bluuebunny/crossref_metadata_2025_split huggingface dataset:
Bistability of Molecular Excitons
Authors: Antonyuk B. P. , Bussemer P. , Leine L. , Mukhamedov A. A. 
Published: July 2007

AbstractThe behaviour of a molecular crystal at high pumping level is described by a coupled system of two nonlinear differential equations for the density of excitons and the temperature. Analysis shows that a bistability and a step‐lie change of the temperature distribution and the quantum efficiency of luminescence occur for continuously varying pump intensity.
Thermal Effects on Thermophysical Properties of Alkali Cyanide Crystals
Authors: Chauhan S. K. S. , Agrawal R. , Pathak P. C. , Gupta B. R. K. , Goyal R. P. 
Published: July 2007

AbstractAn interionic lattice potential is developed on the basis of phonon‐pressure theory. This interionic potential is used to investigate the thermodynamic and thermoelastic properties of NaCN, KCN, and RbCN crystals. Thermal contribution is taken into account within the framework of HZ type short range overlap interactions, effective upto second neighbour ions. Cohesive energy and six distinct values of third order elastic constants (TOEC) for these crystals are calculated at both temperatures 0 and 300 K. Values of pressure and temperature derivatives of SOEC are also calculated corresponding to both temperatures. The results are discussed and compared with the available experimental data.
Multiple‐Scattering‐Theory Calculation of Elastic and Dielectric Properties of Alkali‐Halide Solid Solutions
Authors: Middya T. R. , Roy D. , Basu A. N. 
Published: July 2007

AbstractWith increasing concentration of defects in a crystal, when the defect–defect interaction starts dominating, the absence of a smallness parameter makes all the low concentration theories inapplicable. This interaction is treated here approximately via a mean field generated self‐consistently as envisioned in the multiple‐scattering‐theory. For the different alkali‐halide solid solutions considered, this approach reproduces both elastic and dielectric data at all concentrations without any parameter. The results thus obtained compare satisfactorily with experiment in most cases. Finally some of the limitations of this approach are pointed out.
The Behavior of Active Centers in a Laser Host. A Crystal Field Investigation on Dy3+ in LaF3 Single Crystal
Authors: Neogy D. , Purohit T. 
Published: July 2007

AbstractThe analysis of the observed absorption spectra of Dy3+ ion in the widely used laser host LaF3 is improved upon for D3h site symmetry and the magnetic thermal and hyperfine properties of this system are investigated using the crystal field parameters obtained. Except for one excited state, the fit to all the observed Stark states of the ground multiplet is quite good. The results of the study are discussed in the light of available experimental information.
Versatility of the Two‐Ion Quartic Spin Model of Spontaneous Metamagnetism
Authors: Szymański J. , Cofta H. 
Published: July 2007

AbstractThe interesting spin model, proposed in 1980 by Onyszkiewicz, involving a special two‐ion coupling of fourth order, is known to describe well and easily the spontaneous metamagnetism. Here one examines its possible power to describe also other classes of metamagnetism. For this purpose, thermodynamic properties of the model are calculated for many different values of interaction parameters. Indeed, two further classes of magnetism are found in some regions of the interaction space. Such a result confirms the supposed versatility of the model.
Temperature Dependence of Two‐Dimensional Optical Energy Gap for Germanium Selenide Single Crystals
Authors: Elkorashy A. M. 
Published: July 2007

AbstractThe interband absorption coefficient of GeSe single crystals near the fundamental absorption edge is analyzed on the basis of a two‐dimensional model and is found to correspond to an indirect forbidden transition. The optical energy gap at room temperature is found to be (1.066 ± 0.011) and (1.078 ± 0.010) eV for the a‐ and c‐crystallographic axes, which lie in the plane of cleavage. The temperature dependence of the two‐dimensional optical energy gap is studied from room temperature to near liquid nitrogen temperature. This dependence is found to be linear in the range from 96 to 250 K with a negative temperature coefficient dEg/dT equal to −0.48 and −0.46 meV per K for the a‐ and c‐axes, respectively. The results are compared with those obtained on basis of three dimensional model and agreement is found to be within the limits of experimental error.
Resonatorless Dissipative Optical Bistability. Influence of External Fields and Sample Thickness
Authors: Kochelap V. A. , Sokolov V. N. 
Published: July 2007

