bluuebunny/crossref_metadata_2025_split huggingface dataset:
New Decoupling of the Green Function in the Heisenberg Ferromagnet with Single‐Ion Anisotropy
Authors: Duda A. , Puszkarski H. 
Published: July 2007

AbstractA Heisenberg ferromagnet with a Hamiltonian involving single‐ion anisotropy gives rise to an elliptic precession of the spins; the equations of motion of the single‐particle Green functions can no longer be solved by existing decoupling methods. Here, a new type of decoupling of single‐ion functions of the form \documentclass{article}\pagestyle{empty}\begin{document}$ \langle \langle \hat S_f^z \hat S_f^ \pm |\hat S_g^ - \rangle \rangle $\end{document} is proposed, consisting in a generalization of Lines' decoupling to the case of elliptic spin precession.
Diagonalization of the g‐Matrix of Low Symmetry Transition Ions
Authors: Soliverez C. E. , Belorizky E. 
Published: July 2007

AbstractThe explicit operations — rotation of coordinate axes and transformation of basis functions — required in order to diagonalize the g‐matrix of a Zeeman spin‐Hamiltonian linear with respect to the magnetic field and the effective spin operators, are given for low symmetry complexes. The case where the g‐matrix is singular, is carefully analyzed.
Optical Properties and Collective Excitations in MoS2 and NbSe2 in the 1.7 to 30 eV Range
Authors: Martin L. , Mamy R. , Couget A. , Raisin C. 
Published: July 2007

AbstractOptical functions of two layer type compounds MoS2 and NbSe2 are calculated in the 1.7 to 30 eV range from reflectivity data by means of a Kramers‐Kronig analysis. The positions of the plasmon peaks given by the energy loss function, — Im \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \varepsilon _ \bot ^{ - 1} $\end{document}, are in good agreement with those evaluated directly by electron energy loss experiments. The optical effective number of electrons saturates at the high energy boundary of the experimental interval, for a value corresponding to all the valence electrons.
Functional Integral Approach for an Extended Hubbard Model: N‐Centre Coupling
Authors: Albani B. , Röpke G. , Pöpelt P. 
Published: July 2007

AbstractGeneralized susceptibilities are investigated by the functional integral formalism for an extended Hubbard model. This model includes the parts of intra‐atomic and inter‐atomic Coulomb interaction which can be written in terms of occupation number operators. Phase transitions from the paramagnetic phase to long‐range ordered phases which can be ferromagnetic, antiferromagnetic, or charge ordered, are obtained. In the paramagnetic phase, the behaviour of the local‐moment magnitude is studied, and no sharp phase transition associated with the disappearance of the magnetic moments is found.
Study of Disordered Crystals by Optical Second Harmonic Generation
Authors: Vogt H. 
Published: July 2007

AbstractOptical second harmonic generation (SHG) can be used to study the short‐range order still being present in disordered crystals. This is demonstrated for the paraelectric state of NaNO2, the disorder of which is due to ionic flipping motions. Second harmonic light has been observed under phase‐matching conditions and its intensity I(2) has been measured as function of temperature between Curie‐ and melting point. The observed SHG is mainly determined by short‐range correlations between the polarizabilities of the NO ions, so that the temperature dependence of I(2) reveals the decrease of short range order between Curie‐ and melting point. The relative values of I(2) found in different phase‐matching directions can be interpreted in terms of simple models of the disordered state.
On the Formation of Aggregate Centres by Optical Bleaching of Electrolytically Coloured KCl Crystals
Authors: Pandey R. K. , Bose H. N. 
Published: July 2007

AbstractThe decay of F centers and the growths of F3(R), F4(N) centres in electrolytically coloured KCl crystals under F light stimulation have been found to exhibit a two stage process; the fast exponential change is followed by a slow linear change. Appropriate mechanisms and rate determining processes have been suggested to explain the transformation of F centres to higher aggregate centres. It has been possible from the present measurements to estimate the oscillator strengths for the R1 and R2 centres. The dissolving action of X‐rays on F aggregate centres has been noticed which is attributed to the interaction of Cl°, produced by X‐rays, with the existing aggregate centres.
Size Dependence of Magnetoresistance of Many‐Valley Semiconductors
Authors: Mitin V. V. , Prima N. A. 
Published: July 2007

