bluuebunny/crossref_metadata_2025_split huggingface dataset:
Lattice dynamics of several A NB8–N compounds having the zincblende structure. II. Numerical calculations
Authors: Kunc K. , Balkanski M. , Nusimovici M. A. 
Published: July 2007

AbstractThe formalism of the deformable‐bond approximation, described in the preceding article of this series, is now applied to 15 compounds with zincblende structure. The phonon dispersion curves and densities of phonon spectra are calculated for ZnS, ZnSe, GaP, GaAs, InSb, SiC, GuCl, and CuI, the method developed for determination of 15 parameters characterizing the model is discussed. Analogous results obtained with an 11‐parameter version of the rigid ion model are presented for comparison.
Lattice dynamics of several ANB8–N compounds having the zincblende structure. III. Trends in model parameters
Authors: Kunc K. , Balkanski M. , Nusimovici M. A. 
Published: July 2007

AbstractThe parameters of the deformation‐dipole model which were determined in the preceding article of this series for 15 compounds with zincblende structure are now systematized into a regular scheme. It is found that short‐range forces between first neighbours vary as monotonic functions of interatomic distance and that static ionic charge varies linearly with Pauling's ionicity. The physical meaning of these regularities is discussed.
Bandwidth anisotropy in metallic ferromagnets
Authors: Eagles D. M. 
Published: July 2007

AbstractA simple model has been used to investigate what may be termed bandwidth anisotropy in ferromagnetic metals. This type of anisotropy arises in a tight binding model for energy bands when the bandwidths of the partial densities of states associated with particular atomic levels and a weighted mean width for occupied states depend on magnetisation direction. In the model used, the partial densities of states are assumed to be uniform over bandwidths centred on the atomic energy levels. General expressions for the bandwidth anisotropy subject to this assumption are found. Some insight into the dependence of bandwidths on energy overlap integrals is obtained by consideration of a linear chain system with two energy levels per atom. Very different dependencies are obtained according to whether atomic energy level separations are large or small compared with bandwidths. Plausible generalisations are made for the bandwidths in many level, three‐dimensional systems for the two cases. This allows simplifications to be made in the general expression for the bandwidth anisotropy energy in the case of relatively small atomic level separations.
Ion–ion potential and correlation functions in liquid Na
Authors: Cubiotti G. , Giuliano E. S. , Ruggeri R. 
Published: July 2007

AbstractFrom recent accurate data on the static structure factor S(q) for liquid Na, quantities as the pair correlation function g(r) and the direct correlation function c(r) are computed. An attempt is made to extract some informations on the pair potential φ(r), deduced from the experimental data on S(q) by means of Percus‐Yevick and hyperchain approximations. The potential so derived is compared to that one given by the model potential approach.
Vibrational spectrum of an isotopically disordered diatomic chain with long‐range order in the coherent potential approximation
Authors: Braeter H. , Priezzhev V. B. 
Published: July 2007

AbstractThe vibrational spectrum of a linear diatomic chain with long‐range order is calculated in the coherent potential approximation. Defect bands are formed inside the gap between the acoustic and optical bands and outside the bottom of the optical band. The energy gaps disappear long before the chain becomes completely disordered. The density of states calculated in the coherent potential approximation is compared with the exact vibrational spectrum of the Markov chain for the same values of the short‐range order parameter.
Absorption edges for cyclotron wave propagation in silver
Authors: Saermark K. , Lebech J. 
Published: July 2007

AbstractExperimental evidence for the existence of absorption edges for cyclotron waves propagating in metals, here silver, is given. The edges occur close to the ‘turning points’ of the infinite‐ωτ dispersion curve for (extraordinary) cyclotron waves. The experimental results are compared with numerical calculations based on a finite value of ωτ and on Halse's Fermi surface of silver. A physical mechanism accounting for the observations is presented.
Electric‐field‐induced gradient at the aluminium nucleus in corundum
Authors: Flyagin M. Ya. , Nikiforov A. E. , Men A. N. 
Published: July 2007

AbstractIn the ion shift model the influence of an external constant electric field on the quadrupole interaction of Al27 in Al2O3 is considered. The gradient changes induced by the electric field are calculated with overlap included. Four independent parameters are found defining the tensor of ion shift produced by the electric field. Using the found ion shift tensor an estimation of the metal‐ligand distance changes is performed. Tensor components of the dielectric constant are evaluated and agree with the measured values.
Experimental and theoretical effects of surface layers and spatial dispersion on the free exciton reflectance of ZnO
Authors: Lagois J. , Hümmer K. 
Published: July 2007