AbstractThe influence of sample thickness and carrier surface recombination on the characteristics of resonatorless dissipative optical bistability (RDOB) is investigated. The RDOB under consideration is caused by the excitation density nonlinearities of band‐edge absorption of laser radiation in semiconductors. The diversity of multivalued spatial carrier distributions in the sample of thickness d with arbitrary velocity of surface recombination S are considered. It is established that strong recombination can considerably affect the structure and properties of possible carrier distributions and hysteresis of the transmission.
Optical Bistability in Composite Media
Authors: Jungk G. 
Published: July 2007

AbstractOptically bistable behaviour is demonstrated for systems consisting of optically nonlinear microcrystallites embedded in an optically linear metal or dielectric. The most simple version of the local‐field correction serves as feedback mechanism thus avoiding external mirrors. The dielectric function for macroscopic samples is calculated with Garnett's effective medium theory. Numerical examples for the reflectivity and the transmissivity are given for an embedded dielectric with exciton‐like nonlinearity.
Picosecond Plasma Dynamics and All‐Optical Data Processing
Authors: Collet J. H. , Pugnet M. 
Published: July 2007

AbstractUsing time resolved pulse and probe spectroscopy, the transmission modulation of CdSe platelets is investigated under excitation and generation of a dense electron–hole plasma. Induced absorption is related to exciton screening, band‐gap renormalisation, and energy relaxation in a high density electron–hole plasma. These investigations exhibit also interest from the point of view of optical signal processing because an all‐optical picosecond inverter gate and more generally a many input NOR gate can be built based on the induced absorption which occurs in the presence of a dense electron–hole plasma. The operation of such device is described and discussed in relation with the possible development of optical coprocessors (compatible with a parallel processing of data) and especially in relation with the critical problem of associating and cascading optical NOR gates. A first step is reported toward cascadable picosecond NOR gates by demonstrating the possible operation of an all‐optical inverter gate which uses an intermediate frequency doubling of the output signal. This implies that a cascadable all‐optical NOR gate operating in an effective one‐wavelength mode is possible. Experiments are carried out at liquid nitrogen temperature. The operation wavelength is around 673 nm in CdSe. The switching energy of the NOR gate is 30 μJ/cm2 per pulse at 337 nm.
Phonon Dispersion Curves of the Superconducting A15 Compounds Nb3Ge and Nb3(Al–Ge)
Authors: Gupta V. B. , Gupta H. C. 
Published: July 2007

AbstractThe phonon dispersion relations of superconducting compound Nb3Ge are calculated using an angular force constant model, the analysis being limited to third neighbour interactions only. The force constants are evaluated by fitting them to the measured values of phonons at the zone centre. The calculated results are found to be in very good agreement with the experiment in the case of Nb3Ge. With the help of this analysis the complete set of phonon dispersion relations in Nb3(Al–Ge) along the principal symmetry directions is determined. The obtained results nearly reproduce the measured Raman active phonons for Nb3(Al–Ge).
Single‐Ion Anisotropy and ESR Linewidth in Quasi‐Low‐Dimensional Heisenberg Magnets
Authors: Martynov S. N. 
Published: July 2007

AbstractOn the basis of the Kubo and Tomita theory the influence of single‐ion anisotropy on the ESR linewidth in a low‐dimensional Heisenberg magnet is estimated in the high‐temperature limit. Two spin correlation functions in r‐representation are used. The variation of the linewidth Δ𝒦(K) depends on the sign of anisotropy K, the value of spin S, and the magnetic dimension of system d. The linewidth can decrease only to a limited value, equal at S 𝒦, Δ𝒦(K) = Δ𝒦(0) · (2/5)(d+1)/3.
The Coherent Field in Infinite Disordered Systems of Point‐Like Scatterers. Refractive Index and Amplitude
Authors: Solbrig H. 
Published: July 2007

AbstractThe coherent field which is generated by a planar source is calculated as a sum of contributions arising from definite trajectories. The sum is performed by means of a method recently described by Warner and Gubernatis. Generalizing to effective propagators being capable to include returns and loops, comments are made on the primary‐field extinction and the quasi‐optical behaviour of the coherent field is analysed.
Infrared Reflectivity of High‐Tc YBa2Cu3O7–x and Related Ceramics
Authors: Petzelt J. , Kamba S. , Pačesová S. , Pollert E. , Šrámek J. , Smrčková O. , Sýkorová D. 
Published: July 2007