AbstractThe dependence of the magnetoresistance (MR) is studied of a many‐valley semiconductor with thickness 2 d (‐ d ≦ y ≦ d) comparable with the length of the intervalley scattering and/or the cooling length when the correlation between them is arbitrary. If a weak electric current flows along the x‐axis the MR monotonously reduces in a magnetic field H = Hz when the thickness decreases and for d → 0 it becomes equal to zero. In the magnetic field H =Hy the MR can both increase and reduce and it always remains non‐negative when the thickness decreases.
X‐ray photoemission studies of silicon and germanium
Authors: Vesely C. J. , Kingston D. L. , Langer D. W. 
Published: July 2007

AbstractX‐ray‐induced electron‐emission measurements were used to study the electronic structure of silicon and germanium in a range extending from the top of the valence band to about 1250 eV below the Fermi level. Measured core level values agree very well with previously reported values except for the splitting of the germanium 3p level and the location of the silicon 2s level. “Plasma replicas” were observed for several of the core levels and their valence band photoemission spectra are compared with theoretical density‐of‐states distributions. Two maxima in the germanium conduction‐band density of states are located by comparing the photoemission data with UV absorption data.
Dispersion properties of silver chloride
Authors: Upadhyaya K. S. , Mahesh P. S. 
Published: July 2007

AbstractThe three‐body force shell model of Verma and Singh [1] has been modified to include the van der Waals term in the short‐range interaction potential. Further, the shell charge parameter Y has been multiplied by a factor as demanded by the rigorous theory. The model thus obtained has been applied to the calculation of the dispersion curves of silver chloride. A very good agreement has been obtained with the experimental data.
Optical properties and energy gap of GeTe from reflectance studies
Authors: Lewis J. E. 
Published: July 2007

AbstractThe reflectance spectra of bulk samples of the germanium telluride (GeTe) system have been measured in the wavelength range 13 to 0.2 μm. The principal optical constants of the system have been obtained by inverting the spectra using the Kramers‐Kronig technique. Two distinct types of behavior were encountered depending on whether or not a sample exhibited a free carrier absorption peak. Samples rich in germanium lacked such a peak and had optical properties very similar to amorphous GeTe films. Tellurium rich samples always showed a large free carrier peak and from analysis of their optical Constants the effective mass and optical energy gap of the system was obtained. The variation of the effective mass and energy gap with composition (i.e., hole carrier density) is consistent with the existence of two sets of valence‐conduction bands in the system. A multiple band model based on experimental and theoretical data is outlined and its parameters are found to be in good agreement with published work.
Electric field effect on two‐photon transitions near thresholds in solids. II. Excitonic effects
Authors: Yang E. 
Published: July 2007

AbstractThe excitonic effect on the dielectric properties of solids has been included in the calculation of the electric‐field‐induced change of the coefficient of two‐photon absorption. It is shown that the magnitude and line shapes are drastically changed when the exciton binding energy EB varied from EB < ℏΩ to EB ≫ ℏΩ, where ℏΩ is the effective characteristic electro‐optic energy which is calculated by including the electron‐hole interaction in solids.
Time‐dependent non‐ohmic conductivity and magnetoresistance in p‐tellurium at 2 K
Authors: Bauer G. , Kahilert H. , von Klitzing K. , Landwehr G. 
Published: July 2007

AbstractThe recently observed resonance structure in the high field magnetoresistance of p‐Te was investigated with dc pulses ranging from 20 ns to 100 μs. Time dependent current‐field strength characteristics and the temperature dependence of the conductivity and the Hall coefficient under ohmic conditions are explained by assuming, that an impurity band is located about 0.7 meV above the valence band.
Influence of a strong electric field on the microwave conductivity of semiconductors
Authors: Iwainsky A. , Richter Th. 
Published: July 2007

AbstractFor semiconductors the distribution function of hot charge carriers in the presence of a microwave field is investigated by means of the Boltzmann equation. Bragg reflection is taken into consideration by an expansion of the distribution function in a Fourier series. In case of constant τ approximation and sinusoidal energy bands the current density and from this the microwave conductivity are computed. For sufficient strong dc fields below the Zener frequency a significant field‐induced change of the microwave absorption results which can lead to an amplification of the microwave.
The spin‐flop transition in some two‐sublattice uniaxial antiferromagnets
Authors: Morrison B. R. 
Published: July 2007

