bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy transfer by the normal modes in the exciton resonance region
Authors: Selkin A. 
Published: July 2007

AbstractFor 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

AbstractLie 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractRatios 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

AbstractThermodynamic 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

AbstractBy 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

AbstractThe 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

AbstractThe 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

AbstractPseudopotential 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

AbstractThe 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

AbstractPolycrystalline 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractDispersion 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

AbstractThe 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

AbstractThe 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

AbstractAn 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

AbstractThe 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

AbstractHyperfine 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

AbstractOptical 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

AbstractIn 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

AbstractThe 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.