bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effective Reduction of the Unit Cell for the Interpretation of the Optical Properties of Monoclinic Molecular Crystals with Four Molecules in the Unit Cell: 1,5‐Dichloroanthracene
Authors: Flakus H. 
Published: July 2007

AbstractIt is shown that symmetry considerations allow to interpret the spectral properties of the 1,5‐dichloroanthracene molecular crystal which contains four molecules in the unit cell by using an effective optical unit cell with only two molecules. The numerical computations performed for the models of vibronic coupling in the crystals with respectively two and four molecules in the unit cell confirm the equivalence of the models. For the crystals having the Cj site symmetry group and four molecules in the unit cell of the monoclinic system the equivalence of considerations of those crystals is demonstrated.
Galvanomagnetic Properties of Bi2Se3with Free Carrier Densities below 5 × 1017 cm−3
Authors: Köhler H. , Fabbicius A. 
Published: July 2007

AbstractBi2Se3, Bi2Te3, and Sb2Te3 tend to grow non‐stoichiometric, the defects causing high free carrier densities. Here it was achieved, in the case of Bi2Se3, to produce crystals of n‐and p‐type conductivity showing concentrations down to 4 × 1016 cm−3. The temperature and magnetic field dependences of their galvanomagnetic properties are investigated. At low magnetic fields and low temperatures negative magnetoresistance occurs, at high magnetic fields no saturation of the positive magnetoresistance is observed which is assumed to originate from the influence of an impurity conduction band. The temperature dependence of the Hall constant enables the evaluation of the energy gap for thermal excitations to (160 ± 10) meV for T  0. The temperature dependence of the mobility between about 50 and 200 K supports the assumption of dominating scattering of the free carriers by acoustical phonons in this temperature range.
Anisotropy of Electroreflectance Due to Orbital Degeneracy of Energy Bands
Authors: Enderlein R. , Renner P. , Scheele M. 
Published: July 2007

AbstractA theory is developed for the electric field induced change, Δϵ, of the dielectric function for interband transitions between orbitallydegenerate bands like the E0−transition of tetra‐hedrally coordinated semiconductors. The strong dependence of oscillatorstrengths on the direction of quasimomentum and on the polarization of light for such transitions, together with the field influence on the quasimomentum direction results in strong polarization and field direction anisotropies of Δϵ which can explain the experimentally observed anisotropies of electroreflectance. Explicit results are given for the E0−transition and a valence band model with warping parameter C 0. The results are compared to those from an asymptotic approximation introduced by Keldysh et al. and by Aspnes. Lifetime broadening is taken into account and the first term of a power series for Δϵ with respect to the field strength is obtained. The low field expression of Δϵ consists of two contributions, the third‐derivative term being isotropic and a second term which depends on directions of light polarization and of field strength.
De Haas‐van Alphen‐Type Oscillations in the Interband Faraday Effect of Tellurium
Authors: Lutz M. , Bangert E. , Mizrah T. , Stolze H. 
Published: July 2007

AbstractMeasurements of the interband Faraday rotation were performed on samples with p  1014 to 1018 cm−3 in magnetic fields up to 6 T. Furthermore an oscillatory contribution to the interband rotation at 4 K in fields up to 15 T is observed. For a calculation of the interband rotation a band model is used which takes into account the strong coupling between the H4 and H5 valence bandsand the H61 and H62 conduction bands. The good agreement of theory and experiment gives evidence for the validity of the band model near point H of the Brillouin zone. The oscillations are explained in this model by the inversion asymmetry splitting of the H4 band.
Calculation of the Dislocation Damping Constant in NaCl
Authors: Paetsch B. 
Published: July 2007

