bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of nuclear quadrupolar relaxation in liquid metals
Authors: Halder N. C.
Published: July 2007

The nuclear quadrupolar relaxation (NQR) rate W1q in liquid metals, in 69Ga, 115In and 201Hg is calculated by using the Warren theory. In order to do this, the experimental static interference function I(K) is considered and the dynamic interference function I(K, 0) is generated in the zero frequency limit. W1q is calculated in the temperature range 20 to 150 °C for 69Ga, 170 to 650 °C for 115In, and ‐‐‐35 to 80 °C for 201Hg. It is found that in all the three cases, the temperature dependence of the calculated W1q is in very good agreement with that of the experimenta W1q, but numerically the magnitudes of W1q for 115In and 201Hg deviate considerably from the experimental values. It is concluded that the antishielding factors used may be in error and that the present theory is superior to the earlier classical diffusion theory.
On the theory of thermal conductivity in quantum crystals
Authors: Pushkarov D. I.
Published: July 2007

The influence of defects on the specific heat and thermal conductivity of quantum crystals is considered. As a rule, the presence of defects decreases the thermal conductivity due to phonon (or electron) scattering. However, at sufficiently low temperatures the point defects turn into quasi‐particle defectons which can transport heat by themselves. The defecton thermal conductivity may be significant, due to the other temperature dependencies obtained in this work. Defecton statistic effects in the low‐temperature heat capacity and thermal conductivity are considered, and analytic formulae for all interesting cases are obtained.
Reaction field and dipolar interaction in quasi‐onedimensional heisenberg systems. Application to CsNiF3
Authors: Scherer C. , Barjhoux Y.
Published: July 2007

An improved mean field theory, the reaction field approximation, which was previously used for a Heisenberg paramagnet, is extended to include the magnetic dipolar interaction. It is shown that in spin chains, when the intra‐chain interaction is ferromagnetic, the dipolar inter‐chain interaction plays an important role in explaining the phase transition to an antiferromagnetic three‐dimensional order at low temperature. A study of the magnetic susceptibility of CsNiF3 is presented as an explicit example. Values for the intra‐chain J and inter‐chain J' exchange parameters are derived from TN and χ⊥(TN). The temperature dependence of χ⊥ is very well reproduced by the model.
Field‐induced phase transitions in a randomly diluted antiferromagnet. Mean‐field description of classical spin model
Authors: Moreira F. G. Brady , Fittipaldi I. P. , Rezende S. M. , Tahir‐Kheli R. A. , Žekš B.
Published: July 2007

Within a mean‐field virtual crystal approximation, a phase diagram is given in the H–T plane of a randomly diluted, uniaxial Heisenberg antiferromagnet with classical spins and single ion anisotropy. The temperature dependence of the critical fields, separating the antiferromagnet (af), the spin‐flop (sf), and the paramagnetic (pm) phases are analyzed as a function of the dilution. For systems where the anisotropy is of crystal‐field origin and where the non‐magnetic impurity does not perturb the crystal field, the phase boundaries show an interesting dependence on the dilution. On the other hand, in systems such as (Mn)x(Zn)1‐xF2, where the anisotropy is due to two particle interactions, such as dipolar forces, the present theory would predict only a linear scaling of the phase diagram with magnetic dilution. Thus, the study of magnetically diluted systems would seem to offer a possible method of characterizing the anisotropy.
Thermal Reorientation Kinetics of I in RbI
Authors: Delbecq C. J. , Stokes J. P. , Yuster P. H.
Published: July 2007

Experimental studies of the thermally activated reorientation of I in RbI: AgI single crystals are carried out by measuring the decay of the optical anisotropy of preferentially oriented I at fixed temperatures. These studies show that in the temperature range investigated, contrary to previous results by others, thermally induced changes in orientation of I in RbI result almost totally from elementary jumps in which the orientation of the I molecule ion changes by 60°. The activation energy for this 60° jump is determined to be (0.306 ± 0.005) eV.
Surface Density of States in a Simple Model of Nickel
Authors: Kolář M.
Published: July 2007

A simplified model of a semi‐infinite transition metal crystal is investigated. The changes at the crystal surface are represented only by the reduction of the number of nearest neighbours. The surface density of states is calculated, which includes the deviations from the bulk behaviour in all atomic layers. In the investigated model these deviations can be summed explicitly. The results indicate that also the further layers behind the surface one may have appreciable influence on the surface quantities.
Bond Orbital Model for Tetrahedral Semiconductors on the Base of Molecular Parameters. Covalent Crystals and Perturbative Treatment of Partially Ionic Semiconductors
Authors: Dyachkov P. N. , Levin A. A.
Published: July 2007

A new version of the BOM is proposed on the base of the CNDO procedure. The standard values of the molecular integrals describing a wide range of molecular systems are used. The method allows to calculate the valence band structure, some features of the interband transitions, and the cohesive energies of crystals in accordance with the experiment and with no adjustable parameters. Correlations between the bond orders in the isoelectronic series and some properties of the semiconductors such as the bond lengths, cohesive energies, and the hole effective masses are obtained.
Theory of the Excitonic Mechanism of Crystal Defect Formation
Authors: Anselm A. I. , Lang I. G.
Published: July 2007

