bluuebunny/crossref_metadata_2025_split huggingface dataset:
NMR Study of Ferromagnetic Gd Intermetallic Compounds with CsCl Structure
Authors: Oppelt A. , Dormann E. , Buschow K. H. J. 
Published: July 2007

AbstractBy means of spin echo techniques the zero field resonance of Gd155, Gd157, Ag107, Ag109, and Zn67 nuclei could be detected in the ferromagnetically ordered CsCl type compounds GdAg1−xInx, GdAg1 ‐ xZnx, and GdZn1−xInx. In some cases well resolved lines could not be obtained due to broadening. Both, the hyperfine field of the magnetic and of the nonmagnetic ions are negative. No significant hyperfine field variation with conduction electron concentration is observed for the nonmagnetic ions while for the Gd ions it is appreciable. This cannot be explained by the simple RKKY approach, in which the interaction between the localized Gd‐moments and the conduction electrons is considered to be a δ‐function interaction and in which the Fermi surface is assumed to be spherical. The variation of the hyperfine fields was also studied in the systems GdCdZnx and Gd1no.5 (1−x)‐Ag0,5 (1−x)Znx, in which the number of conduction electrons remains constant. Essentially no change is found in GdCdZnx, where divalent Cd is replaced by divalent Zn. In Gd1n0,5 (1−x)Ag0,5 (1−x)Znx, where divalent Zn is replaced by monovalent Ag and trivalent In, the hyperfine fields become smaller with decreasing x. Apparently the reduction of the conduction electrons' mean free path due to potential scattering modifies their polarization.
Work Function and Surface Structure of Simple Metals
Authors: Paasch G. , Eschrig H. , John W. 
Published: July 2007

AbstractThe work function is calculated for ten simple metals in the framework of model potentials. The Shaw model potential is used. The surface structure is described by a model containing a decrease of the conduction electron density and a change of the lattice constant. The parameters of this model are determined from the requirement of a minimum surface energy. The calculated work functions agree well with the experimental ones. The calculated increase of the lattice constant at the surface is about 1%. It gives rise to a decrease of the total surface dipole potential. The latter is much smaller than that calculated for the usual model where the ions are replaced by a homogeneous background. The agreement of the work function with experiment justifies the use of the mean value of model potentials in the calculation of physical properties.
Effect of Crystalline Anisotropy on the Magnetostatic Spin Wave Modes in Ferromagnetic Plates. I. Theoretical Discussion for Infinite Plates
Authors: Schneider B. 
Published: July 2007

AbstractThe dispersion equations of magnetostatic spin wave modes in thin ferromagnetic plates are derived within the linear phenomenological theory for a general anisotropic tensor of the dynamic susceptibility (Polder tensor). The equations are discussed for the two special cases of the effective magnetic field being parallel or perpendicular to the surface of the plate. In the Appendix the Polder tensor is derived for crystals with an anisotropic gyromagnetic ratio.
Ginsburg‐Landau Theory of the Vortex Lattice in Type‐I1 Superconductors for All Values of ϰ and B
Authors: Brandt E. H. 
Published: July 2007

AbstractA two‐dimensional variational method is established which permits the solution of the Ginsburg‐Landau equations for type‐II superconductors to any desired accuracy. Free energy, magnetization curves, GL function, microscopic magnetic field and current density are calculated for hexagonal and tetragonal lattice symmetries and for all values of the GL parameter ϰ and the induction B. The hexagonal lattice is found to be stable throughout. As a further result simple approximate expressions are found for the free energy (with en error <5 × 10−5) and for the magnetization as functions of ϰ and B or the external field He.
Raman Relaxation Times in Electronic Systems
Authors: Shrivastava K. N. 
Published: July 2007

AbstractThe theory of Raman relaxation is developed without the approximation of small wave vectors for the lattice phonons and it is shown that the accurate theory reduces into the known one if the expressions are expanded into infinite series and only the first term retained. The temperature dependence of electronic relaxation times is found to be more involved in the correct expressions than in the approximate ones. Eight different cases depending on the structure of energy levels are derived starting from a single formula. Four of these cases are new. Efforts are made to compare the theory with the experiments on Pr3+, Ce3+, Nd3+, Yb3+, and Mo5+ ions. A new T5 behaviour predicted for Kramers ions is apparently observed in Nd3+: LaCl3 and Mo5+:TiO2 and for the non‐Kramers Pr3+: LaCl3 system. Faster temperature dependence than the usual T9 one is predicted and probable indications of the same are noted on Ce3+, Nd3+, and Yb3+ in CaWO4.
Umorientierung von Hantel‐Zwischengitteratonien in verformtem Kupfer und Nickel
Authors: Wanger F. J. 
Published: July 2007

