bluuebunny/crossref_metadata_2025 huggingface dataset:
Band structure of TlGaSe2
Authors: Abdullaeva S. G. , Mamedov N. T. , Orudzhev G. S.
Published: July 2007

A model of the TlGaSe2‐I band structure of the monoclinic modification is proposed. The monoclinic modification of TlGaSe2‐I is considered as a deformed tetragonal one. The band structure of the metastable tetragonal modification of TlGaSe2‐II is calculated by the pseudopotential method. The irreducible representations of the D184h group (TlGaSe2‐II) are expanded in irreducible representations of the C4s group (TlGaSe2‐I). It is found that the top of the valence band is located at the point Y (0,2π/bm, 0) of the TlGaSe2‐I Brillouin zone and is formed from band states N2, N4 of the tetragonal modification. The bottom of the conduction band is located at the point Γ of the TlGaSe4‐I Brillouin zone and is formed from T4 states. An additional extremum of the valence and conduction bands is located at the points Γ(A2) and Y(N6, N8), respectively. An interpretation of the absorption spectra of TlGaSe2‐I near the fundamental edge is proposed.
Ionic and electronic processes during frenkel defect annealing in KBr crystals above LHeT
Authors: Ziraps V.
Published: July 2007

The relaxation of radiation‐induced optical absorption spectra and a set of thermally stimulated curves (current, depolarization current, diffusion current, emission‐desorption of gas, thermal bleaching) of KBr crystals X‐irradiated at 4.6 K are recorded synchronously or step by step. A multistage annealing of defects is detected: (i) from 4.6 to 32 K about ten substages controlled by the I centre delocalization are accompanied by electron and energy release, (ii) from 32 to 60 K about six substages controlled by the H, centre delocalization are accompanied by corresponding electron and energy release and gas desorption peaks. A recombination of the migrating I or H centres within such complementary defect systems (pairs, triplets et al.) takes place: (i) {I…α}, {I… F… Vk}, {I… F'… Vk… Vk}, (ii) {H… F} and {H… F'… Vk}. Following phenomena in KBr at 4.6 K are observed and discussed: (i) the “F‐photostimulated” electron tunneling transfer within the excited pair {F* γ. HA} → (α… IA}, (ii) the effect of dc field (3kV/cm) caused Frenkel defect formation efficiency increase (by ≧ 10%), (iii) the photoelectrical dissociation of the α centres. Comments about the anion Frenkel primary defect generation mechanisms are made.
Lattice thermal conductivity of crystals with dislocations
Authors: Ansari M. A. , Kumar A. , Srivastava B. K.
Published: July 2007

The thermal conductivity of crystals containing dislocations is calculated using Kubo's relation. The correlation function is obtained using the projection operators as suggested by Wilson and Kim. It is observed that the phonon scattering by the strain field due to the dislocations is proportional to the phonon frequency and is anisotropic in nature.
Solitons in ferroelectric liquid crystal
Authors: Pires A. S. T. , Sá Barreto F. C.
Published: July 2007

The stable phases of liquid crystals whose constituents contain chiral molecules are studied. Starting from the Landau free energy for the order parameter the soliton‐lattice solution of the incommensurate phase is analysed in detail. The solutions of the normal and commensurate phases are obtained by the transfer matrix method.
Plasma reflection and hole optical effective mass in p‐HgTe
Authors: Ivanov‐Omskii V. I. , Sh. Mekhtiev A. , Rustambekova S. A. , Ukraintsev E. N.
Published: July 2007

The results of plasma reflection investigations in p‐HgTe are described. It is shown, that the plasma reflection spectrum shift towards small energies and the decrease of their depths with increasing temperature cannot be explained by the dielectric anomaly of the zero gap semiconductor due to the Γ8 → Γ8 transition only. The analysis of the experimental results shows that the optical effective mass value has both, a temperature and concentration dependence. Such a dependence may be explained in the frame of the Kane theory, if the renormalization of the energy spectrum of holes due to exchange electron—electron interaction is taken into account.
Two types of F centres in alkaline earth fluoro‐halides II. Double ENDOR experiments
Authors: Niklas J. R. , Bauer R. U. , Spaeth J.‐M.
Published: July 2007

In crystals containing simultaneously F(Cl−) and F(F−) centres in certain frequency regions the ENDOR spectra are superimposed to such a degree that an analysis is not possible. With double ENDOR experiments the ENDOR spectra of both centres can be separated. The ratio of the ENDOR signal heights compared to the double ENDOR signal heights is found to be characteristic for the shell of the nuclei involved. It can be used to discriminate between ENDOR lines of different shells. The effect cannot be understood on the basis of rate equations with reasonable assumptions for the spin—lattice relaxation times. It is probably due to spin diffusion.
K and L absorption spectra of aluminium from APW calculation
Authors: Ojala E.
Published: July 2007

The K and L X‐ray absorption spectra of Al are computed on the basis of an APW band structure calculation over an energy range of 40 eV. The results are compared with experiments. The overall shapes and especially the positions of the main maxima and minima of the experimental spectra are well reproduced by the theory. An appreciable lifetime broadening for the excited electrons is needed to smear out the structures to the extent found in the experiments. Discrepancies in the relative magnitudes of the spectral features between theory and experiment suggest that the core hole screening effect in the final state is primarily to scale the intensities in an energy dependent manner.
Elastic constants of the layered compounds Gas, GaSe, InSe, and their pressure dependence I. Experimental part
Authors: Gatulle M. , Fischer M. , Chevy A.
Published: July 2007