AbstractThe temperature dependence of the dislocation damping constants B for NaCl is calculated, using Brailsford's theory of the anharmonic drag force. The calculation of B consists in the numerical evaluation of integrals over the first Brillouin zone (i.e. over the phonon coordinate space) for the energy dissipated in the dislocation motion. The integrand contains the generalised viscosity ξijkl(s, ω) as a factor, and the major part of the work is the evaluation of ξ.ξ itself is another integral ‐ a thermodynamic average ‐ over the Brill'ouin zone. Shell model calculations of the phonon dispersion curves and the mode Grüneisen parameters are used in evaluating these integrals. Separate calculations are made for the high temperature (T/θD > 1) and low temperature (T/θD)‐≪1) regimes. In each temperature regime the longitudinal and transverse phonon modes are assigned single lifetimes τL, τT. The temperature dependence of the lifetimes follows the theories of Herring (low temperature) and Klemens (high temperature), and the temperature coefficients are fitted to the experimental thermal conductivity data. Numerical results show that the optical modes contribute the major part of B. The calculated values are much larger than the experimental values; the agreement can be improved if smaller values of τL, τT are assumed than those which fit the thermal conductivity data.
Ultraviolet Photoemission Spectroscopy of Solid Nitrogen and Oxygen
Authors: Himpsel F.‐j. , Schwentner N. , Koch E. E. 
Published: July 2007

AbstractPhotoelectron energy distribution curves and constant final state spectra of polycrystalline films of solid N2 and O2 have been measured for photon energies 10 eV ≤ hω ≤ 30 eV by use of synchrotron radiation. Varying the photon energy and final state energy, respectively, the observed transitions originating from the valence bands could be attributed to the known molecular orbitals (MO's) of the gaseous samples. The binding energies of the various MO's are lowered by 1.5 eV for solid N2 and 1.6 eV for solid O2. This is explained by electronic relaxation via the dielectric polarization of the medium. Results on other nonpolar molecular crystals are included to check a simpleestimate of the polarization energy based on the Mott‐Littleton model.
Plasmon dispersion relation in zero‐gap semiconductors
Authors: Goettig S. 
Published: July 2007

AbstractFrequency‐dependent, long‐wavelength random‐phase approximation (RPA) polarizabilities of a n‐type zero‐gap semiconductor are calculated for parabolic and spherical bands. The long wavelength plasmon dispersion relation is established. The dispersion of plasmons is mainly determined by the wave vector dependence of the intraband (free‐electron‐gas type) polarizability. For high electron concentrations, due to Γ8–Γ8 interband polarizability, in the standard plasmon dispersion relation, the concentration‐dependent background dielectric constant has to be used. For low electron concentrations the frequency and wave vector dependence of the Γ8–Γ8 interband polarizability makes the above procedure inaccurate. A more accurate procedure is derived.
Non‐adiabatic calculation of the binding energy of the excitonic molecule ion
Authors: Munschy G. , Stébé B. 
Published: July 2007

AbstractA non‐adiabatic calculation is made of the binding energy of the excitonic molecule ion. In the limiting case of the hydrogen molecule ion good agreement is obtained with very accurate calculations. The results confirm the stability of the complex for all values of the electron‐to‐hole mass ratio.
Spectral representation of nonlinear susceptibilities of solids
Authors: van Hieu Nguyen 
Published: July 2007

AbstractThe analytical properties of nonlinear susceptibility of solids are investigated and its spectral representation is obtained. The nonlinear susceptibility is compared with the amplitudes of multiphoton processes. One‐dimensional dispersion relations and the threshold behaviour of the susceptibility are considered. On the basis of analyticity asymptotic relations are derived of nonlinear susceptibility at high frequencies.
Effective phonon energies in optical spectra of deep impurities in wurtzite‐type crystals
Authors: Boyn R. 
Published: July 2007

AbstractEffects of electron–phonon interaction optical absorption bands due to electronic transitions in deep substitutional impurities in wurtzite‐type crystals are discussed, assuming that the electronic states are coupled only to the displacements of the impurity and the nearest‐neighbour atoms forming a regular tetrahedron. The question is considered under what conditions such effects in experimental spectra may be analysed as for “pure cubic” systems. Referring to the second moment 〈E2〉 of an A → T absorption band it is shown that this should be possible if experimentally 〈E2〉 is found to be not significantly polarisation dependent. Expressions are given for effective frequencies of the tetrahedral cluster vibrations of the various symmetry types.
Polariton‐ and directional dispersion of polar modes in orthorhombic KNbO3 investigated by light scattering
Authors: Winter F. X. , Wiesendanger E. , Claus R. 
Published: July 2007

