bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Magnetic Surface Reconstruction in Spin‐Wave Resonance: Twofold Resonance of the Surface Mode
Authors: Puszkarski H.
Published: July 2007

Feasible techniques are considered, aimed at the production of a magnetic surface reconstruction (MSR) effect in thin ferromagnetic films as a result of which the surface spins orient themselves obliquely to the uniformly oriented bulk spins. It is shown that this kind of MSR admits of twofold resonance excitation of the same surface mode (in intrinsically different physical conditions): once previous to MSR, and once again subsequent to MSR.
Solution of the Onsager Ion Recombination Model in One Dimension
Authors: Singh J. , Baessler H.
Published: July 2007

Applying Onsager's theory in one dimension it is shown that there is a finite probability for a carrier to escape geminate recombination with its image charge a t the interface of a molecular crystal and metal even in the absence of an external field. In contrast to the three dimensional case, however, the escape probability is not a strictly linear function of the field and depends on the initial pair separation, even at low fields. Good agreement between theory and experiment is obtained.
Bound States in Electron—Exciton Collisions
Authors: Gerlach B.
Published: July 2007

An electron‐exciton process is considered for arbitrary positive electron and hole masses. The existence of a bound state can be proved under the following assumptions: The system can be described as a polaronic three‐particle system, the two‐particle interactions are of Haken's type.
Exciton Interaction in Photoluminescence from ZnO
Authors: Hvam J. M.
Published: July 2007

The luminescence spectra of ZnO platelets at 1.9 K have been investigated under 3371 Å Insor excitation from 5 to 500 kW/cm2. New details in the P‐band luminescence (exciton‐exciton scattering) reveal the separation between A and B excitons in ZnO and the stimulated emission in this band is found to be strong for exciton scattering to higher excited states (n > 2). At the highest excitation powers a new emission line is observed which is associated with the decay of excitonic molecules. The molecule binding energy is estimated to be 15 to 20 meV.
Exciton Exchange and Lyddane‐Sachs‐Teller Relations
Authors: Schütz O. , Uihlein Ch.
Published: July 2007

A consistent treatment of excitons interacting with light shows that the nonanalytic part of the exchange interaction leads to a longitudinal‐transverse splitting that equals the energy shift of transversal excitons due to polariton effects in cubic crystals (K = 0). This holds true also if excited exciton states are included.
Relaxation Process in U3+:Lacl3 Detected by Resonant Self‐Absorption of Fluorescence
Authors: Furrer R. , Seiff F. , Pelzl J.
Published: July 2007

Using a pulse saturation method with optical detection of the recovery of fluorescent light, the temperature dependence and angular anisotropy of the spin‐lattice relaxation rate T of U3+ in anhydrous lanthanum trichloride has been measured. In the range 1.5 K ≦ T ≦ 4.2 K the direct relaxation process was found to be dominant, leading to a relaxation rate which can be fitted to the expression T = 4.16(5)T s−1 with the anisotropy coefficient of the direct process A (θ = 90°)/A (θ = 0°) ≈ 10. Both results are comparable with the data measured for the corresponding 4f3 ion, Nd3+ in LaCl3.
Comparison of an Amorphous CBS Structure Model of Amorphous Silicon with X‐Ray and Electron Scattering Data
Authors: Boyce J. F. , McCloughrey J. K.
Published: July 2007

An amorphous CBS‐structure model of the form employed by Kramer et al. has been compared to electron and X‐ray scattering data from amorphous silicon. The results tend to support a distorted wurtzite structure.
Temperature Genesis of the Curves of Absorption and Dispersion of Light by Ionic Crystals in the Exciton Region
Authors: Lubchenko A. F. , Nitsovich V. M. , Tkach N. V.
Published: July 2007

The temperature dependence of the curves of absorption and dispersion of light of ionic crystals in the exciton region is investigated taking into account the interaction of excitons with acoustic and optical phonons. It is shown that the shape of the curves depends on the sign of an effective exciton mass m. In the approximation linear in the phonon operators the maxima of the curves shift with increasing temperature towards lower energies when m > 0, and towards higher energies when m < 0.
Optical Phonons in CdSiP2
Authors: Bettini M. , Bauhofer W. , Cardona M. , Nitsche R.
Published: July 2007

Raman scattering and reststrahl measurements are presented on single crystals of the chalcopyrite CdSiP2, grown from the vapour phase by iodine transport. The spectra of the zone centered phonons from 60 to 520 cm−1 show a strong similarity to those of ZnSiP2. The mode distribution of the spectra can be completely interpreted by considering that the Γ‐phonons of the chalcopyrites derive from the zincblende modes of the Γ, X, and W points. A qualitative comparison of the simpler modes in CdSiP2, to those in ZnSiP2, shows that the force constants do not deviate much from those of the corresponding binary and elemental semiconductors.
Effect of Interatomic Interactions on the Zero‐Bandwidth Hubbard Hamiltonian
Authors: Tu R. S. , Kaplan T. A.
Published: July 2007

We consider the linear‐chain zero‐bandwidth Hubbard Hamiltonian with added nearest‐neighbor interactions, with a magnetic field present. Exact expressions for thermo‐dynamic quantities are obtained in simple closed form for the half‐filled band. The qualitative behavior of the expressions for susceptibility and specific heat is discussed with a view toward recent studies of the organic salt NMP‐TCNQ.
Hyperfine Properties of Capper Metal
Authors: Abati S. , Borchi E. , Coppi P. G. , de Gennaro S.
Published: July 2007

