dislocation cores in Si
Authors: Mergel D. , Labusch R.
Published: July 2007

Photoconductivity spectra and Hall‐effect measurements of deformed Si samples with different doping concentrations and dislocation densities suggest a dependence of the dislocation energy levels on their occupation. This is in contrast to the widely used rigid‐band‐model. A Peierls‐like structural transition in the core of a 90°‐partial is discussed as the most likely explanation of this effect.
Electron spin resonance of doped glow‐discharge amorphous germanium
Authors: Stutzmann M. , Stuke J. , Dersch H.
Published: July 2007

ESR‐spectra and transport properties of a‐Ge:H, prepared by decomposition of GeH4 in a glow‐discharge, are investigated for different phosphorus and boron doping. Undoped and moderately doped samples show the usual resonance at g = 2.0225 due to dangling bonds. For high doping levels, however, two new lines are found: high boron doping leads to a resonance at g = 2.0535 with a linewidth of 112 G, high phosphorus doping to a line at g = 2.0120 with ΔHpp = 33 G. These lines are attributed to localized holes and electrons in the tails of valence and conduction band, respectively. The linewidth of all resonances is the combination of a g‐spectrum ΔHpp(g) and a considerably smaller residual linewidth ΔHpp(r) caused by unresolved hyperfine interaction. A simple relation between the g‐shift g–g0 and ΔHpp(g) is found which explains the similar doping dependence of g‐value and linewidth. From the dependence of the spin density Ns on the conductivity activation energy Eσ a model for the density of states in the mobility gap is deduced. A comparison between the ESR‐data of a‐Ge: H and a‐Si: H shows that g‐shifts and linewidths of the three resonances are for a‐Ge: H by a factor of about six larger than for the corresponding lines of a‐Si: H.
Infrared absorption spectra of Cd0.2Hg0.8Te crystals
Authors: Gurauskas E. , Kavaliauskas J. , Krivaite G. , Šileika A.
Published: July 2007

The investigations of infrared absorption spectra of Cd0.2Hg0.8Te crystals containing inclusions of tellurium surrounded by semimetal phase are carried out in the spectral region 10 to 40 μm at 120 and 300 K. An increase of absorption coefficient and a change of its spectral dependence due to the inclusions are found. Numerical calculations are made of free‐carrier absorption spectra taking into account scattering by acoustic phonons, optical phonons, and ionized impurities. In addition, the extinction spectra due to the absorption and scattering of light by Te and HgTe particles in Cd0.2Hg0.8Te crystals are calculated, too. It is shown, that infrared absorption spectra of Cd0.2Hg0.8Te can be explained by free‐carrier absorption including the extinction of light by inclusions of a semimetal phase (HgTe).
Recherches théoriques sur l'autodiffusion dans les cristaux ioniques
Authors: Guerin P. , Laforgue A.
Published: July 2007

Nous présentons une méthode, déduite de la théorie de la vitesse absolue de réaction, pour étudier la migration du cation dans les cristaux d'halogénures alcalins. Cette méthode est appliquée dans le cadre de trois approximations décrivant les propriétés des ions. La première approximation nous permet de tracer des lignes équipotentielles dans des sections planes de la surface de potentiel. Contrairement à l'idée admise, la voie de diffusion n'est pas rectiligne; en effet on met en évidence deux voies de diffusion planes. Ce résultat est confirmé avec NaCl, dans le cadre de notre troisiàme approximation (la plus fine). Ces deux voies sont alors séparées par une barriére de potentiel de 0,01 eV, les énergies de Schottky et de migration sont respectivement égales à 2,03 et 0,69 eV.
Some optical and electrical measurements on Cs3Bi2Br9 single crystals
Authors: Timmermans C. W. M. , Cholakh S. O. , van der Woude R. L. , Blasse G.
Published: July 2007

The optical properties of Cs3Bi2Br9 in the spectral range 2 to 10 eV are reported and discussed. The results are interpreted with the aid of the calculated band scheme of CsPbBr3. The band edge is situated at 2.707 eV and is ascribed to a transition from the bismuth (6s) valence band to the bismuth (6p) conduction band. The intracationic exciton transition, which takes place at 2.699 eV, is allowed but with a low transition probability. Photoconductivity measurements reveal that the electrons are the more mobile charge carriers.
Electroconduction and thermoelectricity of terbium oxide films
Authors: Smolin M. D. , Andreeva A. F. , Ogorodnik V. A.
Published: July 2007