AbstractExperimental data on polaritons of type A1(TO), B1(TO), and B2(TO) in orthorhombic KNbO3 with wave vectors propagating in the xy‐, yz‐ and xz‐planes, respectively, have been compared with dispersion curves calculated on the basis of the phonon data. Data on mixed mode polaritons to type B1 + B2, A1, + B2, and A1 + B1 in the xy‐, yz‐and xz‐planes are shown to fit numerically calculated corresponding dispersion curves. Finally all directional dispersion branches of polaritons in the phonon limit (k → ∞) are calculated for the three symmetry planes. Experimental data on the 45°‐phonons are presented in order to check the theoretical curves.
EPR investigations on lithium manganate powders
Authors: Scharmann A. , Vitt B. , Hoppe R. , Meyer G. 
Published: July 2007

AbstractEPR powder measurements above 77 K on two forms each of Li3MnO4 and Li2MnO3 are reported. Here the systems Mn5+/Td and Mn4+/Oh are presented. The new spin‐resonance data are compared with those of analogous systems, the g‐shift is discussed. The analysis of lineshapes indicates that exchange effects for Li3MnO4 as well as for Li2MnO3 must be taken into account. Magnetic susceptibility measurements down to 4 K show antiferromagnetism for the hexagonal β‐Li2MnO3 below 50 K. Examining the temperature dependence of intensity and linewidth of the EPR signal one can identify this magnetic behaviour already above TN. The antiferromagnetic coupling between the Mn4+‐ions (3d3) is realized by superexchange through O2‐‐ligand orbitals.
On the instability of paramagnetism in the extended Hubbard model
Authors: Vedyaev A. V. , Kondorskii E. I. , Mizie E. 
Published: July 2007

AbstractThe Hubbard Hamiltonian, including the interatomic exchange terms, is studied in the framework of the alloy analogy. The static paramagnetic susceptibility of the system is calculated and the criterion for instability of paramagnetic state is obtained.
Computer simulation of the phase transition in ammonium halides
Authors: Trikha S. K. , Jain S. C. 
Published: July 2007

AbstractComputer simulation studies were made in ammonium chloride at low temperature. Using the Lennard‐Jones (6–12) potential as the representative interaction between NH and Cl−, the value of the rotational phase transition temperature is obtained which agrees with the experimental results of Schwartz.
Flux pinning by grain boundaries in anisotropic superconducting materials
Authors: Khanra B. C. 
Published: July 2007

AbstractFlux pinning by grain boundaries in anisotropic superconducting materials is studied by assuming that pinning comes through the local change in the Ginzburg‐Landau free energy. The experimental results of Kes and others on the Nb sample, containing grain boundaries as lattice defects, are explained by Labusch's theory. The results are discussed.
The determination of X‐ray dispersion corrections from measurements of X‐ray linear absorption coefficients
Authors: Creagh D. C. 
Published: July 2007

AbstractThe X‐ray linear absorption coefficients have been measured for the alkali halides LiF, NaF, and NaCl using characteristic radiations ranging from SnKα1 to CrKα1. From these measurements values of the imaginary part Δf″ and the real part Δf′ of the X‐ray dispersion correction have been deduced for each atomic species. A comparison is made with the values of μ/ϱ, Δf′, and Δf″, which have been calculated by Cromer and Liberman using relativistic quantum mechanics.
On the scattering of conduction electrons by magnetic impurities in semiconductors of InSb‐Type and HgTe‐Type band structure
Authors: Kossut J. 
Published: July 2007

AbstractThe relaxation time and the mobility are calculated for the case of the conduction electrons scattered by impurities possessing localized magnetic moments. The Kane model is used to describe the energy bands. The perturbing potential is assumed to be of a short range. The numerical estimations are made for two special forms of perturbation: delta‐like and square‐well form. The estimations show that the contribution of the considered mechanism to the mobility is small compared to that of other scattering mechanisms. However, there exist transport phenomena which seem to be strongly influenced by magnetic impurity scattering.
Quantum size effect in SnTe observed by tunnel experiments
Authors: Altwein M. , Finkenrath H. 
Published: July 2007

AbstractIn SnTe–Al2O3–Al tunnel junctions with size quantization of the electron states in the SnTe electrode, oscillations of the 2nd derivative of the I(U) characteristics were observed at 4 K. From this it was possible to determine a series of points of the E(k) relation perpendicular to the film plane in a range of ±100 meV around the Fermi energy. The results are in agreement with values calculated from band structure data.
The influence of local‐field corrections on the collective behaviour of the homogeneous electron gas
Authors: Brosens F. , Lemmens L. F. , Devreese J. T. 
Published: July 2007