A detailed theoretical analysis is made of the hyperfine properties of metallic copper in the framework of the orthogonalized plane‐wave method and pseudopotential formalism using a reliable model for the Fermi surface of copper. The direct and exchange core polarization (ECP) contributions to the conduction electron spin density are evaluated and the role of the ECP contribution is discussed. The exchange and lattice enhancement effects on the Knight shift and the spin‐lattice relaxation time are taken into account accurately. The agreement between the theoretical results and the available experimental data is quite satisfactory.
The Spectral Distribution of Acoustical Phonons in a Plate
Authors: Iosilevskii Ya. A.
Published: July 2007

A general expression is obtained for the spectral density, g(ω2), of acoustical phonon states in the elastically isotropic plate of arbitrary thickness, d, with stress‐free boundaries using the Green‐function method. The cases are analysed where the phonon mean free path, lph, is much larger and conversely much less than d. It is shown that the surface contribution to the thermodynamical values is additive at T ≫ πħc/kd (c is a characteristic velocity of sound) while the exact function g(02) is not additive for lph ≫ d. In this case a simple analytical expression is found for the effective distribution which allows to calculate the surface contribution to the thermodynamical characteristics of the sample of arbitrary configuration.
Behaviour of Microwave Absorption of. Tellurium at Temperatures below 10 K
Authors: Herrmann Karin , Herrmann R. , Braune W.
Published: July 2007

Two characteristic lines have been found in the differential microwave absorption spectrum of tellurium in low magnetic fields at temperatures below 6.4 K. These lines were observed independently of the hole concentration between p = 3.7 × 1013 and 3.4 × 1018 cm−3. The magnetic field dependence of the lines as well as their amplitudes were investigated between 1.4 and 6.4 K and showed a strong temperature dependence. The anisotropy of the lines in the magnetic field is not determined by the crystallographic orientation of tellurium, but by the orientation of the magnetic field with respect to the sample surface. A frequency dependence of the lines has not been observed within the spectral range investigated.
Investigation of Pair Interactions of Er3+ Ions in YVO4 by Paramagnetic Resonance
Authors: Kugler H. , Hillmer W. , Kahle H. G.
Published: July 2007

EPR measurements on YVO4 doped with 1% of the erbium isotope 170 were carried out in the K‐band at T = 1.7 K to investigate the Er3+ pair interaction in this compound. The experimental results for five different neighbour pairs are described by an effective spin Hamiltonian, the parameters of which were obtained by a fit to the measured resonances for various angles between magnetic field and crystal c‐axis. The pair interaction is mostly due to magnetic dipole and only a small part is due to superexchange interaction.
Use of X‐Ray Diffuse Scattering Data for the Construction of the Fe‐Si Equilibrium Diagram
Authors: Semenovskaya S. V. , Umidov D. M.
Published: July 2007

The Fourier‐transforms of interchange energies were determined by the method of X‐ray diffuse scattering from single crystals of disordered Fe‐Si solid solutions. The values of Fourier transforms obtained were used to calculate the free energy in the self‐consistent field approximation and to construct equilibrium Fe‐Si diagrams within the composition range of 0 to 50 at% Si. The diagram calculated contains a two‐phase α1 + α2 field at high temperatures and a two‐phase α + α1field at low temperatures. The situation of the phase boundaries is in good accordance with the experimentally observed one.
Explanation of the Magnetic Structure of Er2Ni7 with a Point‐Charge Crystal Field Model
Authors: Yakinthos J. K. , Rentzeperis P. J.
Published: July 2007

The magnetic structure of the intermetallic compound Er2Ni7, predicted by Bertaut's macroscopic theory, is plausibly explained by a crystal field model. The Er ions occupy two sites with anisotropy directions perpendicular to each other. The exchange energy forces the moments to deviate from these directions and form a canted structure.
Infrared Absorption and Ferroelectricity of Small BaTiO3 Crystals
Authors: Ruppin R.
Published: July 2007

Infrared absorption spectra of small BaTiO3 crystals which have been reported by various investigators are compared with detailed theoretical calculations performed for spherical samples. The porperties of the infrared active modes and the soft mode in small crystallites are discussed. It is found that a shift of the transition temperature is not expected to occur in grains of the size usually employed in infrared absorption measurements, but will appear in extremely small crystallites, less than about 120 Å in size.
Shubnikov‐de Haas Effect in the Longitudinal Magnetoresistance of p‐Type Germanium
Authors: Carrère G. , Ulmet J. P. , Askenazy S. , Barbaste R.
Published: July 2007

Shubnikov‐de Haas oscillations in the longitudinal magnetoresistance of heavily‐doped p‐type germanium (p ≈ 3.9 × 1018 cm−3) are observed for H parallel to 〈100〉, 〈110〉, and〈111〉. The direction‐dependent behaviour of these oscillations shows the anisotropy and non‐equidistance (Luttinger effect) of the light‐hole Landau levels.
Generalization of Fick's Law for Non‐Local Complex Diffusion in Semiconductors
Authors: Jacoboni C.
Published: July 2007