The dependence of electric conductivity and thermoelectric power of terbium oxide films TbO1.75−x upon the temperature (250 to 600 °C) and oxide composition (x = 0 to 0.25) is investigated. The results obtained are interpreted in terms of the small polaron theory. The thermoelectric power of TbO1.75 does not depend on temperature and is equal to −35 μV/K and the dependence of thermoelectric power of TbO1.75−x on the parameter x is in accordance with Heike's formula for the hopping. Also, some parameters of charge transfer mechanism motion are determined. The activation energy of drift mobility of small polarons in TbO1.75 is equal to 0.40 eV and the small polaron radius is equal to 0.21 nm. The concentration of small polarons in TbO1.75−x does not depend on temperature and is equal to the concentration of trivalent terbium ions.
Soliton excitations in one‐dimensional molecular systems
Authors: Brizhik L. S. , Davydov A. S.
Published: July 2007

It is shown that the presence or the absence of a soliton excitation threshold and the threshold values depend on the character of the excitation distribution at the initial time moment. A study is made of the time evolution of an excitation given at the initial moment as a hyperbolic secant, in form of a rectangular step, and as a decreasing exponential.
Nonlinear susceptibility of impurity centres in semiconductors
Authors: Drozhov Yu. P.
Published: July 2007

The nonlinear susceptibility coefficient, responsible for the rotation of the light polarization vector in electromodulation experiments, is shown to exhibit a certain structure at a frequency corresponding to the impurity ionization energy. The influence of a strong modulation field and a random field upon the phenomena considered is discussed.
Thermal conductivity of germanium containing dislocations at low temperatures
Authors: Ansari M. A. , Kumar Anil , Srivastava B. K.
Published: July 2007

To study the frequency dependence of the phonon–dislocation scattering, the phonon conductivity of germanium with dislocations is analysed using Holland's model and Kumar and Sinha's relaxation time for the phonon–dislocation scattering processes. A reasonably good agreement is observed between present theory and experimental data of Sato and Sumino.
Temperature dependence of the structure factor of liquid metals
Authors: Joarder R. N. , Rao R. V. Gopala
Published: July 2007

An expression for temperature dependent packing density is obtained and it is shown that this temperature dependent packing density can account the temperature dependence of structure factor, self‐diffusion coefficient, and shear viscosity with good accuracy. Other observations at higher temperatures also confirm this procedure.
A binary ferromagnetic alloy with magnetic moments dependent on local environment. II. Neutron magnetic elastic scattering and hyperfine field distribution
Authors: Zaborov A. V. , Medvedev M. V.
Published: July 2007

The neutron magnetic elastic scattering and the hyperfine field distribution are calculated for a collinear ferromagnetic alloy with magnetic moments which depend on their local chemical environment in a step‐like manner (Jaccarino‐Walker model). The question is analyzed how, using experimental data, one can discern the model under study from a binary magnetic alloy with competitive nearest‐neighbour exchange interactions and permanent magnetic moments. In the framework of the Jaccarino‐Walker model the concentration dependences of the spontaneous magnetization and Curie temperatures for Ni and Co alloys with elements of the Pt as well as Pd group are considered.
Molecular‐type channeling of relativistic electrons in crystals
Authors: Vyatkin E. G. , Filimonov Yu. M. , Taratin A. M. , Vorobiev S. A.
Published: July 2007

Channeling of relativistic electrons in 〈110〉 direction in a diamond crystal and the channeling radiation spectra are investigated using computer simulation by the binary collision model and using the model of a continuum potential of the atomic rows. In a computer experiment the atomic‐ and molecular‐type states of channeled electrons are revealed, and the orientational dependence of the electron trapping probability in these states is obtained. The peculiarities revealed of the angular distributions and radiation spectra of electrons in the molecular‐type states allow to discover these states in the experiment.
Reflectance and thermoreflectance spectra and energy band structure of GeSe crystals
Authors: Valiukonis G. , Gashimzade F. M. , Guseinova D. A. , Krivaitė G. , Kulibekov A. M. , Orudzhev G. S. , Šileika A.
Published: July 2007