To look into the factors responsible for a large efficiency of defect formation in alkali halides, an excitonic mechanism with an exciton localized at a single lattice site is discussed. The exciton contains a hole forming a small polaron. The nonradiative annihilation of the exciton necessary for defect formation is shown to be improbable due to the fact that the polaron displacements of atoms are small. The large efficiency of the excitonic mechanism is due to the localization of the hole forming the exciton at two lattice sites, since such a localization gives rise to large equilibrium displacements of atoms (of the order of an interatomic spacing).
Generalized Boltzmann Equation with Field‐Dependent Collision Terms Inclusive of Interband Transitions
Authors: Holzhütter H.‐G. , Mocker M.
Published: July 2007

A coupled system of generalized Boltzmann equations with field‐dependent collision terms is derived, taking into account both intraband and interband transitions of the charge carriers. The field dependence of the collision terms is shown to be of importance if the field‐induced energetic uncertainty becomes comparable with the mean energy of the charge carriers. The interband part of the one‐electron density operator is investigated which is associated with the occurrence of an interband current.
Low Temperature Specific Heats of TCNQ Complexes
Authors: Kondow T. , Mizutani U. , Massalski T. B.
Published: July 2007

The low temperature specific heats of NMP(TCNQ), Q(TCNQ)2, DAD(TCNQ)2, TEA(TCNQ)2, Li(TCNQ), and Rb(TCNQ) are measured at temperatures between 1.5 and 4.2 K. The specific heats (C) can be expressed in the form C = γT + αT3 + HT−2. Both γ and H originate from lattice defects and/or impurities in the specimens and they can be discussed in terms of their concentration as determined from the magnetic susceptibilities of the same specimens. With increasing impurity concentration, both γ and H become measurable at an impurity concentration exceeding 2.2 × 1021/mol TCNQ and increase at a rate which keeps the ratio H/γ almost constant. These behaviors are consistent with a scheme of randomly distributed impurities. The Debye temperature for each specimen is also determined from the α‐coefficient. The velocity of sound, and hence the rigidity of the lattice, can be estimated from the Debye temperature.
Carrier‐Assisted Radiation Pumping of Spin Waves in Ferromagnetic Semiconductors under a Strong Magnetic Field
Authors: Pereira P. R. R. , Amato M. A. , Miranda L. C. M.
Published: July 2007

The carrier contribution to the generation of spin waves in ferromagnetic semiconductors under the simultaneous presence of a laser field and a strong dc magnetic field is discussed. It is shown that for degenerate semiconductors this electronic contribution predicts a threshold pumping power within the experimental reach. Also, this carrier‐assisted mechanism excites spin waves in a broad range of relatively high valves of q and may be useful as a new tool for investigating the s–d exchange constant.
The EPR Fine Structure Spectrum of Dislocations in Silicon
Authors: Bartelsen L.
Published: July 2007

The EPR fine structure spectrum of dislocations in plastically deformed silicon single crystals is measured using pure, boron, and phosphorus doped silicon. The EPR transitions of chains of coupled spins Si = 1/2 are calculated using the following spin Hamiltonian:

The results reveal that the fine structure spectrum can be explained as caused by transitions taking place in chains of 2, 3, 4, and 5 coupled spins of unpaired electrons in the dislocation core. In heavily boron doped, plastically deformed crystals the EPR signal of boron atoms located in the stress field of the dislocations is found. The main axis of the stress field is nearly parallel to the Burgers vector of the primary glide system.
Orthogonalized Augmented Plane‐Wave Method for the Solution of the Energy Band Problem in Solids
Authors: Chatterjee S. , Sinha P.
Published: July 2007

The advantages of the OPW and the APW methods are combined to formulate a method for the solution of the energy band problem in solids. Here the valence and the conduction electrons described by the APW's are orthogonalized to the core states constructed by the tight‐binding approximation. The method is applied to wurtzite ZnS and used to obtain the energy values at the highest‐symmetry points – Γ and A – inside the Brillouin zone. The preliminary results appear to indicate that wurtzite ZnS is a semiconductor. The various band gaps calculated from these band energy data are compared with experiments and other theoretical calculations.
Hyperfine Field on the Nucleus of an Atom of a Transition d Element (Metal), Magnetie Moment and Electron Structure of a Ferromagnetic Metal Atom
Authors: Galperin F. M.
Published: July 2007

Some fundamental characteristics of the unfilled d shell of a transition d element are calculated at first time and then a number of the most specific properties of the d substance (an isotope, a free atom (ion), and the transition d metal atom) are calculated with those characteristics taken into consideration.
Impurities and Positronium Formation in Molecular Crystals
Authors: Goworek T. , Rybka C.
Published: July 2007

The influence of impurities on the positronium formation in molecular crystals is investigated. The intensity of the narrow component in the momentum spectrum is measured as a function of the admixture mole fraction and discussed in the framework of a simple two‐process model. The specific positronium formation times for various admixtures are found to be 3 × 10−13 to 10−11 s.
The Effect of Non‐Transition Metals on the Magnetic Susceptibility of Aluminium Solid Solutions
Authors: Lakner J.
Published: July 2007

Non‐transition metals, the effect of which is analysed in binary and ternary solid solutions of aluminium, decrease the magnetic susceptibility of aluminium to a higher degree than to be expected on the basis of spin susceptibility of conduction electrons in metals generally accepted. Using the pseudopotential method and a nearly free‐electron approximation, the present model explains the experimental results as the change in the Landau diamagnetic susceptibility resulting from the electrons and holes on the Fermi surface of the third zone, having the two potential parameters W(111) = 0.018 Ryd and W(200) = 0.053 Ryd.
Correlations and Plasmons in Quasi‐One‐Dimensional Metals
Authors: Campos V. B. , Hipolito O. , Lobo R.
Published: July 2007