AbstractDurch Verwendung genügend kleiner Frequenzen (f ≈ 10−1 Hz) konnte die versetzungsunabhängige Umorientierung der Hantel‐Zwischengitteratome (Hantel‐ZGA) getrennt von der versetzungsinduzierten Umorientierung beobachtet werden. Der versetzungsunabhängige Prozeß besitzt die von der Theorie vorhergesagte Orientierungsabhängigkeit und läßt sich nur mit dem Bild des Hantel‐ZGA mit 〈100〉‐Vorzugsrichtung verstehen. Beim Anlassen der Proben baut sich die versetzungsinduzierte Umorientierung der Hantel‐ZGA auf. Dieser Prozeß zeigt keine wesentliche Orientierungsabhängigkeit. Bei höheren Frequenzen (f ≈ 1 Hz) tritt er gleichzeitig mit der versetzungsunabhängigen Umorientierung der Hantel‐ZGA auf und maskiert dadurch teilweise deren Orientierungsabhängigkeit. Experimentelle Ergebnisse früherer Arbeiten finden durch die versetzungsinduzierte Umorientierung der Hantel‐ZGA ihre Erklärung.
Anharmonicity of Infrared Vibration Modes in Beryl
Authors: Gervais F. , Piriou B. , Cabannes F. 
Published: July 2007

AbstractThe anharmonicity of infrared vibration modes has been studied in beryl crystals by measurements of the temperature dependence of reflectivity. The infrared reflectivity data from 280 to 1500 cm−1 in the temperature range 300 to 1300 °K, were analyzed with the classical oscillator model. The comparison between the results of quantum theories and the dependence of observed damping on temperature shows that the cubic and quartic terms contribute to the anharmonicity of a set of modes, whereas another set of modes exhibits only a cubic anharmonic behaviour. In the latter case, a part of the modes are known as internal.
Analysis of the R2 Absorption Structures in LiF, KCl, and KBr
Authors: Giesecke P. , von der Osten W. , Röder U. 
Published: July 2007

AbstractStarting from the linear and quadratic electron‐phonon coupling for defects in crystals, a method is described to obtain the one‐ and multiphonon contributions to the optical absorption structure for the case of intermediate coupling strength. It is based on the explicit knowledge of the absorption shapes of the zero‐phonon line and the vibrational sideband, which have to be measured with high spectral resolution as function of temperature. The analysis is performed for the R2, transition of the R‐centre in Lip, KCl, and KBr. The peaks of the spectral density of coupled modes resulting from the analysis are compared with characteristic phonon energies at the Brillouin zone boundary and to results of circular dichroism measurements of the R2, band. The linear coupling coefficient S is obtained (a) from the Fourier transform of the one‐phonon absorption spectrum, (b) from the ratio of the integrated absorption of the zero‐phonon line and the total band, and (c) from a calculation using the density of states obtained by the sideband analysis. Taking into account the quadratic coupling, from the spectral density of coupled modes both thermal shift and broadening of the R2, zero‐phonon line are predicted and prove to be in good agreement with the experimental data. A static contribution to the line shift as well as a residual line width at low temperature thereby have been taken into account.
On the Theory of Electroabsorption in Disordered Semiconductors
Authors: Ewer B. 
Published: July 2007

AbstractThe interband absorption coefficient is calculated for a disordered semiconductor subjected to an external electric field in the low‐temperature case. Two kinds of random fields are considered, the “smooth” random field possessing a correlation function which is continuous and differentiable as many times as needed and that of the Coulomb field produced, for example, by the randomly distributed charged centres (compensated samples are considered). If the external field is low enough, the change in the exponential tail (ħω < ϵg) may be expressed in the form of a shiftedband edge Δϵg. When the external field becomes strong enough, the absorption coefficient takes on the well‐known form deduced by Franz and Keldysh. In case of a “smooth” field the tail range in the electroabsorption coefficient is considered while in the Coulombic case graphs of the electroabsorption coefficient are plotted in the range of interest. The Redfield representation of an absorption coefficient of a disordered semiconductor is discussed.
EPR Studies of Impurities in Electron‐Irradiated CaO
Authors: Henderson B. , Tohver H. T. 
Published: July 2007