AbstractThe well‐known exciton reflectance spectra of ZnO at helium temperatures are changed drastically by surface layers. To explain the experimentally observed structure the reflectivity of a medium with harmonic oscillators (nonspatial dispersion) or with spatial dispersion and with an additional surface layer is calculated. The surface layer is either an intrinsic exciton‐free layer or an impurity layer in which multiple reflections and interference take place. From the comparison of experiment and calculation the following results are obtained: firstly, spatial dispersion of free excitons in ZnO has to be considered, secondly, the lineshapes of the closely spaced Al and Bl excitons are changed in a very different way, thirdly, the numerical evaluation yields the longitudinal resonance energies of the Bl and Cl excitons. By heavily doping a surface layer an enlarged exciton‐free layer could be produced which is demonstrated by the reflectance spectra.
Luminescence of high‐density excitons in CdS
Authors: Liebing H. , Mengel P. , Ruppel W. 
Published: July 2007

AbstractThe luminescence emission of CdS single crystals is investigated in the exciton spectral region for high one‐photon and two‐photon excitation at 1.8 K. The inelastic scattering of two excitons (P‐line) is resolved into three distinct lines, corresponding to the transition of one exciton from the 1s state into the states 2p, 3p, and n = ∞. The latter transition leads to photoconduction. The lines M and PM, discovered by Shionoya et al. and ascribed by him to the recombination and inelastic scattering of exciton molecules, are interpreted as Stokes acoustical phonon wings of the exciton–impurity complex lines I2 and I1. The M and PM‐lines are stimulated at high intensities. The onset of several laser modes is observed for the PM‐line. The mechanism of this two‐boson (photon and acoustic phonon) laser is discussed.
Generalising the Anderson model
Authors: Carroll C. E. , Lukes T. , Ringwood G. A. 
Published: July 2007

AbstractAn attempt is made to generalise the Anderson approach to localisation in order to handle other forms of disorder. After some discussion an approximate criterion for determining the mobility edge is adopted because it is simple and intuitive. The theory is applied to positional disorder at negative energies using a model derived from the Lloyd matrix expansion. The criterion is used on this model to compare localisation in s‐and p‐states.
On the energy band structure and the phase transitions in a cuprous chloride crystal
Authors: Rusakov A. P. 
Published: July 2007

AbstractTo explain the break‐down of the Lyddane‐Sachs‐Teller (LST) relation and the compressibility anomaly for CuCl a new energy band model is proposed to include a small indirect gap comparable to the binding energy of direct excitons. It is found for such a model that the condensation of excitons can result in the break‐down of the LST relation as well as a number of other anomalies of properties, i.e. CuCl is a high‐temperature excitonic dielectric with an effective mass ratio mh/mc ≈ 102 of electrons and holes. It is also shown that high pressure can induce a possible excitonic dielectric‐superconductor phase transition.
Photoconduction in CdS under high laser excitation
Authors: Mengel P. , Liebing H. , Ruppel W. 
Published: July 2007

AbstractThe photoconduction in CdS single crystals is investigated up to generation rates of 5 × 1028 cm−3 s−1 for one‐photon and two‐photon excitation by a giant pulse ruby laser. The initially generated hot carriers thermalize into excitons within about 10−13 s. At 1.8 K a strongly supralinear dependence of the photoconduction on excitation intensity (power 4 for two‐photon excitation and power 2 for one‐photon excitation) indicates that the free carriers observed by the photoconduction are generated by inelastic exciton–exciton scattering (P‐process). At higher temperatures (80 K, 300 K) the free electrons are due mainly to dissociation of the excitons. At generation rates higher than 1027 cm−3 s−1 up to the temperature of 80 K the condensation of the excitons into an electron‐hole plasma is observed. The phase transition is accompanied by a discontinuous increase in conductivity, strong fluctuations at the transition point, and a square‐root dependence of the photoconductivity at higher excitation intensities.
Pressure dependence of elastic constants and optical phonon frequencies in Se and Te. A study of the homological relationship between the vibrational modes in these materials
Authors: Fjeldly T. A. , Richter W. 
Published: July 2007