Electron diffusion in semiconductors is studied at short distances and/or times. An extension of Fick's law with higher derivatives is introduced, and a numerical method determines how many terms are to be kept and the values of the corresponding coefficients. At high applied electric fields the asymmetric diffusion introduces an imaginary part of the Fourier‐transformed diffusion coefficient, that is, even order derivatives in the diffusion equation.
On the Chemical Potential of the Hubbard Model
Authors: Monecke J.
Published: July 2007

On the basis of a cluster approach to the Hubbard model it is proved that the chemical potential μ as a function of the number of electrons per atom n has a jump at n = I on the insulating site and that n as a function of μ has a jump at μ = I/2 on the metallic site of the metal–insulator transition. In terms of an one‐electron picture this corresponds to an infinite density of states at the Fermi level in the metallic phase.
The Magnetic Field Distribution of Superconducting Alloys with Small Magnetization
Authors: Brandt E. H.
Published: July 2007

The magnetic field distribution of type‐II superconductors with arbitrary concentration of non‐paramagnetic and paramagnetic impurities is calculated near Hc2 for 0 ≦ T < Tc. At a certain temperature T, depending on the impurity concentration, minimum and saddle point of H interchange their positions. At very low T, the reduced magnetization |4πM| B/N〈|Δ|2〉 reaches a maximum at a certain impurity concentration, but even a small concentration of paramagnetic impurities inhibits this effect. The new H leads to a correction of the χ2‐theory.
Single‐Particle Excitations in Mott Insulators
Authors: Monecke J.
Published: July 2007

On the basis of a moment‐generated solution of the Hubbard model the quasiparticle energies of an extra electron or a hole in an otherwise half‐filled band are calculated in the limit of small hopping probability. In first order in the hopping probability the quasi‐particle bandwidth is zero in the case of a simple up–down spin configuration and small if zero‐point spin deviations are included. In the paramagnetic case the bandwidth is reduced to one half of the ferromagnetic one.
Correlation of Magnetic Susceptibility, Electrical Conductivity, and Structural Parameters of Ti3O5 via EPR Spectroscopy
Authors: Houlihan J. F. , Mulay L. N.
Published: July 2007

Variable temperature EPR studies of polycrystalline Ti3O5 have been interpreted in relation to magnetic susceptibility, electrical conductivity and X‐ray diffraction data obtained on samples from the same chemical run. Electronic mobility and electrical conductivity values have been calculated in both the metallic and semiconducting regions of Ti3O5 and have been discussed in relation to the experimentally determined conductivity values of Bartholomew and Frankl. In addition, the multiplet structure observed in the EPR spectrum has been interpreted in terms of an anisotropic g‐factor and the randomness of the polycrystalline sample.
Pseudopotential Calculation of the Piezoelectric Constant of GaAs
Authors: Zeller G. R.
Published: July 2007

A theory of the piezoelectricity of zincblende type lattices is presented. The piezoelectric polarization in a uniformly strained crystal is produced by the internal displacements of the two core ion sublattices and the distortion of the valence electron distribution. The latter is calculated applying first order perturbation theory to the empirical pseudopotential method. The computational work has been carried out for GaAs. Satisfactory agreement between theoretical and experimental values of the piezoelectric constant is achieved if the deformable ion model is applied to empirical pseudopotentials. As a test for the accuracy of the numerical calculation the transverse effective charge is evaluated.
The Electronic Structure of Chalcogenide Solids: A Photoemission Study of Sulphur
Authors: Williams R. H. , Polanco J. I.
Published: July 2007

Photoelectric yield and energy distribution curves for photon energies up to 11 eV, have been studied for evaporated sulphur films. For clean films the threshold occurs at 7.2 eV. The EDC's, as well as the form of the yield near threshold, agree well with calculated curves derived from the molecular orbital calculation of Chen, with levels broadened by taking into account intermolecular interactions.
Magnetic Ordering in HoAsO4 Investigated by Means of Specific Heat and Spectroscopic Measurements
Authors: Schwab M. , Berkhahn W. , Kahle H. G. , Simon A. , Wüchner W.
Published: July 2007

The heat capacity, optical absorption spectrum, and Zeeman effect of HoAsO4 have been measured at low temperatures. The specific heat shows a lambda anomaly at TN = = (0.557 ± 0.005) K which has to be attributed to a magnetic phase transition from the paramagnetic to the antiferromagnetic state. The anomaly is superimposed on a large contribution from the nuclear hyperfine interaction of holmium. Spectroscopic measurements indicate that the lowest lying crystal field state of the Ho3+ ions in HoAsO4 is a doublet Γ5 (gc = 15.3 ± 0.3) and that the first excited state is a singlet Γ1 at about 7.5 cm−1. The magnetic interaction in HoAsO4 has the form of Ising interaction. A simple two sublattice structure with magnetic moment direction along the optical axis is proposed for the ordered state. Assuming this structure, an estimation of exchange interaction to nearest (nn), next nearest (nnn), and third nearest (tnn) neighbours was performed. It shows that the contributions to the total exchange field of the nn and nnn are nearly compensated by those of tnn. Dipole–dipole interaction was found to be dominant for the antiferromagnetic ordering in HoAsO4.
Green's Functions Approach to the Theory of Hopping Conductivity in Disordered Semiconductors
Authors: Manucharyants E. O. , Zvyagin I. P.
Published: July 2007

