bluuebunny/crossref_metadata_2025_split huggingface dataset:
Non‐Linear Ultrasound Absorption in an Electromagnetic Wave Field under Quantum Size Effect Conditions
Authors: Arakelyan M. M. 
Published: July 2007

AbstractNon‐linear effects in dimensionally quantized films are considered at amplification of hypersound waves in an electromagnetic wave field with frequency Ω under the condition ql ≫ (q'quasimomentum of a phonon, l free‐path length of an electron). Strong resonance singularities in the sound absorption are observed under the condition qvov ∼ ω (v = 1, 2, 3, …, V0v Fermi velocity of the electron of the vth subband of the film, ω sound wave frequency). The momentum distribution function of electrons is distored. Under certain conditions the absorption is reversed to amplification.
Ground State Energy of Bound Polarons
Authors: Lépine Y. , Badder G. , Matz D. 
Published: July 2007

AbstractThe properties of the ground state of the bound large polaron within the Fock approximation are investigated by using a new model spectrum which is known to describe better the polaron at low external fields. The results are compared with those of Matsuura who used the same approximation, obtained from a path integral formulation, with the usual Gaussian and Coulombic trial spectra, Both formulations are variational. It is found that the new spectrum gives better results (lower variational energies) than those of Matsuura for intermediate or large values of electron–phonon coupling and small values of the electrostatic coupling of the electron to the defect. This is related to that fact that the polaron seems to prefer a cylindrical symmetry to a spherical one in the absence of external fields.
Optical Investigation of Atomic Hydrogen Centres in Alkali Chlorides Doped with I− or Br−
Authors: Lohse F. , Reuter G. , Spaeth J. M. 
Published: July 2007

AbstractIn alkali chlorides doped with I− or Br− atomic hydrogen can be trapped in an interstitial configuration where one of the four Cl− neighbours is replaced by I− or Br−. These H‐centres in “mixed configuration” have several absorption bands in the near UV region. The optical bands are due to charge transfer transitions which is shown by magnetooptical experiments as well as by a simple theoretical model.
Exact Derivation of a Generalized Master Equation for the Motion of Excitons
Authors: Kühne R. , Reineker P. 
Published: July 2007

AbstractStarting from the Nakajima‐Zwanzig equation, by an exact evaluation of its kernel, a generalized master equation, describing the coherent motion of excitons is derived. The time dependence of the kernel is discussed in detail. From this coupled set of integro‐differential equations a set of second order differential equations is obtained. In the continuum approximation, this set results in a partial differential equation in space and time variables, which only for large times assumes the form of a wave equation. Explicit solutions of the various equations of motion are presented and discussed.
Residual Resistivity of Disordered Systems
Authors: Richter J. , Schiller W. 
Published: July 2007

AbstractBy using the method of force‐force correlation functions the residual resistivity is calculated for disordered systems in second order of the fluctuating quantities. Going in this order beyond the single‐site character of usual CPA calculations diagonal disorder as well as off‐diagonal disorder, fluctuations of atomic distances, and coupling between the several disorder fluctuations are taken into account. In the weak scattering limit the distance fluctuations yield no direct contributions to the resistivity. In general, the off‐diagonal disorder gives rise to an important term in the residual resistivity and may cause a deviation from Nordheim's rule for binary disordered crystalline alloys.
Dispersion of Dielectric Axes in Monoclinic and Triclinic Crystals Related to Lattice Dynamics
Authors: Merten L. , Claus R. 
Published: July 2007

AbstractIt is shown that the infrared orientational frequency dispersion of dielectric axes in monoclinic and triclinic crystals is determined by those polartion wave vector directions in which purely transverse modes exist. Simultaneously the wave vector directions of purely longitudinal phonons are obtained.
On the Dielectric Function of the Hubbard Model
Authors: Gasser W. 
Published: July 2007

AbstractThe dielectric function of the Hubbard model is discussed in the strong correlation limit. It is shown that there are in general two zero points of the real part of the dielectric function outside the absorption region which correspond to different types of charge density oscillations. In the zero frequency limit an expression for the dielectric function is derived which allows the explanation of the screening of the potential of a donor ion in a narrow energy band system.
Influence of Halogens on the DC Conductivity and Magnetoresistance of (SN)x
Authors: Philipp A. , Seeger K. 
Published: July 2007

