bluuebunny/crossref_metadata_2025_split huggingface dataset:
Raman Spectroscopy of Sr Phases in Potassium Halides
Authors: Zaldo C. , Orozco E. 
Published: July 2007

AbstractThe formation of two different types of precipitates along 70 °C thermal annealing is deduced from the study of the Raman spectra of KCl, KBr, and KI single crystals doped with Sr. One of them is responsible for the Raman peaks present in X (YX) Z configuration and the other for the Raman peaks in the X(YY) Z one. At room temperature, peaks in X (YX) Z configuration are observed at 156, 93, and 66 cm−1 and those appearing in X (YY) Z at 189, 111, and 84 cm−1 for KCl, KBr, and KI matrices, respectively. The Sr precipitation is additionally monitored by the photoluminescence of Eu added in little quantities (<20 ppm). It is observed that Eu is only incorporated in the precipitates responsible for the Raman peaks observed in X(YX) Z configuration.
Frequency Dependence of the Soft Mode in KNbO3
Authors: Fontana M. D. , Ridah A. , Kugel G. E. 
Published: July 2007

AbstractThe analysis of the low‐frequency Raman spectrum of KNbO3 with a phonon–relaxator coupling model allows to reconcile the infrared results with the dielectric data. The discrepancies previously reported can be therefore explained in terms of the frequency dependence of the soft modefrequency and‐damping. The existence of the successive cubic–tetragonal–orthorhombic–rhombohedral phase transitions in KNbO3 can be coherently interpreted in this framework which emphasizes the major role of the ionic fluctuations associated with the central peak in the mechanism of transitions.
Nonlinear Propagation of Laser Pulses in GaAs‐GaAlAs MQWS
Authors: Manzke G. , Henneberger K. 
Published: July 2007

AbstractA theoretical approach is presented for the description of nonlinear optical properties of GaAs‐GaAlAs MQWS connected with the blue‐shift of the 1s‐exciton resonance at high excitation. An experimental situation is considered where the MQWS is resonantly excited by a pump pulse of narrow spectral width creating a high density of excitons, and where the optical spectra are recorded by an ultrafast broad‐band probe pulse at different time delays. The densities of excitons in the single GaAs wells are determined by the simultaneous solution of the exciton diffusion and the optical problem of wave propagation. With increasing pump intensity jumps of the exciton densities in certain layers appear when the maximum of absorption crosses the pump frequency due to the blue‐shift of the resonance. The mechanism of the corresponding bistabilities in the reflection and transmission spectra is similar to that observed in connection with the quantum‐confined Stark effect.
The Complete Set of Polar Optic Phonon Modes of Superlattices. Confined Bulk and Interface Modes
Authors: Enderlein R. , Bechstedt F. , Gerecke G. 
Published: July 2007

AbstractThe complete set of polar optic phonon modes of an infinite superlattice is considered within the macroscopic theory. Eigenfunctions are given for confined bulk modes as well as interface modes. Interface modes arise from forbidden longitudinal and p‐transverse confined bulk modes. It is shown that the totality of confined bulk and interface modes form a complete orthonormalized set of basis functions.
Magnetic Polarons in EuO
Authors: Samokhvalov A. A. , Viglin N. A. , Gizhevskii B. A. , Arbuzova T. I. , Chebotaev N. M. 
Published: July 2007

AbstractThe existence of spin‐polarized complexes or magnetic polarons in magnetic semiconductors has been suggested theoretically some time ago, but up to now there is little experimental evidence of this phenomenon. With this in view, electron paramagnetic resonance is investigated (g‐factor and linewidth temperature coefficient) and also several other electrical and magnetic parameters of EuO‐based magnetic semiconductors with various donor concentrations (oxygen vacancies or Gd3+ ions substituting Eu2+ ions). The results of the investigation show that in a limited region of donor concentrations and temperatures there occurs a localization of charge carriers which may be attributed to the formation of magnetic polarons.
The Improper Ferroelastic Phase Transition in Superionic Rb3H(SeO4)2 Crystals
Authors: Plakida N. M. , Salejda W. 
Published: July 2007

AbstractA phenomenological theory of improper ferroelastic Phase Transition, that superionic hydrogen‐bonded Rb3H(SeO4)2, crystals undergo at Tc = 447 K, is presented. The temperature dependences of components of the spontaneous deformation tensor, elastic coefficient tensor, and electric conductivity in the vicinity of phase transition are calculated. A possible microscopic model of order–disorder type of the phase transition in the proton subsystem is discussed briefly.
On the Einstein Relation in n‐Channel Inversion Layers on Ternary Semiconductors under Magnetic Quantization
Authors: Ghatak K. P. , Mondal M. 
Published: July 2007