The elastic constants of the lamellar compounds Gas, GaSe, InSe are measured on several samples using an ultrasonic method, and the results are compared with previous publications. The variations of C33, C11, and C66 with an hydrostatic pressure are measured up to 3 kbars. In the range of pressure studied, the variations of these constants are perfectly linear. The results on the three compounds are very similar, and typical of the lamellar structure: an important variation of C33 caused by the weakness of the interlayer bond; on the other hand, C66 is very few affected by pressure.
Electronic structure of impurities in dilute alloys of Al
Authors: Mahajan S. , Prakash S.
Published: July 2007

Self‐consistent ”︁nonlinear screening calculations for point‐ion nonmagnetic impurities in jellium are made using the Hohenberg‐Kohn and Kohn‐Sham density functional formalism. The Friedel criterion is used to estimate the effective impurity valency and the Blatt correction is used for the size effect. The impurity induced charge densities in the Al matrix are calculated and an analytical expression, valid in both the preasymptotic and asymptotic regions is proposed. The net impurity effective charges are calculated and it is found that the matrix makes a substantial charge transfer in the impurity cell for all the alloys except AlMg and AlSn. The estimated residual resistivities agree fairly well with available experimental data.
Non‐linear optical susceptibilities of molecular crystals in the region of vibronic excitations. I. The three‐dimensional case
Authors: Lalov I. J. , Stoychev K. T. , Penchev S. P.
Published: July 2007

The quadratic non‐linear susceptibility in the single phonon vibronic spectra of molecular crystals is calculated (one Frenkel exciton + one non‐totally symmetric phonon). The following results are obtained: (a) the exciton—phonon interaction is decisive for the appearance of non‐linear susceptibility resonances in the vibronic region; (b) the two‐photon absorption spectrum is almost entirely of a one‐particle spectrum. The last result is experimentally confirmed elsewhere.
Effect of combined mechanical load and irradiation on hydrogen stabilization in LiF:OH
Authors: Akhvlediani Z. G. , Akhvlediani I. G. , Kalabegishvili T. L.
Published: July 2007

γ‐irradiated LiF:OH crystals are investigated by the ESR and IR‐spectroscopic methods. The possibility of stabilization of atomic hydrogen in a cation vacancy is shown. It is established that applied mechanical load during the irradiation process does not affect the hydroxide decay, but has essential influence on the redistribution of the decay products (conditions are created for stabilization of hydrogen atoms in two different positions).
On the problem of an intermediate order in amorphous semiconductors
Authors: Iskra V. D.
Published: July 2007

Simplest structural models are considered which take account of an intermediate order in both, homopolar and heteropolar, amorphous semiconductors. These models are used to study the influence of an intermediate order on the nature of an intrinsic random field in the material, hence, on the frequency dependence of the light absorption coefficient in the optical tail region.
Circular phonons and optical magnetization of crystals
Authors: Kiselev A. A.
Published: July 2007

The possibility of optical magnetization of crystals by excitation of circular optical vibrations is considered. An expression for the g‐factor of this sort of vibrations is derived for a crystal of rocksalt structure. Directions allowed for circular phonons in three‐dimensional lattices are studied, and the magnetization of an ionic crystal caused by circular resonance light wave is estimated using the classical approach and a point ion lattice model. A quantum approach to the problem is discussed, and creation and destruction operators for circular phonons are written.
Thermodynamic properties of uniform and dimerized hubbard chains
Authors: Lorenz B.
Published: July 2007

The thermodynamics of the uniform and dimerized half‐filled Hubbard chains is investigated within an approximation which takes into account the correlations in a given cluster of sites and which is valid for all model parameters and for all temperatures. The dependence of the specific heat on temperature changes from a one‐peak to a two‐peak structure with increasing interaction strength. The influence of dimerization is discussed. The local magnetic moment is found to decrease with increasing temperature in the high‐temperature region. It is suggested that this qualitative behaviour is more general and independent on dimension and band filling.
Determination of the charge carrier concentration and mobility in n‐gap by Raman spectroscopy
Authors: Irmer G. , Toporov V. V. , Bairamov B. H. , Monecke J.
Published: July 2007

A method is presented which allows the determination of the charge carrier concentration and mobility in n‐GaP by Raman spectroscopy of coupled plasmon‐phonon modes. The results are compared with those of electrical measurements.
Optical properties of TiN single crystals
Authors: Köhler K. , Geserich H. P. , Wolf Th. , Müller‐Vogt G.
Published: July 2007

The spectral reflectance of TiN single crystals in the energy range between 0.5 and 6.2 eV is investigated. The crystals are grown by chemical vapour deposition. They have a very exact stoichiometric composition and a low content of impurities. The reflectance spectra are obtained on grown surfaces and analyzed by means of the Kramers‐Kronig dispersion relations as well as by fitting a Lorentz‐Drude model. It results that, in addition to the well‐known intraband and interband absorption, also direct transitions between the conduction subbands contribute to the optical absorption.
A binary magnetic alloy with competing exchange interactions and magnetic moments dependent on local environment
Authors: Zaborov A. V. , Medvedev M. V.
Published: July 2007