AbstractThe EPR spectrum of Mn4+ ions in CaO has been observed in single crystals containing Li+ or Na+ ions. Electron irradiation of such crystals results in Mn4+ being converted to Mn2+, a concomitant reaction being the formation of [Li]° and [Na]° trapped hole centres. Several other paramagnetic species are produced by the irradiation including atomic hydrogen, [Li]− and [Li+H]+ centres. The spin Hamiltonian parameters which describe the EPR spectra of these impurity centres are reported and discussed. Optical or thermal bleaching of the [Li]° or [Na]° centres above 77 K results in the annihilation of Mn2+ and [Li2+H]+ centres: atomic hydrogen and the [Li] centres are less readily bleached by these treatments.
Optical Spectra and Energy Band Structure of the Monoclinic Crystals ZnP2 and ZnAs2
Authors: Sobolev V. V. , Syrbu N. N. 
Published: July 2007

AbstractThe anisotropic spectra of edge absorption, reflection, and photoconductivity of the monoclinic ZnP2, and ZnA2, crystals have been experimentally investigated at 293 and 77 °K. The group‐theoretical calculations of selection rules for interband transitions and the dispersion relaxation are discussed. Energy band structure models at the band gap are proposed on the basis of known theoretical and obtained experimental data. Polarisation investigations allowed to determine the subband symmetry responsible for transitions in the energy range from 1 to 12.5 eV.
Faraday and Kerr Effects of Hot Electrons in n‐Type InSb in the Infrared (I)
Authors: Vorobev L. E. , Komissarov V. S. , Stafeev V. I. 
Published: July 2007

AbstractThe nonparabolicity of the conduction band and the anisotropy of the hot electron distribution function have been shown to lead to the anisotropy of the refraction index in strong electric fields (Kerr effect). Nonparabolicity causes also a decrease of the Faraday rotation angle θ when the electron gas is heated. A comparison of the calculated values of Δθ with the experimental ones allows to determine the temperature of hot electrons for n‐type InSb with concentrations of the electrons in the range of 3 × 1014 to 4.9 × 1015 cm−3 at T0 = 80 °K in longitudinal magnetic fields. The interaction constant with polar optical phonons was also determined. The influence of e‐e collisions on the distribution function of hot electrons is discussed.
Structural Phase Transitions in Langbeinites
Authors: Dvořák V. 
Published: July 2007

AbstractUsing Landau theory of phase transitions it is shown that in the langbeinite structure (space group T4) four spatially homogeneous structural phase transitions are possible. Theree of them are improper ferroelectric and result from a phonon instability at the M point of the Brillouin zone. The fourth one is very similar to structural phase transitions in hig temperature superconductors of the β‐tungsten structure. The free energies providing a basis for macroscopic description of these phase transitions are presented.
Theory of Disordered Systems.II. Functional Equation of tho Self‐Energy
Authors: Puff H. 
Published: July 2007

AbstractThe self‐energy of a disordered system, which is described as an ensemble average of non‐interacting electron systems of well‐defined configurations, is defined as a functional of the one‐particle funestion of the disordered systems as well as of the totality of its potential correlations by means of a functional‐differential equation. Relations to the theory developed in the first part of this series [1] are outlined.
Cluster Scatterings and Spectra. I. Electrons in Substitutional Alloys
Authors: Čàpek V. 
Published: July 2007

AbstractA diagrammatical analysis of a previously reported set of approximations for the averaged electronic resolvent 〈[z ‐ H]−1〉 in substitutional alloys is made. The results are found to be in correspondence with applicability of the method revealed in a numerical analysis beyond the coherent potential approximation. A new modification of the method is suggested which is free of shortcomings of the original version. It enables relatively very easy and in principle almost exact computer calculations of spectra of substitutional alloys, at least for simplest model Hamiltonians. A simple numerical example is solved.
Core Levels of III‐V Semiconductors
Authors: Gudat W. , Koch E. E. , Yu P. Y. , Cardona M. , Penchina C. M. 
Published: July 2007