AbstractInfrared reflectivity of several Y–Ba–Cu–O‐type ceramic samples with Tc ≈ 90 K is measured from 10 to 4000 cm−1 at room temperature and from 10 to 650 cm−1 as a function of temperature. The room temperature spectrum of the best‐surface Y–Ba–Cu–O sample is fitted using the model for the dielectric function consisting of the sum of the Drude term and five classical phonon oscillators. Due to the strong anisotropy the Drude term belongs predominantly to the E ‖ c electric field orientation whereas the phonon modes solely to the E ⊥ c orientation (Blu modes). The low temperature spectra in all samples show a systematic reflectivity increase below ≈ 180 to 220 cm−1 in agreement with recent results of other laboratories. The interpretation of this feature is discussed in terms of the gap opening below Tc. The smaller gap values than those obtained from tunneling experiments give an evidence for the strong gap anisotropy.
Raman Scattering from Vibrational Modes in Layered CdxHg1–xI2 Solid Solution
Authors: Anedda A. , Bongiovanni G. , Fortin E. 
Published: July 2007

AbstractRaman spectra at 15 and 300 K of CdxHg1–xI2 solid solutions for 0 < x < 0.44 are investigated. One mode behaviour is observed for the low frequency phonons and the A1g mode, while a two mode behaviour is exhibited by the higher frequency vibrations. The expected mass, force, and temperature induced splitting of the B1g2 and Eg1 degenerated modes is not observed. This fact proposes another different experimental evidence of the low Raman cross section of the B1g2 rigid layer mode. For the first time the degeneracy of the Eg2 mode with the A1g is lifted and experimentally found.
Model for Orientational Defects in Quasi‐One‐Dimensional Ice Crystals
Authors: Yanovitskii O. E. , Kryachko E. S. 
Published: July 2007

AbstractA model of the orientational kink defect in quasi‐one‐dimensional crystals of ice is proposed which takes the relaxation of the basic sublattice of the ice crystal into account. Some physical properties of the kink‐defect are evaluated within the continuum approach and compared with the experimental data. An original interpretation of the orientational defect is presented and discussed in view of elucidating the nature of the H‐bonding in ice and water. In particular, it is argued that a central H‐bond of the kink defect can be interpreted as a monomeric OH‐group.
On the Theory of Linear Diatomic Polymers
Authors: Fedyanin V. K. , Osipov V. A. 
Published: July 2007

AbstractThe model of linear diatomic polymers is considered. Both, the ground state and the phonon spectrum of a discrete AB chain are investigated. Taking into account the electron–phonon coupling as well as the finite‐band corrections, an expression for the sound velocity is derived. The continuum equations of motion in the finite‐band scheme are constructed. The kink solution is analyzed when the effects of an order of O(l/L), where L is a chain length, are included. It is shown that the low‐lying electronic excitations of an AB chain with fixed end atoms are the kink pairs where each kink has an irrational charge and a spin σ = 0 (or σ = 1/2). Depending on the value of the parameter α the odd‐membered chain with a free boundary is obtained to contain either a soliton or an electron–hole pair in its ground state. To compensate the irrational charge the creation of one soliton should be accompanied by a chain relaxation. In this case exactly one electron is removed from the valence band. At α = 0 all results obtained are valid for the trans‐polyacetylene model.
Theory of H(I) Centers in Vitreous Silicon Dioxide
Authors: Dianov E. M. , Sokolov V. O. , Sulimov V. B. 
Published: July 2007

AbstractElectronic structure, optical, and ESR properties of a hydrogen atom bound in an oxygen vacancy in vitreous silicon dioxide are investigated by means of a semiempirical LCAO MO method (MINDO/3) in a molecular cluster approach. Three charge states of the defect are considered. The properties of the neutral defect prove to be in good agreement with experimental data on H(I) centers in silica glass.
New Aspects on the Electronic Conduction in Semi‐Insulating CdF2: Gd3+ Single Crystals
Authors: Triberis G. P. , Guskos N. , Kiriakidis G. 
Published: July 2007

AbstractExperimental results on the DC conductivity of semi insulating CdF2:Gd3+ single crystals are presented. The effect of the electric field and the temperature is analysed using In σ versus E1/2 and T−2/5 plots as suggested in the multi‐phonon‐assisted small polaron hopping model introduced by Triberis. The agreement of the experimental results with this model is very satisfactory giving a meaningful interpretation of the transport properties of this material.
Extinction Length Variation in a Distorted Crystal
Authors: Smirnova I. A. , Entin I. R. 
Published: July 2007