AbstractExposure of (SN)x to the vapor of Br2, I2, and ICI leads to the componds (SNBr0.4)x, (SNI0.02)x, and [SN(ICI)0.1]x, respectively. The second and the third one of these compounds have not been reported before. In the temperature range 4.2 to 300 K the de conductivity is increased in (SN Br0.4)x and (SNI0.02)x and lowered in [SN(ICI)0.1]x compared to the pristine (SN)x. The anisotropy of the de conductivity in (SN Br0.4)x is about 10 times higher than in (SN)x. Measurements of the transverse magnetoresistance at 4.2 K in all three materials indicate increased disorder due to doping with the halogens.
Multiple Scattering in Condensed Materials. II. Calculation of the Density of States for Non‐Crystalline Copper and Iron
Authors: Blaudeck P. , Lenk R. 
Published: July 2007

AbstractBased on a previously developed theoretical scheme for the treatment of multiple scattering in dense disordered media, the dispersion relation for the effective medium is evaluated numerically. The result is employed to determine the electronic density of states for two non‐crystalline transition metals. The theory yields satisfactory results for the width of the d‐band and for the total number of states in the d‐band.
Interband Faraday Ellipticity in Germanium
Authors: Kessler F. R. , Metzdorf J. , Zurek W. 
Published: July 2007

AbstractThe interband Faraday ellipticity (IFE) at the E0 transition in germanium single crystals is measured at T = 300 K and B = 2 T. Even at room temperature the oscillatory IFE is due to transitions to exciton states. This is proved as follows: firstly, the IFE of intrinsic material shows characteristic deviations from a theoretical Landau spectrum. Secondly, the doping dependence of the IFE shows the screening influence of free carriers. The IFE is no longer oscillatory at doping concentrations above 1017 cm−3. Only the effect of the spin‐splitting of the band edge in the magnetic field is to be seen. By measuring the interband Faraday rotation (IFR) and comparing it with the rotation calculated from ellipticity according to a Kramers‐Kronig relation, the IFR at the E0 transition is shown to be due to the same origin as the IFE.
The Exchange Charge Shell Model Analysis of Strain Derivatives of Static Polarizability and Effective Charge Parameter of Alkali Halides
Authors: Jain V. C. , Shanker J. 
Published: July 2007

AbstractAn analysis of the strain derivatives of static polarizability and effective charge parameter of alkali halides under hydrostatic pressure is performed within the framework of the exchange charge shell model. Details of the model. Details of the model and relevant theoretical expressions are given. The magnitudes of exchange charge polarization and its volume dependence are evaluated from the overlap integrals recently computed by Hafemeister and Flygare. It is found that the inclusion of the exchange charge polarization in the formulation of the shell model improves significantly the agreement between theory and experiment.
The Properties of Ni Ions in NaCl Single Crystals. I. Vacuum Ultraviolet, Ionic Conductivity, and X‐Ray Diffraction Studies
Authors: Földvári I. , Voszka R. , Morlin Z. 
Published: July 2007

AbstractThe vacuum ultraviolet (VUV) absorption spectra of NaCl:Ni single crystals are studied, and the absorption band data belonging to the monomer Ni2+ ions are determined at RT and 110 K. A new absorption band is observed at 54200 cm−1 (RT). The previously published spectrum completed with another new band at 55800 cm−1 seems to suggest the presence of NaNiCl3 double salt. The supposition that a NaNiCl3 phase might be present is confirmed by ionic conductivity measurements and X‐ray phase analyses.
Line Broadening in the ESR Absorption Spectra of Fe3+, Cr3+, and Gd3+ in the Oxide YAIG
Authors: Serway R. A. , Yang F. H. , Marshall S. A. 
Published: July 2007

AbstractThe ESR absorption spectra of substitutional Fe3+, Cr3+, and Gd3+ ions, in flux grown singlecrystal yttrium aluminum garnet, are reinvestigated. An anisotropic line broadening is observed for Fe3+ in both octahedral and tetrahedral sites and for Cr3+ in octahedral sites. Various linebroadening mechanisms are considered including the mosaic effect, strain broadening, fluctuations in crystal field parameters and variations in the relative slopes of the Zeeman‐split levels. A reasonable fit to the experimental data is possible using a hybrid model wherein the mosaic effect, strain broadening, and fluctuations in the crystal field parameters are assumed to be the dominant mechanisms.
Piezo‐Optical Properties of Alkali Halides and Their Temperature Dependence
Authors: Meseguer F. , Sánchez C. 
Published: July 2007

