bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvable Model for Random Systems Including Off‐Diagonal Disorder
Authors: John W. , Schreiber J.
Published: July 2007

Anderson's model for disordered systems is considered in the case where the transfer matrix elements fluctuate according to a Lorentzian distribution. It is shown that the exact ensemble averaged Green's function may be obtained if the energy level on each site depends linearly on the overlap integrals. Using this result the stability of amorphous Heisenberg ferromagnets is studied. The Weaire model of an amorphous covalent semiconductor with fluctuating matrix elements is considered. Numerical results for the density of states are given.
Modes de relaxation électriques dans les cristaux ioniques de type NaCl dopés par des impuretés trivalentes
Authors: Brun A. , Dansas P.
Published: July 2007

On a utilisé un modéle á puits de potentiel pour déterminer les modes de relaxation électriquement actifs et les temps de relaxation correspondants pour un cristal de type NaCl dope par une impuretk trivalente en substitution. On a calculi! Bgalement les polarisations correspondantes A chaque mode de relaxation et discuté de l'intbrbt particulier des mesures „diélectriques”︁ dans ce cas.
Exciton‐Polariton Luminescence in Cuprous Halides
Authors: Suga S. , Koda T.
Published: July 2007

Resonance luminescence spectra of the free exciton are investigated in cuprous halides by uniaxial‐stress and magneto‐optical measurements. Apart from the perturbation‐induced structures due to the longitudinal and spin‐triplet excitons, the characteristic‐doublet structure is conclusively ascribed to the exciton‐polariton effect. Temperature‐dependent spectral shapes of these polariton luminescences are analyzed to derive the distribution of polaritons in the upper and the lower branches. The bottle‐neck effect of the polariton decay process is also discussed.
Influence of Magnetic Fields on the Lineshape of the Exciton‐Polariton Reflectance in GaAs
Authors: Fischbach J. U. , Dreybbodt W. , Suga S. , Willman F.
Published: July 2007

The influence of high magnetic fields on the line shape of the exciton‐polariton reflectance is investigated in epitaxial GaAs at helium temperature. The changes of the line shape can be understood from the variation of the Bohr radius, the binding energy, and the oscillator strength of the exciton in a magnetic field. From the analysis of the spectra it is shown that the longitudinal‐transverse splitting of the ground state can be obtained from the experimental reflectance curves. In addition to the well‐known exciton free layers at the crystal surface, the influence of a transition layer, where the excitons get partially ionized, is established.
The Fine Structure of Wannier‐Mott Excitons in a Cubic Crystal and Its Behaviour in an Electric Field
Authors: Agekyan V. T. , Monozon B. S. , Shiryapov I. P.
Published: July 2007

The anisotropy of the forbidden transitions in the absorption spectrum of the exciton yellow series of cuprous oxide in an electric field has been studied. The group‐theoretical analysis of electric field effect on transitions with different orbital quantum numbers l makes it possible to reveal the P‐, S‐, and D‐series of exciton levels on the basis of experimental data. The position of dipole‐active and ‐inactive levels has been determined by extrapolation to zero field, which enables one to study the l‐splitting in a cubic crystal. With the l‐splitting, the P‐series remains exactly hydrogen‐like, the D‐series preserves an approximate hydrogen‐like behaviour, and the S‐series looses its hydrogen‐like behaviour.
Spin Wave Amplification in Ferromagnetic Semiconductors: Magnetoresistance and Non‐Ohmic Behavior
Authors: Filho M. D. Coutinho , Miranda L. C. M. , Rezende S. M.
Published: July 2007

The influence of magnon amplification on the magnetoresistance of ferromagnetic semiconductors is analyzed microscopically. A simple model for the non‐linear relaxation of magnons is proposed which can explain the existing measurements in CdCr2Se4.
Cross‐Relaxation Studies of a Four‐Spin Flip Transition
Authors: Mai Chan Khanh
Published: July 2007

A theoretical investigation of the process of four‐spin flip cross‐relaxation transitions has been carried out in the case of three types of spins in the crystal 2ω1 = ω2+ω3+Δ; |Δ|≪ ω1, ω2, ω3, The dipole–dipole reservoir is taken into account. Using the method of the nonequilibrium statistical operator of Zubarev, a system of equations describing the evolution of the temperatures of the Zeeman subsystem and of the dipole–dipole reservoir is obtained. The stationary solution in the case of saturation of one of the Zeeman spin subsystems is studied.
Effect of Spatial Correlations on Hopping Transport
Authors: Pollak M.
Published: July 2007

Spatial fluctuations of the potential are inherent in hopping transport. The effect is considered of the stochastic dependence between the potentials of hopping sites, specifically the case when the correlation length of the potential is considerably longer than the hopping distance. The percolation approach is used. The behavior is found to depend on certain characteristics of the equipotential surface at the Fermi energy. If this surface is non‐connected, an activated conductivity is expected. If the surface consists of parts connected through narrow channels, a T−l/2 dependence is predicted. For stronger connectivity, a T−1/3 dependence is deduced. The T−1/4 dependence, characteristic of hopping in the absence of spatial correlation, is not expected to occur.
Band Structure and Fermi Surface of In3Sn Alloys in the ß‐Phase
Authors: Maciejewski S.
Published: July 2007