It is shown that care should be taken when the phonon‐assisted hopping conductivity is evaluated using the formal expansion in powers of the electron–phonon coupling constant g. This expansion appears to be divergent in the low‐frequency range, and the divergent terms should be partially summed up to give a correct lowest‐order contribution to the conductivity. The resulting expressions can be obtained as well by using a standard decoupling procedure in the equations for the higher‐order Green's functions.
Anisotropy in the Optical Transitions from the π and σ Valence Bands of Graphite
Authors: Klucker R. , Skibowski M. , Steinmann W.
Published: July 2007

The dielectric tensor \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^ \wedge $\end{document}(ω) of pyrolytic graphite, cleaved with its surface perpendicular to the c‐axis, has been determined using polarized synchrotron radiation of photon energies 3 eV ≦ hω ≦ 40 eV. For this purpose the reflectivities were measured for nine angles of incidence between 15° and 75°, the electric field vector E lying parallel and perpendicular to the plane of incidence. The complex dielectric functions \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^ \wedge $\end{document} ⊥and \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^ \wedge $\end{document} ‖ for E perpendicular and parallel to the c‐axis were obtained by a least squares fitting of the measured reflectivities to the calculated ones. In addition \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^ \wedge $\end{document} ⊥ was determined independently by a dispersion analysis of the reflectivity at 15° angle of incidence. The results showing strong anisotropy over the whole energy range are compared with previous optical data for \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \varepsilon \limits^ \wedge $\end{document} and electron energy losses. On the basis of recent two and three dimensional band structure calculations the spectra are attributed to direct interband transitions from the π‐ and σ‐type valence bands to π(σ)‐and σ(π)‐conduction bands for E ⊥ c (E ‖ c), respectively.
Herzberg‐Teller Interaction and Vibronic Coupling in Molecular Crystals. III. Influence of the Phonon‐Assisted Exciton Transfer and Phonon Propagation on the “False” Origin of the Absorption Spectrum
Authors: Zgierski M. Z.
Published: July 2007

The influence is discussed of the phonon‐assisted exciton transfer induced by the Herzberg‐Teller interaction, and of the propagation of phonons participating in the Herzberg‐Teller coupling on the creation of vibronic and scattering states in the region of a “false” origin of an absorption spectrum of an one‐dimensional molecular crystal.
Observation of High‐Temperature Heisenberg Ferromagnet Spin Relaxation by EPR
Authors: Cusumano C. , Troup G. J.
Published: July 2007

The previously observed exponential lineshape in the wings of an exchange‐narrowed system, namely solid solution of DPPH in polystyrene, is interpreted as being the result of the spin‐relaxation function of the high‐temperature Heisenberg ferromagnet. This is possible because the exchange frequency was of the same order as the hyperfine width, and because the system is amorphous and disordered.
The Kinetics of Recombination Radiation and the Temperature of the Electron–Hole Plasma in Silicon
Authors: Dite A. F. , Lysenko V. G. , Timofeev V. B.
Published: July 2007

The temperature of the excitonic gas as well as the temperature and the carrier density in electron‐hole drops (EHD) under intense optical excitation are determined from the analysis of the spectrum of recombination radiation in silicon. The temperature range within which EHD exist is determined (the critical temperature is Tc ≈︁ 33 °K) and the upper branch of the state diagram, excitonic gas–excitonic condensate, is constructed. The kinetics of the radiative recombination spectrum of EHD and excitons is studied, as well as that of the decay of the electron‐hole plasma at temperatures above Tc. The dependence of the boundaries of the radiative recombination spectrum on the carrier density in the plasma is analysed.
A Non‐Parametric Model Pseudopotential and Electronic Transport Coefficients
Authors: Srivastava S. K.
Published: July 2007

A non‐parametric model pseudopotential for metals is proposed. The cut of region of the form factors lies between 1.4<q/kF<2. The various electronic transport coefficients of liquid metals obtained for this model potential agree reasonably well with experiment. The influence of electron–lattice interaction parameter K on the various electronic transport coefficients is discussed.
Localization of Electron Wave Functions in One‐Dimensional Disordered Systems
Authors: Hermann H.
Published: July 2007

Results are presented of numerical calculations of the eigenvalues and eigenvectors of one‐dimensional systems with diagonal and off‐diagonal disorder. Two possible definitions of the localization length are discussed. Using a formula of Thouless the quantity Lexp(E) describing the exponential decay of a wave function far away from the localization centre is calculated for chains containing up to 400 lattice sites. The energy dependence of Lexp for off‐diagonal disorder differs from that of the well‐known case of diagonal disorder. It is found that the wave functions of an infinite one‐dimensional system with statistically distributed hopping terms are extended at least over an half infinite part of the complete infinite system.
Long‐Wavelength Optical Phonons of AgBrxCl1−x Mixed Crystals
Authors: Bootz B. , von der Osten W. , Uhle N.
Published: July 2007

Infrared reflectivity and second‐order Raman scattering in the mixed crystal system AgBrxCl1−x, were studied at 77 K. The TO(Γ) and LO(Γ) phonon frequencies obtained by means of a Kramers‐Kronig analysis shift continuously between their energetic positions in the pure components indicating one‐mode behaviour of the system. This result is discussed in view of a recent model that describes the optical modes of a mixed crystal. The LO(Γ) phonon frequencies taken from the Raman spectra are found to be slightly lower. A resonant enhancement of the 2L0 scattering process that occurs for z = 0.42 with shorter‐wavelength excitation is used to determine the longitudinal optical phonon frequency.
A New Radiochemical Method for Determining the Oscillator Strength of F Centres in Alkali Halides
Authors: Bosi L. , Lazzarini A. L. Fantola , Lazzarini E.
Published: July 2007