The longitudinal dielectric function for a quasi‐one‐dimensional electron gas is obtained in a self‐consistent field approximation (SCFA) scheme. Short range correlations arising from both, Coulomb and exchange effects through a local field correction depending on the pair‐correlation function are included. The structure factor and the dielectric function are determined in a self‐consistent scheme. Numerical results are presented for the structure factor, the static pair‐correlation function, the plasmon dispersion and the compressibility. It is found that in the high density limit the results for the structure factor and the pair‐correlation function are the same as given by Girardeau's exact expression. The plasmon dispersion curves indicate the onset of a minimum for xs ⪆ 7, in contrast with the RPA results. For values of xs in this region, the compressibility values become negative, indicating a possible instability of the system. In both RPA and SCFA results the plasmon modes are not Landau damped in contrast to isotropic metals.
Contribution of the d‐Electron Screening to the Properties of Impurities in Transition Metals
Authors: Lannoo M. , Allan G.
Published: July 2007

The screening around impurities in narrow band systems is analyzed in a linearized approximation, leading to a dielectric constant formalism. The response matrix is determined, first using a Green's function technique and then a method of moments. An application is made to the case of d‐electrons in transition metals. Near the centre of the series it is found that most of the screening occurs on the impurity site. Finally the results are used to calculate the dissolution energy of the impurity.
On the Bose‐Einstein Condensation of Excitons
Authors: Kotelnikov Yu. E. , Kochelaev B. I.
Published: July 2007

The Bose‐Einstein condensation of Frenkel excitons in molecular crystals is investigated by means of the Green's function method without the use of the effective mass approximation, the kinematic interaction between excitons being taken into account accurately. The spectrum of elementary excitations at low temperatures in the case of dipole–dipole interaction between molecules is obtained. The temperature of the phase transition and its dependence on the concentration of excitons are estimated.
Magnetic and Thermodynaniic Properties of Heisenberg Open Finite Chains II. Specific Heat
Authors: Rakitin Tu. V. , Kalinnikov V. T. , Hatfield W. E.
Published: July 2007

Calculations are made of spin specific heat, C'M,, as a function of reduced temperature, kT/J, for Heisenberg open finite chains containing up to 11 ions (N ≦ 11) with spins Si of 1/2 to 5/2 on the basis of the model described in a preceding communication. Cha‐acteristic features of the C'M(kT/J) curves corresponding to various Si‐ and N‐values are discussed. These can be used in interpretation of experimental temperature dependences of C'M for the respective ferro‐ and antiferromagnetic chains.
Weak‐Field Thermomagnetic Coefficients of a Trigonal Bismuth Sample
Authors: Hansen O. P.
Published: July 2007

The weak‐field thermomagnetic coefficients of bismuth with heat current in the trigonal direction have been measured in the range of temperatures from 10 to 65 K. The data are analysed in terms of the symmetry restrictions imposed by the 3m point group. The temperature dependences of the coefficients point to a diffusion thermoelectric power down to temperatures which depend on the coefficient in question, the highest temperature being around 40 K.
The Influence of Antiferromagnetic Order on the Evaluation of Spin Disorder Resistivity
Authors: Gratz E. , Poldy C. A.
Published: July 2007

It is known that the parameters ϱ spd (spin disorder rosistivity) and θp can be used for estimating the effective mass m* of the conduction electrons and exchange constant G. ϱspd is the temperature independent contribution to the electrical resistivity in the paramagnetic temperature range whereby this contribution is due to the disordered localized spins, and θp is the paramagnetic Curie temperature. The above procedure is appeared in the literature for the rare earth melalv (R) and their interinetallic compounds not only for ferromagnetic (Ral2) but also for antiferromagnetic (Rau2, Rag2, Rcu2) systems. It is tested whether such a procedure for estimating m* and G is valid irrespective of whether the ordering is ferromagnetic or antifcrromagnctic. This investigation is carried out for the system Gd(Ni, CU)2 and it is shown that the above procedure is principally inapplicable for θspdi values obtained from antiferromagnctic samples. In addition a discussion is given of the validity of the free electron approximation in the Gd(Ni, Cu)2 and the GdZn2 systems, whereby it is concluded that the s‐character deteriorates with increasing electron concentration.
Ultrasonic Absorption in the Intermediate State of Gallium and Indium
Authors: Singh O.
Published: July 2007

The absorption is stndied of longitudinal ultrasound in the intermediate state of indium and gallium single crystal cylinders at 1.17 and 0.39 K, respectively, for frequencies ranging between 10 and 70 MHz. The results are analysed with a theory developed by Andreev which is extended to allow for the arbitrary values of ql. The width of the normal regions calculated from the results is in reasonable agreement with that calculated using Landau's unbranched model with surface energy parameters equal to 3.5 × 10‐5 and 8 × 10−5 cm for indium and gallium, respectively. The nonlinear variation of the high‐frequency‐limit attenuation is perhaps due to the curvature of the N‐S interfaces near the surface of the specimen.
Temperature Dependence of the Grüneisen Constant of Si and Ge
Authors: Soma T.
Published: July 2007