Band structure calculations are performed for In3Sn alloys with Sn concentrations from 16.2 to 30 at% by the pseudopotential method with the Animalu‐Heine model potential. From the dispersion relations E(k) obtained, the Fermi surface parameters of the alloys are determined. In the point R of the Brillouin zone, the influence of Spin–orbit interaction is calculated.
Determination of optical constants: Derivative spectra from ellipsometric data
Authors: Jungk G.
Published: July 2007

Derivative spectra calculated from the ellipsometrically determined dielectric function are discussed. The line‐shape analysis enables the determination of type, energetic position, and broadening parameter of van Hove singularities. The procedure is demonstrated for the E1structures in Ge and GaP. For Ge critical points are found near 2.0 eV (M0‐type); 2.1 eV (Mo‐type), and 2.3 eV (M1‐type). The optical structure near 3.7 eV in GaP consists mainly of a M1‐transition. The influence of final‐state interactions is discussed.
Second‐order effects in the anisotropic elastic field of a straight edge dislocation
Authors: Seeger A. , Teodosiu C. , Petrasch P.
Published: July 2007

A method is described for the calculation of the non‐linear anisotropic elastic field of a straight edge dislocation lying along a two‐fold axis of material symmetry taking into account the continuity conditions at the boundary of the dislocation core. The results may be applied to improve the study of the dislocation core by using semi‐discrete techniques.
Extension of the coherent potential approximation for disordered alloys with random transfer integrals
Authors: Eifrig Th. , Kolley E. , Kolley W.
Published: July 2007

The one‐electron Hamiltonian which takes into account the diagonal and off‐diagonal randomness is used to describe disordered binary alloys. The k‐dependent self‐energy is calculated in an extended single‐site coherent potential approximation based on the cluster concept. The behaviour is found to be correct in the dilute alloy limit. The formalism is quite tractable for numerical calculations and yields continuous self‐energies. The difference to the method proposed by Brouers is demonstrated by a numerical example.
The critical voltage effect in transmission electron microscopy. II. A theoretical study neglecting absorption effects
Authors: Gevers R. , Serneels R. , David M.
Published: July 2007

Using degenerate perturbation theory a study is made of the critical voltage effect in electron microscopy. In particular the dependence on the accelerating electron voltage of the profile of the second order bend contour for centrosymmetric crystals is considered. The case of non‐centrosymmetric crystals is included in the formulae. It is found for centro‐symmetrical crystals that the more the critical voltage is approached, the second order bend contour appears as a kinematical like bend contour which disappears completely in a small interval around the critical voltage. Also the symmetry properties are discussed of the Bloch waves corresponding to the branches of the dispersion surface which touch at the critical voltage. It is shown that away from the critical voltage and when the second order reflection is not exactly satisfied the Bloch waves are nearly symmetric or antisymmetric. When the voltage is varied towards the critical voltage a continuous change appears of the Bloch wave symmetry in a given interval around the critical voltage. The smaller the mis‐orientation the smaller is the voltage interval. For non‐centrosymmetric crystals a similar effect is observed.
Galvanothermomagnetic effects in anisotropic media
Authors: B1Abanskii P. I. , Buda I. S. , Dakhovskii I. V. , Samoilovich A. G.
Published: July 2007

From a uniform point of view the possible transverse (with respect to J or ▽) and longitudinal (directed along J or ▽) galvanomagnetic and thermomagnetic effects arising in anisotropic media are considered. Presenting the tensors magnified image as sums of the symmetrical and antisymmetrical parts a classification is supposed which is convenient for the experimental investigations of the galvanomagnetic and thermomagnetic effects (transverse and longitudinal, even and odd with respect to H) arising in anisotropic media. The classification of the galvano‐ and thermomagnetic effects based on the physical considerations shows that effects which have not been investigated formely should arise in anisotropic media together with the known phenomena. The phenomenological considerations developed in the work are valid for any anisotropic medium. Detailed calculations of the kinetic coefficients describing the effects considered in the work can be made (for such crystals as germanium, silicon et al.) on the basis of the anisotropic scattering theory.
Experimental and theoretical study of Raman scattering in Co++‐ and Fe++‐doped magnesium oxide
Authors: Billat A. , Mon J. P. , Voisin M.
Published: July 2007

Raman spectra of iron or cobalt doped MgO single crystals have been obtained. A strong band which lies between 250 and 330 cm−1 for the cobalt doped crystal and between 100 and 350 cm−1 for the iron doped sample appears in the spectrum and has been assigned to resonance modes. Using experimental frequencies, the central and shear force constant changes βr. and γr have been calculated.
The electronic thermal conductivity of strained aluminium below 1 K
Authors: O'Hara S. G. , Anderson A. C.
Published: July 2007

The thermal conductivity of strained, high purity Al has been measured in the temperature range 0.04 to 1 K. It is concluded that electrons do not interact significantly with the localized phonon states which are associated with dislocations in Al.
Radiative and non‐radiative transitions of electrons in local centers involving hot phonons
Authors: Kovaeskii V. A. , Popov E. A. , Chaikovskii I. A.
Published: July 2007

A theory of multi‐quantum radiative and non‐radiative (NRT) transitions in local centers under a high power laser radiation is presented. A model of local center having local phonons (LP) is used. When the interaction between the LP and the thermostat (crystal vibrations) is weak, the quantum transitions of electrons involving hot phonons result in a heating of the LP to an effective temperature Tv well above the equilibrium one. An explicit expression for Tv based on the method of nonequilibrium statistical operator is derived. The latter is determined self‐consistently in terms of quantum correlation functions, which contain standard multiphonon structures. This allows the shape function of absorption and NRT involving hot phonons to be found. In particular one displays the LP heating by means of a sharp asymmetry (flattening) of the absorption band and an essential activation of NRT.
Non‐conservation of frenkel excitons and bound states
Authors: Maeinković M. M. , Tošarić B. S.
Published: July 2007

