bluuebunny/crossref_metadata_2025_split huggingface dataset:
Piezoemission Study of I1‐and I2‐Type Bound Excitons in CdS
Authors: Goede O. , Blaschke M. , Klohs K. H. 
Published: July 2007

AbstractDeformation effects for I1‐ and I‐type bound excitons in CdS are calculated in the framework of an effective‐mass approximation using the Bir‐Pikus Hamiltonian for wurtzitetype crystals and taking into account exchange and relativistic hole‐hole interactions. By piezoemission measurements at 4.2 K for uniaxial stress, σ ∥ c and σ ⊥ c, significant differences are obtained for the stress‐induced energy shift and polarization of the two types of bound excitons. From the comparison with the corresponding calculated results the electron‐hole exchange and the relativistic hole‐hole interaction parameters for the I1 bound exciton are determined. The occurrence of a low‐energy emission line which is not observed until now, the expected stress dependence of this line, and fine‐structure effects for the I1, bound exciton are discussed.
Exciton Binding Energies in Polar Crystals
Authors: Hattori K. 
Published: July 2007

AbstractThe ground state of Wannier excitons in polar crystals, such as TlCl, TlBr, GaAs, and II‐VI compounds, is investigated. The exciton‐phonon coupling is taken into account by the Fröhlich‐type interaction. Exciton binding energies are evaluated by a combination of a variational method and the second‐order perturbation theory. The theoretical results are compared with those obtained experimentally. A good agreement between theory and experiment is found.
Logitudinal Anisotropy of Conductivity of n‐Si in Heating Electric Fields at Low Temperatures
Authors: Papunashvili K. I. , Frolova E. K. 
Published: July 2007

AbstractThe longitudinal anisotropy of the conductivity in n‐Si is studied in heating electric fields a t temperatures ranging from 35 to 77 K. The data concerning the dynamics of electron redistribution between the equivalent valleys with the temperature and electric field are reported. It is shown by extrapolation of experimental results to lower temperatures that a t these temperatures a homogeneous sample must exhibit NDC due to the electron repopulation.
Spectroscopic Investigations of Magnons in Weak Ferromagnetic RbMnCl3
Authors: Belyaev A. I. , Kotlyarskii M. M. 
Published: July 2007

AbstractThe temperature dependence of the RbMnCl3 absorption spectrum is investigated in the region of the C and D group bands. The detailed analysis of the fine structure allows to distinguish two characteristic intervals, 75 and 46 cm−1, corresponding to the excitation of optical and acoustic modes in the spin‐wave spectrum of RbMnCl3. In general the spectrum structure is analogous to that of isostructural CsMnF3.
Jahn‐Teller Coupling Parameters from Stress‐Dichroism Spectra for CdS:Ti2+ and CdS:Co2+
Authors: Ruszczynski G. , Boyn R. 
Published: July 2007

AbstractStress‐induced dichroism spectra are measured for the mA2→mT1(mF) and mA2→mT1(mP) transitions of CdS:Ti2+ (m = 3) and CdS:Co2+ (m = 4), at 80 K, for stress parallel and perpendicular to the hexagonal axis. The stress‐induced changes of the first moments of the absorption bands are discussed on the basis of a tetrahedral cluster model. Coupling parameters to ϵ‐ and τ2‐type distortions are estimated, and comparable values are obtained for these two symmetries in all cases. The coupling behaviour of “corresponding” levels of the two impurity systems turns out to be rather similar. The data obtained are consistent with an earlier quantitative interpretation of the absorption band shapes for CdS:Ti2+. It is found that a point‐ion model can give the correct sign of the coupling parameters, but leads to wrong results with respect to their (relative) magnitudes.
The Thermodynamic Behaviour of Charge‐Ordered Crystals
Authors: Lubimov V. S. , Manakova L. A. , Makarov E. F. , Ionov S. P. 
Published: July 2007