Magnetic states of an alloy with competing nearest‐neighbour exchange interactions, IAA > 0, IAB > 0, IBB < 0, in which the magnetic moments depend on their local chemical environment in a step‐like manner (according to the Jaccarino‐Walker model), are studied in the vicinity of the transition temperatures from the paramagnetic states. It is shown that in the cases of s.c. and b.c.c. lattices the existence of the magnetic moment dependence on the local environment does not create any additional possibilities for the occurrence of the spin‐glass state in such alloys as compared to usual Heisenberg alloys with nearest‐neighbour competing exchange interactions of permanent magnetic moments.
Interaction of free hole intraband excitations with the A1‐phonon in highly doped (p‐type) tellurium
Authors: Goetz K. H. , Richter W.
Published: July 2007

A remarkable asymmetry is observed in the Raman lineshape of the A1‐phonon of Te. This asymmetry increases with increasing hole concentration. The E‐symmetry phonons on the other hand always exhibit symmetric lineshapes. The A1‐phonon asymmetry is interpreted as a Fano‐type interaction between the A1‐phonon and a continuum of excitations. With the aid of various experiments this continuum is identified as intraband excitations of holes in the upper valence band.
Hyperfine magnetic field at 181Ta in YFe2
Authors: Akselrod Z. Z. , Budzynski M. , Komissarova B. A. , Kryukova L. N. , Ryasny G. K. , Sorokin A. A.
Published: July 2007

The hyperfine magnetic field at 181Ta in the ferromagnetic intermetallic Laves compound (Y0.97Hf0.03) Fe2 is measured by the perturbed angular correlation method. A value Hhf(Ta) = –14.4 T is obtained at 300 K. The temperature dependence of this field, measured in the range from 78 K up to and above, the Curie point, is shown to be different from that of the bulk magnetization.
Impurity potentials in diamond
Authors: Ortu≁o M.
Published: July 2007

The total screened potential produced by a static point‐charge impurity in diamond is calculated using a real‐space formalism. In this formalism a two‐flat‐band model is assumed. The potential is calculated for substitutional and “tetrahedral” and “hexagonal” interstitial positions. A very large cavity potential is obtained for interstitial impurities.
Chemical shift and EXAFS studies of some copper complexes
Authors: Bhattacharya P. , Chetal A. R.
Published: July 2007

Chemical shifts of the X‐ray K‐absorption edge of copper along with EXAFS are studied in the complexes CuC2O2(ammonia)2, Cu(LH)SO4 · 4 H2O, Cu(LH)2SO4 · 4 H2O, and Cu(phen) (xant)2. From the shift of the edge, it is concluded that the copper ion behaves as a cation in the complexes. It is also observed that as the partial charge on an absorbing ion increases, the chemical shift also increases. Bond lengths using Lytle et al.'s and Levy's methods are determined in the above complexes.
Macroscopical dielectric permittivities of nonhomogeneous media
Authors: Fokin A. G.
Published: July 2007

Based on concepts of the functional analysis and treating the fields E and D as elements of the real vector Hilbert space H, a method is exploited for the calculation of macroscopical permittivities εM of nonhomogeneous media, and bounds within which they acquire their values. Series expansions are obtained for εM and γM = εM–1, and the convergence conditions are found for the series. Bounds are constructed for εM, which are contracting with the introduction of additional information on the medium used in the calculation. A comparison with the results of other authors is performed.
The Ground State Energy of Small Polaron Gas
Authors: Weger M. , Entin‐Wohlman O.
Published: July 2007

The ground state energy of the small polaron model is calculated as a function of the electronic density. The applicability of the small polaron model to metals of the TTF‐TCNQ family is discussed.
Polarized At Emission of In+ Centres in KI:In
Authors: Nagirnyi V. , Soovik T. , Zazubovich S. , Janson N.
Published: July 2007

Spectral, polarization, and kinetic characteristics are investigated at 4.2 to 300 K for the blue luminescence of KI:In crystals with various In+ concentrations. It is established that the At emission band of In+ centres is elementary and its decay consists of only one component. The At emission is polarized only at excitation in the B and A absorption bands, the polarization degree is positive and grows as the energy of exciting photons decreases. The doublet structure of the At emission band as well as the second slower decay component which shows in KI:In with large In+ concentration a large negative polarization on excitation in the C band at 80 K, are caused by the presence, besides of In+, of some anisotropic indium centres in the crystal. Their emission is shifted by about 0.04 eV to lower energies with respect to the At emission of In+ centres.
On the Influence of Amorphization on Atomic Diffusion in Condensed Systems
Authors: Milchev A. , Avramov I.
Published: July 2007

A simple theoretical model which examines the effect of structural disorder on the mobility of diffusing particles in amorphous materials is suggested. So, following Anderson's ideas, it is assumed that structural disarray results in a probability distribution of energy barrier heights for diffusion around some mean value, Eer, with a given dispersion δE, used as a measure of disorder. In the framework of the mean field approximation and using the concept of random walks, it is shown that the impact of dispersion δE on the mean stay time of the diffusing particles leads to corrections of several orders of magnitude, as compared to mean stay time values, calculated traditionally in terms of the average activation energy Ecr only.
On the Connection between Isofrequency Temperature Dependences of Inelastic Light Scattering and Dielectric Anomalies in Lithium Niobate Crystals
Authors: Gorelik V. S. , Umarov B. S. , Umarov M.
Published: July 2007