A new method which is characterized by a clean approach and high efficiency is proposed for the analysis of bound states in systems that do not conserve the number of quasiparticles. This approach observes fully the specific commutation relation belonging to Pauli operators. Its efficiency made it possible to calculate the energy spectrum of biexcitons in a higher order of approximation than in previous papers and avoids some of their errors. The calculations are performed using the Dyson‐Maleev Bose representation of Pauli operators as well, in order to settle a fair estimate of the fitness of this representation for the analysis of non‐conservative systems of Prenkel excitons. In the linear approximation a full agreement is found with the Dyson‐Maleev approach. In higher approximation, as could be expected, Dyson‐Maleev representation does not yield correct results. Finally the critic is directed to the uncritical application of some previous approaches and the possible cause of errors to which such applications led is discussed.
Multiple inelastic scatterings in electron diffraction
Authors: Ohtsuki Y. H.
Published: July 2007

A multiple inelastic‐scattering theory for electron diffraction is developed in terms of the transport equation formed from the Bloch waves. The diffusion of the Bragg spot due to inelastic scattering is represented by a coupled diffusion equation with a generalized diffusion coefficient which is the Fourier component of the position‐dependent diffusion‐coefficient function appearing in dechanneling theory. The two‐wave case is studied in detail, in which the coupled diffusion equation may be separated into two single diffusion equations with two modified diffusion coefficients. The modified diffusion coefficients are calculated for many‐phonon and single‐electron excitations.
Electron‐hole drops in Ge: Correlation effects in the emission line shape
Authors: Rösler M. , Zimmermann R.
Published: July 2007

The effect of exchange and correlation on the density of states and the emission intensity of highly excited Ge is investigated. The self‐energy is calculated using the single‐plasmon pole approximation for the dielectric function. At the equilibrium density rS = 0.58 an enhancement factor of about three and a line narrowing of 5% are found for the emission line shape.
Electric field effects on anisotropic excitons in lead iodide
Authors: Bordas J. , Ricco B.
Published: July 2007

The electric field dependence of the ground state of the exciton at the fundamental absorption edge of PbI2 is interpreted by means of a theoretical approach including anisotropy. As a result of the theoretical fit to the experimental curves the following exciton parameters are obtained: R = 16 meV, ε⟂ = 36, ε‖ = 24, a⟂ = 20 Ǎ, a‖ = 13 Ǎ. These estimates are used to explain satisfactorily the behaviour of the electroabsorption spectrum as well as the temperature dependence of the width of the exciton line which previous published values failed to do.
Resonant tunnelling into a size‐quantised metal film
Authors: Cottey A. A.
Published: July 2007

The theory of resonant tunneling through a thin oxide layer into a size‐quantised metal film has hitherto been given only by modelling the metal film as a free‐electron well with infinite potential barriers at the surface. It Is shown that the essential feature of the theory, namely the “bunching of commensurate energy levels near the middle of the Brillouin zone”, is retained if allowance is made for the lattice potential, for a finite surface potential, and for relaxation of the surface layers of atoms. There is, however, a uniform displacement of the energy levels, and this has an influence on the phase of the observed oscillations of conductance versus bias.
The kinetics of dislocation pinning at low temperatures
Authors: Thompson D. O. , Buck O.
Published: July 2007

A further analysis is made of earlier, low‐amplitude internal friction results (εmax ≈≈ 1.2 × 10−7) for a γ‐irradiated copper single crystal. It is shown that the minimum in the rate at which irradiation produced defects arrive at dislocations in Cu at 170 to 180 K is obtained not only from measurements at the irradiation temperature, but also from measurements made at lower temperatures following irradiation. This is in direct contrast to the results obtained by Lücke et al. The unusual temperature–internal‐friction relation of the crystals used by Lücke et al. suggests that dislocation pinning by impurities affected the results of the Aachen group.
Excitation spectra of exciton luminescence in CdTe
Authors: Hiesinger P. , Suga S. , Willmann F. , Dreybrodt W.
Published: July 2007

Measurements of the excitation spectra of the photoluminescence lines below the band gap of CdTe led to the identification of emissions originating from excitons bound to neutral donors, to ionized donors, and to neutral acceptors, as well as from two‐electron transitions. Oscillatory variations of the free and bound exciton emission intensity with the energy change of the exciting light above the band gap and their changes with temperature indicated details of the exciton relaxation mechanisms.
The stress‐induced anisotropy of the dielectric function in α‐Sn
Authors: Blinowski J.
Published: July 2007

The interband Γ8‐Γ8 part of the dielectric function and its anisotropy for α‐Sn samples under [001] compressional stress have been calculated by the BPA method. In the high stress region, the anisotropy of the dielectric function in the long‐wavelength limit depends neither on stress nor on material parameters and results entirely from the p‐character of the Γ8 Bloch functions and the breakdown of the cubic symmetry. The result obtained: εvc⟂/εvc ‖ = 57/46 should therefore remain valid also for HgTe and HgSe. The wave‐vector, electron concentration, and frequency dependence of the dielectric function in stressed a‐Sn also has been investigated.
Uniaxial stress dependence of the EPR spectrum of Ir4+ in Y3Ga5012
Authors: Hodges J. A.
Published: July 2007