AbstractThe dynamic structure factor of the electron gas is studied using different static local‐field corrections, and a comparison is made with the Hartree approximation. The plasmon dispersion relation is rather insensitive to the approximations used. However, the relative contributions of the plasmon branch and the pair creation region change substantially from one approximation to the other. This point is investigated by a Kramers‐Kronig analysis of the different approximations.
Optical spectra and local order in amorphous TlCl and TlBr
Authors: Heidrich K. , Kramer B. 
Published: July 2007

AbstractThe optical absorption spectra of amorphous TlCl and TlBr films are investigated in the photon energy range near the absorption edge. Annealing the films and tracing the spectra with annealing temperature discontinuous changes in the shape of the spectra are observed at two different annealing temperatures. A continuous random network model is presented for the structure of the amorphous TlCl and TlBr films, which is in agreement with recent structural data. The annealing dependence of the spectra is understood by assuming the existence of two disordered modifications with different short‐range order.
Modulated spectroscopy of zincblende semiconductors. Calculation of piezomodulation parameters for indirect semiconductors
Authors: Mathieu H. , Auvergne D. , Merle P. 
Published: July 2007

AbstractRelative modulation parameters are calculated for the absorption coefficient in the indirect absorption edge region for periodically stressed zincblende‐type semiconductors. The calculations are made for [001] and [111] uniaxial and coplanar stresses, for all possible phonons, for all independent polarizations of light, and through all relevant intermediate states (Γ1c, Γ15c and X5v or L3v).
Dispersion relation for cyclotron waves as a spatial condition for minimum collisionless damping
Authors: Saermark K. , Lebech J. 
Published: July 2007

AbstractIt is shown that the dispersion relation for cyclotron waves of the extraordinary type (ECW) propagating in a metal with a cylindrical Fermi surface may be found in a simple way from the zero's of certain Bessel functions. This follows as a consequence of a requirement of minimum damping of the waves by direct collisionless interaction between the electrons and the waves. A detailed comparison with calculations based on a realistic silver Fermi surface and a spherical Fermi surface, in both cases for a finite value of ωτ, is performed. The behavior of the dispersion relation in the limit of vanishing magnetic field is considered in some detail and a number of interesting features are pointed out.
Piezoresistance of tellurium and selenium–tellurium mixed crystals
Authors: Takagi T. 
Published: July 2007

AbstractThe piezoresistance coefficients of Te, Se5Te95, and Se10Te90 are measured in the temperature range from 220 to 410 K, and at liquid nitrogen temperature. They are analyzed by a simple deformation potential model. The stress coefficients of energy gap, ϵ1 and ϵ3, are obtained. The nonlinearity of ϵ1 for tellurium is restricted to the pressure range smaller than 106 dyn/cm2. The electron mass as well as the hole mass possibly increase with selenium content.
Theory of slow neutron scattering on parametrically excited polarization waves in hydrogen‐bonded ferroelectrics
Authors: Stamenković S. 
Published: July 2007

AbstractThe coherent neutron scattering of slow neutrons on polarization waves excited by a possible parametric resonance in hydrogen bonded ferroelectrics is considered. The expressions for the cross sections both in the case of the parallel and transversal “pumping” are obtained. The experimental possibilities to observe such a scattering and a scattering under an arbitrary oriented “pumping” are discussed.
Ionic size effect on the radiation‐induced dynamic motion of interstitial halogen in alkali halides
Authors: Saidoh M. , Itoh N. 
Published: July 2007

AbstractBy pulse radiolysis technique, the interaction volume between the dynamic replacement sequence and the H center in RbBr and CsBr has been obtained between liquid nitrogen and room temperatures. Similarly as the previous results on KBr, the temperature dependence of the interaction volume follows an Arrhenius equation, with an activation energy smaller than that for the H center migration in each salt. The activation energies for the interaction volume in RbBr and CsBr and also those in KCl and KBr, previously obtained, are compared with the ionic size of each salt and it is found that the activation energy is smaller in salts with smaller ratio of the halogen‐ion radius to the space between two neighboring alkali ions. The amount of the interaction volume at liquid helium temperature in KI and CsBr is also measured and found to be larger than that in KBr and KCl, which is in accordance with the fact that the ratio of the halogen ion radius to the space between two neighboring halogens is larger in the former salts.
F–FA(Na) conversion by repetitive light pulses in additively coloured KCl crystals
Authors: Mladenova M. , Yankov R. , Georgiev M. 
Published: July 2007

