bluuebunny/crossref_metadata_2025_split huggingface dataset:
Exciton Model of Spin Excitations in Magnetic Dielectrics. Biaxial Antiferrodielectrics with One‐Ion Anisotropy
Authors: Ostrovskii V. S. , Petrov E. G. 
Published: July 2007

AbstractA generalized exciton model is used to investigate the acoustic and optical spin excitations of magnetic dielectrics in a magnetic field. Equations which determine the equilibrium spin configuration and which are valid for an arbitrary spin magnitude are derived. The frequencies and the polarization properties of the antiferromagnetic resonance (AFMR) for biaxial antiferrodielectrics with one‐ion anisotropy are considered in detail.
X‐Ray Study of PbxSn1−xTe
Authors: Bocchi C. , Ghezzi C. 
Published: July 2007

AbstractThe temperature and the (sin θ)/λ dependences of Bragg and diffuse scattering intensities have been measured in Pb0.63Sn0.37 Te powder samples. The X‐ray Debye temperature has been determined. Low temperature anomalies in the Debye‐Waller factor have been observed and are briefly discussed. An upper limit to the mean‐square static displacement of the atoms is established and possible causes for it are discussed.
The Influence of Low‐Frequency Oscillations on the Storage Time of Ultracold Neutrons The Relation to the Low‐Tebmperature Specific Heat Anomaly Of Disordered Solids
Authors: Ignatovich V. K. 
Published: July 2007

AbstractThe influence of the low‐frequency spectrum of solids on the heating of ultracold neutrons (UCN) per single interaction with the wall is reported. It is shown that the anomalously short storage time in traps can beexplained by UCN heating if there are clusters near the surface of the solid matter, vibrating at a frequency of 1010 Hz. The comparison withthe experimental data of the measurements of the heat capacities of disordered solids carried out at low temperature permits to make the conclusion that the clusters should be of about 70 to 60 Å. The influence on the UCN heating of other possible low‐frequency vibrations inside the solid matter is considered as well.
Vacancy Relaxation and Phonon Band Calculations for Solid Argon
Authors: Guse M. P. , Kunz A. B. 
Published: July 2007

AbstractThe relaxation about a vacancy in solid argon is calculated by approximating the crystal by a cluster of atoms and using the extended Hückel method to calculate the repulsive energy of the cluster while the Van der Waals attractive energy is added explicitly. The nearest neighbor atoms arc found to relax inward by about 0.06% of the original nearest neighbor distance. In addition the effect of a superlattice of vacancies on the phonon band structure of solid argon is calculated using the nearest neighbor Born‐von Kármán model.
Semi‐Empirical Calculations for Stacking Faults in Diamond‐Type Lattices
Authors: Weigel C. , Alexander H. , Corbett J. W. 
Published: July 2007

AbstractThe stacking fault energies, γ, for intrinsic, extrinsic, and twin faults in diamond and silicon are calculated by the extended Hückel theory. For both materials γint > γext > >γtwin. and γtwin ≈︁ γext/2. The number and distance of plane sequences containing boat‐shaped six‐membered atomic rings appear to determine the ordering of the γs. Charge distribution analyses allow intrinsic and extrinsic faults to be interpreted as superpositions of twin faults and suggest that lattice distortions occur in the vicinity of the faults.
The Luminescence Spectra of Bi3+ Ions in MgO and CaO
Authors: Hughes A. E. , Pells G. P. 
Published: July 2007

AbstractThe details of the luminescence spectra of Bi3+ in MgO and CaO have been examined over a wide range of temperature. It is found thatemission from both 3P0 and 3P1 excited states to the 1S0 ground state occurs, the former forbidden transition dominating at low temperatures and the latter at high temperatures. The bands can be resolved from one another, thus permitting the intensities and lifetimes of each transition to be followed. These measurements show that over a substantial temperature range the two excited states are in thermal equilibrium, and the decay characteristics are examined with theaid of a model developed in the Appendix. The radiative lifetimes of the 3PI and 3P0 states are found to be 4 × 10−8 and 2.9 × 10−3 s, respectivelyin MgO, and 2 × 10−9 and 3.8 × 10−3 s in CaO. With the aid of the discovery of the true zero‐phonon line for the 3P0 → 1S0 transition in CaO: Bi at 383.98 nm, the form of the vibronic series for this transitioncan be interpreted. The forbidden transition is made slightly allowed by a combination of a static perturbation and coupling to a vibrational mode which mixes the 3P0 and 3P1 states. This interpretation is consistent with the results of magnetic‐field enhancement of the zero‐phonon line intensity reported by Runciman et al.
Relaxation Time of Phonons Interacting with Charge Carriers in GaAs
Authors: Kravchenko A. F. , Magarill L. I. , Romanov A. A. , Semchukov N. F. , Skok E. M. 
Published: July 2007