The Grüneisen parameters of the individual modes of phonons and the mean Grüneisen parameters are obtained for Si and Ge in the higher‐order perturbation method. Calculations are made for normal modes propagating in the [l00]‐, [110]‐ and [111] Indirections.
Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO
Authors: Vance E. R. , Manson N. B.
Published: July 2007

Magnetic circular dichroism (MCD) measurements are made on neutron‐irradiated MgO to examine the sharp 524, 642, and 649 nm optical absorption lines, which are known to be A ↔ E transitions within trigonal centres. The MCD of the 524 nm line is strong at low temperatures, as expected from previous Faraday rotation measurements, and vibrational sidebands are observed in the MCD measurements. The MCD signals from the 642 and 649 nm lines are very weak, but temperature‐dependent: assuming the centres are paramagnetic the magnitudes of the spin‐orbit splittings of the 2E‐states are calculated as ⪅ 0.1 cm−l from the MCD data. In neutron‐irradiated CaO some sharp, strong temperature‐dependent MCD signals are observed after the samples are annealed at moderate temperatures; the corresponding absorption lines at 465, 573, 602, 672. and 678 nm are not previously reported. The centres responsible for these lines appear to be paramapnctic and to possess at least axial symmetry.
Magnetic Properties of a' Random Binary System
Authors: Hattori K.
Published: July 2007

The quenched random system composed of A and B magnetic atoms is theoretically investigated. The exchange integrals between A‐A and B‐B atoms, JA, JB, are assumed to be positive, bnt no restriction is imposed on the exchange integral between A‐B atoms, JAB. By using effective Hamiltonians, the transition temperature and specific heat above it are calculated in the Heisen‐berg and Ising models. It is found that the transition temperature depends on the sign of JAB in the Heisenberg model, while it does not in the Ising model. The expression of the specific heat obtained is applicable to any paramagnetic phase, without restrictions on the magnitude and sign of JA, JB, and JAB.
Elastic Interaction between Crowdions and Extended Atomic Defects in Isotropic Media
Authors: Scheu W. , Frank W. , Kronmüller H.
Published: July 2007

The interactions between the partners of crowdion‐vacancy, crowdion‐crowdion, and crowdion‐dumbbell pairs in face‐centred cubic crystal sare calculated within the framework of the isotropic linear elasticity theory, taking into account the finite extensions of these defects. It is found that among all types of pairs there are configurations which can form bound states at small defect separations. For a number of crowdion‐vacancy and particularly of crowdion‐crowdion configurations, pairs of neighbouring bound states exist which are capable of giving rise to relaxation phenomena.
Phase Transitions in Spin‐One Ferromagnetic Systems
Authors: Kumar V. , Sharma K. C.
Published: July 2007

A theory is given for the spin‐one ferromagnetic system containing all the major interactions. The anisotropy term is evaluated exactly whereas the bilinear and the biquadratic exchange terms are evaluated in RPA. A closed form expression for the critical temperature Tc at which the quadrupolar to paramagnetic phase transition occurs, is given. The variation of Tc vs. the biquadratic exchange parameter is presented and is compared with the earlier results.
Mössbauer Effect of γ‐Irradiated CaF2 Doped with 57Co
Authors: Sato Y.
Published: July 2007

Mössbauer spectra of 57Co doped into CaF2 single crystals are decomposed into a single line and a quadrupole doublet. The former is ascribed to Fe2+ ions on Ca2+ sites with cubic symmetry, and the latter to Fe2+ ions on axially symmetric interstitial sites associated with an adjacent cation vacancy, γ‐irradiation further produces a new line which arises from Fe2+ ions on interstitial sites with cubic symmetry. When the irradiated crystals are quenched from 500°0, only two lines appear which arise from Fe2+ ions on interstitial sites associated with two neighbouring cation vacancies.
Energy transfer by the normal modes in the exciton resonance region
Authors: Selkin A.
Published: July 2007

For a dielectric medium with isolated exciton resonance, expressions for the energy density and energy flux density of a radiation field, given in a general form, are derived, taking into account energy absorption and spatial dispersion. The energy fluxes associated with individual normal modes are considered. It is shown that in the case of nonzero damping the interference between different normal modes gives rise to interference energy fluxes. The behaviour of the energy flux on the crystal surface is analysed. It is found that the energy flux across the medium boundary is continuous, if the coefficients in the relation between exciton polarization at the surface and its derivative along the normal are real. The numerical calculations of energy distribution between the modes, including the interference fluxes, are carried out for the case of light transmission through the CdS crystal boundary in the energy region of An=1 exciton resonance.
Octupole and hexadecapole dynamics in solids state physics
Authors: Kh. Kopvillem U. , Prants S. V.
Published: July 2007

Lie algrebras generated by commutation of operators are closely related to the algebraic structure of dynamical physical effects. This feature may be explored to search for new physical effects by means of algebraical methods. This principle is illustrated by the formula describing the general properties of superradiance effects in quantum systems of high rank radiating multipoles. It is shown that electromagnetic and acoustic hexadecapole and octupole superradiance may be detected by existing experimental technique.
Soft mode and Debye relaxations in Thiourea
Authors: Khelifa B. , Delahaigue A. , Jouve P.
Published: July 2007