AbstractThe optical conversion of F centres to FA(Na) centres induced by repetitive light pulses is investigated above room temperature. The conversion kinetics is investigated as a function of the pulse intensity and repetition rate, as well as of the temperature. The dependence of the initial FA formation rate and the FA density at saturation are found to fit fairly the predictions of a F–FA conversion model in which the thermal dissociation of the ionized FA centre is rate determining. Data on the binding energy of the latter are obtained and subsequently discussed.
α‐Oxygen doublets. I. Theory
Authors: Gaididei Yu. B. , Loktev V. M. , Prikhotko A. F. , Shanskii L. I. 
Published: July 2007

AbstractA group‐theoretical classification is made of the α‐O2 states. It is found that the bimolecular absorption lines must be polarized. It is shown that the bimolecular absorption lines split into doublets by taking into account the collective nature of excited crystal states. The electron‐vibrational structure of bimolecular transition bands is studied.
Crystal field levels of Pr3+ in PrFeO3 and PrGaO3 determined by inelastic neutron scattering
Authors: Feldmann K. , Hennig K. , Kaun L. , Lippold B. , Lukina M. M. , Matthies S. , Matz W. , Warming E. 
Published: July 2007

AbstractThe crystal field splitting of the 3H4 ground state of the Pr ion in PrFeO3 and PrGaO3 has been investigated by inelastic scattering of thermal neutrons. At several temperatures the transitions have been measured by TAS and TOF methods for polycrystalline PrFeO3 and by the TOF method for polycrystalline PrGaO3. Energy level schemes which are different for these materials are given.
Dynamical Theory of Neutron Diffraction by Magnetic Crystals for Laue Geometry
Authors: Schmidt H. H. , Deimel P. 
Published: July 2007

AbstractThe dynamical diffraction of thermal neutrons by magnetic crystals is being considered for the case of the symmetrical Laue‐geometry. It is shown that the results depend on the angle between BA, the average value of the magnetic field in the crystal, and BH, the Fourier‐transform of the magnetic field. The neutron wave function in the crystal consists of four wave‐fields, which have the form |Ψ> =[Ψ0|χ0> +ΨH|χH> · exp (i Hr)] · exp (ik′r), where |χ0> and |χH> are spin states. In the general case |X0> and |X)H> differ from each other. The ratio of the reflected to the incident intensity is derived for the incidence of neutrons polarized parallel or antiparallel to BA and for the incidence of unpolarized neutrons, for the case of pure magnetic diffraction, and for the case that BA is normal to BH. In both cases, for each of the two spin‐directions of the incident beam, there are up to two Braggpeaks. For unpolarized neutrons there are up to three peaks.
Theory of Resonance Raman Scattering near Critical Points
Authors: Bechstedt F. , Enderlein R. , Kolpakov A. , Peuker K. 
Published: July 2007

AbstractThe resonance behaviour of the Raman scattering cross section for allowed one‐phonon scattering is investigated as a function of incident frequency. Both the cases of uncorrelated and of interacting electron‐hole pairs as intermediate states are considered. The scattering amplitude is calculated for uncorrelated pairs for all types of critical points of the interband electronic energy. The dispersion of the scattering cross section is shown to be independent of the type of the critical point. The effect of Coulomb electron‐hole interaction is studied for M0 and M3s critical points. As a result of this interaction the resonance enhancement is much more stronger and the scattering cross section as a function of incident frequency shows more pronounced structures as compared to the case of uncorrelated pairs.
Theoretical Investigation of the Hopping Conductivity in Disordered Systems in a Large Frequency Range
Authors: Böttger H. , Bryksin V. V. 
Published: July 2007

AbstractThe frequency dependence of the hopping conductivity in disordered systems is investigated taking electron‐phonon interaction of arbitrary strength into account. The underlying theoretical concept is based on the density matrix formalism. The hopping conductivity is shown to be essentially governed by two parameters with a dimension of a time τ, the average time between two consecutive hops from site to site and δt, the average time of restoring the equilibrium on a site after a hop. The dimensionless parameter ωτ, governs the relative contribution of the diffusive and the Ohmic currents to the total current. The quantity ω δt is responsible for presence or absence of memory in hopping processes.