AbstractIt is shown theoretically and confirmed experimentally that the correction to the extinction length τ depends on the displacement wavelength λs in a typical dispersion manner, which is associated with the resonance interbranch scattering at λs = τ. For long waves, the correction is negative and tends to zero as λ in the limit λs/τ → ∞. For λs < τ, the correction is positive and, in the limit λs/τ → 0, it is described by the Debye‐Waller factor. For arbitrary distortions the sign and the magnitude of the correction are determined by the characteristic size of the deformation field. Short‐wave distortions make dynamical reflections narrower and long‐wave distortions broaden them. In the former case diffraction occurs from the average lattice and is characterized by a reduced structure amplitude, whereas in the latter case scattering from the average lattice cannot be taken into consideration.
Elastic, Dielectric, and Photoelastic Constants of Alkali Halide Crystals
Authors: Hans D. , Sangachin A. A. S. , Kumar M. 
Published: July 2007

AbstractThe interionic potential model recently developed by taking account of the composite effect of short range repulsive and dispersion forces along with three body interactions is extended in the present work to calculate the (i) third and fourth order elastic constants, (ii) first and second order pressure derivatives of second order elastic constants, (iii) photoelastic constants, volume derivatives of dielectric constants, and optic mode Grüneisen parameters. The calculated values of various physical quantities are found to present close agreement with experimental data.
Single‐Electron Deexcitation of Volume Plasmons Induced by Heavy Ions in Thin Solid Foils
Authors: Burkhard M. F. , Rothard H. , Groeneveld K.‐O. E. 
Published: July 2007

AbstractIn heavy ion (C+, N+) induced electron spectra (Ee < 30 eV) differential in angle and energy, of thin C (x = 370 nm), Cu (x = 100 nm), and Al(x = 160 nm) targets pronounced structures in forward direction superimposed on the flanks of the “true” secondary electron peak are observed. The energies of these structures are well correlated with the plasmon energies hωp of the materials in question. The present data offer good evidence for the existence of the single‐electron deexcitation process of volume plasmons in thin solid films.
The Hyperfine Fields and Their Pressure Derivatives in (Y0.9Hf0.1)Fe2 and (Zr0.9Hf0.1)Fe2
Authors: Budzyński M. , Subotowicz M. , Niezgoda H. , Spustek H. , Tańska‐Krupa W. , Wasiewicz R. 
Published: July 2007

AbstractMeasurements of the hyperfine fields Bhf on Ta nuclei in (Y0.9Hf0.1)Fe2 and (Zr0.9Hf0.1)Fe2 are performed by means of TDPAC (time‐differential perturbed angular correlation) method. The pressure derivatves of Bhf are also calculated. The results confirm that on R site a magnetic moment exists. It stems from a diffuse negative spin density induced from Fe.
Local and Quasi‐Local Modes of Fe and Zn in CdTe
Authors: Lu W. , Ye H. J. , Yu Z. Y. , Zhang S. Y. , Fu Y. , Xu W. L. , Shen S. C. , Giriat W. 
Published: July 2007

AbstractThe far infrared absorption and reflection spectra of the mixed crystals Cd1−xFexTe and Cd1−x. ZnxTe with small x in the temperature range of 4.2 to 300 K and the wavenumber range of 20 to 350 cm−1 are reported. Local modes induced by Fe and Zn (196 and 173 cm−1), quasi‐local modes induced by FeTe and ZnTe (56 and 54 cm−1) in CdTe and CdTe‐like 2TA two‐phonon absorption (72 cm−1) are observed and estimated by use of the Green's function method. The random‐ element‐isodisplacement (REI) model is also used to calculate the two‐mode‐behaviors of the mixed crystals and to best fit the reflection spectra. Besides, the temperature dependences of the phonon absorptions are calculated and well explained.
Impurity Effects in the Raman and Luminescence Spectra of Nitrogen‐Doped GaP
Authors: Dos Santos M. P. , Hirlimann C. 
Published: July 2007