Measurements have been made by EPR techniques of the stress dependence of the lowest Ir4+ (5d5; 2Ta2g) levels in yttrium gallium garnet. The results confirm the comparable behaviour of Ir4+ arid Ru3+ (4d5; 2T2g) ions at the octahedral sites in garnets in contrast to recent results obtained by ferrimagnetic resonance. On a simple molecular field model the Ir4+ energy levels in YIG: Ir4+ introduce a negative contribution to λ100and a smaller positive contribution to.λ111.
Effect of potential scattering on the spin disorder resistivity
Authors: Troper A. , Gomes A. A.
Published: July 2007

An extension of the calculation of the temperature‐independent resistivity performed by Blackmann and Elliott is prrsented. in order to describe transition metal‐like hosts. An integral Dyson‐like equation is derived for the one eleetron s‐s propagator using the Green's function formalism. The integral equation is solved to second order in the exchange parameters. in the absense of magnetic order. Out of this solution an expression for the temperature‐independent resistivity is obtained in terms of phase shifts and exchange copulings.
Spectral distribution of nonlinear oscillators with nonlinear friction due to a medium
Authors: Dykman M. I. , Krivoglaz M. A.
Published: July 2007

The analog of a quantum kinetic equation for the description of time correlation functions of singled out nonlinear oscillators interacting with a medium (e.g. local or quasi‐local vibrations in crystals) is obtained. Both the frictions, the linear and nonlinear one (the latter is due to decays with participation of two quanta of singled out oscillators), were taken into account. Spectral distributions of the correlation functions of the coordinates are found as well as those of occupation numbers. The essentially non‐Lorentzian shape of the spectral distribution and its dependence on interaction parameters and temperature are analysed.
Determination of the crystal field parameters for Pr3+ in LaF3 from least‐squares fits
Authors: Matthies S. , Welsch D.
Published: July 2007

The results of least‐squares fits of the energy levels of Pr3+ in crystalline LaF3, are reported. Especially, by carrying out a 23‐variable fit involving the crystal field parameters as well as the free‐ion parameters, second‐order effects due to J‐mixing are taken into account. From testing the resulting energies and eigenfunctions by inelastic neutron scattering experiments the reported crystal field parameters are expected to describe the physics of Pr3+ in LaF3 sufficiently well.
Raman‐active phonons in α‐boron
Authors: Richter W. , Ploog K.
Published: July 2007

Single crystals of the α‐rhombohedral boron modification, prepared by chemical vapor deposition, were investigated by Raman scattering. Twelve quite strong peaks were observed in the Raman spectrum. The results are compared with group theoretical predictions and a lattice dynamical model, recently developed for α‐boron.
Bond charge model for the transverse dynamic effective charge and the piezoelectric constant of zincblende‐type crystals
Authors: Hübwer K.
Published: July 2007

A bond charge model for the transverse dynamic effective charge and the piezoelectric constant of the zincblende structure is presented. It works with analytical pseudopotentials and leads to simple analytical results, which yield numerical agreement with experimental data. The transverse dynamic effective charge is obtained from ionic and electronic polarizations arising in an optic‐mode sublattice displacement. The piezoelectric constant is calculated regarding additionally the effects of a macroscopic strain in a deformable‐ion model. It consists of an internal‐displacement term and a charge redistribution term, the latter comes from the bond charge. Both are of the same order of magnitude but of opposite sign and confirm quantitatively Lawaetz' qualitative local‐field analysis of Martin's theory of the piezoelectric effect.
ESR investigations of Tl2+‐ and Pb3+‐centres in alkali halides
Authors: Frey W. , Huss R. , Seidel H. , Werkmann E.
Published: July 2007

The results of ESR investigations of Tl2+‐centres in NaCl, KCl, RbCl, KBr and of Pb3+‐centres in KCl are reported. The g‐factor and the heavy‐metal hyperfine (hf) constant as well as superhyperfine (shf) constants for the six nearest neighbour halogen nuclei are determined. The quadrupole interaction is analyzed in KBr:Tl2+. An interpretation of the shf‐ and the hf‐interaction is presented. Model calculations show that the complex formed by the centre ion and the six halogen ligands is strongly determined by covalent bonding.
On the theory of interband ε(ω) and External Electric Field Modulated Δε(ω) in Disordered Semiconductors
Authors: Esser B. , Kleinert P.
Published: July 2007

The interband dielectric function ε(ω) and its external field induced change Δε(ω) are calculated in the one‐electron approximation for a semiconductor with fluctuating band edges presented by smooth random Gaussian fields. The influence of different random bending of the valence and conduction band edges onε(ω) and Δε(ω) is investigated. The corresponding plots are presented. A broadening of the line shape Δε(ω) is obtained if the band bending becomes different. This broadening might be obtained in electromodulation experiments of crystals propagated by an internal acoustic noise flux. Application of the model to amorphous semiconductors is discussed.
An analysis of the fine structure in the main band of a disordered chain with dilute light impurity concentrations
Authors: Wu Shi‐Yu , Chao M.
Published: July 2007

The main band of a disordered binary chain is studied, using Dean's method of negative mode counting, Schmidt's method of functional equation and an approximate theory of Wu. It is found that even for dilute light impurity concentrations, the main band is not smooth as previously believed, but has fine structure. By comparing the results of these calculations, the nature of the construction of the frequency spectrum in the form of a histogram is discussed. The truncation scheme in the approximate theory of Wu is also examined.
EPR of Gd ions in Jahn‐Teller‐distorted rare‐earth crystals TbAsO4, TmAsO4, and TmVO4
Authors: Schwab G.
Published: July 2007