A new radiochemical method for determining the concentration of F centres in X‐ or γ‐ray coloured crystals of alkali halides is described. The method is based on the fact that crystals containing F centres upon dissolution in water give rise to the formation of an equal amount of hydrated electrons. Hydrated electrons are able to reduce [Co(NH3)6]3+complex ions to Co2+; thus, by dissolving crystals containing F centres in a water solution of [Co(NH3)6]3+ions, the amount of formed Co2+ions is related to the concentration of F centres in the crystals. The method was applied to the determination of F centre concentration in NaCl, KC1, and KBr crystals coloured by50Co γ‐ray irradiation. The use of these concentrations in the Smakula absorption formula enables to calculate the optical oscillator strength / of F centres. The / values obtained are as follows: ƒNaCl = 0.64 ± 0.02; ƒKCl = 0.68 ± 0.02; /KBr = 0.72 ± 0.02. These results are compared with those already reported in literature.
Modifications of KN Theory for Weak Resonance and Dislocation Scattering of Phonons
Authors: Singh M. P. , Verma G. S.
Published: July 2007

The resonance scattering of phonons in Co++ doped CdS is investigated both in the presence and absence of magnetic fields on the basis of modified expressions of Kozakov and Nagaev, which include weak resonance scattering of phonons for the off‐resonance situations T<TR, where TR is the resonance temperature. It is possible to establish that the major contribution to resistivity due to resonance scattering of phonons comes from the virtual transition between the magnetic field split states | + 1/2 〉 and | −1/2〉. Similar modifications of KN expressions for the weak dislocation scattering as well as the weak resonance scattering for ω>ωr lead to plausible explanations for the unexplained depressions in the phonon conductivity of CaF2 in the temperature range 2 to 4 °K.
Inelastic Scattering of Slow Electrons from a Heisenfoerg Ferromagnet
Authors: Harriague S. , Majlis N. , Makler S. A. , Olmedo A. M.
Published: July 2007

The magnetic inelastic scattering of low‐energy electrons by the (001) surface of a b.c.c. Heisenberg ferromagnet is considered. As a first step, the spectrum of elementary magnetic excitations is obtained, revealing the existence of optical and acoustic surface magnons, as well as a continuum of bulk magnons. The differential cross‐section for inelastic scattering shows the contribution of those types of excitations; localized niagnons give a delta function contribution, while the contribution due to the bulk states is qualitatively different depending on the value of the ratio of the exchange integrals for surface and bulk spins, thereby providing a method for investigating the exchange interactions at the surface.
Computer Modelling of the Order–Disorder Transformation in Cu3Au and Ni3Fe
Authors: Golosov N. S. , Dudka B. V.
Published: July 2007

The problem of the type definition of phase transition is discussed on the example of order–disorder transformation modelling by the Monte‐Carlo method. The discrepancy of the transition temperature of stationary state distributions which were obtained from high‐and low‐temperature chains defines the phase transition type. Necessity of metastable phase overcooling or overheating at the first‐order transition results in the dependence of the critical state of the substance on that one preceeding the transition in the critical one.
Luminescence and Excitation Spectra of Exciton Emission in GaAs
Authors: Heim U. , Hiesinger P.
Published: July 2007

Luminescence, excitation, and photoconductivity spectra of epitaxial n‐ and p‐type GaAs at 1.6 K are measured with high resolution, using very low intensity of excitation. Under this condition, it is found that the (D°, X) bound exciton emission is split into two components, and that further structure is observable between the (D°, X) and the luminescence band, recently attributed to lower branch polariton emission. In particular, two very sharp lines are seen ≈︁ 0.5 meV above the (D°, X) emission. These lines are observed in all our samples, and their intensities seem to be related to the strength of the (D°, X) line. Analysis of the excitation spectra lead to the identification of three emission lines. These are the n = 2 free exciton transition, the two‐electron transition of the (D°, X) complex with the donor left in then = 3 excited state, and most probably the n = 2 donor‐to‐valence band transition.
The Critical Voltage Effect in Transmission Electron Microscopy. I. Eigenvalue Degeneracy in the Three‐Beam Case
Authors: Gevers R. , Serneels R. , David M.
Published: July 2007

Given an orientation where only one reflection h is nearly in exact Bragg condition, the others being far from the Ewald sphere, the various physical processes are discussed which may lead to an anomalously low intensity of the reflection h. It is found that exact eigenvalue degeneracy is neither a sufficient nor a necessary condition for an anomalous intensity reduction. Absorption effects being neglected, a general three beam case 0, h, g is discussed. It is shown that a reflection being far from the Ewald sphere may induce a weak intensity for a reflection ft close to the Ewald sphere. The necessary conditions are discussed. The usual linear second order critical voltage effect follows as a particular case. Then it is shown that for the linear three beam case, absorption effects taken into account, particular diffraction conditions exist, i.e. voltage and orientation conditions, for which two complex eigenvalues are equal.
The Linear Dielectric Response of a Semiconductor: A New Analytic Form for the Dielectric Function
Authors: Schulze K. R. , Unger K.
Published: July 2007