AbstractThe spectral dependence in the visible and vacuum ultraviolet of the piezo‐birefringence coefficients of NaF and KF is measured at room temperature. From these data the shear deformation potentials for the valence band are obtained. A piezo‐dichroic effect is also detected in the piezo‐optical experiments and the corresponding coefficients for KF are now reported. Finally the temperature dependence of the photoelastic coefficient Q44 in KBr is measured between room temperature and 700 K. Q44 remains practically constant in the visible range and shows a strong decrease on increasing temperature for photon energies close to the absorption edge. It can be concluded that there is no significant variation of the deformation potential in that temperature region.
Magnetic and Thermal Properties of Nd3+ in Scheelite‐Type Crystals
Authors: Fakioǧlu S. , Erkoç Ş. 
Published: July 2007

AbstractUsing the effective spin Hamiltonian formalism, crystal field calculations of the temperature dependent susceptibilities and the Schottky specific heat are performed over the temperature range 10 to 300 K besides the dipolar and quadrupolar transition probabilities of Nd3+PbMoO4 and Nd3+CaWO4 using different sets of known parameters. Two different methods are employed for the susceptibility calculations and the Curie‐Weiss law is checked. Emphasis is given of the effects of different sets of crystal field parameters.
On the Surface Electrical Resistivity of Thin Semiconducting and Semimetallic Films
Authors: Dimitrov H. D. 
Published: July 2007

AbstractIn the effective‐mass approximation, taking into account the image forces, the low‐energy electronic states in thin crystalline films, the transport relaxation time of current carriers, and electrical resistivity of the films are calculated when the scattering mechanism is connected with the randomly distributed on their boundary surfaces screened Coulomb and electric dipole impurity centres. It is shown that in some cases of thin solid film materials the image forces influence essentially the electronic states and their surface electrical resistivity.
Exciton Contribution to Oscillatory Magnetophotoconductivity in Ge
Authors: Kessler F. R. , Olsowski W. 
Published: July 2007

AbstractIn the region of direct interband transitions, two types of oscillatory magnetophotoconductivity (MPC) spectra of lightly n‐doped Ge samples of various thickness exist. First, there is a correlative spectral dependence, wherein the MPC maxima correspond to magnetoabsorption peaks, and secondly, anticorrelative spectra, which have mimima at positions of absorption maxima. From this the contribution of excitons to the MPC in Ge is deduced. Further analyses of the temperature dependence of MPC, and investigations of external influences such as additional illumination support the assumption that primarily excitons are generated by the absorption mechanism. In a secondary process these excitons may either dissociate into free carriers or recombine instantaneously. If the recombination overbalances the dissociation there is a selective defect in the quantum efficiency of the MPC at each exciton absorption line. Calculations based on the continuity equation and taking the different generation and reduction processes into consideration confirm the spectral conversion of MPC spectra.
Anderson Transition in FeSi2 Films
Authors: Theiner W. A. , Geserich H. P. 
Published: July 2007

AbstractThin films of FeSi2 are prepared by electron beam evaporation. Electron and X‐ray diffraction investigations show that the structure of these films can be varied between an amorphous state and polycrystalline α‐FeSi2, depending on the substrate temperature. As found from the temperature dependence of the dc conductivity and from ir reflectance measurements the structural transition is accompanied by a metal‐insulator transition of the Anderson type with a minimum metallic conductivity of about 400 Ω−1cm−1.
Temperature‐Dependent Mn2+‐6S State Splitting in Scheelites
Authors: Biederbick R. , Hofstaetter A. , Scharmann A. 
Published: July 2007

AbstractMeasurements of the EPR spectrum of seven scheelite structure crystals doped with Mn2+ ions are performed in the temperature range from 4.2 to 350 K. Experimental data of a preceding paper [1] are completed. The temperature dependent zero field splitting is discussed in terms of implicit and explicit contributions. Being the most important one the temperature effect of b is described with an Einstein model where the impurity force constant is related to regular cationic vibrations by a Born‐Mayer potential.
Magnetic Dipole–Dipole Hyperfine Integrals for Slater‐Type Orbitals
Authors: Ammerlaan C. A. J. , Wolfrat J. C. 
Published: July 2007