EPR measurements on TbAsO4, TmAsO4, and TmVO4 doped with 1 to 2% Gd ions were carried out at temperatures below TD, the critical temperature for the Jahn‐Teller phase transition. A spin‐Hamiltonian allowing for lower symmetry and magnetic interaction is fitted to the spectra of the Gd ions. Low symmetry parameters B are determined. The parameters of the magnetic interaction for Gd ions in the Tb and Tm host crystals are extremly different.
On the theory of hyper Raman scattering in molecular crystals
Authors: Zavorotnev Yu. D. , Ovander L. N.
Published: July 2007

Hyper Raman scattering in crystals with an inversion centre is investigated using polariton theory. I n the Hamiltonian of a crystal, terms are taken into account, which are responsible for the intermolecular, kinematic interactions and the interaction of charges with the transverse photon field. The explicit form of the hyper Raman scattering tensor is calculated. For groups with an inversion centre the way of identification of the contribution of the intermolecular interaction to this effect is shown.
Effects of high pressure on the optical absorption spectra of colloid centers in KCl and NaCl
Authors: Mamola K. C.
Published: July 2007

The spectral behavior of colloid centers in additively colored KCl and NaCl has been studied at room temperature over the pressure range 1 to 10 kbar. The optical absorption bands associated with non‐scattering colloidal particles in these salts are found to exhibit blue shifts with increasing pressure. A plasma resonance model is used to account for the pressure shifts. The increase with pressure in the density of conduction electrons in the colloidal particles is the dominant effect. Pressure shifts of the peaks of the optical absorption bands are calculated using available compressibility data and the known pressure dependences of the refractive indices of the two salts. Agreement with the measured data is good.
Die rolle des drucks bei der untersuchung der zwischenbandstreuung in n‐GaAs
Authors: Sviridov I. F. , Presnov V. A. , Sonnenburg K.
Published: July 2007

Die Druckabhängigkeit der elektrischen Leitfähigkeit von n‐GaAs wird theoretisch untersucht. Es wird gezeigt, daß bei geringen Drücken der Größenordnung 2,5 kbar und Zimmertemperatur die Zwischenbandstreuung im n‐GaAs nicht in Erscheinung tritt. Für beste Übereinstimmung von Theorie und Experiment muß angenommen werden, daß das Matrixelement der Zwischenbandstreuung 10 bis 28% des Matrixelementes des „intravalley”︁‐ Effekts im Minimum (000) ausmacht.
Formation of substitutional hydrogen centers in LiF:OH crystals
Authors: Kamikawa T.
Published: July 2007

ESR and infrared absorption are measured on neutron‐irradiated LiF:OH crystals. In unirradiated specimens an sharp absorption at 2.8 μ;m is observed and attributed to OH− ions coupled to a Mg impurity. After neutron irradiation the 2.8 μm band disappears and a broad band appears at 1930 cm−1. When the irradiated crystal is annealed at higher temperatures, the 1930 cm−1 band increases a little and then decreases. In parallel with the decay of the 1930 cm−1 band, another absorption band grows peaking at 1013 cm−1, which is attributed to U centers in LiF. H0 centers are observed by ESR in neutron‐irradiated crystals. On annealing at higher temperatures the H0 centers increase as the 1013 cm−1 band grows. While the 1013 cm−1 band increases monotonously with temperature, the H0 centers decrease above 270 °C. It is concluded that hydrogen centres are produced by dissociation of OH‐ ions in neutron‐irradiated LiF:OH crystals and that immediately after irradiation U centers do not exist, which are formed in the course of annealing from the precursor suggested to be U1 centers and afterwards from H0 centers.
Exchange and correlation in nearly free‐electron metals
Authors: Brosens F.
Published: July 2007

On the basis of the Hohenberg‐Kohn theorem, an effective exchange and correlation potential is derived for nearly free electron metals. Using the one particle approach, derived by Kohn and Sham, on variational grounds, an expansion of the total energy with respect to deviations from the uniform density allows for calculating the effective potential.
Experimental and theoretical study of Pr3+ in CaF2
Authors: Bhola V. P.
Published: July 2007

Calculations of Pr3+ ion in CaF2 crystal were performed for octahedral symmetry. Tensor operator technique was used to calculate the matrix elements of the crystal field. The crystal field parameters are found to be B = −397 cm−1; B = −799 cm−1. The values of F2, F4, F6, and ζ are found to be 309, 55.4, 6.8, and 716 cm−1, respectively. The r.m.s. deviation for 25 Stark split components is 13 cm−1; if one of the components 1Γ1(3F4) is neglected, the r.m.s. deviation reduces to 10 cm−1. Due to agreement between experimental and theoretical results, it is suggested that the symmetry surrounding the Pr3+ ion in the matrix of CaF2 is octahedral, but the presence of another symmetry is not excluded.
Excitonic emission in cadmium telluride
Authors: Taguchi T. , Shirafuji J. , Inuishi Y.
Published: July 2007