The isofrequency temperature dependence of inelastic light scattering intensity in ferroelectric LiNbO3 crystals at frequencies of Ω = 15 to 100 cm−1 and at different scattering geometries is discussed. The parameters of an effective soft mode are obtained. A comparison is made of the experimental results with the simplest variant of the light scattering theory near the phase transition point. The Curie‐Weiss constant and temperature shift coefficient of the soft mode frequency are determined. The applicability is shown of the approximation of one effective soft mode in the region of low frequencies near the phase transition point in the crystal examined.
Theory of Free‐Carrier Magnetoabsorption in Mixed Zincblende Narrow‐Gap Semiconductors
Authors: Szatkowski J. , Sierański K.
Published: July 2007

The influence of chemical disorder on the magneto‐optical properties of mixed semiconductors with zincblende structure and dispersion relations typical of III‐V and II‐VI compounds is considered theoretically. The magneto‐optical transitions between Landau levels are described, treating the potential due to the chemical disorder as a perturbation. It is found that all transitions between any two Landau levels are allowed (Δn δ 2). The absorption coefficient is non‐vanishing for any polarization and frequency due to the presence of chemical disorder. For parallel and perpendicular light polarization additional Kohn or logarithmic‐like divergencies of the absorption coefficient are obtained at photon energies equal to hω = E – E.
Spin‐Polarized Compton Profile of Nickel
Authors: Rennert P. , Carl G. , Hergert W.
Published: July 2007

The expression for the spin‐polarized Compton profile is calculated using relativistic corrections in a nonrelativistic time‐dependent perturbation theory. The spin polarization of the Compton profile for ferromagnetic materials can be measured by circularly polarized X‐rays. The calculated spin‐polarized Compton profile of nickel is presented. The anisotropic profiles and the isotropic one are considered. The curves show a characteristic shape caused by a positive spin polarization of the d‐electrons and a negative one of the s, p‐electrons.
Structure and Thermodynamics of the Liquid Noble Metals from the Generalized OPW and Resonant Model Approaches
Authors: Regnaut C. , Fusco E. , Badiali J. P.
Published: July 2007

The influence of the d electrons on the structure factor, entropy, and heat capacity of liquid noble metals is analyzed. This is done by using the generalized orthogonalized plane wave (OPW) and resonant model potential (RMP) approaches in connection with the optimized random phase approximation technique. The results of the OPW and RMP equally well compare to the data on Cu and Ag. In the case of gold, only very qualitative OPW results are obtained.
Role of Thermal Transport in the Non‐Equilibrium Behaviour of a Cylindrical Ferromagnetic Superconductor
Authors: Chashoo G. J. , Dharmadurai G.
Published: July 2007

The steady state temperature distribution in a long current carrying cylindrical wire of a ferromagnetic superconductor at bath temperatures below the magnetic transition is obtained using some simplifying approximations. It is found that dissipation in this ferromagnetic resistive state would lead to the onset of a non‐equilibrium intermediate resistance state almost simultaneously in the entire cross‐section of the wire at a critical power level. The results indicate the dominant role of surface thermal transport in deciding the gross electrical behaviour of this non‐equilibrium reentrant superconductor under various thermal environments involving gaseous and liquid helium. The significance of these findings is discussed.
Luminescence of Ga‐Doped α‐Al2O3 Crystals
Authors: Jansons J. L. , Kulis P. A. , Rachko Z. A. , Springis M. J. , Tale I. A. , Valbis J. A.
Published: July 2007

The introduction of Ga3+ ions into α‐Al2O3 crystals results in a new complex luminescence band at about 5 eV. The main optical excitation maximum is observed at 8.5 eV indicating the bound exciton nature of the excited state. In the cathodoluminescence spectra at 80 K three sub‐bands can be detected with maxima at 4.61, 5.15, and 5.58 eV. The luminescence decay in all the three bands has two exponential components — a short one with τ = (15 ± 1) ns and a long one with τ about 1 ms. The Ga‐related luminescence can be observed under the excitation of X‐rays up to 600 K; a drop in intensity takes place at about 210 K corresponding to the Ga‐related TSL peak. The activation energy of the release of electrons at this peak as determined by the fractional‐glow method is (0.70 ± 0.02) eV and the frequency factor 1014 s−1.
Oscillations of Local Magnetization in the Non‐Ideal Antiferromagnetic Chain
Authors: Gildenblat G.
Published: July 2007

A Green's function technique is used to obtain the exact form of the Friedel type oscillations of local magnetization in the spin 1/2, XY antiferromagnetic chain induced by the presence of a boundary or impurity. It is shown that, in addition to the amplitude, the functional form of the oscillations is determined by the strength of the perturbation caused by the impurity and intensity of the external magnetic field. Comparison is made with the earlier perturbation theory treatment in the free‐fermion approximation.
Correlations between OH(OD) Stretching Vibrations and Structural Characteristics of Hydrogen Bonds in K4Fe(CN)6 · 3 H2O Crystal
Authors: Savatinova I. , Anachkova E.
Published: July 2007

The OH and OD uncoupled stretching vibrations of the K4Fe(CN)6 · 3 H2O (KFCT) crystal obtained by isotope dilution technique are discussed and connected with the H bond structural characteristics. The force constants Fo‐H and the anharmonicity constants 2XoH of the different OH oscillators in KFCT are calculated using the uncoupled Raman frequencies. No significant differences are found between the correlations of spectroscopic and crystallographic data in the case of weak O‐H …︁ N and O‐H …︁ O hydrogen bonds.
Study of a Bound Surface Optical Polaron by a Non‐Perturbative Method
Authors: Bhattacharya M. , Mukhopadhyay S. , Mitra T. K.
Published: July 2007