A simple analytic form for the linear dielectric function ϵ(q) of a semiconductor is constructed, which accounts correctly for the limiting cases q = 0 and asymptotic large q values. For ϵ(0) → ∞ a Weizsäcker type of screening is obtained. Numerical comparison with dielectric functions calculated on the basis of exact band structures shows very good agreement. Analytical expressions for the r‐dependent dielectric function, for the filter function, and for the convolution function are also given and their implications are discussed.
Influence of Uniaxial Pressure on Hot‐Electron Conductivity in n‐Si at Low Temperatures
Authors: Asche M. , Russu E.
Published: July 2007

Investigations of current‐voltage characteristics of n‐Si along 〈100〉 direction at low temperatures when nde should occur are repeated in detail. In order to verify the inter‐valley carrier transfer as origin of current saturation and accompanying oscillations, the valleys are split energetically by uniaxial pressures parallel and perpendicular to the 〈100〉 direction.
On the Determination of the Phonon Density of States by Means of Coherent Inelastic Scattering of Neutrons on Polycrystalline Samples
Authors: Eschrig H. , van Loyen L. , Ziesche P.
Published: July 2007

The possibility, the accuracy, and the necessary experimental conditions for a direct determination of the phonon density of states by means of coherent inelastic neutron scattering on polycrystalline samples in the case of non‐cubic material is tested by model calculations of both the phonon density of states and the spectrum of scattered neutrons corresponding to measurements carried out by Eremeev et al. on Mg. It is found that apart from the low energy range the necessary experimental conditions can easily be fulfilled and that for correcting the experimental results a method earlier proposed by Gorbachev et al. is appropriate.
Electron–Hole Liquid above the Critical Point
Authors: Martin R. W.
Published: July 2007

At sufficiently high pumping levels, carrier densities of more than 2 × 1017 cm−3 can be produced in doped Ge or small crystals of intrinsic Ge. As a result, the crystal can be entirely filled with a degenerate electron–hole plasma. With increasing temperature this system does not undergo a phase transition to a low density free exciton gas, even at temperatures above the critical point.
Lattice Dynamics of Some Transition Metals by an Elastic Force Model
Authors: Goel C. M. , Pandey B. P. , Dayal B.
Published: July 2007

A modified Sharma‐Joshi model has been employed to compute the phonon dispersion relations in three transition metals, namely gold, palladium, and platinum. The results have been computed in three principal symmetry directions and are found to agree very well with recent experimental data.
A study of spin‐phonon interactions in ferromagnetic insulators. II. The phonon spectrum and spin‐lattice relaxation bate
Authors: Jones M. J. , Cottam M. G.
Published: July 2007

The formalism of a previous paper is employed to evaluate the renormalised energy and damping of phonon excitations due to spin–phonon interactions in a Heisenberg ferro‐magnet. The calculations make use of a field theoretical perturbation technique and an expansion in powers of the high density parameter 1/z. The results are applicable over a wide range of temperatures below Tc. Calculations are also presented for the spin‐lattice relaxation rate, which can be closely related to the phonon damping.
Lattice dynamical study of NaF and RbF by the shell model
Authors: Sneh  , Dayal B.
Published: July 2007

The simple shell model with both ions polarizable has been used to calculate the dispersion relations for NaF and the Debye‐Waller factors for RbF and NaF. Satisfactory agreement between the theoretical and experimental results has been obtained.
Radiative recombination from non‐equilibrium states in cathodo‐excited semiconductors
Authors: Zehe A. , Röpke G.
Published: July 2007

Application of the theory of non‐equilibrium processes is made to the relaxation and recombination of high‐excited electrons in semiconductors. Following the scheme of a hot‐electron theory based on the method of nonequilibrium statistical operator, developed by Zubarev and, independently, by McLennan, expressions for relaxation and recombination times of hot carriers has been established, resulting directly from a microscopial description of the system. As an example a free‐to‐bound transition is discussed (transitions between donor levels and the valence band or between acceptor levels and the conduction band) in (Ga, Al)As. With this en the one hand the concept of hot electrons is shown to be related to the general theoretical description and on the other hand, intensity curves of time‐resolved spectra, i.e., the time‐variation of spectra after an excitation pulse, are given.
Shock waves of excess carrier concentration in a doped semiconductor
Authors: Almazov A. B.
Published: July 2007

A generalization is derived and discussed of the usual equations of ambipolar diffusion, which describes the behaviour of the extra electrons and holes. It is shown that a special quasilinear version of the equations can describe the formation and development of shock waves of electron density.
Influence of E–E scattering on the phenomenological energy relaxation time in nonpolar semiconductors
Authors: Dienys V. , Kancleris Ž
Published: July 2007

The dependence of the phenomenological energy relaxation time τε on carrier concentration in n‐and p‐type Ge and n‐type Si at 77°K was measured. The measurements were carried out in a warm electron region using the harmonic mixing technique. A considerable decrease of τε with concentration was observed. It is shown that this effect can be explained by the influence of interelectronic scattering on the energy losses caused by the emission of high energy (optical or intervalley) phonons.
Linear magnetoresistance, hall effect, and the geometry of anisotropic quantum scattering
Authors: Hsu W. Y. , Falicov L. M.
Published: July 2007