The photoluminescent properties of free and bound excitons in high‐purity p‐ and n‐type CdTe single crystals are investigated in detail in connection with the dependence on excitation intensity, temperature, impurity‐doping, and uniaxial stress. Intense free‐exciton emission is observed in the single crystals grown from solutions and vapor epitaxial methods. From the thermal dissociation processes and the analysis on the basis of the effective mass argument developed by Sharma and Rodriguez and by Hopfield, an assignment of exciton emission lines is tried.
Crystal dynamics of silver bromide at 80 and 295 K
Authors: Bührer W.
Published: July 2007

The dispersion relation for the normal modes of vibration in AgBr have been determined at 80 and 295 K along symmetry directions. The neutron inelastic scattering experiments have been performed on a triple axis spectrometer at the “Diorit” reactor. The phonon frequencies show a strong temperature dependence. The dispersion curves can be well fitted by an eleven‐parameter model with deformable electronic shells of the silver ions.
On the theory of optical absorption in narrow‐gap disordered semiconductors
Authors: Fedirko V. A.
Published: July 2007

The interband absorption coefficient in disordered semiconductors is calculated for the frequency range below the threshold value (“optical tail”). A model with three doubly degenerate bands is considered containing one conduction band and two valence bands with very different effective masses. Disorder is taken into account in terms of a random field present in the sample including both smooth and Coulomb fields. The energy gap is assumed to be comparable with the r.m.s. value of the random field, so the scheme of the interacting adjacent bands is used. Expressions for the absorption coefficient are obtained for various relations between the parameters of the system and the photon energy. Theoretical results are compared with some experimental data available.
Microwave‐optical double resonance and magnetic circular dichroism of photochromic centres in CaF2:Ce
Authors: Komarov A. V. , Ryabchenko S. M. , Terletsrii O. V.
Published: July 2007

The dispersion of magnetic circular dichroism (MCD) and Faraday rotation (FR) for Ce3+ photochromic centres in CaF2 is measured at T = 1.7 °K. The optical technique of EPR detection is used to separate the paramagnetic and diamagnetic contributions to the MCD. From the phenomenological analysis of the MCD paramagnetic and diamagnetic parts is found that the optical transition to the lowest excited state is split into at least three components. This splitting as well as the presence of the paramagnetic contribution into the MCD are assumed to be due to an exchange interaction of 4f electron of Ce3+ with the photochromic centre, other electrons involving spin‐orbital interaction.
Tunnel spectroscopy of thin bismuth films
Authors: Lutskii V. N. , Nikitin N. E. , Pinsker T. N. , Volkov V. A. , Elinson M. I.
Published: July 2007

A quantitative study is made of the band structure in Bi by tunnel spectroscopy of size‐quantized films. The positions of quantum energy levels are determined for a wide range of film thickness (700 to 3200 Å). On the basis of the two‐band model the electron energy spectrum in size‐quantized bismuth films is calculated. From the comparison of the experimental results with the theory the energy gap and the effective mass at the extremum of the conduction band in bulk Bi are determined. The dispersion law of charge carriers is defined by these parameters.
Strain‐modulated electron spin resonance spectra of 3d5 ions in ZnSe
Authors: Henning J. C. M. , Boef J. H. Den
Published: July 2007

Flexural‐strain‐modulated ESR spectra of Mn2+ and Fe3+ in ZnSe are reported. A study of the relative phases of the fine structure lines and of the angular dependence of the SMESR amplitudes reveals that first‐harmonic SMESR signals arise mainly through modulation of the off‐diagonal elements (g̃xz + g̃zx) of the tensor g̃. A slight amount of A‐modulation is present in ZnSe:Mn2+. Weak quadrature signals are observed in ZnSe:Fe3+; they are attributed to anelastic losses. A second‐harmonic SMESR signal is also present. It arises from second‐order line shifts. The angular dependence of the second‐harmonic signal reveals that, apart from (g̃xz + g̃zx), g̃zz‐modulation also plays some part in producing these signals. The component g̃xx normal to the bending plane, however, is not modulated.
k · p parameters for HgSe based on a relativistic energy band calculation
Authors: Bliek L. M. , Overhof H.
Published: July 2007

For HgSe the k · p parameters of Kane's theory have been derived from energy bands calculated by means of the relativistic Green's function (KKR) method. They compare reasonably with published experimental data.
The theory of Raman scattering by surface polaritons
Authors: Strizhevskii V. L. , Yashkir Yu. N.
Published: July 2007

The fluctuation–dissipation theory of spontaneous Raman scattering (RS) by surface polaritons (SP) is carried out. The appearance of scattering emission is treated as the result of mixing of the pump emission and the fluctuating surface electromagnetic field. A calculation is made of the surface electromagnetic fluctuations near the interface between the two dissipative media and in the absorbing layer surrounded by transparent dielectrics. Expressions are found and analysed describing the frequency–angular distribution of the scattering emission intensity in the layer, taking into account the interference of SP localized near different surfaces (and near the interface of two absorbing media). The RS intensities are expressed by the phenomenological parameters of the media. It is also shown that the RS intensity decreases essentially with increasing scattering angle θ (when the scattering frequency approaches the frequency of the surface phonon) due to the increasing localization of SP. Various essential physical features of the phenomenon are analysed in detail: scattering line shape and its angular dependence; the Fermi resonance of SP and its display in the RS; mixing of SP of different media and RS by appearing mixed modes; RS by SP connected genetically with the linear medium which borders with the transparent nonlinear medium; evolution of RS in a layer by decreasing its thickness, and others.
Diffraction due to substitutional disorder in garnets and in spinels
Authors: Szabó P.
Published: July 2007