AbstractGallium phosphide doped with up to 1019 cm−3 nitrogen atoms, is used to show impurity induced effects in both, Raman and luminescence spectra. A simple way to distinguish between Raman and luminescence is given, which incidently allows to observe an intermediate behavior for some peaks. The results can be split into two different classes: the ones which are dependent on the nature of the impurity, like trapped exciton lines in the luminescence spectrum or localized Raman modes, and the ones which are independent from the nature of the impurity, like disorder activated first‐order Raman scattering (DAFORS). The Raman results are obtained under resonant conditions.
Modified Thomas‐Fermi Approximation for Accumulation Layers in MIS Structures
Authors: Übensee H. , Paasch G. , Zöllner J.‐P. , Handschack S. 
Published: July 2007

AbstractA modified Thomas‐Fermi method for the calculation of the electronic structure of accumulation layers in MIS‐systems is presented which in spite of the simplicity of a “classical” method yields results in good agreement with extensive self‐consistent quantum mechanical calculations. It is shown that the modified Thomas‐Fermi method is suitable to describe the characteristics of bound and mobile carriers in accumulation layers. The method is used to calculate accumulation layers in n‐ and p‐type GaAs and to discuss the influence of the quantum effects at the interface on the dependence of the total charge in the accumulation layer on the surface band bending. Comparison is made with the classical results.
Effects of Electron–Electron Interactions on the Nature of the Eigenstates in Amorphous Materials
Authors: Brezini A. , Bendaoud M. , Sebbani M. , Behilil F. 
Published: July 2007

AbstractA Hubbard model with random site energies is used to investigate the electron localization effects associated with both disorder and electron‐electron interactions on a Cayley tree. Extensive results are reported on the ground states properties of the system. In particular, two regimes are found: if the electron–electron interaction parameter U is greater than the disorder parameter W, it appears to be significant in localizing electronic states otherwise it may be viewed as weakly perturbating the effects of the disorder.
Localization in a System of N Coupled Chains. Recursion Method. I. General Approach
Authors: Weller W. , Kasner M. 
Published: July 2007

AbstractThe localization length of an electron is studied, which moves under the influence of a random potential in a system of N coupled chains. First, a closed recursion relation for the reflection amplitude for one chain is derived. It is based on a simple diagrammatic interpretation of this amplitude in the framework of Berezinskii's real space diagram analysis. Then, the method is generalized to the case of N coupled chains. The calculation of the localization length is reduced to the determination of a stationary probability density of some parameters characterizing the reflection properties of such a system in the asymptotic regime for large length. The general method provides a numerical algorithm for the calculation of interesting quantities too.
Stability of a Superconducting Condensate of Bisolitons
Authors: Davydov A. S. , Ermakov V. N. 
Published: July 2007

AbstractAn equation for the condensate wave function of a bisoliton gas is derived when the current carrier density is finite. The stability of a bisoliton condensate generating high‐temperature superconductivity in ceramic oxides is studied. The values of the critical condensate velocity and the electric current density are found. The weak dependence of the critical temperature on the heavy ion mass is explained. The dependence of the critical temperature on the charge‐carrier concentration is elucidated.
Calculation of Matrix Elements for Optical Transitions
Authors: Fritzsche V. 
Published: July 2007

AbstractA careful analysis of the formulas for optical interband matrix elements yields expressions in which additional surface integrals appear. These additional terms cannot be neglected in calculations of optical properties, because both the initial and the final states have extended wave functions. – A calculation of the imginary part of ϵ(0, ω) for copper shows that an application of the ∇V‐matrix element leads to considerable errors at low exciting energies.
Electron Raman Scattering in Semiconductor Quantum Wells
Authors: Riera R. , Comas F. , Giner C. Trallero , Pavlov S. T. 
Published: July 2007

AbstractDifferential‐cross‐section for a type of electronic Raman‐scattering process in a semiconductor quantum‐well heterostructure is calculated considering inter‐valley transitions between the system of sub‐bands provided by electron confinement. The intermediate states consist of virtual electron–hole pairs belonging to sub‐bands from the conduction and valence bands respectively. The T = 0 K case is assumed and transitions with the participation of phonons are not included in the analysis. The quantum‐well is modeled with infinite potential barriers at the interfaces and the effective‐mass‐approximation is utilized. The dependence of the cross‐section with respect to the incident and emitted photon frequencies is analyzed for arbitrary polarization of the secondary radiation field. Singularities in the emission spectra as well as the step‐like character of the emission and excitation spectra are predicted. Numerical results and estimations of the process quantum‐efficiency for the case of a GaAs/AlxGa1−xAs heterostructure are reported.