The infrared reflection spectra of a single crystal of thiourea polarized along the ferroelectric axis [100], are extended up to very low frequencies with the help of ε data obtained by some authors. The whole reflectivity spectrum is fitted with a generalized dielectric constant model at several temperatures. So for the first time all the dispersion parameters of the soft mode (a comparison with Raman data is made) are obtained and the existence of two Debye relaxations is shown. One is attributed to domain wall motion and the other to reorientation of molecules. With the help of a generalized LST relation, it is proved that the discontinuity of ε(0) at 169 K is due to a relaxation process.
The phonon spectra of β‐CuI3)
Authors: Haxaton M. B.
Published: July 2007

The phonon dispersion curves of the wurtzite structured β phase of CuI are calculated, using the second neighbor ionic model of Banerjee and Varshni. This calculation based on a previous paper where the phonon spectra of the zinc blende structured γ phase of CuI is calculated. The long range. Coulomb interaction is calculated directly and the short range interaction matrices are calculated by a transformation of the previously calculated short range matrices of γ‐CuI with some additional corrections because of the nonideal nature of the wurtzite lattice of β‐CuI.

Es wird die Berechnung der Phononendispersionskurven der Wurtzit‐β‐Phase von CuJ mit dem Tonenmodell zweiter Nachbarn von Banerjee und Varshni beschrieben. Sie beruht auf einer früheren Arbeit, in der die Phononenspektren der Zinkblende‐γ‐Phase von CuJ berechnet wurden. Die langreichweitige Coulombwechselwirkung wird direkt berechnet und die Matrizen der kurz‐reichweitigen Wechselwirkungen werden durch eine Transformation der früher berechneten kurz‐reichweitigen Matrizen von γ‐CuJ berechnet mit einigen zusätzlichen Korrekturen wegen der nicht‐idealen Natur des Wurtzitgitters von β‐CuJ.
Second‐order Raman spectra and phonon spectrum of CsF
Authors: Kumar M. , Mahesh P. S. , Sharma T. P.
Published: July 2007

The phonon dispersion curves, and the one‐phonon density and two‐phonon density of states for CsF crystals at 300 K are calculated. The phonon dispersion curves calculated with the deformation dipole model are found to be in good agreement with the experimental data. The two‐phonon density of statees agres with the peak positions of the observed second order Raman spectra.
Effect of the mixing between Frenkel excitons and charge transfer states on charge carrier transfer in molecular crystals
Authors: Petelenz P.
Published: July 2007

The recently suggested increased mixing of Frenkel excitons and CT states in the neighbourhood of a charge is discussed. It is demonstrated that such mixing results in renormalization of the charge carrier hopping integral as it provides an additional channel of crystal polarization. The effect is expected to be highly anisotropic and different for electrons and holes. It is supposed to be especially important in the crystals of charge transfer complexes where the low‐lying excited states are present.
Site ordering and magnetostriction in garnet
Authors: Muller M. W.
Published: July 2007

The symmetry is determined and the magnitude is estimated of the noncubic magnetostriction of mixed rare‐earth iron garnet, associated with the preferential occupation of non‐equivalent dodeca‐hedral lattice sites by the different ionic species. The site preference is assumed to be connected with the kinetics of growth on different crystal surfaces. The calculation is carried out by evaluating weighted sums of the local orthorhombio magnetoelastic tensors. It is found that the symmetry of the magnetoelastic tensor associated with growth along several crystal directions is: tetragonal for (001); trigonal for (111); orthorhombic for (110); and monoclinic for (112). A sample calculation of the relative magnitude of the cubic and uniaxial magnetoelastic tensors associated with partial site‐ordering of Er3+ suggests that the noncubic contribution should be readily observable for the modest preferences experienced in practice for some growth directions and might be below detection for others.
Electric field gradient in biotite mica
Authors: Chandra R. , Lokanathan S.
Published: July 2007

The angular variation of the quadrupole doublet ratio for the two resolved sites in biotite single crystals is studied by Mössbauer absorption. It is shown that the angular variation is consistent with the assumption that the EFG principal Z‐axis for the prominent M2 site is distributed on a cone of half angle 20°, the axis lying normal to the layer. This model is justified from the known poly‐typism of biotite. The lower and higher energy lines of the quadrupole doublet are identified with I3/2 and I1/2 respectively, indicating that the quadrupole coupling constant in M2 site is negative.
Computation of force constant change of dilute Mössbauer impurities in metallic hosts from Mössbauer Parameters
Authors: Puri R. K. , Gupta L. R.
Published: July 2007

Ratios of impurity‐host to host‐host spring force constants are computed from the temperature variation of recoilfree fraction and second order Doppler shift of Mössbauer Sn119 atoms present as dilute impurity in hosts of various masses. The weighted mean temperatures θ(−2), θ(−1), θ(+1), and θ(+2) of Mössbauer impurity atoms in various hosts are calculated from the temperature variation of recoilfree fraction and second order Doppler shift. Such calculations of force constant change for various impurity‐host systems are made on the basis of nearest neighbour central forces model assuming Debye spectrum by taking account of relative weighting of resonance and localised modes of vibration. The computed results are interpreted to discuss how temperature, mass, and force constant changes would affect the Mössbauer parameters.
Thermodynamic properties of a finite disordered hubbard cluster
Authors: Löser W.
Published: July 2007

Thermodynamic and magnetic properties of a four‐site Hubbard ring with random site, and correlation energies as well as hopping integrals are investigated in a wide range of temperature and correlation parameter in the half‐filled band case. A qualitative behaviour similar to finite ordered systems is found in case of not too large fluctuations. Deviations from the crystalline values in the temperature dependence of the configurational averaged quantities of the specific heat, the magnetic susceptibility, and the spin correlation functions are caused by an increased antiferro‐magnetic exchange and enhanced single‐particle excitations due to disorder. The validity of the Bogolyubov theorem for the free energy of amorphous magnets is verified for this model.