The problem of an extrinsic surface optical polaron, bound to an impurity atom, near the surface is investigated for the first time in the framework of a recently developed canonical transformation method. Energy and wave function for all ranges of the coupling constant (α) and the impurity parameter (β) are obtained numerically following the canonical transformation method. For a strong impurity the relatively unimportant effect due to the image potential is neglected. The dependence of the energy, the wave function, and the induced potential on α, β are obtained and shown graphically.
Picosecond Recombination of Photoexcited Carriers in Glow Discharge a‐Si:H Films of Different Deposition Temperature
Authors: Bergner H. , Brückner V. , Dietrich D. , Nowick W.
Published: July 2007

The temporal behaviour is studied of photoexcited charge carriers in GD‐a‐Si: H in the ps range, the amorphous films are produced at different substrate temperatures up to 200°C. The recombination is characterized by a two‐step relaxation process. The relaxation times are 32 to 165 ps and 0.75 to 1.4 ns, respectively. The sample prepared at 200°C shows a superposition of linear and bimolecular recombination. The recombination rate was found to be γ = 3,3 × 10−10 cm3 s−1.
Non‐Linear Optical Susceptibilities of Molecular Crystals in the Region of Vibronic Excitations. II. Totally Symmetric Vibrations
Authors: Lalov I. J. , Stoychev K. T. , Penchev S. P.
Published: July 2007

The quadratic non‐linear optical susceptibility of molecular crystals in the vibronic region (one Frenkel exciton + one totally symmetric phonon) is calculated. A one‐dimensional model is used to find an analytic expression for the non‐linear susceptibility and numerical calculations of its frequency dependence are made. The two mechanisms of exciton‐phonon interaction — frequency shift of the vibration and shift of the equilibrium positions — have: (1) a cummulative action for excitons of positive effective mass (one‐particle resonances of the non‐linear susceptibility appear); (2) a compensating effect for excitons of negative mass (many‐particle resonances appear).
Acoustic Phonon Anomaly in Mixed‐Valent TmSe
Authors: Ichinose S. , Tamura I.
Published: July 2007

The excitation spectrum of mixed‐valent TmSe is investiagted in terms of the Sherrington‐Riseborough model. The numerical calculations reproduce the anomalous feature of the acoustic phonon band, provided that the d‐f electronic excitation energy is comparable to the energy of the LA phonon.
Screening of a Proton Moving through the Electron Gas
Authors: Stachowiak H.
Published: July 2007

The problem is considered of a positively charged particle moving through the electron gas with a velocity comparable to the Fermi velocity vF or smaller. The problem is solved in the static RPA approximation. It is found that up to a velocity of about 0.8vF the positive particle is completely screened. For higher velocities oscillations appear in the screening charge distribution — different from the Friedel oscillations — and complete screening does not occur. For velocities lower than about 0.8vF the anisotropy of the screening is studied. The relevance of this problem to positron annihilation in metals is pointed out.
Structure and Bonding in Cubic IV–VI Crystals. III. On the Cubic to Rhombohedral Phase Transition
Authors: Enders P.
Published: July 2007

The cubic to rhombohedral phase transition is considered within the Girshberg‐Tamarchenko approach for the vibronic anharmonicity of the soft TO modes and a Landau free energy expansion for the strictive coupling of optic and acoustic modes. It turns out, that one needs a not too simple model for the lattice repulsion forces for a comprehensive description of the phase transition. The spontaneous rhombohedral shear below Tc appears as secondary (induced) order parameter. The relation to the work of Littlewood is discussed.
Spin Waves in an Easy‐Axis Ferromagnet with a Finite Concentration of Impurities with Competing Anisotropy
Authors: Lukanin A. I. , Medvedev M. V.
Published: July 2007

The spin‐wave spectrum is investigated of a Heisenberg ferromagnet with an easy‐axis single‐ion anisotropy in which a number of host atoms is substituted by impurities with an easy‐plane singleion anisotropy. It is shown for both, the case of a weak local perturbation (when one impurity atom does not break the stability of the ground state with completely parallel spins) and the case of a strong local perturbation (when a single impurity creates one or even several localized spin deviations in the ground state) that there is a gap in the spectrum of low‐lying spin excitations and the spin‐wave damping Γk is proportional to the wave vector k near the bottom of the quasicontinuous band. The critical concentration up to which the ferromagnetic state possessing a gap in the spin‐wave spectrum remains stable is estimated.
Thermodynamical Behaviour of the Uniaxial Crystals of “Easy‐Axis” Type with Diverse Origins of Magnetic Anisotropy
Authors: Kazakov A. A. , Kleshev S. M.
Published: July 2007

Instead of solving the set of the 2s equations for the Green function of a ferromagnetic with singleion and two‐ion anisotropy of arbitrary order, it is suggested to solve a single equation for the parametric Green function. Green's function, low‐temperature spectrum, and magnetization are calculated for an anisotropic ferromagnet with arbitrary spin. In the low‐temperature region one spin wave branch is excited and as a consequence the thermodynamical behaviour of the system is particularly simple.
Magnetic Anisotropy of Disordered Magnetics with Jahn‐Teller Ions
Authors: Ivanov M. A. , Mitrofanov V. Ya. , Fishman A. Ya.
Published: July 2007

The influence of random strain fields on the magnetic anisotropy of systems with Jahn‐Teller (JT) ions of the iron group is investigated. The averaging of the free energy over the randomly distributed sources of the strain fields is carried out using two‐ and three‐dimensional distribution functions obtained in the paper. It is shown that the value and sign of the first and second cubic anisotropy constants are determined by the ratio of the molecular field parameter to the width of the random field distribution.
Wannier Functions from Landau Functions?
Authors: Wannier G. H.
Published: July 2007