AbstractThe spin‐flop transition has been reconsidered in the molecular‐field approximation at 0 K. A general free energy expression, which includes isotropic and anisotropic exchange, first and second uniaxial single‐ion anisotropy, and Dzyaloshinsky‐Moriya antisymmetric exchange, is the starting point for the calculations. Both exact and approximate relations are derived for the critical fields. By setting the appropriate parameters equal to zero these relations are readily reduced to the corresponding equations for better known simpler systems. An expression is found for the angular dependence of the spin‐flop transition, and the importance of second uniaxial single‐ion anisotropy is discussed.
Elastic properties and superconductivity of V3Si at high pressure
Authors: Carcia P. F. , Barsch G. R. 
Published: July 2007

AbstractThe pressure derivatives of the single crystal elastic constants of V3Si have been measured at 77 and 298 °K, and the pressure derivative of the shear modulus cs = (c11 – c12)/2 has been measured between 37 and 298 °K. The V3Si crystal was of the kind which undergoes the Batterman‐Barrett low temperature structural transformation. The results differ significantly from those for nontransforming V3Si in that for transforming crystals (∂c/∂p)T shows a negative minimum at about 80 °K and is positive below about 50 °K. This is in semiquantitative agreement with the negative pressure coefficient for the structural transformation measured by Chu. From the elastic data the elastic Grüneisen parameters and the pressure coefficients (∂Tc/∂p) of the superconducting transition temperature were calculated in the anisotropic Debye approximation. For nontransforming V3Si the calculated value of (∂Tc/∂p) shows the correct sign, but is two and one‐half times larger than the experimental value. The calculated (∂Tc/∂p) is for transforming V3Si considerably smaller than for nontransforming samples.
On the nature of color center absorptions
Authors: Papazian H. A. 
Published: July 2007

AbstractEvidence is presented for absorptions in halide molecules and halide solutions analogous to color center absorptions in crystals. With similar observations by Hersh, several color center absorptions appear not to be unique to the solid state. Color center absorptions in any one crystal may be correlated in a manner similar to energy level correlations in molecules. Some comments are made on the F absorption band in view of these results.
Bound states of Frenkel excitons
Authors: Popović‐Božić M. , Tošić B. S. 
Published: July 2007

AbstractThe dependence of the energy of bound states of Frenkel excitons (biexcitons), in molecular crystals with one molecule per unit cell, on the number of neighbours is studied. It is shown that the number of bound states depends on the number of neighbours taken into account and the signs of their interaction energies with a fixed molecule.
One phonon versus multiphonon absorption in small ionic crystals
Authors: Ruppin R. 
Published: July 2007

AbstractSome recently published experimental absorption curves of small crystallites of NaI, KI. and MgO are shown to be in qualitative agreement with calculated absorption spectra of small spheres. Although the calculation allows for resonant, one phonon absorption only, it reproduces all the main features of the experimental data. It is shown that allowing for multiphonon processes has a very small effect on the calculated spectra. It follows that measurements on small samples are not a sensitive tool for obtaining information about multiphonon processes in the reststrahlen region.
On the optimization of local model pseudopotentials
Authors: Krasko G. L. 
Published: July 2007

AbstractThe problem of the optimization of local model pseudopotentials (m.p.) is considered. Some aspects of optimization are discussed to stress the importance for a local m.p. to be optimized. An optimization criterion for local m.p. is suggested. To illustrate the “relative optimalness” of various local pseudopotentials the parameters of several ones were found from the same “fitting” equation and the values of the optimization criteria calculated.
The optical absorption by the frenkel exciton in V2O3
Authors: Andrianov G. O. , Aronov A. G. , Smirnova T. V. , Chudnovskii F. A. 
Published: July 2007

AbstractResults are presented of an experimental study of the polarized‐light absorption spectra of V2O3 at the fundamental absorption edge. It is shown that the absorption spectra can be described within the framework of the theory of optical absorption of Frenkel excitons which strongly interact with phonons.
Absorption edge and urbach's rule in ZnO
Authors: Hvedstrup Jensen G. , Skettrup T. 
Published: July 2007

AbstractMeasurements of the absorption edge of ZnO up to 4000 cm−1 for 11 K < T < 878 K are presented. The results are analyzed in terms of Urbach's rule. For T < 350 K, Urbach's rule is reasonably well fulfilled, and the parameters of the rule are derived. Both LO‐phonons and acoustic phonons seem to be involved in the absorption process. For T > 350 K, Urbach's rule in its usual form is no longer valid.
Convergence of lattice sums in hartree‐fock LCAO calculations
Authors: Stoll H. , Preuss H. 
Published: July 2007