AbstractThe hydrostatic pressure coefficients for the elastic constants of Se and Te are shown to be large and positive in contrast to the negative mode Grüneisen parameters reported earlier for the optical vibrations. These results can be understood on the basis of the idea that the hydrostatic pressure promotes transfer of electrons from intrachain bonding orbitals to interchain bonding states. This strengthens the interchain “elastic” force constants while simultaneously weakening the intrachain “optic” force constants. The great similarity in the Se and Te lattices suggests that their vibrational spectra can be related by homology. Of the three lattice parameters describing each crystal, the intrachain bond angle is nearly identical and relatively pressure insensitive in the two lattices. Therefore, by a two‐step transformation in which the c/a‐ratio is first made equal by applying hydrostatic pressure, and secondly by properly scaling the mass and the unit cell, the two lattices can be made structurally identical. With this transformation and the available pressure data all zone centre phonons studied in the two materials could be related by a universal scaling factor for the “mode” force constants. Rough estimates for the higher‐order pressure coefficients can also be made.
Symmetries of the displacement field of a dislocation lying normal to a twofold axis
Authors: Blümel K. H. , Pongratz P. , Skalicky P. 
Published: July 2007

AbstractIt is shown that the displacements of a dislocation in an anisotropic crystal lying normal to a twofold axis and whose Burgers vector has components normal or parallel to it are even or odd functions in polar coordinates.
Thin film with anisotropically coupled internal layer defect. Localized states
Authors: Maciejewski W. , Duda A. 
Published: July 2007

AbstractThe model of a thin film with an internal anisotropically coupled layer defect is consider ed. Symmetric and antisymmetric defect coupling is assumed. An analytical method is proposed for the study of localized states. The conditions for the existence of acoustic and optical localized states are determined.
The cluster coherent potential approximation in the presence of off‐diagonal disorder
Authors: Herscovici C. 
Published: July 2007

AbstractOff‐diagonal randomness is included in the cluster CPA using the “matrix‐locator” approach, suitably generalized for the many‐site approximations. At the limit of vanishing off‐diagonal disorder, the usual cluster CPA is reobtained; it is also shown that in a special case of off‐diagonal randomness, important simplifications are possible.
Magnetoplasma effects in semiconducting Bi1–xSbx alloys
Authors: Oelgart G. , Herrmann R. 
Published: July 2007

AbstractThe magnetic‐field‐dependent microwave absorption is measured in semiconducting Bi1–xSbx alloys in the frequency ranges of 36 and 70 GHz at helium temperatures. In addition to cyclotron resonance, tilted orbit resonance ist observed. A theoretical lineshape analysis is carried out. The comparison with the experiment shows good agreement. Moreover the hybrid resonance and the dielectric anomaly are discussed and their influence on the magnetic‐field‐dependent microwave absorption in the present material is investigated. The results can in effect be understood on the basis of a model of the isoenergy surface, which is composed of three ellipsoidal parts. Moreover there are indications of deviation from this model.
The electric conductivity and its anisotropy in the quasi‐one‐dimensional conductor K2Pt(CN)4Br0.3 · xH2O at variation of the water content
Authors: Drosdziok S. , Engbrodt M. 
Published: July 2007

AbstractBy varying the external water vapor partial pressure all three of the water molecules of the quasi‐one‐dimensional conductor K2Pt(CN)4Br0.3 · 3 H2O can be extracted and added again. This has been measured by gravimetrical means. The electric conductivity and its anisotropy vary over more than four orders of magnitude. This strong dependence under laboratory atmosphere conditions gives an explanation for the variety of results reported in the literature.
Asymptotic approximations of non‐linear mechanics in the dynamical theory of X‐ray scattering
Authors: Tikhonova E. A. 
Published: July 2007

AbstractA dynamical scattering theory is developed in a crystal with distortions of second class. The method of movement separation is used to derive the equations determining the wave field in imperfect crystals. For a perfect crystal the obtained equations have an additional solution corresponding to an oscillation process of the overtone type with a frequency two times larger than the pendellösung one.
Energy–momentum relations in polaron problems with arbitrary coupling
Authors: Sheka V. I. , Khazan L. S. , Mozdor E. V. 
Published: July 2007

