bluuebunny/crossref_metadata_2025 huggingface dataset:
New Decoupling of the Green Function in the Heisenberg Ferromagnet with Single‐Ion Anisotropy
Authors: Duda A. , Puszkarski H.
Published: July 2007

A 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

The 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

Optical 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

Generalized 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

Optical 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

The 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

The 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

X‐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.
Cluster approximations in alloys
Authors: Mertsching J.
Published: July 2007

Several cluster approximations in alloys are discussed and numerically studied for the one‐dimensional tight‐binding model. The approximations are compared, and some interrelations are found to exist. The shortcomings of the cluster approximations such as broadening, shifting, and diminishing of various cluster bands are exhibited.
Dispersion properties of silver chloride
Authors: Upadhyaya K. S. , Mahesh P. S.
Published: July 2007

The 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

The 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

The 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

The 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

For 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

The 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

The 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

Evidence 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

The 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

Some 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

The 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

Results 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

Measurements 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

It 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

A 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

A 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

The 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

The 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

Identical 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

An 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

On 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

The 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

The 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

The 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

The 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

It 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

Le 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

The 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.
Sulphur Impurity Level Dependence on Band Structure Variation with Pressure in GaxIn1−xSb
Authors: Campos M. D'olne , Gouskov A. , Gouskov L.
Published: July 2007

Results of Hall effect and resistivity measurements under hydrostatic pressure (up to 15 kbar) between 300 and 77°K are given for three samples of GaxIn1−xSb. The values of x are 1; 0.96; and 0.865. These results are interpreted on the basis of a simple model of charge exchange between one donor level ES and one occupied band minimum. It is shown that for x = 1 andx = 0.96 the sulphur level ES localized in the forbidden band gap has the same pressure coefficient as the X1 minimum: E d (E − E)/dP = d (ES − E)/dP = 16 meV/kbar. For x = 0.865, ES is merged in the conduction band and is no more rigidly associated to the X1minimum. ES shows various pressure coefficients depending on its position in the band structure.
Dynamical Susceptibility for Interacting Electrons in a Narrow Energy Band
Authors: Nolting W.
Published: July 2007

An approximate expression for the dynamical susceptibility χ (q, ω) of the paramagnetic phase of an interacting electron system in a narrow energy band is derived. The treatment is based on the “method of spectral moments”, χ (q, ω) is used to look for instabilities of the paramagnetic state toward ferromagnetic ordering. If the number of electrons is less than the number of sites, χ (q, ω) signals the possibility of two different types of ferromagnetism for both the s.c. and b.c.c. structure, while the paramagnetic system is stable for the f.c.c. structure. For the f.c.c.‐lattice a single ferromagnetic solution exists, if the band is more than half‐filled.
Theory of the Hopping Heat Conductivity in Amorphous Semiconductors and Semiconducting Glasses
Authors: Čápek V. , Marvan M.
Published: July 2007

The lowest order (singl‐phonon) contribution to the hopping heatconductivity of amorphous semiconductors and semiconducting glasses with harmonic lattices is found and discussed.
Extension of the Unsaturable Trap Model to One‐Dimensional Interstitial Migration
Authors: Gösele U. , Frank W.
Published: July 2007

For most f.c.c. metals the increase of the electrical resistivity due to stage‐II temperature irradiation may be interpreted in terms of trapping of mobile interstitials at unsaturable traps. This model originally elaborated for interstitials migrating in three dimensions predicts that the inverse of the production rate of defects retained in an irradiation‐exposed sample increases linearly with the defect concentration. Using three different methods (rate equation approach, diffusion theory, and a computer simulation technique) it will be shown that this relationship remains valid if the mobile interstitials are off‐line crowdions and thus constrained to a long‐range migration in one dimension. It is therefore not admissible to draw any conclusions about the nature of self‐interstitials in f.c.c. metals from radiation damage curves measured at stage‐II temperatures, contrary to what has repeatedly been done in the literature.
Influence of Acoustic Lattice Vibrations on Dynamical X‐Ray Diffraction
Authors: Köhler R. , Möhling W. , Peibst H.
Published: July 2007