A series of model calculations for the magnetoresistance and Hall effect of free‐electron‐like metals is presented. They involve anisotropic‐quantum‐flucfruation coherent scattering and yield linear longitudinal and transverse magnetoresistances. The models here studied exhibit various anisotropic behaviors for the magnetoresistance slopes. The Hall coefficient is not affected by the coherent scattering and is a constant equal to the free‐electron model in all cases. The results are compatible with existing experimental data for potassium.
Improved model for the stark effect of the F‐center in KCl
Authors: Bonciani M. , Grassano U. M.
Published: July 2007

The results of the Stark effect measured with light polarized parallel to the field, have been reexamined using a more complete theoretical model which takes into account four excited states of the F‐center. The very good agreement between the fitted curve and the experimental data confirms the evaluation of the energy position of the even parity states 2s and 3d already known from previous works.
Shape and structure of phonon sidebands of the ν1 stretching‐vibration absorption band of the NCO− impurity ion in alkali halide crystals
Authors: Tsyashchenko Yu. P. , Pinkevich I. P. , Zaporozhets V. M.
Published: July 2007

Experimental data are compared of the shape and structure of the phonon sidebands (PS) of the ν1 intramolecular vibration (IMV) of the impurity ion NCO− in KCl, KBr, and KI matrices obtained at 10 °K with their theoretical calculations. It is shown that in f.c.c. host lattices the shape (distribution of absorption coefficient K(ωk)) of the acoustic branch of PS is in quantitative agreement with the expression magnified image × × 〈sin2 (κ, ι)〉 g (ωκ), where ωk is the frequency displacement from the centre of the IMV absorption band (frequency of lattice phonon), ωm the maximum frequency of the phonon spectrum of the matrix, g(ωk) the density of its phonon states, k the quasimomentum of matrix phonon, and l the vector of the nearest neighbour of the impurity. In the optical branch of PS the agreement is qualitative. Assuming a change of the force constants of the two nearest axial neighbours of the linear impurity conditions are obtained of the appearance of local vibrations (LV) separating into the gap of the phonon spectrum of the matrix crystal and above ωm· The treatment is given of a sharp structure observed at the short‐wave side of PS and in the region of PS corresponding to the frequency gap of the f.c.c. matrices (doublets). Corresponding limiting changes of the force constants in the impurity crystals are evaluated.
A study of structurally disordered single‐element magnetic materials beyond the homogeneous molecular field approximation
Authors: Šlechta J.
Published: July 2007

A usual assumption in the theory of amorphous magnetic materials is 〈Si〉 = S. It is shown that in amorphous materials this is a very crude approximation. The magnetic properties of small random clusters of localised spins were studied beyond the above simplification. It is found that in this case the fluctuations of the exchange interaction around its crystalline mean value cause an increase of the Curie temperature. In the light of this conclusion most of the experimental results in this direction should be reinterpreted. It is argued that the magnetic properties inherently depend on higher spatial correlations than pair–pair ones.
Transfer matrix approach to axial hyperchanneling
Authors: Corciovei A. , Apostol M.
Published: July 2007

The mechanism of channeling is reviewed and a discrete model for hyperchanneling is formulated, which holds both for planar and axial channeling. It is assumed that the moving particle in the crystal interacts with the atoms in the planes of the lattice which are perpendicular to its direction of motion. An axial channel consists in a succession of parallel rings of atoms. The interaction between hyperchanneled particle and these atomic rings is described in terms of classical collisions. From the linearized equations of motion in the momentum approximation one obtains that the interaction with the rings can be described by a transfer matrix which relates the mechanical states of the particle before and after a collision with an atomic ring. Then by induction the mechanical state parameters of the particle after the n‐th collision are obtained in terms of the initial conditions. Minimum critical hyperchanneling energies are derived. Expressions are given for the critical angles as well as for the energy losses in the case of the f.e.e. 〈100〉 channel. Numerical evaluations show a good agreement between the theoretical results and the experiment.
Isothermal electrocaloric effect in KCI:OH
Authors: Korrovits V. , Liidja G. , Mihkelsoo V.
Published: July 2007

The entropy of a noninteracting dipole system with the equilibrium dipole moment μ ‖ 〈100〉 in a rigid cubic lattice is obtained for the external electric field E‖ [100] of arbitrary strength, taking into account the 90° tunneling. In thermostating experiments the KCI‐OH monocrystals were illuminated with light pulses of rectangular shape (single 10−2 s shots, absorbed power ranging 3 × 103 erg/s cm3) and the electric field applied to the crystal was simultaneously removed. The electric field pulse shape, needed for maintaining the crystal temperature constant within some millidegrees at 0.75 K, is close to the theoretical one assuming a “working” dipole number NE ≈ 0.1Nopt.
Surface energies of simple metals (I)
Authors: Paasch G. , Hietschold M.
Published: July 2007

The surface energy and the dipole barrier are calculated for eleven simple metals. The potential of the positive ions is replaced by Ashcroft‐model potentials. The decrease of the electron density at the surface and the relaxation of the top lattice plane are described by two parameters. Using the theory of the inhomogeneous electron gas the surface energy is calculated and minimized with respect to the surface parameters. The influence of the inhomogeneity of the electron density on the exchange and correlation energy has to be taken into account in order to obtain reasonable results for both the surface energy and the dipole contribution to the work function.
Calculation of nonradiative multiphonon capture coefficients and ionization rates for neutral centres according to the static coupling scheme: I. Theory
Authors: Pässler R.
Published: July 2007