AbstractIn molecular orbital calculations a linear combination of Slater‐type atomic orbitals is frequently used as a trial wavefunction. To determine hyperfine energies, certain matrix elements, involving the electron orbitals and the hyperfine interaction operator, are required. In the paper explicit expressions are reported, which are valid for Slater‐type orbitals with principal quantum number n = 3, and s‐ or p‐type angular dependence. The expressions allow for a fast computation of the integrals.
Electronic Structure of 4d and 5d Transition Metal Clusters
Authors: Seifert G. , Mrosan E. , Müller H. 
Published: July 2007

AbstractThe electronic structure of Nb6‐, Mo6‐, W6‐ and Ta6‐ clusters is calculated using the scattered wave‐Xα‐method (SW‐Xα). Results for the energy eigenvalues of the clusters and the density‐of‐states are presented. Comparing with results obtained from band structure calculations for the bulk materials the small clusters already a reflect a great part of the bulk electronic structure.
Luminescence from Electron–Hole Drops in Undoped GaP
Authors: Schwabe R. , Thuselt F. , Weinert H. , Bindemann R. , Unger K. 
Published: July 2007

AbstractThe radiative recombination from electron–hole drops (EHD) is investigated in highly excited pure GaP in the temperature range of T = 2 to 77 K. The fundamental parameters of the electronhole condensate are obtained by comparison of the experimental and calculated lineshape of the luminescence band, taking into account the “camel's back” density of states of the lower conduction band near the X‐point. The influence of the real density of states of the lower conduction band near the X‐point. The influence of the real density of states on the lineshape is discussed. The equilibrium carrier density and the work function are determined to be neq = 7.4 × 1018 cm−3 and Φexp 15.5 meV, respectively. The “camel's back” depth is estimated to be δ = (9.1 × 1) meV. It appears that the critical temperature of the EHD is Tc ≧ 70 K. Decay measurements led to the conclusion that the luminescence in the high energy tail of the band is caused by a radiative recombination from a gas‐like plasma.
Aggregation of Point Defects in Highly Doped NaCl:Ca++Crystals
Authors: Dubiel M. , Berg G. , Fröhlich F. 
Published: July 2007

AbstractThe spin‐lattice relaxation time T1 measured by pulsed nuclear magnetic resonance is determined by the concentration of the Me2+‐cation vacancy dipoles alone contrary to the dielectric loss measurements. Therefore this method is used to investigate isothermal anncaling proceses via the dipole concentration also for long times when higher aggregates are formed. It is found that aggregation processes take place in NaCl: Ca2+ crystals in the range up to 200°C. For higher temperatures no change in the dipole concentration can be measured. Whereas for small doping levels (up to 500 ppm) mainly dimers and trimers are formed, at high concentrations (over 1000 ppm) the formation of the Suzuki like phase is shown and described theoretically. The enthalpy for the dipole motion (0.60 eV) and the dimer association enthalpy (0.31 eV) are calculated analysing the experimental curves.
On the Electron–Phonon and Exciton–Phonon Interaction in ZnTe
Authors: Friedrich H. , Ozerov Yu. V. , Strehlow R. , Georgobiani A. N. 
Published: July 2007

AbstractThe λ‐modulated reflectance spectrum of ZnTe is investigated in the spectral region above the energy of the exciton ground state Eex1. Structures are observed near the energy Eex1 + ℏωo (ℏωo is the LO‐phonon energy) and at the energy Eg + 1.3 ℏωo (Eg is the gap energy). In accord‐ance with similar results obtained in a lot of other II‐VI compounds the first of these structures is interpreted from the point of view of the exciton‐phonon complex formation. The second struc‐ture not observed experimentally so far is ascribed to a polaron effect. In order to support this interpretation the existing theory of this polaron effect is extended to the determination of the spectral dependence of the real part of the dielectric function. This allows the determination of the behaviour of the reflectance R and of the derivative dR/dλ. The theoretical results are in quite good agreement with the experimentally observed spectrum.
Model for Trapping Rates for Sensitized Fluorescence in Molecular Crystals
Authors: Kenkre V. M. 
Published: July 2007

AbstractExpressions for trapping (and detrapping) rates, which describe transitions from (and to) the ϰ‐states of the host crystal to (and from) the trap states, and which serve as inputs in a theory of sensitized fluorescence in molecular crystals, are derived from the point of view of their ϰ‐dependence. They are shown to be consistent with experimental observations.
Theory of Resonant Raman Scattering in Systems with Localized Electron States
Authors: Böttger H. , Bryksin V. V. , Kleinert P. 
Published: July 2007