It is well known that the set of functions in a Bloch band is equivalent to copies of a single function placed on a lattice. The possibility of establishing a similar relation in the presence of a magnetic field is investigated. Landau ground state wave functions for the symmetric gauge placed on a square net serve as trial objects. The result is equivocal. It is proved that the standard procedures of the Bloch—Wannier transformation are feasible for some fields, but impossible for other fields. Both sets of fields are infinite and all members of the second set are smaller than those on the first. The resulting situation is discussed.
Positron Annihilation in Zinc. Theory and Experiment
Authors: Pawłowska Zoja
Published: July 2007

The electron—positron pair momentum density distribution in zinc is computed in the independent particle model (IPM) approximation on the basis of band structure calculations and the positron wave function. Some additional results of the application of the selection rule for positron annihilation to the h.c.p. structure are given. The angular correlation curves are calculated along three main crystallographic directions [100], [110], [001]. The overall shapes of the angular correlation curves can not be well explained in terms of the IPM approximation. Introducing the enhancement factor proposed recently by Arponen and Pajanne and the momentum independent enhancement for the core states and the umklapp components, the disagreement between IPM theory and experiment is reduced.
Computer Simulation of Glancing Scattering of a Fast Proton Beam by a Crystal Surface
Authors: Vyatkin E. G. , Taratin A. M. , Vorobiev S. A.
Published: July 2007

With the help of the binary collision model the glancing scattering of 10 MeV protons incident at the (100) and (110) surfaces of a tungsten crystal is investigated. The angular distribution and the dependence of the angular spread of the reflected protons on the incidence angle and crystal temperature are calculated.
Thermal Expansion and Mode Grüneisen Parameters in Lithium
Authors: Prasad B.
Published: July 2007

The mode Grüneisen parameters γqj, the mean value of Grüneisen parameter γM, and the thermal expansion coefficient γM are computed for b.c.c. lithium employing the model potential proposed by Krasko and Gurskü. The present results are compared with available experimental data. Reasonable agreement between theory and experiment is obtained.
Thermal Conductivity and Optical and Paramagnetic Absorption Spectra of Reactor Irradiated MgO Crystals
Authors: Abramishvili M. G. , Altukhov V. I. , Kalabegishvili T. L. , Kvachadze V. G.
Published: July 2007

The effect of reactor irradiation on thermal and optical properties of MgO is studied experimentally and theoretically. Optimum regimes of irradiation and annealing are selected, which promote the accumulation of a great number of F‐centres in the crystal and allow to reveal a relevant bend on the thermal conductivity curve x(T) at T ≈ 10 K. A general expression for the resonance inverse relaxation time of phonons' on F‐centers is obtained. Parameters of the model and basic mechanisms of thermal resistance of n,γ‐irradiated MgO crystals are determined. It is shown that the low temperature irradiation leads to more effective accumulation of F‐ and F+‐centres and hence to a stronger radiation change of the thermal conductivity.
Investigation of Z Center Conversions in KCl:SrCl2. II. Dielectric Relaxation Caused by Mutual Conversions of Z1, Z2, and F Centers
Authors: Kessler A. , Kizler P.
Published: July 2007

Loss angle measurements in the course of linear heating are carried out during a mutual conversion of F, Z1, and Z2 centers in KCl:SrCl2 one into the other and of KCl:SiCl2 containing no colour centers. It is found that besides the known losses caused by reorientation of impurity vacancy complexes, point defect surface polarization, and conductivity, also the Z2 → F conversion causes losses. A thorough analysis of the results leads to the conclusion that a transport of electrons takes place during the Z2 → F conversion whereas the other conversions are realized by a „transport”︁ of ions or neutral components.
The Validity of the Low‐Temperature Expansions for the Anisotropic Heisenberg Models
Authors: Škeinjar M. T. , Kapor D. V. , Šetrajčić J. P.
Published: July 2007

The low‐temperature expansion of the relative magnetization of two anisotropic Heisenberg models is discussed. It is shown that due to the existence of the temperature independent energy gap, the dimensionless temperature cannot be applied as the small parameter. In other words the low‐temperature expansion is completely invalid for the Heisenberg model with the Ising anisotropy. For the anisotropic model proposed by Belorizky et al. one can make calculations in the first order in the anisotropy parameter, but there is no term of order T3 as previously stated.
Evaluation of the First Relaxed Excited State Energy of the Free Fröhlich Polaron
Authors: Lépine Y. , Charbonneau M.
Published: July 2007

The first relaxed excited state energy of the free Fröhlich polaron is evaluated using the Fock approximation of Matz and Burkey. From a discussion of a variational‐type expression of this energy, it is concluded that a singularity in the space of variational parameters separates the ground state from the first relaxed excited state and that other singularities separate the other excited states. From this interpretation, this energy is evaluated, using two complete model spectra; a Gaussian spectrum and a second one involving an internal magnetic field in addition to the Gaussian part. It is found that for α ≦ 1.2 no excited state is observed. For 1.2 ≦ α ≦ 4.2, a two‐dimensional excited state is obtained while for α ≦ 4.2 a Gaussian excited state is obtained. These results are, however, limited by the choice of the model spectrum. For large α's, an asymptotic limit is obtained of ‐0.053α2ħω, a value close to that obtained from the best product ansatz calculations.
Control of Carrier Concentration by Stimulated Emission in Highly Excited Direct Gap Semiconductors
Authors: Egorov V. D. , Müller G. O. , Weber H. H. , Jacobson M. A. , Dite A. F.
Published: July 2007