Capture coefficients and ionization rates for nonradiative multiphonon transitions of charge carriers at neutral imperfections in semiconductors are calculated within the framework of the static coupling scheme. The temperature averaging connected with the thermal distribution of free carriers over the band states is performed on the basis of a three‐param‐etrical overlap‐factor formula which enables a reduction of the analytical expressions for the quantities in consideration to simple functions of five imperfection parameters.
Domain structure at ferromagnetic‐antiferromagnetic phase transitions as superheated‐undercooled state of a solid
Authors: Popov V. A. , Polyakov M. L.
Published: July 2007

A theory is developed for the coherent domain structure at the temperature‐induced phase transition FM⇄AFM. The domain sizes, energy, and thickness of the domain boundaries and their temperature dependences have been calculated. Heat capacity of the FM‐AFM‐domain structure is shown to depend on temperature and achieve very high values. The possibility of a libration phase transition between two FM‐domain structures is discussed.
Ferromagnetism for the hubbard model in the alloy analogy approximation
Authors: Schneider J. , Drchal V.
Published: July 2007

The stability of the strongly ferromagnetic ground state in the Hubbard model for U → ∞ is investigated, using the Hubbard alloy analogy result. In this approximation the saturated ferromagnetic state is not stable for a nearly half‐filled band. In fact, for n < 1, only the paramagnetic solution exists.
Linear electroabsorption due to substitutional deep impurities in wurtzite‐type crystals: 1. Theoretical analysis for A → T absorption bands
Authors: Boyn R. , Gardavský J.
Published: July 2007

A theoretical study is made of the first‐order effects of an external electric field on optical transitions between strongly localized electronic states of isolated substitutional impurities in a wurtzite‐type crystal taking into account the near‐tetrahedral symmetry of these centers. The field effect on optical absorption bands due to A → T type transitions is treated on the basis of the method of moments which is extended to include mixing with the A ground level. The field‐induced changes of the zeroth and first moment (corresponding to changes in transition probability and to the Stark splitting, respectively) are calculated as functions of field and polarization direction. The polarization dependence of the zero‐field moments which is due to the axial part of the impurity potential is also discussed. On the basis of a rigid‐shift approximation a brief semi‐phenomenological treatment is given of the field‐induced changes in band shape.
Cluster of four superexchange‐coupled Cu2+ ions in copper‐doped triglycine fluoroberyllate. A theoretical interpretation of the EPR spectrum
Authors: Kurzyńaski M. , Kowalewski L.
Published: July 2007

The unusual EPR spectrum of copper‐doped triglycine fluoroberyllate is shown to originate in large complcx molecules of the hypothetical formula [Cu2+·(NH2CH2COO−)2·H2BeF4]4, built into the distorted host lattice. Superexchange couplings of Cu2+ ions in the cluster through CuFBeFCu paths are discussed. The isotropic superexchange interactions split the 16‐fold degenerate ground crystal state of the four Cu2+ ions into multiplets the further splitting of which, originated by anisotropic interactions, can be described in terms of effective spin Hamiltonians with S = 2 and 1. The observed spectrum is attributed to transitions within the quintet and one of the triplets. By second‐order perturbation procedure, the phenomenological parameters of the effective spin Hamiltonians are expressed in terms of parameters directly describing anisotropic as well as isotropic superexchange interactions in the cluster. The magnitudes of these interactions are thus determined from experimental data.
Effect of piezoelectric potential scattering on the microwave faraday effect in n‐CdS
Authors: Srivastava G. P. , Mathur P. C. , Shyam R. , Kataria N. D. , Chandra I.
Published: July 2007

The theory of free‐carrier Faraday effect in semiconductors previously developed by Donovan and Webster for lattice and impurity scattering is extended to the case of n‐type CdS, taking into account piezoelectric and deformation potential scattering. The high frequency magnetoconductivity tensors are derived considering the detailed structure of the conduction band of CdS. The Faraday rotation measurements in n‐type CdS along the c‐axis are performed at liquid nitrogen temperature and room temperature at 24 GHz. The experimental results are found to agree well with the theory, for β = p/a = 1.9, where a and p being the deformation potential and piezoelectric scattering coefficients at T = 90 °K, respectively.
Theory of diffraction due to substitutional disorder
Authors: Szabó P.
Published: July 2007

It is shown that existing theories of the diffraction due to substitutional disorder are partly wrong and partly do not correspond to the case of a large number of crystallographically equivalent positions in the unit cell. The pertinent diffraction theory is elaborated. Certain striking differences are found between the new results and those of former theories. So it is shown, that some experimental inferences referring to spinels and reported in the literature are unfounded.
Debye‐Waller factors of MgO and CaO by a modified shell model
Authors: Sneh  , Dayal B.
Published: July 2007

Calculated values of the average mean square displacements for MgO and CaO have been plotted as functions of temperature, while the Debye‐Waller B‐values and the mean square displacements for Mg and O, and Ca and O ions have been tabulated. The calculations have been done by a modified shell model, in which three‐body forces have been incorporated on the lines given by Verma and Singh, and Singh and Upadhyaya. The calculated values of the Debye‐Wailer factors and the mean square displacements are compared with available experimental values and good agreement is obtained.