AbstractOn the basis of a general theory of the resonant Raman scattering ordered and disordered systems with strong electron‐phonon interaction are investigated in systems with localized electron states. For a system with a single narrow band a resonance peak occurs, which is analogue to the optical intraband absorption peak of small polaron systems. For models with two bands a luminescence peak occurs with a large Stokes shift besides an analogous resonant Raman peak.
IR‐absorption of OH and OD centres and OH/OH, OD/OD, and OH/OD complexes in Cu‐doped ZnO single crystals. II. Discussion and quantitative interpretation of models
Authors: Gärtner F. G. , Mollwo E. 
Published: July 2007

AbstractIR‐absorption line spectra observed with ZnO:Cu:H, ZnO:Cu:D, and ZnO:Cu:H:D are discussed. The main peaks are due to vibrations of OH and OD centres in ZnO:Cu. The side bands assigned to vibrations of complexes of the form OH/OH, OD/OD, and OH/OD are calculated by means of a threedimensional, lattice‐induced model.
Biexcitons, impurity transitions, and electron–hole plasma in highly excited II–VI compounds?
Authors: Schrey H. , Klingshirn C. 
Published: July 2007

AbstractTo determine the origin of the so‐called M‐ and P‐emission bands, observed in highly excited II–VI compounds at low temperature, CdS is investigated at 5 K by means of excitation spectroscopy. The results are discussed in terms of inelastic scattering processes, biexcitons, impurity transitions, and electron–hole plasma (EHP), including in the discussion also results published by other authors. It is shown that in II–VI compounds the high‐excitation luminescence is mainly due to inelastic scattering processes and to impurity transitions at low temperature and that it is only to a smaller extent caused by EHP and biexciton decay.
Isothermal compressibility for various liquid metals by harmonic model potential
Authors: Sahay B. B. , Srivastava P. L. 
Published: July 2007

AbstractIsothermal compressibility of different groups of liquid metals (Na, Rb, Zn, Cd, Al, Tl, In) is calculated using the harmonic model potential. The computed values are better in agreement with the experimental measurements than other theoretical values.
Low‐temperature renormalization group and symmetry breaking perturbations
Authors: Nattermann Th. 
Published: July 2007

AbstractA skeleton graph approach in d = 2 + ϵ dimensions is used to calculate the low‐temperature properties of a n‐component classical Heisenberg model. The approach allows a unified description of both Goldstone and critical singularities. The influence of small symmetry breaking fields on these singularities is investigated. The main results are closed form expressions for the correlation functions, the equation of state, and the free energy. If n ≧ 3 these fields are stongly relevant. In particular, a small cubic perturbation leads to a first‐order transition. For n = 2 perturbations with the symmetry of a p‐fold axis may become irrelevant for a certain temperature region.
Analysis of phonon frequencies of alkali metals
Authors: Ramamurthy V. , Satishkumar M. 
Published: July 2007

AbstractInvestigations of the dependence of phonon frequencies of alkali metals on their atomic mass reveal that they satisfy an empirical relation Mγv = D(q), a constant which depends on q, with γ = 0.754 ± 0.019 at all wave vectors and hence homologous. It is shown that the observed cross‐over of [ζ00] branches of lithium is not an anomalous property, but only manifests itself in alkali metals of smaller atomic mass due to small variations of γ. The role played by atomic mass which is closely related to that of electron gas constant, C′, in producing a cross‐over is discussed.
A Green's function investigation of nearly two‐dimensional and nearly one‐dimensional Heisenberg ferromagnets
Authors: Treplin W. , Jelitto R. J. 
Published: July 2007

AbstractA Heisenberg model of a ferromagnet is considered, which is modified such that one can change continuously from three to two and to one dimensions. The thermodynamic properties of this model are evaluated by application of Green's functions. A special decoupling procedure is needed in order to provide a Stanley transition for the two‐dimensional model. The results of the theory are evaluated both asymptotically and numerically. By these results an assertion drawn from a former paper is substantiated in so far, that already a tiny interaction between different two‐dimensional magnetic layers is sufficient for producing essentially three‐dimensional magnetic behaviour. By consideration of the various nearest neighbour spin correlations one gets some insight into the question of the reliability of the applied decoupling scheme.