AbstractA theory of transmitted phonon drag effect (TPDE) in a sandwich n‐i‐n‐structure (n is an electron layer and i a semiinsulating one of the semiconductor) is considered. The solution is performed on the basis of a kinetic equation system for electrons and phonons. The calculated results enable a good qualitative description of the main dependences of TPDE in GaAs. An analysis of experimental results shows that the transmitted phonons are dragged by electrons mainly through the deformation potential. The transmitted phonons are represented by long‐wavelength acoustic ones the relaxation of which is described by Herring's theory, the constant of the phonon‐phonon interaction B is equal to 2 × × 10−17 cm deg−3.
The critical voltage effect in transmission electron microscopy. V. The case of non‐centrosymmetric crystals
Authors: Serneels R. , David M. , Gevers R. 
Published: July 2007

AbstractThe critical voltage effect is discussed for the case of noncentrosymmetric crystals (systematic reflections only). An appropriate measure of the noncentrosymmetry is found in terms of the phase angles θg and θ2g of the lowest order extinction distances i.e. Δ = θ2g–2θg. When |Δ| ≦ 3° a critical voltage effect is still expected. For larger |Δ|‐values usually no critical voltage effect will occur. As an application the case is considered of interstitial alloys such as V2H, Nb2H, and Nb2O. For these alloys the critical voltage effect may be used to determine whether the H or O are in octahedral or tetrahedral interstitial positions.
Additional waves and Fabry‐Perot interference of photoexcitons (polaritons) in thin II–VI crystals
Authors: Kiselev V. A. , Razbirin B. S. , Uraltsev I. N. 
Published: July 2007

AbstractWithin the frame work of Pekar's model for a spatially dispersive, bounded medium, optical spectra of thin crystals, reported by us recently, are analyzed, showing a structure due to the interference of additional waves and the Fabry‐Perot structure deviating markedly from the one expected in the classical case in the neighbourhood of exciton resonances. A comparison of the experimental reflectivity with that obtained numerically provides values for all physical parameters defining dispersion and damping of the photoexcitons. Some additional experimental results for excitons in thin crystals are reported: evidence for the induced surface wave, an effect of the forbidden exciton, etc.
Strain‐modulated electron spin resonance of MgO:Fe2+
Authors: Henning J. C. M. , den Boef J. H. 
Published: July 2007

AbstractFlexural‐strain‐modulated ESR spectra of Fe2+ in MgO are reported. No first‐harmonic SMESR signal is detected. The angular dependence of the second‐harmonic signal is consistent with modulation of the tensor D by the periodic strain field. The magnitude of the effect is calibrated against the known width of the spin packets. The value of G11 = 680 cm−1 (±25%), deduced from this experiment, agrees well with literature data.
On the influence of a smooth gaussian random field on optical thresholds in disordered semiconductors and external electric field induced change
Authors: Esser B. , Kleinert P. 
Published: July 2007

AbstractThe interband dielectric function ϵ(ω) and its external field induced change Δϵ(ω, E) are calculated in the one‐electron approximation for optical transitions between initial and final states shifted randomly in space by a smooth Gaussian random field. All types of critical points are considered. Using an exact semi‐classical approach up to the order of h2 the influence of the smooth Gaussian random field in ϵ(ω) and Δϵ(ω, E) is shown to be representable as an averaging of the corresponding Franz‐Keldysh expressions. The M3 and M2 problems are shown to be reducible to the M0 and M1 ones. The plots for ϵ(ω) and Δϵ(ω. E) are presented for the isotropic M0 point and the M1 point with equal transversal masses.
Soft mode in thiourea by Raman measurements
Authors: Delahaigue A. , Khelifa B. , Jouve P. 
Published: July 2007

AbstractRaman measurements have been carried out on the soft mode of thiourea and the wave vector and temperature dependence of the soft mode frequency (ωS) are observed. The longitudinal (ωSL) and transverse (ωST) frequencies are used to derive a comparison between the dielectric constant obtained from the LST relation and that obtained from standard measurements of the capacitance.
Band structure and density of states in SiP2
Authors: Farberovich O. V. , Domashevskaya E. P. 
Published: July 2007