AbstractAn attempt is made to investigate the Einstein relation for the diffusivity–mobility ratio of the carriers in n‐channel inversion layers on ternary semiconductors in the presence of a quantizing magnetic field, on the basis of the fourth order in effective mass theory and taking into account the interactions of the conduction, heavy‐hole, light‐hole and split‐off bands, without any approximations of weak or strong electric field limits and incorporating the influences of electron spin and Dingle temperature, respectively. It is found, taking n‐channel inversion layers on Hg1−xCdxTe as an example, that the Einstein relation exhibits an oscillatory magnetic field dependence and the oscillatory spikes are much sharper with large numerical magnitudes even in the presence of broadening in inversion layers. The nature of oscillation is quite different to those obtained for bulk specimen of the same material under magnetic quantization. In addition, the corresponding well‐known results for isotropic parabolic energy bands form the special cases of the generalized expressions.
The Application of Thermally Stimulated Processes to the Study of Defects in Perovskite Type Fluorides
Authors: Kristianpoller N. , Trieman B. , Chen R. , Kirsh Y. 
Published: July 2007

AbstractThermally stimulated processes are studied in nominally pure, as well as in doped, perovskite type fluorides. The TSL and TSC, induced by X and monochromatic UV irradiations, are measured and their relations with other thermally stimulated processes are discussed. The excitation and emission spectra of the main TSL peaks are also measured, and their kinetic parameters are evaluated. From the comparison of the TSL and TSC curves, valuable information concerning the processes which occurred during the irradiation and the subsequent heating of the samples is obtained. The main TSL peaks seem to arise in this type of crystals from the thermal release of holes and their subsequent recombination with electron centres.
Mass Defects in Polyethylene and the Role of Mass Weighting in Phonon Green's Function Theory
Authors: Hägele P. C. , Wolf S. , Schmid C. 
Published: July 2007

AbstractIn phonon Green's function theory two representations of the defect matrix operator are used, a frequency‐dependent and a frequency‐independent. Care has to be taken when changing from one definition to the other: Not all results are independent of the definition chosen. Since such dependencies are not mentioned in the literature, one may be easily misled, e.g. in numerical calculations. Therefore a systematic comparison of the two representations is given. The theory is applied to the calculation of the defect density of states for mass defects in the polyethylene chain.
Optical Constants of Germanium Selenide Single Crystals in the Transparency Region
Authors: Elkorashy A. M. 
Published: July 2007

AbstractThe refractive index, n, of GeSe single crystal is measured by the interference method in the transparency region. An effective thickness smaller than the true one is introduced to account for the photon scattering by the layer structure of the material in this region. The effective thickness hypothesis gives satisfactory explanation of the pronounced rise in reflectance for E < Eg. The extinction coefficient, k, is calculated from the measured absorption coefficient. Measurements are performed at room temperature using plane polarized light with the plane of polarization parallel to the a and c crystallographic axes which lie in the plane of cleavage. The real and imaginary parts of the complex dielectric constant (ϵr, ϵi) as well as the reflectance and its phase change (R, φ) are calculated from the values of n and k. It is shown that GeSe single crystal exhibits birefringence. Assuming that GeSe binding is partly ionic and partly covalent, the results of the optical constants are satisfactorily fitted to the model of a single effective oscillator.
Optical and Electrical Studies of GaP: Ti
Authors: Ulrici W. , Friedland K. , Eaves L. , Halliday D. P. , Payling C. A. 
Published: July 2007

AbstractThe results of optical absorption, photoluminescence, electrical conductivity, and Hall‐effect measurements on LEC‐grown GaP: Ti are reported. The positions of the Ti/Ti acceptor level at Ec − 0.50 eV and the Ti/Ti donor level at Ev + 1.0 eV are established and the photoionization transitions from and to these deep levels are identified in the absorption spectra and from recharging processes. The internal transitions 3A2 → 3T1(F) and → 3T1(P) of Ti are found in absorption and the 2E ↔ 2T2 transitions of Ti3+Ga are seen in the absorption as well as luminescence spectra. The interpretation of these bands and sharp lines requires to take into account different kinds of electron–phonon coupling. Photoinduced recharging experiments at low temperatures on high‐resistance GaP:Ti and GaP:Ti:Zn are consistently explained including the three charge states of TiGa, of the PGaP4 antisite defect, and of the residual FeGa impurity.
An Effective‐Field Theory of Mixed Ising Spin Systems in Transverse Fields
Authors: Kaneyoshi T. , Sarmento E. F. , Fittipaldi I. P. 
Published: July 2007

AbstractAn effective‐field theory is proposed and used to derive the thermodynamic properties of the mixed Ising spin system consisting of spin‐½ and spin‐1 with different transverse fields. The effects of transverse fields on the critical behavior and the magnetization curves are studied. It is shown that the present formalism, in spite of its simplicity, yields results which represent a remarkable improvement on the mean‐field treatment.
Digital Optics: Architecture and Systems Requirements
Authors: Streibl N. , Prise M. E. 
Published: July 2007

AbstractDigital optics is a technology for (i) processing, (ii) transport, and (iii) storage of optical digital information. Digital optics offers both, the high temporal bandwidth known from fiber‐communications, as well as the high connectivity and information density of optical imaging. Digital optical circuits may be constructed by cascading two‐dimensional planar arrays of optical logic gates. The connection between these arrays is performed by light beams going through the free space above a chip. The system design and the architecture have to be adapted to optics. Some requirements for optical systems and for optical logic gates will be discussed.
Semiconductor Cells with N‐Type Photocurrent–Voltage Characteristic. A New Class of Nonlinear Optical Elements
Authors: Ryvkin B. S. 
Published: July 2007