AbstractThe absorption and reflection spectra of GaP, GaAs, GaSb, InP, InAs, and InSb in the region from 15 to 40 eV are presented and compared with those obtained from characteristic electron energy losses. From these data, information about the density of conduction states, the matrix elements for transitions from the outermost core levels to the conduction bands, and the energy and spin‐orbit splittings of those core levels is obtained. X‐ray photoemission data (ESCA) in the region from 10 to 1487 eV are also presented and compared with the results of absorption and emission spectroscopy.
Optical Absorption Edge of Heavily Doped Semiconductors at Low Temperatures
Authors: Van Cong H. , Mesnard G. 
Published: July 2007

AbstractUsing the Green function methods in the Feynman‐path integral formulation, applied to disordered systems and developed already in previous papers, the optical absorption edge of heavily doped semiconductors in zero magnetic field at low temperatures is investigated. Two band‐structure models are used here and the absorption coefficeint is calculated directly from the Greenwood formula in the one‐electron approximation. Two examples concerning Ge and GaAs are discussed assuming that one excess electron is provided by each donor.
Dependence of the Phenomenological Energy Relaxation Time on Electric Field in n‐Si and n‐Ge at 77 °K
Authors: Dargys A. , Banys T. 
Published: July 2007

AbstractA method to measure the dependence of energy relaxation time of hot carriers τϵ in semiconductors is proposed. The method is based on the dependence of the phase of the second harmonic ϑ on τϵ. The relationship between ϑ and τϵ was found from equations of the momentum balance and conductivity relaxation. The dependence of τϵ on the de electric field in n‐Go and n‐Si at a lattice temperature of 77°K was measured. In n‐Si the influence of redistribution of electrons among different valleys on the phase of thosecond harmonic was observed.
Electronic Structure and Spectrum of the (TmF8)6− Cluster
Authors: Anisimov F. , Dagys R. 
Published: July 2007

AbstractThe crystal field splitting and g values in SrF2:Tm2+ were calculated within the framework of the (TmF8)6− cluster using the Heitler‐London method. Two sorts of numerical Hartree‐Fock functions for Tm2+ electrons were used in the calculations. It is shown, that a better agreement between the values calculated and observed may be obtained using more expanded radial functions of the Tm2+ electrons.
The Electronic Structure of NaCl:Cu
Authors: Yip Kwok Leung , Fowler W. B. 
Published: July 2007

AbstractThe one‐electron energy levels of NaCl:Cu have been calculated by a charge‐consistent extended Hückel approach. The lowest transition is found to be electric‐dipole forbidden and its energy agrees with experiment when a Hückel constant of 2.0 is used. The ground state of the center is found to be partially covalent, with a net charge of +0.75 e on the Cu. Similar calculations on KCl:Cu tend to yield transitions too high in energy, although an estimate of the KCl valence band structure yields reasonable results. These results support the idea that the primary transition for on‐site Cu is the crystalline analogue of S → D, and that the cases of apparent allowed transitions arise from admixture of P into D through the electric field associated with a stable off‐center position of the Cu. It is suggested that the crystalline increase of the d10 → d9s transition energy arises in part from the penetration of d‐orbitals into the ligand cores, with consequent lowering of their energies.
Optical and Photoelectric Properties of the Lead Chalcogenides
Authors: Cardona M. , Penchina C. M. , Koch E. E. , Yu P. Y. 
Published: July 2007

AbstractThe reflection spectra of PbS, PbSe, and PbTe have been measured in the 14 to 26 eV region using as a source the DESY‐synchrotron. Three peaks are observed for PbSe and PbTe and two for PbS, corresponding to transitions from the outermost d‐levels of the Pb core to the conduction bands. In order to find out the relationship between these peaks and the spin–orbit splitting of the d‐core levels, X‐rays photoemission measurements (ESCA) have been performed. It is concluded that this spin‐orbit splitting of the d‐core levels does not appear directly in the optical spectra. A simple model is given to interpret these spectra. The valence band, and other core levels of binding energies in the range of the Kα line (1486.6 eV) aluminium‐X‐ray source have also been studied. The binding energies, splittings, and chemical shifts obtained are discussed and compared with theoretical calculations. The most intense ESCA lines exhibit satellites which can be associated with lines produced by the excitation of valence plasmons. The effects of adsorbed oxygen on the ESCA spectra are also discussed.
On the Exciton Binding to Neutral Impurities in Semiconductors
Authors: Munschy G. 
Published: July 2007