A definite method, based on a previously given diffraction theory, is suggested for the diffraction investigation of order–disorder in substituted garnets and spinels. The formulae for the calculation of the pertinent disorder parameters are derived and the calculated values of the parameters are tabulated.
Calculation of the spin‐disorder resistivity in disordered magnetic alloys
Authors: Goedsche F. , Richter R. , Richter A.
Published: July 2007

The CPA‐conductivity theory is applied to the s–d exchange model to calculate the spin contribution to the electrical resistivity of disordered binary alloys containing two kinds of localized spins. The second‐order resistivity is expressed in terms of alloy magnetization, spin‐pair correlation functions, atomic spin difference, and mean spin value. Striking deviations from a Nordheim‐like behaviour of the spin‐disorder residual resistivity are obtained, if the magnitude of the atomic spins depends on alloy composition and local environment.
The probability of bi‐exciton formation in crystals
Authors: Borbrysheva A. I. , Vybornov V. I.
Published: July 2007

In first order of perturbation theory the probability of binding of two excitons into a bi‐exciton accompanied by light emission and caused by exciton–exciton collisions is calculated. Firstly the case is considered where two excitons of 1s‐type and n‐p‐type form a bi‐exciton in the ground state consisting of two excitons of 1s‐type. The binding is accompanied by light emission. The luminescence begins with a threshold and the luminescence band has the form of a sharp asymmetrical peak. The exciton–exciton collisions lead to the binding of two excitons of 1s‐type with no internal excitation. Numerical estimations of the processes under consideration are made for a set of crystals.
Stress absorption spectra of V3+ in α‐Al2O3: Jahn‐Teller effect in the 3T2 state
Authors: Champagnon B. , Duval E. , Louat R.
Published: July 2007

Study under stress of 3T2 state of the V3+ ion in corundum (α‐Al2O3) was performed at 77 K. Absorption spectra are made on 3A2(3T1)→3T2 zero‐phonon lines and on the 3A2(3T1) → → 1A1 sharp line. Under uniaxial stress, the degeneracy of vibronic levels is removed: curves show the shifts and splittings of 3A2 → 3T1 and 3A2 → 1A1 transitions as a function of the applied pressure. The strain interaction with 3T2 state only is deduced. The stress spectra cannot be quantitatively explained without taking into account a large S15* local elastic compliance. The local deformation around the V3+ ion is similar to this one around the Cr3+ or Co3+ ion. Studying the 3A2 → 3T2 band by the method of moments a coupling between the V3+ ion and a breathing mode A1 is found. Results of measurements cannot be interpreted by a Jahn‐Teller effect with only the pure tetragonal modes.
Angular dispersion of phonons in biaxial NaNO2
Authors: Bonilla I. R. , Rutt H. N.
Published: July 2007

Angular dispersion curves for polar phonons in biaxial crystals are experimentally verified for sodium nitrite. Both crossing and repulsion of phonons are expected and observed, and an important difference between biaxial and uniaxial crystals is noted.
Third‐order contribution to the energy and the elastic constants of simple metals
Authors: Benckert S.
Published: July 2007

The energy of a metal is usually calculated by second‐order perturbation theory in the pseudopotential. It is, however, also possible to calculate the third‐order contribution. This contribution is calculated to the energy and the elastic constants of Al and Na using two different types of local model potentials in order to see how the choice of the potential will influence the magnitude of the third‐order contributions.
Order parameters of the phase transition in bismuth titanate
Authors: Holakovský J.
Published: July 2007

It is shown that the phase transition in bismuth titanate (at 675 °C) cannot be caused by the onset of parameters belonging to an irreducible representation. The parameters of the transition are divided into two sets; one leading to the symmetry B2cb, the other being responsible for the onset of the polarisation Pc. The former set of parameters is found and is shown to belong to a reducible representation.
Cyclotron resonance in plastically deformed germanium
Authors: Rusiniak L.
Published: July 2007

Measurements are made on samples of n‐type germanium with resistivities of about 30 Ωcm subjected to plastic deformation about the [100] axis in the microwave X‐band in the temperature range from 1.7 to 4.2 °K. Results are given of the dependence of relaxation time and carrier lifetime on temperature and dislocation density. The effective mass of carriers was found to be affected also by dislocations. The experimental results are interpreted assuming deformation and Coulomb potentials and taking into account Morrison's theory.
Lineshape of the biexciton luminescence in CuCl
Authors: Ostertag E. , Levy R. , Grun J. B.
Published: July 2007

A comparison has been made between the experimentally observed spectral lineshape of the luminescence due to the recombination of excitonic molecules in CuCl at 4.2 K and a theoretical calculation involving transitions from the biexciton ground state to the longitudinal and transverse excitonic states. A new value of the binding energy of the excitonic molecule in its ground state is deduced and compared with theoretical evaluations.
Optical Absorption Spectra of Co2+ Ion in KCl and NaCl Crystals
Authors: Nasu T.
Published: July 2007

Optical absorption spectra of Co2+ ion in KCl and NaCl crystals were measured in the region of energies from 4000 to 60000 cm−1. Temperature dependences of the spectra were also measured from 80 to 810 K for KClCo and from 80 to 650 K for NaClCo. The assignment of transitions concerned with absorption bands and the crystal field acting on the Co2+ ion in these crystals were discussed on the basis of the analysis of the intensity of absorption bands and the temperature dependence of the spectra.
Multiphonon Processes of the 4T1g(I) Absorption in RbMnF3
Authors: Heumüller R. , Gebhardt W. , Von Der Osten W.
Published: July 2007