A dynamical X‐ray diffraction theory for crystals with acoustic lattice vibrations is presented. The fundamental equations derived for the general case are specified to monochromatic vibrations of wavelengths shorter than the pendellösung distance taking into account higher harmonics. It is shown that each satellite reflection, caused by these vibrations, can be treated as a usual dynamical two‐beam case using a modified dielectric susceptibility χ which depends on the amplitudes and phases of the vibrations.
Temperature Dependence of the 1S Line of the Yellow Series of Cu2O
Authors: Merle J. C. , Nikitine S. , Haken H.
Published: July 2007

The behaviour of the shape of the quadrupolar n = 1 line of the yellow serie of Cu2O is described as a function of temperature. The total anisotropy ofthis line enables one to use a highly sensitive polarization modulation technique, with ahigh dispersion set‐up. The behaviour of the line being inexplainable in the scopeof previous theories, is discussed with a new theoretical model in which both the defectsof the crystal and the phonons are taken into account, and which leads to an absorption having a Voigt shape. The analysis of the experimental results enables one to evaluate theparticipation of the defects and shows that both acoustic and optical phonons scatter theconsidered exciton, in good agreement with the theory.
Shubnikov‐de Haas Oscillations in CdvHg1−vSe
Authors: Stankiewicz J. , Giriat W. , Dobrowolski W.
Published: July 2007

Oscillations of the magnetoresistivity in CdvHg1−vSe for v = 0.05; 0.075; 0.10 and 0.19 are reported. From the temperaturedependence of the amplitudes of the oscillations, the effective mass of electrons was determined. The effective mass is dependent on concentration. In order to find the energy band gap, E0, and the matrix element, P, the k · p method, including interaction with higher bands and finite Δ, was used. It was foundthat the dependence of E0 on composition at 4.2 K is linear for v < 0.2.
Calculated Infrared Absorption Due to Impurities in Cesium Halides
Authors: Ram P. N. , Agrawal Bal K.
Published: July 2007

A next‐nearest neighbour perturbation model has been employed to remove the discrepancies observed earlier in the infrared absorption data of cesium halides containingsome impurity ions such as Tl+ and Rb+. Improvement in the locations of absorption peaks in CsCl:Rb+ and some structure in the high frequency region in thalium doped CsBr and CsI have been seen but discrepancies still exist in the intensity of absorption. The experimental data in CsI:Na+ and CsI:Li+ have been discussed on the basis of nearest neighbour defect model. Very good agreement between theory and experiment has been found. Further, the earlier calculated absorption in CsI due to impurities using the phonons of Mahler show much improvement when phonons of a better calculation recently performed by Jex are used. The results showthe necessity of a revised calculation for phonons in pure cesium halide crystals.
Linear Magnetic Birefringence in Transition Metal Oxides: CoO
Authors: Germann K. H. , Maier K. , Strauss E.
Published: July 2007

The optical linear magnetic birefringence (LMB) of cobalt oxide single crystals was determined as a function of temperature near the antiferromagnetic Néel point. The measurements were taken from single‐domain crystals with the tetragonal c‐axis oriented parallel or perpendicular to the direction of the light beam. Below TN the change of LMB with temperature in perpendicular arrangement is found to be proportional to the c‐axis contraction and to the square of the long‐range order parameter. Magnetic short‐range order effects have been observed even far above TN. The results are discussed by means of a magnetostriction model.
Crystal Field Splitting of S‐State Ions in Crystalline TeO2 and Amorphous TeO2‐V2O5
Authors: Sperlich G. , Urban P.
Published: July 2007

The parameters of the spin‐Hamiltonian of Gd3+ in TeO2 single crystals are determined and the ESR spectra of polycrystalline TeO2:Gd3+ samples are compared with those of Gd3+ ions on Te sites in amorphous TeO2‐V2O5 compounds. It turns out that the microenvironment of the tellurium ion in the amorphous samples is similar to that in crystalline TeO2. Further ESR signals of Fe3+ ions in amorphous TeO2‐V2O5 are discussed.
Ionic Raman Scattering and Ionic Frequency Mixing
Authors: Martin T. P. , Genzel L.
Published: July 2007