Thermodynamische und magnetische Eigenschaften eines aus vier Gitterpläatzen bestehenden Hubbardringes mit stochastischen Gitterplatz‐ und Korrelationsenergien sowie Hoppingintegralen werden in einem weiten Bereich der Temperatur und des Korrelationsparameters im Fall des halb‐gefüllten Bandes untersucht. Für nicht zu große Fluktuationen wird ein qualitatives Verhalten ähnlich dem in geordneten Systemen festgestellt. Abweichungen von den kristallinen Werten in der Temperaturabhängigkeit der konfigurationsgemittelten Größen der spezifischen Wärme, der magnetischen Suszeptibilität und der Spinkorrelationsfunktionen werden verursacht durch den vergrößerten antiferromagnetischen Austausch und die verstärkten Einteilchenanregungen infolge der Unordnung. Die Gültigkeit des Bogolyubov‐Theorems für die freie Energie amorpher Magneten in diesem Modell wird gezeigt.
Magnetostriction of an electron‐hole drop in Ge
Authors: Wolfe J. P. , Markiewicz R. S. , Furneaux J. E. , Kelso S. M. , Jeffries C. D.
Published: July 2007

By infrared imaging the shape of a strain‐confined electron‐hole drop in germanium is measured in an applied magnetic field, H ≦ 22 kOe. A considerable distortion from sphericity is observed for all field orientations and stress geometries studied. It is found that in most cases the drop flattens along the field direction. For II ‖〈111〉‖ (applied stress) the drop shape depends markedly on the position of the light excitation spot on the crystal. The dimension parallel to the field decays more rapidly than the perpendicular dimension. The data strongly support a dynamic interpretation in which the recombination currents of electrons and holes give rise to the field‐induced distortion.
The thermodynamic theory of domain formation in superionic crystals
Authors: Kharkats Yu. I.
Published: July 2007

The thermodynamics of disordering of the cation sublattice in Superionic crystals is studied. It is shown that along with the homogeneous states of the crystal at which the concentration of interstitial cations is equal to the concentration of cation vacancies in every point, thermodynamically stable nonhomogeneous states are possible. These states correspond to the separation of the crystal into domain regions with increased and lowered concentrations of interstitial cations and vacancies in relation to their average values. Appearance of domain states in certain cases may lead to blocking of phase transitions associated with abrupt changes of defect concentrations in the homogeneous states of crystals. The critical conditions of appearance of domain states at various values of system parameters are studied. Domain states may exist in certain cases at 0 < T < T, and sometimes in temperature ranges limited from both sides, T < T < T.
On the charge carrier mobilities in molecular crystals
Authors: Pesz K. , Luty T.
Published: July 2007

The dynamics of polarons with interaction quadratic in the phonon coordinates is studied. An expression for the charge carrier mobility using the many‐body theory is derived. The temperature dependence of the charge carrier mobility exhibits similar behaviour to that for one‐phonon processes, i.e. the mobility decreases with the temperature. It is found that the higher‐order processes only decrease the value of the mobility but do not change the character of its temperature dependence.
Three‐body interaction and lattice dynamics of metals
Authors: Sarkar S. K. , Das S. K. , Roy D. , Sengupta S.
Published: July 2007

Pseudopotential theory demonstrates that the total energy of a metal consists of an effective two‐body central interaction and a purely volume dependent part. Starting from this basis, a model expression for the energy in a metal is developed. A three parameter expression for the volume energy is used of which only two are necessary for the lattice dynamical properties. It is shown that the interpretation of the volume strain as the local strain, as demanded by the adiabatic hypothesis, leads at once to an effective three‐body ion‐ion interaction which is essential in order to demonstrate the equality of the static and dynamic elastic constants. The present model has no difficulty either with the equilibrium condition or with the symmetry properties of the dynamical matrix as in other phenomenological models. The model is applied to calculate the phonon frequencies of the b.c.c. metal W and the f.c.c. metals Cu, Ag, and Ni. With a suitable expression for the two‐body central interaction the model can be easily extended to calculate a fairly large number of both static and dynamic properties of metals.
On the calculation of the surface density of states in the tight‐binding formalism
Authors: Kolář M.
Published: July 2007

The derivation is presented for a closed formula which gives the surface density of states of an arbitrary semi‐infinite tight‐binding crystal in terms of the Green function of the corresponding perfect (infinite) crystal and the change of the electron potential at the crystal surface. Using this formula, some general contributions to the surface density of states are found. A simple example is presented.
Antiferromagnetic properties of the Li2MnxTi1−xO3 mixed crystal system an EPR investigation
Authors: Hofstaetter A. , Scharmann A. , Vitt B.
Published: July 2007

Polycrystalline samples of antiferromagnetic Li2MnO3, diluted with diamagnetic Li2TiO3, are investigated by means of EPR. Powder spectra of isolated Mn4+‐ions as well as cluster spectra are obtained. The principal EPR features of exchange coupled clusters are discussed and a qualitative model for the concentration dependent EPR linewidth is presented. From this follows that even at high diamagnetic dilution antiferromagnetic clusters are stabilized which can be used to determine the homogeneity of paramagnetic ion distribution.
Mössbauer study of dispersed iron ions in alkali halides
Authors: Anand H. R.
Published: July 2007