The reflectance spectra of orthorhombic GeSe crystals are studied in the region 0.5 to 12 eV in polarized light with E ∥ a and E ∥ b at 290 K and a Kramers‐Kronig analysis is performed. The thermoreflectance spectra are investigated in the region 1 to 5 eV for E ∥ a and E ∥ b at 300 and 90 K. The energy band structure calculations of GeSe are carried out by the pseudopotential method, and the spectra of the imaginary part of the dielectric constant are calculated by the k · p method. A classification and interpretation of main structures of optical spectra of GeSe by direct interband transitions at critical points T, U, T and high symmetry lines B' V, A is presented on the basis of calculated band structure and selection rules for optical dipole transitions of orthorhombic A4B6 crystals.
Influence of directed chemical bonding on the structure of metallic glasses
Authors: Hermann H.
Published: July 2007

Non‐central potentials are used to describe directed chemical bonding due to metalloids in amorphous metal—metalloid systems. Average atomic volume, partial pair distribution functions, local symmetries, and correlation functions of symmetry coefficients are calculated for two‐dimensional model systems produced numerically by simultaneously carrying out density variation and energy relaxation. The model reproduces qualitatively essential experimental results.
A theoretical investigation of the fermi surfaces of bismuth and antimony
Authors: Rose J. , Schuchardt R.
Published: July 2007

The band structure and Fermi surfaces of Bi and Sb are studied with four different semiempirical pseudopotentials. By application of a Golin potential screened by the Heine‐Abarenkov form of dielectric function reasonable band structures are obtained. The calculated Fermi surfaces show very good agreement with the experimental data. Remaining discrepancies with respect to lower lying valence bands are analysed and compared with other calculations.
Decay behaviour of high‐excitation luminescence in Gap:N II. Experimental results on the densities of the coexisting phases and the relevant recombination processes
Authors: Weinert H. , Bindemann R. , Kreher K.
Published: July 2007

The radiative recombination of isoelectronic nitrogen‐doped GaP is investigated under high optical excitation at several temperatures (T = 2, 4.2, 27, and 77 K). A previously reported phase seperation is confirmed. The parameters of the electron—hole liquid and the coexisting low‐density phase are determined both from luminescence spectra and decay measurements. The dominating recombination processes for the carrier system are found to be phonon‐assisted band‐to‐band Auger recombination, phononless radiative recombination with participation of the nitrogen impurities, and phononless Auger recombination with participation of the nitrogen impurities. The respective rate coefficients are determined.
Electron localization on defects and optical nuclear polarization in disordered and semimagnetic semiconductors
Authors: Bagraev N. T.
Published: July 2007

Using optical nuclear polarization (ONP) and nuclear magnetic resonance (NMR) techniques silicon single crystal solid solutions containing impurity defects of more than one kind are studied. The application of the ONP techniques to the investigation of nonuniformly doped silicon is demonstrated. The concentration and energy fluctuations of donor levels are determined in silicon doped with elements of III to V and transition groups. Solid solution decay processes are studied by ONP techniques in silicon crystals doped with deep‐level impurities. Finally, the ONP technique is applied to the investigations of semimagnetic semiconductors.
Study of the Single and Double Groups CS and SS and Properties of Their Two Isomorphisms
Authors: Le Paillier‐Malecot A.
Published: July 2007

The C8 group is isomorphic to the S8 group, so with the inversion isomorphism for these double groups, the character table, the multiplication table, and the full rotation group compatibility table are established. Another isomorphism, Tisza's isomorphism, is defined. The study of the properties in the two isomorphisms shows the main advantages of the inversion isomorphism. Finally for groups having the Sn operation (n = 5, 8, 10 and 12) the correspondence between improper and inverse rotations is given and for a doubly degenerate species (with J = 1) the character value is calculated which is irrational.
Impurity Band Conduction in HgTe I. Experimental Data
Authors: Dziuba Z.
Published: July 2007

The magnetic field dependence of the electrical conduction in HgTe samples at 4.2 K is analysed taking into account the energy dependence of the carrier mobility. The concentration and mobility of the electric carriers are calculated. The electrical conductivity in HgTe samples at low temperatures is described by electrons in the conduction band, holes in the valence band, and electric carriers in an impurity band. The electric carriers in the impurity band behave in the magnetic field like electrons and holes with comparable values of mobility and concentration. The electrical conductivity shows a strong dependence of the mobility on the energy of the conduction band electrons.
On the Configuration Coordinate Diagram of ZnS:Fe
Authors: Zimmermann H. , Boyn R.
Published: July 2007