AbstractThe influence is considered of the low lying collective excitations upon thermodynamic behaviour of charge‐ordered states. The collective mode is found which provides the instability of this state relative to the Mott state when intra‐(I) and intersite (V) interactions of electrons correspond to a repulsion. If I < 0 and is stronger than the intersite repulsion, the collective excitations correspond to the motion of electron pairs. The energy and the specific heat are calculated and the phase diagram is qualitatively discussed as function of the I to V‐ratio.
The Soft X‐Ray Appearance Potential Spectra of Diamond and Graphite
Authors: Lee A. E. , Webster J. 
Published: July 2007

AbstractThe soft X‐ray appearance potential spectroscopic technique has been applied to diamond and graphite. The spectrum obtained from diamond is found to be similar to those obtained from other forms of carbon but to contain important differences. From the differences it is inferred that a graphitic over layer is not present. The similarities in the spectra of diamond and graphite suggest that the multiplicity of peaks observed in spectra from carbon is not due to plasmon effects. Agreement between the observed diamond spectrum and that obtained from a density of electron states model is only possible if an empty surface state band is postulated existing between the valence and lower conduction bands. The spectra of both diamond and graphite are effected by the evaporation of tungsten from the thermionic cathode. The contamination is particularly difficult to observe as the technique is shown to be relatively insensitive to evaporated tungsten films.
Bound States of Fast Electrons in Planar Channeling
Authors: Kaplin V. V. , Popov D. E. , Vorobev S. A. 
Published: July 2007

AbstractA detailed treatment in the classical and quantum‐mechanical approximations of the bound motion of energetic electrons with (011) atomic planes of Si is given. For electrons with energies up to 2.5 MeV only a few resolved quantized states are obtained. The angular distributions of electrons in the ground and exited states are calculated and compared with experimental data. It is shown that electrons are effectively channeled through the crystal along the atomic planes, thereby being mainly in an excited state.
Low and very high magnetic field dependence of the magnetization and the Faraday rotation induced by the magnetic dipole transitions in YIG
Authors: Guillot M. , Le Gall H. 
Published: July 2007

AbstractFrom Faraday rotation experiment performed at 1.15 and 0.6328 μm wavelength under low and very high magnetic fields up to 4 MOe, the magnetic field dependences are analyzed of the magnetization and the Faraday rotation induced by the magnetic dipole transitions in both the octa‐ and tetrahedral sites of YIG. The best agreement between the magneto‐optical data under intense fields and a molecular field model iniplies a choice of the exchange parameters where the intra‐sublattice interactions must be neglected. The magnetic field dependence of the sublattices magnetization, calculated from this molecular field model follows a paramagnetic behaviour up to a spin‐flop critical field as observed in our experiment near 1.7 MOe. It is shown that the Faraday rotation induced by the magnetic dipole transitions is described from two terms having a magnetic field increase of opposite sign. The first term is the usual “magnetic” Faraday rotation proportional to the total magnetization and the second one, having a dispersive character, has a quadratic dependence on the effective field defined by the applied and exchange fields.
Transport theory of ultrasonic attenuation
Authors: Sorbello R. S. 
Published: July 2007

AbstractThe transport theory of ultrasonic attenuation is critically examined. The Pippard expression for arbitrary band structure is derived from the traditional transport approach without introducing Pippard's fictitious forces. The validity of using the relaxation‐time approximation in the transport equation is investigated, and the analysis is extended beyond this approximation. It is pointed out that for the general collision integral the relaxation time appearing in Pippard's theory may have an imaginary part. Pippard's expression for the attenuation is shown to be valid only when the relaxation time is real, and this will occur when the mean‐free path is either much less or much greater than the wavelength of sound, and in the case of “quasi‐catastrophic” scattering. Finally, some inconsistencies in the literature are pointed out.
Steady‐state diffusion of point defects in the interaction force field
Authors: Yoo M. H. , Butler W. H. 
Published: July 2007

AbstractThe diffusion equation including the drift term due to the long‐range interaction between a point defect and a spherical sink is solved numerically for the steady‐state concentration of point defects. The effective capture radius of a sink is calculated from the flux integral of point defects, and the biased flux of interstitials over vacancies to an infinitesimal dislocation loop is obtained from the effective capture radii. Effects of temperature, point defect parameters, and elastic properties of the medium on the kinetics of vacancy‐inter‐stitial recombination and of dislocation loop growth are discussed.
Density of states of layer compounds. 2H‐NbSe2
Authors: Riccò B. 
Published: July 2007