The one‐phonon spectrum K1(ω) is calculated from the 4T1g(I) absorption in RbMnF3, using the method of generating function. In the temperature range of T ≦ 35K the coupling strength S(T) obtained with K1(ω) is in good agreement with directly measured values. S (T) is then used to derive the contribution of hot band processes to the intensity of exciton—magnon transitions at temperatures T > 35 K. We conclude from the temperature variation of the line width of the magnetic dipole transitions that exciton—magnon processes dominate when the exciton decays.
Resonant Raman Scattering in Amorphous As2S3
Authors: Razzetti C. , Fontana M. P.
Published: July 2007

Resonant Raman spectra are presented taken over the whole absorption edge of amorphous As2S3. Results indicate that 1. the electron vibration coupling is the same over all the electron states sampled; 2. band resonance effects are absent, and, rather, resonance is manifested by a monotonic increase of the scattered intensity with increasing exciting energy.
Two‐Phonon Infrared Absorption in CdSexTe1−x Mixed Crystals
Authors: Górska M. , Grad J. , Nazarewicz W.
Published: July 2007

The transmission spectra of CdSexTe1−x(0 ≦ x ≦ 1) mixed crystals have been measured in the spectral range 250 to 400 cm−1 at room temperature. Eleven absorption bands are observed and ascribed to particular two‐phonon combinations. The composition dependence of short‐wavelength optical phonon frequencies (from the edge of the Brillouin zone) is determined.
Optical Charge Transfer Spectra and EPR Spectra of Cr2+(d4) and Cr1+ (d5) in CdTe
Authors: Cieplak M. Z. , Godlewski M. , Baranowski J. M.
Published: July 2007

By EPR experiments is shown that two charge states of chromium impurity are present in CdTe:Cr. The Cr2+(d4) state has an acceptor character and can accept an additional electron into the d‐shell. The energy required to change the charge state of Cr2+(d4) into Cr1+(d5) by transferring one electron from the valence band is about 1.2 to 1.3 eV. The photosensitive EPR spectrum due to Cr1+(d5) shows that the observed intense exponential absorption tail is due to this charge transfer process. The shape of the exponential tail is also investigated. It is suggested that the exponential character of the absorption tail is due to the presence of a Jahn‐Teller effect in the ground state of Cr2+(d4).
The Grüneisen Constant and the Thermal Expansion Coefficient of Crystals with Defects
Authors: Litzman O.
Published: July 2007

The Green's function method to evaluate the frequency sums Σ f(λi) of crystals with defects is generalized to the calculation of the sums Σ f(λi) (δλi/δα). The deduced formulae are used for the calculation of the Grüneisen constant and the heat expansion coefficient of the linear chain with isolated defects.
The Spatial Dispersion Effects in CdS Crystals and Their Temperature Dependence
Authors: Brodin M. S. , Davydova N. A. , Strashnikova M. I.
Published: July 2007

The dispersion of the refractive index of a CdS crystal in the region of its main exciton bands is experimentally investigated at temperatures ranging from 4 to 80 °K. From the change of the birefringence pattern in the region of the lowest state, three specific temperature intervals are revealed. The data obtained show good agreement with the predicted temperature change of the normal wave parameters for media with the spatial dispersion; 1) in the low‐temperature region (T < 15 °K) in different regions of the absorption band either one or the second normal wave is dominating: 2) in the region of intermediate temperatures including the “critical” temperature T0 predicted by the theory (25 to 55 °K), both waves being comparable in intensity within the absorption band halfwidth; 3) in the high‐temperature region (T > 55 °K) one wave dominates all along with the dispersion of the classical character.
Calcul de la largeur des bandes d'absorption optique du rubis excité
Authors: Brunstein S. H. , Tsukerblat B. S.
Published: July 2007

La méthode des moments se généralise pour des bandes optiques provenant de transitions vers des niveaux électroniques quasi‐dégénérés. L'interaction Jahn‐Teller est supposée forte et l'on ne peut pas décomposer la bande optique en sous‐bandes élémentaires. On utilise cette théorie pour analyser les bandes multiphonons d'absorption optique du rubis. On montre que l'effet Jahn‐Teller donne une contribution essentielle à l'élargissement des bandes optiques; on fait une estimation numbérique des demi‐largeur, laquelle est en bon accord avec l'expérience.
Optical Absorption of MnI2
Authors: Van Erk W. , Haas C.
Published: July 2007

The absorption spectra of single crystals of MnI2 in the region 10000 to 30000 cm−1 are reported. At low temperature, 5 K, several of the observed absorption bands show well‐resolved vibronic fine structure. The absorption bands are assigned to electronic transitions of the d electrons of the manganese ion. From the spectra the values of the ligand field splitting 10Dq = 6400 cm−1 and the Racah parameters B = 640 cm−1, C = 3136 cm−1 are calculated. The results are compared with data for MnCl2 and MnBr2, and are discussed in terms of the covalency in the manganese halides.
Electrons in a Well Potential: Density of States, Surface Energy, and Phase Rule for Wave Functions
Authors: Paasch G. , Wonn H.
Published: July 2007

For the problem of electrons in a well potential as a model of a metal with surfaces, exact expressions are derived for the density of states, the surface energy, and for the sum rule for the phases of the wave functions. Incorrect derivations and errors of previous publications are pointed out. The connection between the phase rule and the surface energy is discussed.