Two closely related effects based on the nonlinearity of ionic polarizability are considered, ionic Raman scattering and ionic frequency mixing. A simple model is used to make explicit calculations for quartz. Both effects have two resonances associated with each reststrahl band. In addition, antiresonances may occur due to the interference between the ionic process and the corresponding normal process. Although two resonance conditions may be satisfied simultaneously in quartz, the Raman efficiency seems to be below the present limits of observation. On the other hand, the calculation shows that resonant ionic frequency mixing should be observable in quartz.
Thermally Activated Plastic Deformation of High‐Purity Copper Single Crystals
Authors: Zeyfang R. , Buck O. , Seeger A.
Published: July 2007

Changes in strain rate during dynamical tensile deformation of high‐purity copper single crystals are used to determine the activation volume associated with plastic deformation as a function of flow stress, temperature, and strain rate. In work‐hardening stage I the activation volume is almost constant, whereas in stage II the reciprocal activation volume increases linearly with flow stress. It is shown that the flow‐stress ratios are not constant in any of the stages I to III, in violation of the so‐called Cottrell‐Stokes law. The experimental results are discussed in terms of the work‐hardening model of Seeger and co‐workers and indicate that the strain‐rate dependent (thermally activated) but not the temperature independent contribution to the flow stress is determined by the intersection of dislocations.
Lattice Dynamics of Graphite
Authors: Mani K. K. , Ramani R.
Published: July 2007

The lattice dynamics of graphite have been studied using a tensor force model. Force constant values for interactions between atoms in‐plane have been transferred from the naphthalene molecule. It is shown that the agreement between theory and neutron scattering data is very good, indicating that the molecular constants form a very good approximation to the crystal force constants. Significant differences are, however, found between the present calculations and those of Nicklow et al. using an axially symmetric model, regarding some out‐of‐plane modes. It is assumed that these arise from the restrictions of the axially symmetric model. It is also shown that restoring forces corresponding to twisting motions about CC bonds in the plane are very small.
Study at 108 and 2 × 108 Hz of the Electrical Conductivity and Dielectric Constant of the Highly Anisotropic Organic Semiconductor TEA (TCNQ)2
Authors: Farges J. P. , Brau A.
Published: July 2007

The ac electrical conductivity and anisotropy, including an evaluation of the dielectric constant, at 108 and 2 × 108 Hz in the range 77 to 350°K is investigated. A hopping mechanism is evidenced from the dispersion of conductivity observed below 200°K, and is associated with the existence of tails of localized states at the edges of a band gap. The vanishing dispersion in the upper temperature range is likely to indicate that an interband transition process may then become important for conduction.
Phonon Dispersion Relations and Two‐Phonon Raman Spectra of Cesium Fluoride
Authors: Gupta H. N. , Singh R. K.
Published: July 2007

The phonon dispersion relations and two‐phonon Raman spectra of cesium fluoride have been calculated with a three‐body force shell model employing core and shell charge modifications which account for the Cauchy‐discrepancy and electronic polarizability. The results show an excellent agreement with the observed data. A brief description of the modification introduced by three‐body interactions in the electronic polarizability is also presented.
Antimony Centres in Potassium Halides
Authors: Radhakrishna S. , Karguppikar A. M.
Published: July 2007

Antimony doped potassium halide crystals give one optical absorption band at 205 nm in KCl, 215 nm in KBr, and 243 nm in KI at RT. These bands are ascribed to the divalent state of antimony. Correlated EPR studies show that the paramagnetic antimony (Sb2+) state is associated with a vacancy in the second neighbour position. Experiments have also been performed on the conductivity and dielectric loss. The impurity–vacancy binding energy is found to be ≈ 0.30 eV in these crystals.
Die Bedeutung der Elektron–Elektron‐Wechselwirkung in Halbleitern
Authors: Jäger I.
Published: July 2007