AbstractIt is shown that the complex consisting of an exciton bound to a neutral donor (or acceptor) is not always stable. There is an intermediate range of values of the electron to hole effective‐mass ratio σ for which the complex becomes unstable against the dissociation into an exciton and a neutral donor (or acceptor). Furthermore, in the so delimited H2‐like region of stability of the complex, the binding energy expressed in atomic units is shown to vary monotonically with the mass ratio σ up to a critical value àc (1 < σc < 2) which corresponds to the dissociation limit.
Application of the Quantum Theory of Diffusion to H and D in Niobium
Authors: Sussmann J. A. , Weissman Y. 
Published: July 2007

AbstractA detailed description is presented of the quantum mechanical diffusion mechanism for the case of strong coupling between lattice phonons and the migrating particle. In this mechanism the particle is excited from a localized ground state to time‐dependent Wannier states in several accessible bands, from where it falls down into neighbouring or non‐neighbouring sites. This mechanism leads to a classical‐like expression for the jumping probability, W = ν0 e‐E/kT, where E is the excitation energy to the band and ν0 is related to the width of the band. This theory is shown to account for the experimental data on hydrogen diffusion in niobium, which exhibits two sets of activation energies and pre‐exponential factors, and a highly non‐classical isotope effect. Available diffusion data in V and Ta are also found compatible with the same model.
Recombination in Semiconductors by Excitation of Plasmons
Authors: Tussing P. , Rosenthal W. , Haug A. 
Published: July 2007

AbstractIn a highly doped semiconductor excitation of plasmons is shown to be an effective mechanism for electron‐hole recombination. If the carrier concentration is sufficiently high the corresponding recombination rate overwhelms that for the radiative process by orders of magnitude.
Theory of Disordered Systems III. Single‐Site Theory of Transport in Systems with Randomly Distributed Scatterers
Authors: Fischbeck H. J. 
Published: July 2007

AbstractIn case of systems with randomly distributed scatterers the self‐consistent single‐site approximation of Schwartz and Ehrenreich [1] leads to a simple expression for the kernel of the Bethe‐Salpeter equation in terms of the local self‐energies. Thus, a consistent single‐site theory of linear response of such systems is given. It is applied to the disordered Kronig‐Penney model which permits exact solutions of the Schwartz‐Ehrenreich and the corresponding Bethe‐Salpeter equation. Numerical results of the density of states and the mobility of the electrons are given. The existence of a mobility edge is established.
Fluctuation‐Dissipation Theory of Polariton Raman Scattering in the Vicinity of Phonon Resonance
Authors: Strizhevskii V. L. , Obukhovskii V. V. 
Published: July 2007

AbstractQuantum theory of stimulated Raman scattering (SRS) by polaritons is developed which is valid both far from phonon resonance and in the vicinity of it. The theory is based on treating polariton SRS as an interaction process of pumping radiation with fluctuation (Langevin's) sources of an active medium. The principal peculiarity of a resonance region is the necessity to take into account non‐equilibrium (dependent on pumping field) fluctuations of polarizations. Therefore, the correspondent generalization of the fluctuation‐dissipation theorem was made, the necessary correlators of Langevin forces and fluctuation sources were found up to the second order with respect to the pumping field. On the base of fluctuation‐dissipation theory of scattering the intensity of polariton SRS was determined as a function of macroparameters of the system. The temperature dependence of gain factor and scattering intensity is discussed.
Resonant Absorption of γ‐Rays by a Deformed Crystal
Authors: Srivastava B. P. , Sharma H. N. K. , Bhattacharya D. L. 
Published: July 2007

AbstractThe exponent of the Debye‐Waller factor and the energy shift for γ‐rays resonantly absorbed by a deformed crystal (deformation introduced along a line) have been calculated using the technique of double time Green's function. It has been found that due to deformation there exists an observable broadening of the Mössbauer line and an additional energy shift also appears which however may not be well observed. A quadrupole splitting of the line has been predicted on the basis of deformation of the lattice.