AbstractThe band structure of SiP2 is determined by the orthogonalized plane wave method. This compound is found to be a semimetal. The density of states of the valence electrons n(ϵ) is calculated by the linear interpolation method. The function n(ϵ) is compared with the experimental X‐ray emission L‐ and K‐bands of silicon and phosphorus in SiP2.
UV optical properties of lithium formate
Authors: Baldini G. , Cottini M. , Doglia S. , Grilli E. 
Published: July 2007

AbstractThe optical properties of single crystals of lithium formate monohydrate have been studied between 5 and 10 eV by means of absorption and reflection techniques. The data, which show two peaks at 7.2 and 8.4 eV, are interpreted in terms of n → σ and π → π transitions whose energy, strength, and polarization are consistent with the electronic properties of HCOOLi and H2O molecules.
Anomalies of sound propagation near orientational phase transitions in antiferromagnets
Authors: Mitsek A. I. , Kolmakova N. P. , Sirota D. I. 
Published: July 2007

AbstractThe sound velocity and attenuation anomalies, connected with the orientational phase transitions (OPT) of first and second order, are investigated for antiferromagnets and ferromagnets. For the first order OPT the anomalies are determined by the motion of the domain walls of the transitional domain structure, while for the second order one they are determined by the interaction of the sound wave and nonequilibrium deviations of the magnetization. The consideration of the influence of the magnetic subsystem on the sound propagation is reduced to the renormalization of the elastic constants. The wave equation and general expressions for the renormalization of the elastic constants, sound velocity, and attenuation are received, taking the magnetostriction and piezomagnetism into account. The comparison of the results received and available experimental data reveals the agreement of the theory and experiments.
Molecular Cluster Approach to Magnesium and Calcium Oxide Crystals. II. F+ and F Centres
Authors: Ermoshkin A. N. , Evarestov R. A. , Kotomin E. A. 
Published: July 2007

AbstractThe model of the molecular duster (MC) which has been applied for perfect MgO and CaO crystals is applied now to F+ and F centres in these crystals. A method is proposed to estimate the optical transition energies within the SCCC technique. The calculated values of the absorption maxima of the F+ and F centres are in reasonable agreement with experimental data. The procedure is also suggested to estimate the location of the defect levels within the forbidden gap of the perfect crystal. For comparison the analogous calculations of the ionic KCI crystal (perfect and with F centre) are given.
Modulation Spectroscopy on MoS2, and MoSe2
Authors: Meinhold H. , Weiser G. 
Published: July 2007

AbstractOn MoS2 and MoSe2 crystals electroreflectance, piezoreflectance, thermoreflectance, and wavelength modulated reflectance is measured. From the evaluation of the spectra and from the comparison with bandstructure calculations it is most likely that the first direct transition is at point P of the Brillouin‐zone between bands formed by the non‐bonding states of the Mo atom. The four excitons near the fundamental edge correspond to this transition split by spin‐orbit coupling and by interlayer interaction. From electroreflectance above the absorption edge it is concluded that the valence bands of MoS2 correlated to the covalent bonds are less than 700 meV below the first occupied bands.
The Relationship between Correlation Factor and Isotope Effect of Diffusion in Crystals
Authors: Mehrer H. , Seeger A. , Steiner E. 
Published: July 2007

AbstractFor crystals of orthorhombic or higher symmetry the sufficient conditions for the equality of correlation factor and isotope effect of tracer self‐ or impurity diffusion are critically discussed. For the most important one of these conditions, which concerns the dependence of the correlation factor on the tracer jump‐frequencies, necessary and sufficient formulations are derived covering the general case of more than one tracer jump frequency. This leads to a natural classification of diffusion mechanisms according to whether the correlation factor contains one or more tracer jump frequencies and whether the necessary and sufficient condition is fulfilled or not. The available calculations of correlation factors are assigned to the four cases resulting from this classification.
A Self‐Consistent Cluster Approximation for the Disordered Hubbard Model
Authors: Löser W. 
Published: July 2007

AbstractThe influence of disorder on electronic systems with intra‐atomic electron correlation is considered within a self‐consistent cluster approach. The disorder is represented by random site energies and hopping integrals in the Hubbard Hamiltonian. Configurational averaged properties of these disordered systems like reservoir coupling, local magnetic moments, and free energy per site are calculated for zero temperature and related to those in the ordered case.
Impurity‐Induced Raman Spectra of Potassium Halides
Authors: Siederdissen J. höner Zu 
Published: July 2007