A discussion is given of the configuration coordinate diagram of ZnS:Fe, which is partly based on experimental data obtained by the authors in this and in a preceding paper. Intraimpurity transitions in Fe2+ → Fe3 + e (I) and Fe3+ → Fe2+ + + h (N) are included. For the latter transitions the zero‐phonon threshold energies are found to be 2.45 eV and 1.38 eV, respectively, at 80 K, and a value of 0.375 eV is obtained for the lattice‐ relaxation energies. In the absorption spectrum at 2 K sharp lines are observed above the zerophonon threshold energy of transition I, and these are attributed to 3d‐levels in resonance with the conduction band.
Scattering of Axially Channeled Particles by Atomic Strings in Crystals
Authors: Cherdyntsev V. V. , Pokhil G. P. , Ryabov V. A.
Published: July 2007

The motion of axially channeled particles at shallow depth is analytically treated. It is shown that there may exist finite trajectories for particles with energies of the order of the critical energy. The effect of focusing upon backscattering yield oscillations is examined. For the transition from axial channeling to planar one, the self‐consistent solution of the equations of motion is obtained, which allows to take an exact account of the discrete arrangement of atomic rows making up a planar channel. An analysis of this solution offers a new interpretation of the processes occurring at such transitions.
Carrier Statistics in Superlattices of Small‐Gap Semiconductors in the Presence of a Quantizing Magnetic Field
Authors: Ghatak K. P. , Chakravarti A. N.
Published: July 2007

An attempt is made to formulate the generalized carrier statistics in superlattice structures of small‐gap semiconductors by deriving expressions for the density‐of‐states function and the carrier concentration in such structures in the presence of a quantizing magnetic field. The corresponding expressions for superlattices of large‐gap semiconductors can easily be obtained from the generalized relations derived. These results would be more general than those reported in the literature for such structures of large‐gap semiconductors on the basis of tight‐binding approximations.
Anharmonic Study on the Debye‐Waller Factor for a Vacancy‐Trapped Mössbauer Impurity in Solid Solution of Aluminium
Authors: Acharyya P. K. , Tewari A. K. , Roy S. K. , Bhattacharya D. L.
Published: July 2007

The effect of anharmonicity on the Debye‐Waller factor for a simple cubic lattice containing Mössbauer atoms as impurity along with vacancies at the lattice points is evaluated theoretically using the double‐time Green's function technique in the high‐temperature limit. The contributions of the anharmonic lattice vibrations to the Debye‐Waller factor for 57Fe atoms present as a dilute impurity in solid solution of aluminium are also estimated. The exponent of the Debye‐Waller factor is found to increase much more rapidly at higher temperature for a defect lattice than that for a perfect lattice due to the presence of the anharmonic terms in the expansion of the potential energy of the crystal.
Luminescence et structure vibrationnelle des luminophores CaS:Ce3+ et CaSe:Ce3+
Authors: Asano S. , Yamashita N. , Ogawa Y.
Published: July 2007

Excitation and emission spectra for CaS:Ce3+ and CaSe:Ce3+ phosphors are measured at 6, 80, and 300 K. The vibrational structure on the emission bands 2T2g → 2F5/2, 2F7/2 and the excitation band 2F5/2 → 2T2g is analysed by the use of energy matrices in order to determine the coupling constant of spin‐orbit interaction ζf and the crystal field parameters V, V for the 4f orbit. In addition, some information on the transfer of the excitation energy from the host crystal to an unexcited Ce3+ ion is obtained by the aid of the diffused reflection spectra of CaS, CaSe, SrS, and SrSe phosphors containing no activator.
Picosecond Study of the Exciton Gas Cooling
Authors: Amand T. , Cornet A. , Pugnet M. , Collet J. , Brousseau M. , Razbirin B. S.
Published: July 2007

The exciton gas cooling in several materials. namely CdSe, CuI is investigated. The exciton gas temperature is measured directly from picosecond time resolved luminescence. It is shown that the cooling kinetics are well explained by emission of acoustical phonons by excitons. The lattice temperature is deduced, as well as the density of excitons created after the exciting laser pulse.
Tight‐Binding Cluster Model for C, Si, and Ge. II. Size and Shape Dependence of Some Cluster Properties
Authors: Künne L. , Skála L. , Bílek O.
Published: July 2007