AbstractA theory is derived suitable to obtain the energy‐momentum dependence for an arbitrarily coupling polaron. Using a variational principle general expressions are given which describe this dependence when an arbitrary shape of the electron surface of constant energy is considered and the polarization field is approximated by any number of fictitious particles. This method is able to yield the lowest band of electrons interacting with any mode of lattice vibrations. In particular, numerical results for the energy, depending on the total momentum, are obtained for isotropic optical and piezoelectric polarons in a wide range of a dimensionless coupling constant. Comparison of the results with known ones (only weak and strong‐coupling limits were investigated earlier) shows, that the energy values presented lie lower, though they represent themselves an upper bound for the exact polaron energymomentum relation.
Electron Paramagnetic Resonance of Cr3+ in Chalcogenide Glasses
Authors: Chepeleva I. V. , Zhilinskaya E. A. , Lazukin V. N. , Chernov A. P. 
Published: July 2007

AbstractThe EPR spectra of Cr3+ ions in chalcogenide glasses As2Se3, yM (100 – y) As2Se3 (M = = Cu, Hg, Pb) and TlAsSe2(S2) are observed at X‐, K‐, and Q‐band. It is found that Cr3+ ions may incorporate into the glass network with surroundings forming intermediate or strong crystalline fields. Considerable fluctuations may occur in the coordinational spheres of the Cr3+ ions. The phase transition from the ferromagnetic (or ferrimagnetic) state into the antiferromagnetic one is observed for the Cr3+ spin system in microcrystalline inclusions of CuAsSe glass by EPR and magnetic susceptibility measurements.
Untersuchungen zur anomalen Absorption von Elektronen in Si in der Nähe von Zonenachsen
Authors: Hansen‐Schmidt J. 
Published: July 2007

AbstractDie Schwächung von Laue‐Reflexen aufgrund unelastischer Streuprozesse in Einkristallen wird im allgemeinen dadurch beschrieben, daß man zum reellen Kristallpotential V(r) ein imaginäres „Absorptionspotential”︁ iP(r) = i Ph exp (ihr) hinzufügt. Es wird die Frage untersucht, ob dieses Verfahren auch anwendbar ist bei Experimenten mit einer zweidimensionalen Anordnung von angeregten Reflexen. Experimentell ergibt sich eine Erhöhung der „Asorptionspotentiale”︁ Ph in der Nähe der Zonenachsen. Diese Abhängigkeit der Ph von der Kristallorientierung wird qualitativ durch Rechnungen bestätigt, denen die von Yoshioka eingeführten Parameter C zugrunde liegen. Absorptionsvorgänge können daher nicht mit Hilfe eines Potentials verstanden werden.
On a Structural Phase Transition in Ytterbium
Authors: Ausloos M. 
Published: July 2007

AbstractRecent data on the electrical resistivity and the normal Hall coefficient of ytterbium are used in order to study the temperature dependence of the density of states at the Fermi level, away from as well as in the vicinity of the h.c.p.‐f.c.c. structural phase transition. Two recent theories of the Hall coefficient are discussed when the density of states presents a dip near EF. In the vicinity of the structural phase transition, the influence of the band gap temperature dependence is discussed. The theory based on a model valid in rare‐earth metals predicts the temperature dependence of the Hall coefficient near the critical temperature. The theoretical results compare favourably with the experimental data, thereby giving some insight into the mechanism of the transition.
Effect of Polarization on Charge Carrier Transfer in Molecular Crystals
Authors: Petelenz P. 
Published: July 2007

AbstractThe influence of the polarization of the molecular orbitals of a neutral molecule by the electric field of an ion on the charge carrier transfer integrals is investigated. A simplified crystal structure is assumed and the calculations are performed for a dimer model of the neighbouring molecules. The polarization effects are calculated in a scheme which does not involve the polarizability concept. Some essential differences with previous results are discussed.
Phonons and Quasi‐Periodic Static Fluctuations
Authors: Falicov L. M. , Hsu W. Y. 
Published: July 2007

AbstractElectron wave‐packets moving in an external magnetic field experience a potential caused by quasi‐periodic static fluctuations which arise from electron‐lattice interactions. Such a potential is derived from first principles and the electron scattering probability is estimated. Zero‐point vibrations of the phonon modes are properly included. The importance and consequence of phase‐coherence are also emphasized. The calculation does not include effects due to electron‐electron interaction.
A New Type of Domain Wall NMR Spectrum in Canted Antiferromagnets: YFeO3 and LuFeO3 Crystals
Authors: Zalesskii A. V. , Zvezdin A. K. , Zheludev I. S. , Savvinov A. M. , Lebedev A. F. 
Published: July 2007