AbstractThe density of states of the transition‐metal layer compound 2H‐NbSe2, is calculated for both valence and conduction bands. The solid is studied by means of two different oneelectron models, one where the sandwiches of the structure interact with each other and one where instead each sandwich is treated as isolated. The densities of states given by the two models are compared with that determined by means of experiments of photoemission and transmission of synchrotron radiation. That obtained considering an isolated sandwich agrees better with the experimental curves. This seems to indicate that the oneelectron picture is inadequate to describe the intersandwich interactions in layered structures and therefore that the kz dependence of the calculated band structure E(k) should not be looked at as really reliable.
The influence of phonons and impurities on the broadening of excitonic spectra in gallium arsenide
Authors: Alperovich V. L. , Zaletin V. M. , Kravchenko A. F. , Terekhov A. S. 
Published: July 2007

AbstractThe absorption spectra near the fundamental edge of gallium arsenide are measured in the temperature range of 4.2 to 500 K on samples with concentration of charged impurities from 1014 to 1019 cm−3. The description of the experiment by phenomenologically broadened theoretical spectra enables to determine the broadening parameter as a function of temperature and impurity concentration. The comparison of the results obtained with microscopic theories shows the interactions of excitons with longitudinal optical phonons and charged impurities to be the main broadening mechanisms. A semiempirical equation is obtained which permits to calculate the broadening parameter in gallium arsenide for a given temperature and impurity concentration.
Electric field induced changes of the fundamental absorption and low‐temperature energy relaxation in n‐type GaSb
Authors: Obiditsch M. , Kahlert H. 
Published: July 2007

AbstractThe dependence of the absorption coefficient α on the photon energy is investigated for different degenerately doped samples of n‐type GaSb in the region of the fundamental absorption. A comparison of the experimental data with the theory by Dyakonov et al. shows satisfactory agreement for α > 500 cm−1. The application of pulsed electric fields at a lattice temperature of 6 K gradually removes the Burstein‐Moss shift. A comparison of the experimentally obtained absorption edges for various applied electric fields to calculated curves allows to correlate energy loss rates to certain values of the electron temperature Te. A calculation of this dependence considering acoustical and polar optical phonon scattering reveals that single phonon scattering processes are not capable of explaining the data between 12 and 40 K. Following the treatment by Ngai a calculation of the energy loss rate for two‐phonon deformation potential scattering is performed. The temperature dependence of this mechanism is in good agreement with the experimental values thus allowing to deduce a value of the two‐phonon deformation potential D2TA ≈ 5 × 103 eV.
Low‐temperature diffusion of electrons in ice evidence for polaron bands
Authors: Prince R. H. 
Published: July 2007

AbstractThe hopping activation energy for localized polaron states in ice is measured as (0.065 ± 0.005) eV. Below a transition temperature of ≈ 125 K, polaron band motion is observed with a temperature dependence in accordance with the density of optical phonons effective for scattering, with θ1 = 209 K in agreement with IR absorption determinations. Such observations lend support to the hypothesis of mobile electrons as precursor to fluorescence and thermoluminescence in ice.
Electron interactions and the spin‐wave stiffness constant in ferromagnetic two‐component alloys
Authors: Poo G. S. , Edwards D. M. 
Published: July 2007

AbstractElectron interaction effects and the spin‐wave stiffness constant of an ordered ferromagnetic binary alloy are studied. A general condition for ferromagnetism is obtained for the system. Within a certain approximation, an explicit expression for the spin‐wave stiffness constant is derived in the limit of very weak ferromagnetism.
Exciton interactions in naphthalene single crystals
Authors: Fujiwara S. , Nakayama T. , Itoh N. 
Published: July 2007