Stimulated recombination may result in orders of magnitude reduction of lifetime of photo‐excited carriers depending on the length over which optical amplification of (spontaneous) emission can occur. This effect is verified in the excitation spot dependence of photoconductivity in CdS, in qualitative agreement with model calculations.
EPR Studies of Cu2+ in Anhydrous Lead Tartrate
Authors: Korkmaz M. , Bessege R. , Gavaix A.
Published: July 2007

EPR of Cu2+ doped single crystals of anhydrous lead tartrate grown from silica gel are carried out at 300 K. Four Cu2+ lattice sites, labeled A, B, C, and D are identified. These sites exhibit four sets of four hyperfine lines in all directions except some particular directions. The g‐factor, the hyperfine splitting, the mixing parameters, and Fermi contact interaction constant are calculated for each site and they are compared with those found for other tartrate crystals in the literature. The spectra are found to have different intensities for different sites suggesting a preferential occupation of the sites by the Cu2+ ions.
The Ni Donor and Acceptor Levels in CdSe
Authors: Baranowski J. M. , An Phuong
Published: July 2007

The charge transfer spectra of Ni impurity in CdSe are reported and discussed. It is shown that they consist of the photoionization spectrum of the donor Ni2+ (d8) and the spectrum connected with the transition of an electron from the valence band to the Ni in result of which the Ni1+(d9) charge state is created. An analysis of the charge transfer spectra allows to determine the N2+ (d8) donor level at 0.32 eV and the Ni1+(d9) acceptor level at 1.81 eV above the top of the valence band. These results and other transition metal levels established previously in CdSe allows to discuss chemical trends in their position within the gap.
The Improved Point‐Ion Potentials for the F+‐Centers in LiNbO3 and LiTaO3
Authors: Ho Fat Duen
Published: July 2007

The validity of the point‐ion potentials is very essential and even critical for the study of the F+‐centers in lithium niobate and lithium tantalate. Earlier calculations are based on the assembly of 810 ions of the crystal‐lattices. In order to address to this important subject in much greater detail new potentials are developed by including 3750 ions instead of 810 ions. It is found that the 3750 point‐ion potentials are almost in accord with those of 810 point‐ions for each crystal. This is also true for the resulting values of the transition energies. Thus the convergence of the lattice sums is further demonstrated, and the validity of 810 point‐ion potentials is more convincingly shown.
Photostimulated Uneven Magnetoresistance at Inelastic Scattering of Electrons in Semiconductors
Authors: Tsurkan G. I. , Shmelev G. M. , Shon Nguyen Hong
Published: July 2007

The magnetoresistance (MR) of a nondegenerated polar semiconductor in the presence of linear polarized laser radiation (frequency Ω) is estimated. At low temperatures (k0T ≪ hω0, ω0 is the optical phonon frequency), the contribution in MR is made by two groups of carriers with energies 0 < ε < hω0 and hω0 < ε < ε < 2hω0. In strong magnetic fields besides usual (anomallously large) MR an uneven MR (UMR) appears (due to the laser radiation) depending on the orientation of the wave field relative to the dc electric field. The UMR value may be governed by the difference |Ω – ω0|.
Fluctuation Theory of Magnetic Superconductors
Authors: Savchenko M. A. , Stefanovich A. V.
Published: July 2007

A fluctuation theory of ternary and pseudo‐ternary superconducting compounds of rare‐earth metals is constructed on the basis of phase lamination in complex magnetic systems during phase transitions. It is shown that a high‐temperature superconducting phase may appear in these compounds. The criterion for the emergence of such a phase is obtained.
Kondo‐Like Behaviour of the Electrical Resistivity of the Compound Pro0.25La0.75Al3
Authors: Lippold B. , Müller H.
Published: July 2007

The temperature dependence of the electrical resistivity of Pr0.25La0.75Al3 is investigated. The 4f part of the resistivity shows Kondo‐like behaviour. Using the experimental crystalline electric field splitting scheme and the Hamann factor, the 4f part of the resistivity can be fitted with the parameters ϱ = 0.139 μΩ cm, TK = 83.5 K, and S = 0.004.
Microscopic Model for the Ferroelastic Transition in KLiSO4
Authors: Žekš B. , Lavrenčič B. B. , Bling R.
Published: July 2007

A microscopic model for the ferroelastic transition in KLiSO4 at Tc = 190 K is proposed. The model is based on the ordering of the “top” SO4 oxygen into one of the three off‐centre sites below Tc. It yields a transition of first order and agrees with the predictions of the Landau theory for the hexagonal‐orthorhombic equitranslational transition.
Short‐Range Order Model of GexS1−x Glasses and Utilization of This Model in Calculation of the Optical Gap and the IR 367 cm−1 Band Intensity
Authors: Černošek Z. , Černošková E.
Published: July 2007

A short‐range order model (SRO model) of GexS1−x glasses is proposed and utilized to calculate the optical gap values and the IR 367 cm−1 band intensity. The optical gap values of the SRO model structural units are derived for calculation. Based on the description according to the SRO model it can be stated that the studied IR band is probably due to vibrations of the Ge‐S bonds in the (Ge‐S)n chains.
Theory of the Power Spectrum and Scattering Properties of Three‐Dimensional Disordered Systems Having a Variable Degree of Short‐Range Order. I. Continuous Spectrum
Authors: Celasco M. , Mazzetti P. , Stepanescu A.
Published: July 2007