The analytic description of the electronic structure of the s‐states tight‐binding cluster model of C, Si, and Ge (part I) is used for the investigation of the dependence of some electronic properties of the clusters on their size and shape. Clusters in the form of cubes, plates, and rods are considered. The rate of convergence of the cluster properties to the corresponding limit values is discussed.
Carrier Density near the Semiconductor‐Insulator Interface. Local Density Approximation for Non‐Isotropic Effective Mass
Authors: Paasch G. , Übensee H.
Published: July 2007

For bent bands in a semiconductor near the interface to an insulator, a modified local density approximation is described which takes into account the influence of the interface barrier in the potential. Simple expressions are obtained for the electron and hole densities in a multi‐valley semiconductor with non‐isotropic parabolic bands. Due to the interface barrier the densities decrease to zero towards the interface. The characteristic length for this decrease is determined by the effective mass tensor and by the orientation of the interface. Results of self‐consistent density calculations are reported.
Cooperative Jahn‐Teller Orbital Ordering in KCuF3 and K2CuF4 Crystals
Authors: Nikiforov A. E. , Shashkin S. Yu. , Levitan M. L. , Agamalyan T. H.
Published: July 2007

Two mechanisms of Cu2+ orbital ordering in KCuF3 and K2CuF4 are considered: the Jahn‐Teller and the exchange mechanisms with account of the crystal elastic energy. The Jahn‐Teller coupling constants and exchange parameters necessary for the evaluation of angles characterizing the orbital and crystal structures of crystals are estimated. It is shown that the exchange mechanism alone cannot explain the experimentally observed values of orbital structure angles. Within the frames of the Jahn‐Teller mechanism both, orbital and magnetic structures of KCuF3 and K2CuF4 are explained and the connection between the orbital structure angles of these crystals is established and also a qualitative explanation of the orbital structure angle values experimentally observed in MnF3 and KCrF3 is given.
On the Translation Mode of the Kink Lattice of a Quasi‐One‐Dimensional Peierls‐Fröhlich System with a Nearly Half‐Filled Band
Authors: Fischbeck H. J.
Published: July 2007

The translation mode of the kink lattice of a quasi‐one‐dimensional Peierls‐Fröhlich system in the continuum limit is investigated by means of the Landau expansion of the thermodynamic potential. An expression for the sound velocity is given and numerically evaluated.
The Rôle of Two‐Level Systems in Amorphous Metallic Alloys. I. Induced Anisotropy in Amorphous Ferromagnets
Authors: Kronmüller H.
Published: July 2007

The induced anisotropy in amorphous ferromagnets is attributed to the directional ordering of atom pair axes. The thermodynamics and micromagnetic background of this process is treated on the basis of two‐level systems which are identified with mobile atom pairs in the neighbourhood of free volumes. The rǒle of ‘reversible’ and of ‘irreversible’ relaxation processes is discussed in detail.
Magnetic Properties of Dysprosium Oxisulphate (Dy2O2SO4)
Authors: Hülsing H. , Kasten A.
Published: July 2007

Below TN = (8.05 ± 0.10) K Dy2O2SO4 is a nearly two‐dimensional co‐linear Ising antiferromagnet. At low temperatures (T ≦ 2.4 K) and in external magnetic fields between 1.7 and 2.2 T along the ordering direction there is an intermediate ferrimagnetic phase with a T = 0 magnetization of 1/3 of the saturation value. From Zeeman spectroscopy and theoretical considerations the structure of the magnetic phases are derived. The details of the phase diagram as established by magnetization measurements are not reproduced by mean field calculations. At the boundary between the antiferromagnetic and ferrimagnetic phase there is a hysteresis which is asymmetric with respect to the sweep direction of the external magnetic field. This behaviour can be explained by the occurrence of different magnetization processes. The time constants as determined experimentally are of the order of ten minutes.
A Raman Study of Compositional Disorder in Defect Chalcopyrites
Authors: Razzetti C. , Lottici P. P. , Zanotti L. , Curti M.
Published: July 2007

The growth and Raman scattering results obtained for ZnGa2xIn2(1−x)S4, ZnxCd1−xGa2S4, and CdGa2xIn2(1−x)S4 tetragonal pseudoternary compounds are given and discussed. The phonon spectra display one‐ and two‐mode behaviour. For ZnxCd1−xGa2S4 compounds a virtual crystal model seems to describe the observed one‐mode behaviour.