bluuebunny/crossref_metadata_2025_split huggingface dataset:
Far‐Infrared Reflectivity of Optical Magnons in FeF2 and CoF2
Authors: Häussler K. M. , Lehmeyer A. , Merten L. 
Published: July 2007

AbstractAntiferromagnetic optical magnons are observed by means of their far‐infrared reflectivity for the first time. The structure of these “magnetic reststrahlenbands” is calculated approximately for normal incidence of the infrared radiation. Spin wave damping can be taken into account by introducing complex eigenfrequencies. The tensor elements of the frequency dependent complex magnetic permeability fulfill a generalized Kurosawa relation.
On the Diffusion Theory of Kink‐Pair Formation on Dislocations
Authors: Mann E. 
Published: July 2007

AbstractThe process of kink‐pair formation on dislocations under the action of an applied shear stress is studied by means of the diffusion theory for the limiting cases of high or low kink mobility. In both cases exact expressions for the mean lifetime or formation time of a kink pair are derived. They may be converted into analytical forms under fairly general assumptions. In the case of low kink mobility a general formula is obtained that is valid for applied shear stresses of arbitrary magnitude and thus connects the results known hitherto only for high and low stresses, respectively. From the expressions for the lifetime the rate of kink‐pair formation is calculated by introducing the free energies of the system. The appendix deals with the exactly solvable diffusion equation in the case of high mobility.
Time‐Dependent Momentum Distribution of Polarons at Arbitrary Temperature and Electric Field
Authors: Brosens F. , Devreese J. T. 
Published: July 2007

AbstractThe 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

AbstractA 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

AbstractThe “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

AbstractKnight 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

AbstractPhotoluminescence 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractUsing 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

AbstractAn 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

AbstractThe 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

AbstractDescribing 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

AbstractHD‐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

AbstractIn 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

AbstractThe 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

AbstractQuantumchemical 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

AbstractThe 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

AbstractHydrogenated 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

AbstractA 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractA 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

AbstractPhonon 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

AbstractThe 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.