AbstractIt is shown that in evaluating the elements of the Fock matrix for the case of translational symmetry fast convergence can be achieved using a Gaussian‐lobe basis. The summation over the next‐neighbour shell should be sufficient for many applications. It is pointed out that the separation in volume‐dependent and structure‐dependent terms is not unique.
Longitudinal magnetoresistance of n‐type germanium in the quantum limit
Authors: Baranskii P. I. , Korolyuk S. L. , Samoylovich A. G. , Yagolnik V. D. 
Published: July 2007

AbstractA quantum kinetic equation for the interaction of electrons having effective masses of tensor character with phonons is derived. The longitudinal magnetoresistance of n‐Ge is calculated in the quantum limit taking into account anisotropy of scattering by acoustic phonons. A detailed study of the dependence of relaxation time ratio T1/Tt on the magnetic field direction is made.
On a variational localization of wannier functions
Authors: KertÉsz M. , BiczOá G. 
Published: July 2007

AbstractA new variational localization of the Wannier functions is proposed and the corresponding equation for fixing the free phase of Bloch functions is derived for the case of non‐degenerate bands. The new equation is shown to be equivalent to a modification of the equations given by des Cloizeaux. The relation permits to warrant the exponential decay of Wannier functions. Illustrative one‐dimensional model calculations are presented to show the efficiency of the method.
Effects of anisotropic spin–orbit interaction on the splitting of d2 configuration orbital cubic triplets in a trigonal field
Authors: Lulek T. 
Published: July 2007

AbstractThe form of the anisotropic spin–orbit interaction Hamiltonian for d ions in a trigonal crystal field is discussed, and the independent parameters are classified using the symmetry reduction formalism. In the case of d2 configuration, the spherical, cubic, and two trigonal parameters can be determined assuming the experimentally known energy level structure of three cubic triplets 3P̃T1, F̃T1, and 3FT2.
Raman investigation of the phonon spectrum in single crystalline β‐boron
Authors: Richter W. , Hausen A. , Binnenbruck H. 
Published: July 2007

AbstractThe Raman spectra of β‐rhombohedral boron are reported. Approximately 10 broad structures are found, a number which is much less than the 83 phonons allowed in first order Raman scattering. In addition, the symmetry selection rules do not hold for these broad peaks. They are thought to be groups of close lying phonon modes, which might have different symmetry and are broadened by damping. The Raman density of states is discussed.
Cyclotron Resonance in Plastically Deformed Tellurium
Authors: Ortenberg M. V. 
Published: July 2007

AbstractIdentical magnetotransmission spectra with B ‖ c for deformed and undeformed tellurium with dislocation densities of 105 and 108 cm−2 give no indication for any electsonic dislocation band up to 3.68 meV above the valence band edge using 337 μm wavelength radiation.
The Band Structure of VO by the APW‐LCAO Method
Authors: Kraan D. J. 
Published: July 2007

AbstractAn APW energy band calculation has been carried out for f.c.c. VO for several symmetry points in the Brillouin zone. Interpolation of the energy values and calculation of the density of states is executed in a way proposed by Mattheiss, with the difference that the vanadium 4s orbital is included in the scheme of this paper.
Effect of an Electromagnetic Wave on the Magnon Damping in Ferromagnetic Semiconductors
Authors: Miranda L. C. M. 
Published: July 2007

AbstractOn the assumption of a weak electron‐magnon interaction, a dispersion equation is obtained for magnons in the presence of an electromagnetic wave. It is shown that the correction added to the frequency and damping of a magnon on account of interaction with the electrons may undergo oscillations. Some possible consequences of this effect are discussed, particularly, in connection with the problem of spin wave amplification.
Effects of Degenerate Bands and Different Hole Masses on Direct Excitons
Authors: Khan M. A. 
Published: July 2007

AbstractThe study of the effect of the degeneracy of the valence band on the exciton series, by Baldereschi and Lipari, has been extended to include the possibility when the two series (originating from the two spin‐orbit split off valence bands), in diamond and zinc‐blende structure crystals, do not have the same effective Rydbergs. A theoretical study developed in this paper is applied to the exciton spectrum of CuBr. The calculated values are found to be in good agreement with the experimental data.
Radiation from Free Excitonic Molecules in CdS Single Crystals
Authors: Voigt J. , Mauersberger G. 
Published: July 2007