A new approach to power spectrum calculation of structures characterized by a spatial distribution of localized physical events, which has the advantage of being flexible enough to describe practically any type of local order and of giving the results in a very compact and general form, is given. Starting from a completely ordered three‐dimensional structure, with the elementary events arranged according to an arbitrary crystallographic order, the disorder is introduced by displacing the elementary events from their lattice position. The displacements are described from a probabilistic point of view by means of three general distribution functions which can be related to the interaction forces acting between nearest neighbour events and using a convolution technique for describing self‐consistently the whole structure, whose statistical properties remain translationally invariant. A simple analytic expression is obtained where different degrees of short‐range order with any local crystallographic configuration can be introduced and characterized.
Photo‐Induced Transformation of Close Frenkel Pairs in Strontium Fluorite
Authors: Eshita T. , Tanimura K. , Itoh N.
Published: July 2007

Laser‐induced change is studied of the optical absorption and luminescence due to F‐H pairs generated by an electron pulse in SrF2. It is found that laser irradiation near 2.34 eV at a delay of 26 μs after an electron pulse by which F‐H pairs are generated reduces the component I of the pairs that has a decay time of 59 μs and optical absorption bands at 2.34 and 4.13 eV and enhances the component II that has a decay time of 7.7 ms and has optical absorption bands at 2.7 and 3.35 eV. Laser irradiation near 2.7 eV at a delay of 4 ms after the electron pulse is found to induce the reverse reaction. Studies of dichroism of the laser‐induced reduction and enhancement of the optical absorption bands and the luminescence reveal that the direction of the Σ‐Σ transition of the F molecular ion is converted by the transformation from I to II and vice versa. It is suggested that the component I corresponds to the F‐H pairs at nearest‐neighbour and II to those at the next‐nearest neighbour sites.
Diffusion Model for Charge Transport by Moving Dislocations in Simple Ionic Crystals
Authors: Tóth A. , Keszthelyi T. , Kálmán P. , Sárközi J.
Published: July 2007

A diffusion model for the calculation of the charge transport caused by moving dislocations in simple ionic crystals is developed. The model is based on the assumption that the charge transport originates from the diffusional motion of vacancies under the influence of the potential field of moving dislocations. The numerical results obtained for LiF and NaCl doped with divalent cation impurities show that the assumed mechanism is suitable for the explanation of the main features of the experimental results.
Theory of the Dependence of the Phase Boundary of FeBr2 Metamagnet on Hydrostatic Pressure
Authors: Onyszkiewicz Z.
Published: July 2007

A two‐sublattice Ising model is proposed with special four‐spin interactions added in order to describe the FeBr2 metamagnet. The important fact is that the four‐spin interaction terms are not even functions of spin sublattice operators. Thermodynamical properties of this model are investigated in the molecular field. The complete phase diagrams for FeBr2 as functions of hydrostatic pressure are obtained numerically. Moreover the theoretical results are compared with experiment.
Self‐Consistent Semi‐Classical Dynamic Theory of Non‐Radiative Capture and Emission Statistics in Defect Semiconductors
Authors: Mandelis A.
Published: July 2007

A self‐consistent, semi‐classical theory of the non‐radiative capture and emission rates of carriers in defect semiconductors is presented. The lattice is treated in the configuration coordinate approximation and thermal averages of transition rates and cross‐sections are calculated in the experimentally important statistical limit. The semi‐classical theory gives explicit expressions for the time‐dependent transition rates following optical excitation of carriers initially trapped in defect states within the band‐gap of the semiconductor. In the steady state and in the thermodynamic limit, the predicted rates are in agreement with experiment. The theory bridges the gap in the literature between rigorous but complex quantum‐mechanical theories and rough but experimentally useful semi‐classical calculations, while it makes apparent the connection to Shockley‐Read statistics.
Low‐Temperature Pair Associates of H Centres in KBr
Authors: Akilbekov A. , Elango A.
Published: July 2007

In the course of investigating the annealing of KBr crystals X‐rayed at 4.2 K the creation of new pair associates of interstitial halogen atoms (di‐H centres) with an absorption band at 3.85 eV is detected. The 3.85 centres arise only as a result of thermodestruction of the well‐known di‐H centres with an absorption band at 4.4 eV. The properties of these new centres are studied and the possible ways of halogen association in KBr are discussed. As a model for the 3.85 centres a model of (Br)2 dimers is proposed, in which the optical transition differs from that in Br ions both by size and direction. The range of KBr absorption spectra between Br and Br absorptions (3.6 to 4.4 eV) is attributed to the absorption of (Br)2 dimers. The V4A centres in KBr with Li or Na impurities have obviously also a (Br−2)2 structure.
Ellipsometric Investigation of Lattice Vibrations in Anisotropic Systems. α‐Quartz
Authors: Jungk G. , Röseler A.
Published: July 2007

The ellipsometric equation for uniaxial crystals with the optic axis parallel to the reflecting surface is used to calculate the dielectric function from experimental data on α‐quartz in the range 700 < ω < 1200 cm−1. The experimental arrangement as well as the mathematical procedure are presented. The results confirm the known mode frequencies including the selection rules and show above the influence of many‐particle effects. The latter can be described approximately as for electronic spectra.