The dispersed state of iron ions in LiF and NaF is studied by absorber Mössbauer spectroscopy for the first time. Crystals containing low concentrations of Fe57 and quenched from high temperatures show spectra consisting of symmetric quadrupole split doublets indicative of iron ions in a divalent charge state. The absorber spectra show marked differences with the corresponding source spectra of LiF:Co57 and NaF: Co57. Aftereffects of the decay of Co57 are believed to be responsible for the difference in source and absorber spectra.
Induced absorption due to exciton‐biexciton transitions in CuCl
Authors: Bivas A. , Phach Vu Duy , Hönerlage B. , Grun J. B.
Published: July 2007

The induced absorption due to exciton‐biexciton transitions is observed in CuCl with an experimental set‐up using two dye lasers. The shape of the absorption band is studied. The oscillator strength per exciton of this absorption is estimated and compared to theoretical evaluations. The excitation spectrum of this induced absorption is also measured and its features are tentatively explained by the existence of a superficial layer on the samples, where excitons have a very short lifetime.
Theory of excitons in zincblende‐structure semiconductors in intermediate magnetic fields
Authors: Ekardt W.
Published: July 2007

A simple numerical method is presented for the calculation of excitonic spectra in real semiconductors like GaAs in intermediate magnetic fields. A proper set of intermediate field functions including nonlinear variational parameters is used for the numerical diagonalization of the exciton matrix with inclusion of the electron‐hole exchange interaction. By the calculation of both the transverse and “longitudinal” levels, a basis is developed for a comparison between theory and experimentally observed splitting pattern in magneto‐reflectivity measurements. A good overall agreement is obtained between theory and experiment in case of GaAs and InP in a region of the magnetic field strength not acceissible up to now neither with low‐field methods nor with the usually adopted adiabatic method in the high‐field case.
On the quantum theory of anomalous electromagnetic fluctuations in electron–phonon systems
Authors: van Trong Nguyen , Tan Ha Vinh
Published: July 2007

The fluctuations in electron‐phonon systems in the presence of a long‐wavelength electromagnetic radiation field are investigated. For obtaining the correlation functions of electromagnetic fluctuations a microscopic formalism is proposed in the framework of the random phase approximation. It is shown that the level of collective fluctuations increases anomalously when approaching the borderline of the parametric instability region.
Dispersion relations of elementary excitations when damping is present
Authors: Ritchie R. H. , Hamm R. N. , Williams M. W. , Arakawa E. T.
Published: July 2007

Dispersion relations of elementary excitations in solids when appreciable damping occurs are discussed. It is shown that in order to establish a dispersion relation from experiment it is necessary to specify how the experiment was performed; different experiments may lead to different dispersion relations even when a single kind of elementary excitation is involved. “Momentum‐fixed” dispersion relations and “energy‐fixed” dispersion relations are discussed for the surface plasmon at a single isolated metal‐vacuum interface and for the surface plasmon at the metal‐vacuum interface of a thin metal foil bounded by a transparent dielectric on one side and by vacuum on the other.
Relations between vacancy migration and formation energies, debye temperature and melting point
Authors: Ben‐Abraham S. I. , Rabinovitch A. , Pelleg J.
Published: July 2007

The interaction between a vacancy and the lattice vibrations is represented as a collision between a sine‐Gordon soliton and a breather. The migration to formation energy ratio of a vacancy is found to be ϵ = 2 tgh (π/8≈), where ϵ is a relaxation radius. From general considerations 0.6 < ϵ < 1.2, whence ϵ ≈ 0.91 ± 0.25 modified by crystal structure. The relationship for ϵ implies that both the energies of migration and formation are simply related to the Debye temperature. Through Lindemann's melting formula they are then proportional to the melting point.
Orbital–charge ordering in crystals
Authors: Ionova G. V. , Ionov S. P. , Pachev O. M. , Makarov E. F.
Published: July 2007

The orbital and orbital–charge ordering in crystals with doubly degenerate or quasi‐degenerate orbitals on a centre are considered. A Hamiltonian including the inter‐centre kinetic energy and the intra‐centre and inter‐centre Coulomb interactions is used. It is shown that the transition to the ordered electronic state is possible in these systems, and criteria for the occurrence of these states are obtained.
X‐ray induced luminescence in NaCl: Ca++
Authors: Aguilar M. , Jaque F. , Agulló‐López F.
Published: July 2007

An investigation of the X‐ray induced luminescence in pure and Ca‐doped NaCl crystals is made in the range from liquid nitrogen temperature (LNT) to 200 K. At LNT a band at 415 nm is observed in the doped samples in addition to the typical σ and π emissions. It does not appear to be a perturbed “pure crystal” emission but is associted to a different intrinsic transition of the VK + e system. At higher temperatures the LNT emissions become quenched and two bands at 310 and 330 nm are observed in pure as well as doped crystals. They markedly decrease on increasing Ca‐concentration and on quenching the doped samples. These bands appear to involve electron transitions to excited VK centres.
The diagonal vertex approximation for gaussian random systems as a further test of the CPA
Authors: Fischbeck H. J.
Published: July 2007