AbstractThe emission and transmission spectra of extremely thin high‐quality CdS single crystals are investigated from 1.8 to 77 °K. At low temperatures using stationary excitation by a high‐pressure mercury lamp an emission spectrum with a pronounced peak 3.2 meV below the energy of the A‐exciton is observed, which is characteristic of all thin crystals investigated. The experimental facts, including kinetic phenomena, suggest an explanation of the emission in terms of a system of interacting excitonic molecules. The molecule binding energy is found to be 3.2 meV.
Electrical Resistivity Study of Lattice Defects Formed during Plastic Deformation of Copper Single Crystals
Authors: Buck O. , Schumacher D. , Seeger A. 
Published: July 2007

AbstractThe changes of the electrical resistivity of a high‐purity copper single crystal during tensile deformation at liquid‐helium temperature and after subsequent anneals have been determined. The anneals to 200 °C served as a procedure for separating the resistivity contributions of dislocations and of point defects from each other. It is concluded that during stage‐I deformation both components depend strongly on the impurity content of the material. A model for point‐defect production at dislocation jogs is presented and a linear jog density of about 45 cm−1 is derived from the annealing data. The dislocation resistivity per unit length of about 7 × 10−19 Ω cm3 obtained by Yoshinaga appears to be of the right order of magnitude for stage‐I deformation.
Far Infrared Absorption Due to Electronic Transitions of MgF2:Fe2+ and ZnF2:Fe2+
Authors: Dürr U. , Weber R. 
Published: July 2007

AbstractThe splitting of the spin multiplet of the T2g:A1 ground state of isolated Fe2+ ions (S = 2) in MgF2 and ZnF2 was measured. In MgF2:Fe2+, spectra due to pairs of Fez2+ ions could also be observed. From the data the anisotropy constants D and E can be extracted and for MgF2:Fe2+ the exchange constant as well.
Formulation of Hubbard's Solution Including Scattering and Broadening Corrections in Terms of the Coherent Potential Scheme
Authors: Weller W. , Gobsch G. 
Published: July 2007

AbstractIt is shown that Hubbard's solution of the Hubbard model with inclusion of the scattering and also the broadening corrections can be written as the solution of the single‐site CPA self‐consistency condition with a continuous distribution function for the stochastic variables. The distribution function is determined and discussed.
Pouvoir rotatoire du cinabre (α‐HgS)
Authors: Langlois H. , Ayrault B. , Lefin F. , Toudic Y. 
Published: July 2007

AbstractLe pouvoir rotatoire du cinabre a été mesuré entre la bande interdite (λ ≈ 0,6 μm) et λ = 1 μm à 300, 196,77,20 et 4 K. L'analyse des résultats montre que la dispersion rotatoire est mieux décrite par la formule de Drude (ϱ = A/λ2–λ), déduite d'un modèle à un oscillateur, que par celle de Chandrasekhar (ϱ = Aλ2/λ2–λ)2, déduite d'un modèle à deux oscillateurs couplés. L'étude en fonction de la température montre qu'une même transition est responsable de l'absorption fondamentale au voisinage de Eg et de la dispersion rotatoire.
The Effect of Phonon Heating on the Propagation of Strong High‐Frequency Electromagnetic Waves in Semiconductors
Authors: Gasymov T. M. , Katanov A. A. 
Published: July 2007

AbstractThe propagation of a strong high‐frequency electromagnetic wave has been considered in non‐degenerate semiconductors in the case of electron heating and interaction with long‐wavelength (LW) phonons. Especially a case has been considered where there is no “thermal reservoir” (bath) of short‐wavelength (SW) phonons in the lattice and hence the energy obtained by the LW phonons from the electrons is transferred across the crystal boundaries to the medium surrounding the specimen. It is shown that the heating of phonons leads to an essentially non‐linear problem. Expressions of the field damping and of the temperature damping for electrons (Te) and phonons (Tf) have been established in the weak damping region, and it is shown that the penetration depth is strongly dependent on the amplitude of the incident electromagnetic wave, E0. In the intense damping region (near cyclotron resonance), the surface impedance of the semiconductor has been calculated, which grows with the amplitude of the incident electromagnetic wave.