AbstractWithin the framework of the theory, specially developed for orthoferrites, the experimental results of NMR studies on Fe57 nuclei in domain walls of YFeO3 and LuFeO3 crystals are analysed. It is shown that the origin of two peaks in the wall NMR spectra for orthoferrites is quite different from that in a simple uniaxial ferromagnet and connected with a peculiar angular dependence of NMR frequency in a canted antiferromagnet. This analysis of wall NMR spectra is generally applicable to any other canted antiferromagnet with a similar magnetic structure.
X‐Ray Photoelectron Spectra of Quasi‐One‐Dimensional Conductors
Authors: Kaplunov M. G. , Shuwa Yu. M. , Pokhodnya K. I. , Borodko Yu. G. 
Published: July 2007

AbstractX‐ray photoelectron spectra of several quasi‐one‐dimensional conductors, complexes of tetracyanoquinodimethane (TCNQ) and conductive complexes of iridium, are investigated. Highly conductive TCNQ complexes are characterized by splitting of the nitrogen Is line into two components, and conductive iridium complexes by widening of the metal 4f‐line. This splitting (widening) is possibly due to a variation of charge density along conducting chains.
Calculations of Optical Transitions of the F Center
Authors: Sati R. 
Published: July 2007

AbstractAll the low‐lying complete wave functions for the F center have been obtained in the molecular orbital approach. These functions are further improved by taking into account the interaction between different configurations. It is shown that the ground state is 1A1 and the first excited state is lE. The calculated transition energies are in reasonable agreement with the observed R+ transitions.
Temperature Dependence of the Magnetization of the Ferromagnetic Spinels CdCr2Se4 and CuCr2S4 and Its Calculation in the Spin‐Wave Approximation
Authors: Samokhvalov A. A. , Morozov Yu. N. , Karpenko B. V. , Simonova M. I. 
Published: July 2007

AbstractThe temperature dependence of the magnetization of the ferromagnetic spinels CdCr2Se4 and CuCr2S4 measured in the temperature range of 4.2 to 300 K is described by the expression Δσ/σ0 = C1T3/2 + C2T5/2, where σ0, is the magnetization at T = 0 and C1 and C2 the coefficients determined by the least‐square method. Expressions for C1 and C2 are calculated in a conventional method of the spin‐wave approximation for a ferromagnetic spinel with magnetic atoms in octahedral sites and 36 neighbours up to the fourth coordination sphere are taken into account as well as two exchange parameters I1 and I2. From the experimental values of C1 and C2 the following data are derived: I1 = (15.85±0.05) K, I2 = (0.01 ± 0.02) K for CdCr2Se4 and I1 = (−140 ± 40) K, I2 = (35 ± 10) K for CuCr2S4.
X‐Ray Scattering from Electrons in Random Semiclassical Potential Wells
Authors: Esser B. 
Published: July 2007

AbstractThe scattering of X‐rays is calculated which arises from the electron density fluctuations, due to localization in a semiclassical random field. For an exponential autocorrelation function of the random field the scattering is expressed by the density of states at the Fermi level, the correlation length, and the mean square of the fluctuations of the random field potential.
On the Theory of Electroabsorption in Polar Crystals
Authors: Röseler J. , Henneberger K. , Strehlow R. 
Published: July 2007

AbstractThe present paper is a first step in looking for polaron effects by changing the optical interband absorption due to the presence of an external electric field. It is shown that the dielectric function can be expressed by a generalized one‐particle Green's function GF(k, t). A hierarchy of equations of motion is derived for GF(k, t) and a generalized Dyson equation is found. Finally, an expression for GF(k, t) is obtained, which allows the explicit calculation of the absorption coefficient in the neighbourhood of the absorption edge.
Overlap and Covalency Contribution to the Zero‐Field Splitting of S‐State Ions
Authors: Novák P. , Veltruský I. 
Published: July 2007

AbstractGeneral formulation of overlap and covalency effects for parameters D, E in the spin‐Hamiltonian of S‐state ions using the independent bond approach is developed. The model may be applied to complexes with both arbitrary coordination number and positions of ligands. All local, nonlocal, and distant terms of spin‐spin and spin‐orbit interactions are computed and used both for comparison with previous works on the Mn2+ ion with fluorine ligands and for discussion of the origin of the parameter D of octahedral Mn2+ and Fe2+ ions in garnets. Some of the errors and improper approximations appearing in various papers are discussed.