AbstractImpurity‐induced Raman scattering spectra of monovalent (Cs+, Rb+, Tl+) and divalent (Pb++, Sr++, Ca++) centres in potassium halides are presented. In the caseof the divalent impurities different spectra are found for associations of one divalent impurity plus one vacancy and aggregates of such associations. Contrary to the monovalent defects these impurities cause a strong A1g spectrum. The Raman scattering cross sections of the Tl+ and Pb++ impurities are much stronger than those of the other defects (Cs+, Rb+, Sr++, Ca++).
Lattice Dynamics of ZnTe and CdTe
Authors: Plumelle P. , Vandevyver M. 
Published: July 2007

AbstractThe model used is a rigid‐ion model with an effective ionic charge including general interactions for nearest and next nearest neighbours and long range Coulomb interactions. It provides a good fit with recent neutron data for ZnTe and CdTe on condition that no hypothesis is made a priori on the interatomic forces and that the eleven parameters given by the model are used.
An Investigation of the Electronic Structure of GaSe and GaTe by Photoelectron Spectroscopy, Using a Synchrotron Source, and Electron Energy Loss Spectroscopy
Authors: Williams R. H. , McGovern I. T. , Murray R. B. , Howells M. 
Published: July 2007

AbstractThe valence band structure as well as the location of the Ga 3d levels have been investigated for vacuum cleaved GaSe and GaTe single crystals by photoelectron spectroscopy using a synchrotron source. In addition the conduction band structure has been probed by photoemission partial yield spectroscopy and by electron energy, loss spectroscopy. The experimental results are in reasonable agreement with theoretical predictions although calculated band widths are generally too narrow. A feature in the partial yield and electron loss spectra is tentatively interpreted in terms of surface states.
Anharmonic Effects in Heat Capacity and Saturation Vapour Pressure of Dilute Substitutional Alloys
Authors: Masharov S. I. 
Published: July 2007

AbstractThe contribution to the heat capacity and equilibrium pressure of the vapour over crystals due to the anharmonicity of atomic vibrations is calculated. It is shown that the anharmonicity changes the temperature dependence of the saturation pressure. This change is simply connected with the anharmonic addition to the heat capacity. In the high temperature region the vapour density over the crystal is increased several times due to the anharmonism. The corresponding temperature addition may be about 10%. The temperature dependences of the anharmonic contribution to the heat capacity at high and low temperatures are found. In the high‐temperature region the corresponding addition is linear in temperature. At low temperatures its behaviour is more complicated, especially in crystals containing heavy substitutional impurities an anomalous growth may be observed.
On the Theory of Tunnelling in Disordered Semiconductors
Authors: Belova I. M. 
Published: July 2007

AbstractTunnelling is considered in the metal‐insulator‐disordered semiconductor structure. Current carriers are supposed to tunnel from the metal to discrete deep levels in the gap of the disordered semiconductor and vice versa. The expression for the tunnel‐current density is obtained taking into account electron‐phonon interaction in the semiconductor. The conditions are investigated for the determination the density of states in disordered semiconductors from the tunnel‐current density.
In+vv Centres in a KCl‐In Crystal
Authors: Nagli L. 
Published: July 2007

AbstractA new emission centre (El = 2.43 eV) has been found in a KCl‐In crystal. It is produced by ionizing radiation at 350 K or after thermal destruction of In2+vc− centres. It is assumed that the centre in question is an In+ centre asociated with a bivacancy vv; (In+vv. The spectral, polarization, and kinetic characteristics of this centre are studied.
Molecular Cluster Approach to Magnesium and Calcium Oxide Crystals III. Charge Distribution Analysis of Some Hole Centres
Authors: Evarestov R. A. , Ermoshkm A. N. , Kotomin E. A. 
Published: July 2007

AbstractThe molecular cluster model is applied to the analysis of charge distribution and of symmetry of some hole centres (V−, V0, Li0, Na0), and also to impurity atoms of aluminium and to nitrogen ions in MgO. The calculations yield theoretical evidence of the models of hole centres derived from experimental data. Besides the essential effect of charge redistribution in the vicinity of impurity it is shown, e.g., the aluminium ion may be considered as triply charged only accounting for the charge of its nearest environment.
The Band Edge Excitons in 2HMoS2
Authors: Neville R. A. , Evans B. L. 
Published: July 2007