Der Einfluß der Streuung von Ladungsträgern aneinander auf die Gestalt ihrer Verteilungsfunktion wird mit Hilfe einer erweiterten „Path‐Variable‐Methode”︁ diskutiert. Es wird als Beispiel die Verteilungsfunktion warmer Elektronen in n‐Germanium berechnet und gezeigt, daß unter der getroffenen Wahl eines abgeschirmten Coulombpotentials für die Wechselwirkung der Elektronen untereinander die Rechnungen mit und ohne Berücksichtigung dieser Wechselwirkung kaum differieren, sowie, daß die so berechnete Verteilungsfunktion den Koeffizienten der warmen Elektronen (β), eine von der Gestalt der Verteilungsfunktion stark abhängige Größe, gut wiedergibt, obwohl die Berechnung absichtlich in einem für die Rechnung ungünstigen Bereich der Ladungsträgerkonzentration durchgeführt wird.
Acceptor States in Selenium–Tellurium Mixed Crystals with Low Selenium Concentrations
Authors: Yoshikawa A. , Takagi T. , Mizushima Y.
Published: July 2007

Transport properties of undoped Se–Te mixed crystals containing up to 5 at% Se are investigated in the temperature range from 1.7 to 300 K. The behaviour of the Hall coefficient at low temperatures shows sudden changes at 1 to 2 at% of Se concentration. In mixed crystals with more than about 2 at% Se, the magnetic freeze‐out effect and impact ionization effect of impurities are found. These experimental results indicate that, contrary to undoped Te, shallow acceptor states exist above the top of the valence band in these mixed crystals.
Covariant Forms of the Third‐ and Fourth‐Rank Tensors such as Piezoelectric Moduli and Piezo‐Optical Coefficients
Authors: Iosilevskii Ya. A.
Published: July 2007

Covariant forms of the third‐rank tensor Likl invariant under the permutation k ⇄ I and the fourth‐rank tensor Lpiklm invariant under the permutations i ⇄ k and I ⇄ m are suggested for all the crystal classes. A covariant procedure of reducing the given tensor to principal axes is presented when these axes are not completely determined by the symmetry elements of the crystal. A covariant method of constructing the fourth‐rank reciprocal tensor is given.
Conductivity Tensor Components of n‐Bi2Se3
Authors: Köhler H.
Published: July 2007

From Shubnikov‐de Haas experiments a single‐valley model for the lowest conduction band of Bi2Se3 was derived. The determination of non‐parabolic contributions to the ϵ(k) relation enabled the calculation of conductivity tensor components, which should be zero in an ellipsoidal approximation. The results are compared with own measurements and earlier data, which had been interpreted in an ellipsoidal six‐valley model.
Theory of the Residual Resistivity of Bloch Walls. II. Inclusion of Diamagnetic Effects
Authors: Cabrera G. G. , Falicov L. M.
Published: July 2007

Diamagnetic effects are included in the calculation of the electrical resistivity of a Bloch wall in a ferromagnetic metal. The resulting electron orbits, which are convoluted zig‐zag‐ging trajectories near the wall, produce an increase in the resistivity which is roughly proportional to (ωcτ)2, where ωc is the cyclotron frequency of the electron due to the internal magnetization. Such an effect explains satisfactorily the experimental data reported in the literature.
Crystal Field Determination in Rare‐Earth Oxysulfides. I. Optical Experiments for Tb3+ Ion
Authors: Rossat‐Mignod J. , Souillat J. C. , Quezel G.
Published: July 2007

The crystalline electric field (CEF) acting on a Tb3+ ion in rare‐earth oxysulfides has been determined. By analysing fluorescence spectra of Y2O2S: 1% Tb3+, it is possible to obtain the energy levels of the 7FJ (J = 1 to 6) manifolds of Tb3+ in this host and therefore the CEF Hamiltonian of C3v symmetry. In order to explain the particular magnetization processes in Tb2O2S, particular attention has been paid to the ground state. It has been found that the ground state consists of four levels lying very close one another (0, 6, 15, 26 cm−1). Both the direction and the value of the magnetic moment calculated according to this level scheme agree well with neutron diffraction data.
Analytic Expression for the Electronic Charge Density Distribution in Diamond‐Structure Crystals
Authors: Chadi D. J. , Cohen M. L.
Published: July 2007