The diagonal vertex approximation for Gaussian random systems is defined. It can considered to be the best c‐number approximation of the self‐energy of such systems. Numerical computations of the density of states of the 1D Anderson model in this approximation agree within about one per thousand with the CPA.
An investigation of magnetic ordering in GdxCe1−xRu2 by 155Gd mössbauer spectroscopy
Authors: Ruebenbauer K. , Fink J. , Schmidt H. , Czjzek G. , Tomala K.
Published: July 2007

Hyperfine interactions at 155Gd nuclei in alloys GdxCe1−xRu2 with 0.05 ≦ x ≦ 0.5 are investigated by Mössbauer spectroscopy. For x ≧ 0.11 a linear variation of the magnetic ordering temperature TM with x is found. The temperature dependence of the magnetic hyperfine fields below TM indicates a gradual reduction of the extension of ferromagnetic correlations with decreasing Gd concentration. For dilute alloys (0.11 ≦ x ≦ 0.18), the temperature and field dependence of the hyperfine field is characterized by nearly critical behaviour over a large range of temperatures.
Optical properties of polymorphous thallous halide mixed crystal layers
Authors: Heidrich K. , Schäfer W. , Schreiber M. , Trendel G. , Treusch J.
Published: July 2007

Optical properties of TlClxBr1‐x and TlBrxI1−x mixed crystal layers in s.c. and f.c.c. structure are described. An interpretation of the nonlinear shift of the gap energy with composition is given in terms of an empirical LCAO band model.
On the validity of using the classical elastic line force solution in lattice dislocation calculations
Authors: Barnett D. M.
Published: July 2007

In computer simulations involving dislocations in discrete crystal lattices the use of so‐called flexible boundary conditions requires the elasticity solution for the displacement field due to point forces periodically distributed along a line. It is, of course, much easier to approximate this field as that due to the available classical solution for an infinite line of uniformly distributed forces. The validity of this approximation is investigated for an isotropic medium and it is shown that the approximation is a good one (to terms of order exp [−2πr/b] where r is the normal distance from the line of force and b is the lattice repeat distance along the line of force) even at distances on the order of one lattice repeat distance from the line of force. Anisotropy is not expected to significantly affect this result.
Covalency and hyperfine interaction on the impurity Ion in crystals
Authors: Muravev V. I. , Yunusov N. B.
Published: July 2007

The change of the hyperfine interaction constant A of impurity ions in crystals is considered as resulting from the exchange polarization of the bonding molecular orbitals, containing the 4S‐orbital of the impurity and the s‐ and p‐orbitals of ligands. Apart from terms considering the 3d‐electron covalency of the ground state and the 4S‐orbital of the metal ion the calculated unpaired spin density contains the contribution from ligands, by means of which the dependence of A on covalency for 3d transition metal ions in various hosts is explained.
Resonant secondary radiation in polar semiconductors
Authors: Ivchenko E. L. , Lang I. G. , Pavlov S. T.
Published: July 2007

A theory of secondary radiation in semiconductors is presented. The general expression for the differential cross‐section is derived which in the corresponding limiting cases describes off‐resonant light scattering and luminescence and is valid in resonance conditions. The theory developed is applied to the analysis of two‐phonon resonant Raman scattering by longitudinal optical phonons. It is shown that in this case the secondary radiation contains an additional contribution arising from quantum interference, comparable with the main contribution. The additional contribution may lead to a peculiarity of the radiation angular distribution in the backscattering direction.
On the theory of photon‐induced DC hopping conductivity in disordered semiconductors
Authors: Keiper R. , Schuchardt R.
Published: July 2007

Based on the Kubo formalism a detailed discussion of the static hopping transport is given for the case that the electron transitions between localized gap states are caused by intense illumination of the specimen. The statistical correlations of the excited hopping system taking into account by means of the percolation theory, the resulting conductivities in the low‐frequency and highfrequency limits are analyzed with respect to their spectral dependences and their orders of magnitude in relation to Mott's phonon‐induced variable range hopping. Apart from its purely theoretical interest this new mechanism of photoconduction should also deserve experimental interest, in particular, because of the possibility of a frequency‐controlled „reading”︁ of the electronic density of states within the mobility gap.
On the mass dependence of the energy factor ΔK of the isotope effect for impurity diffusion in copper
Authors: Eckseler H. , Herzig Ch.
Published: July 2007

Using diffusion and isotope effect measurements of Au and Co in Cu a lower limit of the correlation factor of impurity diffusion as well as the corresponding limits of the kinetic energy factor ΔK are calculated from Manning's jump frequency model of vacancies near impurity atoms. The results show that a variation in impurity mass by a factor of three does not change ΔK for impurity diffusion significantly. For Au and Co in Cu ΔK is found to be about the same as for self‐diffusion in copper. This result allows to estimate the jump frequency ratios and the corresponding vacancy flow factors.
On the tight binding eigenvalues of finite S.C. and F.C.C. crystallites with (100) surfaces
Authors: Bílek O. , Kadura P.
Published: July 2007

The properties of small crystallities more or less deviate from those of infinite periodic crystals. An investigation is made of the size and habit dependence of one‐electron states of small crystallites in the Hückel approximation. Analytic formulae for s‐quasi‐bana electronic levels of finite s.c. and f.c.c. crystallites with (100) surfaces are presented. The symmtery classification of the levels for the f.c.c. crystallites possessing C4v point symmetry is also given.