AbstractThe optical constantsof synthetic 2HMoS2 have been determined over the range 0.05 to 3.75 eV. The absorption edge consists of two series of exciton bands due to direct allowed transitions. The Zeeman splitting and diamagnetic shift of these exciton lines are measured together with the effects of crystal confinement and temperature; the derived exciton parameters are different from those of molybdenite. Oscillatory magneto‐absorption occurs at the high energy side of each exciton series which gives a measure of the carrier masses.
On the Electric Field Dependence of the Conductivity of Holes in Si at Low Temperatures 1. The Drift Velocity under Consideration of the Non‐Equipartition of Acoustic Phonon Energy
Authors: Von Borzeszkowski J. 
Published: July 2007

AbstractThe field dependence of the drift pelocity of holes in Si is discussed within a non‐parabolic valence band model. Taking into consideration the non‐equipartition of acoustic phonon energy both in a field‐dependent distribution function and in the relaxation time of holes, a satisfactory description of the experimental drift velocity can be given at low temperatures in a wide field range.
Fermi Surface and Helicons in Semiconducting Bi1−xSbx Alloys
Authors: Oelgart G. , Herrmann R. , Krüger H. , Stegmann R. 
Published: July 2007

AbstractIn semiconducting n‐and p‐type Bi1−xSbx alloys, the propagation of helicons is observed at 35.5 GHz and helium temperatures. Besides the propagation of helicons in the asymptotic limit, helicons are detected also under the influence of time dispersion. Under the influence of time dispersion, the anisotropy of the isoenergy surface of the charge carriers substantially influences the dispersion of helicons. On the basis of this effect we are able to gain information on the Fermi surface and to confirm results obtained from previous cyclotron resonance measurements. The influence of the anisotropy of the isoenergy surface is shown for magnetic fields parallel to special directions of the crystal lattice. This influence can, however, also be found in the anisotropy of the field values of the spatial resonances of the helicon waves.
Scattering of Light by Magnetoelastic Excitations in Ferromagnets
Authors: De Almeida J. R. L. , Rezende S. M. 
Published: July 2007

AbstractInelastic scattering of light by acoustic magnetoelastic excitations in ferromagnets is studied theoretically. The scattering cross‐section is obtained using the Green's function formalism. The radiationis considered to interact with the system both through the elastooptical and the magneto‐optical couplings. The contributions of the various mechanisms are analyzed in detail and estimates for cases of experimental interest are presented.
Annihilation of Incoherent Excitons in Molecular Crystals with Allowance for Their Interaction
Authors: Onipko A. I. 
Published: July 2007

AbstractThe dependence of the exciton annihilation rate on the dynamic interaction between excitons in molecular crystals is studied. The equation of motion for the two‐particle density matrix of incoherent excitons is used to obtain an expression for the annihilation rate constant in one‐, two‐, and three‐dimensional crystals. It is shown that the interaction between excitons may considerably affect the annihilation rate and its temperature dependence, and that the contribution of exciton‐exciton interaction to the annihilation process depends on the dimensionality of the system in which the excitons move.
Optical Properties of a Polydiacetylene Single Crystal Evidence for van Hove Singularities
Authors: Reimer B. , Baessler H. , Hesse J. , Weiser G. 
Published: July 2007

AbstractReflection spectra of polydiacetylene‐bis(toluene‐sulphonate) single crystals have been measured in the temperature range between 9 K and 300 K. By means of Kramers‐Kronig analysis the dielectric constants ε1 and ε2 and the absorption constant a have been derived. As they display van Hove singularities it is concluded that the optical absorption results from transitions between delocalized one‐dimensional states of the polymer chain forming broad bands. All absorption bands are split into doublets, the splitting energy decreases from 32 meV at 9 K to less than 10 meV at 300 K. Splitting is interpreted in terms of a threedimensional band model and finite intermolecular interaction.
Anisotropic Size Effect in Thin Aluminium Films
Authors: Sato H. , Yonemitsu K. 
Published: July 2007

AbstractA practical calculation of the anisotropic size effect of aluminum is made on the basis of the realistic 4‐OPW Fermi surface and the anisotropic relaxation time model. Measure‐ ments are done for three crystallographic orientations. The theoretical prediction is in good agreement with the experimental data.