The explicit dependence of the valence band electronic charge density distribution on the pseudo‐potential form factors for the diamond structure crystals has been obtained using a weighted sum of the charge densities at the symmetry points Γ, X, and L of the Brillouin zone. The charge densities and energies for the bands at Γ X, and L, and their dependence on the lattice constant and pseudopotentials are also discussed.
Optimal CC, CH, and HH Potential Curves for the Naphthalene Crystal
Authors: Mirskaya K. V. , Kozlova I. E. , Bereznitskaya V. F.
Published: July 2007

On the basis of experimental data on the structure and elasticity tensor of naphthalene crystals at low temperatures a more accurate fitting of the parameters of the C‐C, C‐H, and H‐H potential curves was made. The potential curves involved have the form φ − (A/r6)+ Be−αr and the following numerical parameters: Acc= 421 (kcal/mol)Å6, Bcc = 7.16 × 104 kcal/mol, αcc = 3.68 Å−1; ACH = 118 (kcal/mol) Å6, BCH = 1.86 Å 104 kcal/mol, ÅCH = 3.94 Å−1;AHH = 29(kcal/mol) Å6, BHH × 0.49 × 104kcal/mol,αHH = 4.29 Å−1.
Effect of Anharmonicity on Urbach's Exponential Tail in Amorphous Semiconductors
Authors: Bhatia K. L. , Bhat P. K.
Published: July 2007

At the crystal‐glass transition the overall short range order is retained but a distortion exists in the interatomic bonds which introduces a nonlinearity in the interatomic forces. The characteristics of amorphous materials have been studied taking into account the nonlinearity of the interatomic forces. This anharmonicity in amorphous semiconductors is related with the decreased slope of the exponential part of the fundamental optical absorption edge of a semiconductor.
On the Definition of the Current Operator Application to the Hubbard Model
Authors: Monecke J.
Published: July 2007

A disagreement existing in the literature between various calculations of the electrical conductivity y in the single‐band Hubbard model is resolved. It is replied to an article by Cabib and Kaplan who tried to disprove that there are no absorption and emission lines near ω = U due to the translational invariance. The physical reason for this result and some of its consequences are discussed.
Ionized‐Beam Energy Loss to Electron Gas
Authors: Shah S. , Srivastava M. P. , Kothari L. S.
Published: July 2007

The stopping power and other transport coefficients have been calculated for ionized beams channeling through a crystal using a statistical method developed by Chandrasekhar. Only the interaction of beam particles with conduction electrons, treated as a free gas, is considered. Unlike earlier investigations, also the effect on the test particles of encounters with faster field particles is considered. Two types of potentials have been used, a Debye and a Coulomb one. Both potentials give nearly the same results. It is found that the values of energy loss and transport coefficients increase with increasing energy of the incident beam. The stopping power is nearly the same as that obtained by earlier workers.
Crystal Field Determination in Rare‐Earth Oxysulfides. II. Optical Experiments for Ho3+ and Dy3+ Ions
Authors: Rossat‐Mignod J. , Souillat J. C. , Quezel G.
Published: July 2007

The crystal field levels of the 5I8 ground state manifold of Ho3+have been obtained from the luminescence spectrum of Y2O2S:5%Ho3+. The ground state of the Ho3+ ion is found to be a set of five levels (0, 8, 14, 18, 23 cm−1). An attempt has been made to deduce the parameters of the CEF Hamiltonian from the experimental level energies. For Dy3+, the breaking up of the fundamental 6H15/2 multiplet has been obtained by analysing the fluorescence spectrum of Y2O2S:1% Dy3+ and the absorption spectrum of Dy2O2S. The ground state is found to be a doublet relatively well separated (21 cm−1) from the first excited level. In these two compounds, CEF calculations give satisfactory agreement with both the value and direction of the magnetic moment obtained from neutron diffraction experiments.
Static Structure of Liquid Noble and Transition Metals by X‐Ray Diffraction
Authors: Waseda Y. , Ohtani M.
Published: July 2007