AbstractThe decay of the fluorescence and T—T absorption change in naphthalene crystal induced by an intense electron pulse is measured and analyzed. It is shown from the optical absorption measurement that the decay of the triplet excitons follows bimolecular reaction kinetics. The decay of the fluorescence is found to be very much dependent on the intensity of the electron pulse. The result is analyzed with reaction kinetics comprising the singlet—singlet, singlet—triplet, and triplet—triplet annihilation processes. The values of annihilation rate constants are obtained at 77, 200, and 298 K. The singlet—triplet annihilation rate constant is calculated under Förster's long‐range resonance transfer scheme by employing the results of T—T absorption of naphthalene crystal. Good agreement is obtained for γST. It is demonstrated that the annihilation of the singlet exciton is diffusion controlled.
Structure and generation mechanism of Br centres in X‐rayed KBr
Authors: Elango A. A. , Nurakhmetov T. N. 
Published: July 2007

AbstractThe structure of V4 and D3 centres in KBr and KBr:Sr crystals X‐rayed at 80 K is studied by optical methods. The molecular ion Br situated in two anion and one cation lattice sites is shown to be the basic entity of the structure of these centres. The creation mechanism of such Br centres via interaction of H centres is discussed. It is assumed that the interaction of two H centres leads to the local reconstruction of the lattice. The occurrence of one particular process of this kind resulting in a new anion Frenkel pair near a cation vacancy is demonstrated.
Electron transport phenomena in narrow‐ and zero‐gap semiconductors containing magnetic impurities
Authors: Kossut J. 
Published: July 2007

AbstractVarious transport phenomena are studied which are due to scattering of electrons in narrow‐ and zero‐gap semiconductors by magnetic impurities, namely manganese impurities. The formulae for the relaxation time corresponding to this scattering mechanism are derived and the mobility and thermoelectric power are discussed. Numerical estimations for manganese‐doped mercury telluride are presented. They show that the contribution of magnetic impurity scattering to the electron mobility, although smaller than that of ionized centre scattering, cannot be neglected at low temperatures. The thermoelectric power seems to be strongly influenced by the presence of the considered scattering mechanism. The dependence of transport coefficients on the range of the scattering potential is studied. It is particularly pronounced in the case of an inverted band structure at low electron concentrations.
X‐ray compton scattering from LiMg alloys
Authors: Berndt K. , Brümmer O. 
Published: July 2007

AbstractResults are given of X‐ray Compton scattering from a series of LiMg alloys, polycrystalline Li, and Mg. The spectra indicate a contact of the “Fermi surface” of the alloy with the [110]‐Brillouin zone boundary at 30 to 40 at % Mg. LiMg seems to be a good, nearly free electron alloy. Theoretical calculations using a model alloy potential are made and the calculated momentum distributions are compared with the experimetnal results.
Thermal effects in magnetic exchange interaction
Authors: Shrivastava K. N. 
Published: July 2007

AbstractThe temperature dependence in the exchange interaction –2JSi · Sj in ferromagnetic K2CuCl4 · 2 H2O is interpreted to arise from the matrix elements of the phonon‐modulated nuclear potential. It is found that the temperature dependent effect is so large that at a particular temperature, called the null point, the phonon‐induced exchange cancels the static Heisenberg exchange. The contributions of anharmonicity are also estimated and correlations of exchange with interatomic distance and dependence on ionic size are established.
Energy transfer by excitons in a tetracene‐doped anthracene crystal
Authors: Golubov S. I. , Konobeev Yu. V. 
Published: July 2007

AbstractA new interpretation is proposed of experimental data found by Powell on fluorescence decay of donor and acceptor molecules in binary tetracene‐anthracene solid solution. Intense direct excitation of acceptor molecules by X‐rays is assumed and a new exact relation between fluorescence decay laws of acceptor and donor molecules is derived. Using this relation it is shown that the experimental data may be satisfactorily described under the assumption that the direct impurity excitation efficiency is about four orders higher than for molecules of the host substance. Such a difference may be explained by the impurity large capture cross section of charge carriers produced by X‐rays.
Electromagnetic waves in an electron plasma in an alternating external magnetic field
Authors: Lugovoi V. N. 
Published: July 2007