X‐ray diffraction patterns were obtained from liquid noble and transition metals (Cu, Ag, Au, Fe, Co, Ni) at temperatures between melting point mid 1650 °C. After calculating the structure factor (Fourier analysis), the atomic radial distribution function was evaluated from which interatomic distance and coordination number were obtained. Comparing the results with those observed previously by other researchers, adequate agreement was found.
Low‐Symmetry Crystal Fields in CrCl3
Authors: Majer M. , Szymczak H.
Published: July 2007

A crystal field study has been performed for CrCl3 crystals. The energy matrix has been diagonalized within the 120 states of the complete d3 configuration. In the calculation the trigonal field, spin–orbit interaction as well as Tree's correction have been taken into account. The results of the calculations have been compared with the experimental data on the specific magnetic rotation, optical absorption, and EPR spectra.
Lattice Vibrations and Free Carrier Dispersion in PbSe
Authors: Burkhard H. , Geick R. , Kästner P. , Unkelbach K.‐H.
Published: July 2007

The reflectivity of n‐type PbSe has been measured in the infrared region from 10 to 400 cm−1 at 5, 77, and 300 K. A Kramers‐Kronig analysis has been performed as well as a fit with a dispersion formula including a Drude term for free carriers. The transverse phonon frequency was observed at 39 cm−l and found to be independent of temperature. The contribution of the lattice oscillator to the static dielectric constant of 180 is relatively high. The longitudinal excitations are determined considering the coupling between phonons and free carriers. Finally the plasma frequency has also been found to be independent of temperature and in reasonable agreement with Hall effect and effective mass data.
Temperature Dependence of Paraelectric Resonance of OH− Centres in KI Single Crystals
Authors: Hundhausen R. , Osswald R. , Wolf H. C.
Published: July 2007

PER spectra of OH− in KI have been measured in the X‐band (11 GHz) and the Q‐band (35 GHz) with several orientations of the electrostatic field at liquid helium temperatures. The temperature dependence of the Q‐band spectrum (E0 ‖ E1 ‖ 〈100〉) indicates a zerofield splitting in the range of about 120 GHz.
Stark Effect of Wannier‐Mott Excitons Due to Disorder
Authors: Majerníková E.
Published: July 2007

The averaged canonical density matrix of a large orbit exciton with a diameter ϱ in a Gaussian random potential field with the correlation length L and the mean square root of the random potential η, has been calculated using the Feynman path integral approach. It has been shown that in the case when the condition ϱ < L – De is fulfilled the influence of the random field on the internal state of the exciton is equivalent to the Stark splitting of the excitonic levels due to an effective Gaussian random electrical field. The theory permits to interpret the experiments of Nikitine et al. on the excitonic absorption spectra of structurally perturbed Cu2O and to make conclusions about the parameters of the structure of disordered solids (correlation functions, correlation lengths). The respective density of states has been calculated with a strongly marked asymptotic tail behaviour.
Kondo Effect in the Transport Properties of the CsCl‐Type Compounds Fe1−xTi1+x. II. Magnetic Scattering Center due to Atomic Disordering
Authors: Ikeda K. , Nakamichi T. , Yamada Y. , Yamamoto M.
Published: July 2007

Neutron diffraction measurements were made at room temperature for the CsCl‐type compounds, Fe1−xTi1+x, in order to confirm the origins of the anomalous transport phenomena in the titanium‐rich compositions and the superparamagnetism near the stoichiometric one. On the basis of those results, it could be concluded that the former behaviour is due to the Kondo effect, originating in a magnetic cluster of nine iron atoms with a concentration of about 10−2, produced by the atomic disorder. The composition dependence of the residual resistivity also seems to be explainable from this point of view.