AbstractThe generalization of the theory of plane waves in a semiconductor electron plasma located in an external magnetic field, H  H0  H1 cos pt, whose variable component is parallel to the stationary one, is given for the case of relatively low electron density in the conductivity band and for a sample inhomogeneous in the direction of the magnetic field, the waves propagating along this direction. Also the problem of the transformation of a monochromatic incident wave with a frequency co to components with new frequencies ω + np (n = ±1, ±2, …) for the wave reflected from the sample boundary and transmitted into the sample is considered. Several effects which are due to the simultaneous influence of the periodic non‐stationarity and a periodic in‐homogeneity of the medium are noted.
A Theoretical investigation of the specific heats and transport properties of the actinides
Authors: Long K. A. 
Published: July 2007

AbstractThe role of phonons, both optical and acoustic, is examined in the actinide metals. It is pointed out that the complicated crystal structures imply that optical phonon branches exist in the actinide metals. These branches lie at relatively low frequencies, low enough for them to contribute to the low temperature electrical resistively, and to other transport properties. In particular in plutonium it is proposed that there are two temperature regimes. The first below 100 K is that in which plutonium behaves like a normal transition metal, and where conduction is metallic in nature. In this regime the electrical resistivity is due to electron‐phonon scattering mainly by optical phonons. The second regime above 100 K is more akin to an amorphous or non‐crystalline material, than to a metal, due to the disorder caused by the lattice vibrations. In this regime conduction is by hopping of electrons from site to site. Here the interaction is so strong that plutonium can not be described even in a first approximation by a one electron band model.
The critical voltage effect in transmission electron microscopy. VII. The Influence of Absorption
Authors: David M. , Gevers R. , Serneels R. 
Published: July 2007

AbstractThe influence is studied of the absorption on the dynamical behaviour of a linear (2N + 1)‐beam system close to the second order Bragg orientation and around the critical voltage. When a small imaginary potential is introduced into the dynamical matrix for such situations, the eigenvalues and the eigenvectors of the latter are drastically altered. In particular the mechanism of symmetry exchange of the first two Bloch waves, and its consequences on the vanishing of the second order DF bend contour are examined. A qualitative effect of absorption on the behaviour of the bend contour with varying voltage, seems unlikely to be observable in most materials. However, one finds that absorptionless calculations may underestimate the critical voltage of heavy materials by an amount of the order of 50 kV.
Ultraviolet reflectivity spectra of CoCl2 and CoBr2 crystals
Authors: Pollini I. 
Published: July 2007

AbstractThe reflectivity spectra of CoCl2 and CoBr2 magnetic crystals in the 2 to 11eV region, at 300 and 80 K, are reported. The optical constants are derived by means of a Kramers‐Kronig analysis of their reflectivity data. It is suggested that peaks up to ≈ 8eV are due to electron transfer processes, in which halogen p‐ and cobalt d‐states are involved in localized excitations. The structures beyond 8eV are due to onset of band‐to‐band transitions, from p halogen to s metal states.
Isotope effects of diatomic CH and CD complexes in Ni single crystals
Authors: Kronmller H. , König N. , Walz F. 
Published: July 2007

AbstractThe magnetic relaxation spectrum of Ni single crystals containing different amounts of carbon interstitials is measured after charging with hydrogen or deuterium. A relaxation maximum observed after hydrogen charging at 133 K is attributed to the reorientation aftereffect of diatomic C‐H complexes of orthorhombic symmetry. Similarly after deuterium charging a relaxation maximum at 139 K is observed. The relaxation times of both these relaxation maxima show significant isotope effects of the activation energies as well as the pre‐exponential factors. These results are discussed within the framework of an harmonic oscillator model.
Phonon spectra of the laves phase intermetallic compound CaMg2
Authors: Eschrig H. , Feldmann K. , Hennig K. , Matz W. , Paufler P. 
Published: July 2007

AbstractA pseudopotential calculation of the phonon dispersion curves and the phonon density of states of CaMg2 is done using Shaw's model pseudopotential in second order. Moreover on a polycrystal‐line sample the phonon density of states is determined by coherent inelastic neutron scattering following the suggestion of Bredov et al. Good agreement between the calculated and measured results is obtained. The experimental accuracy is investigated by calculating the neutron scattering cross section from the theoretical phonon spectrum.