AbstractElectrooptic effects are shown to result in an N‐type differential negative resistance of semiconductor cells illuminated with radiation. Various types of optoelectronic elements (bistable element included) are created on the basis of these cells.
Amplitude and Frequency Modulation of Light Pulses in Semiconductor Fabry‐Perot Amplifiers
Authors: Müller R. 
Published: July 2007

AbstractThe amplification of Gaussian‐shaped light pulses being incident on the facet of a semiconductor diode laser which is operated below its lasing threshold (Fabry‐Perot or laser amplifier) is studied theoretically. Numerical results are presented showing the time‐dependent intensity behaviour as well as the frequency chirp, caused by self‐phase modulation, of the transmitted and reflected fields.
Exciton Parameters and Electron Miniband Structure of GaAs/AlGaAs Superlattices
Authors: Uraltsev I. N. , Ivchenko E. L. , Kopev P. S. , Kochereshko V. P. , Yakovlev D. R. 
Published: July 2007

AbstractThe linear optical properties of short‐period GaAs/AlGaAs superlattices (SL) with three‐dimensional conduction minibands are studied. Using spectroscopy of p‐polarized reflectance at oblique incidence the enhancement of exciton resonance features and effect of lineshape inversion at Brewster angle for the first time are observed. The heavy‐ and light‐hole exciton oscillator strength is found to decrease with shortening of the SL period. Using the optical orientation technique the splitting of electron minibands linear in wave vector is measured for the first time. An essential increase of the photoelectron spin polarization is observed in the presence of magnetic field in Faraday configuration. The magnetic field effect is described in terms of the suppression of the electron spin relaxation connected with the k‐linear splitting of miniband spin states.
On Lattice Parameters and Enthalpies of Mixing of Alkali Halide Solid Solutions
Authors: Königsberger E. , Schrunner H. 
Published: July 2007

AbstractA simple unified approach to the calculation of enthalpies of mixing and deviations of lattice parameters from Vegard's rule is presented. No experimental information about the solid solution is required and the results are insensitive to small experimental errors of the relevant parameters (r,x) of the pure components. In many cases this model is superior to more sophisticated theories in reproducing experimental data of mixed crystals. The main shortcomings of these theories with respect to lattice spacings are discussed.
Time Evolution of the Refractive Index of CuCl under Picosecond Pulsed Excitation
Authors: Manar A. , Gomes M. J. M. , Hönerlage B. , Levy R. 
Published: July 2007

AbstractBy measuring the dynamical shift of interference fringes in epitaxial layers of CuCl, using picosecond “pump and test” experiments, the temporal evolution of the nonlinear refractive index of this compound at low temperatures is obtained. This time behaviour analysed at different photon energies of the test beam is shown to depend on the intensity and photon energy of the pump beam. This observed index change is closely related to the dynamics of the polaritons created by the pump beam.
Multiple LO‐phonon Raman scattering and hot excitons
Authors: Trallero‐Giner C. , Riera R. 
Published: July 2007

AbstractA theory is developed which describes multiphonon resonance Raman scattering (MPRS) processes in polar semiconductors when the incident radiation frequency ω1 is above the fundamental absorption region. A general formula for the MPRS cross‐section based on the exciton distribution function is presented as a function of ω1, the indirect creation and annihilation probabilities, the exciton lifetime, and the transition probabilities between different exciton states. The different role of hot‐exciton transitions and their main contribution to the cross‐section is investigated. It is shown that the cross‐sections depend greatly on the ratio of the electron and hole effective masses. Using that and earlier theoretical results for the exciton scattering, the scattering lines showing intensity alternation in InI and InBr are explained if the electron and hole effective masses are nearly equal. The final result corresponds to a cascade model of exciton relaxation where the main transitions are between 2p and 2s exciton states. An approximate expression for the MPRS cross‐section is given.
On the theory of roton‐like excitations in amorphous solids
Authors: Handrich K. , Resch J. 
Published: July 2007

AbstractA view ofg(r)andS(k)suggests an approximately separate treatment of the short‐range order in the directk‐space and the “quasi long‐range order” (which follows from a high and sharp peak of the static structure factor) in thek‐space. The employment of structure functions leads to a widening of the reciprocalk‐space. With an approximated orthonormal and complete set of functions the Green function is decoupled from the force constants. A set of hierarchy equations follows, which is solved to obtain a structure averaged Green function. With that function the density of states is calculated and the specific heat of an amorphous system.
IR reflection spectra of manganese‐zinc ferrites
Authors: Katsnelson E. Z. , Karoza A. G. , Meleshchenko L. A. , Bashkirov L. A. 
Published: July 2007

AbstractIR reflection spectra of the Mn1−xZnxFe2O4 ferrite‐spinel system are investigated in the range from 200 to 800 cm−1. The spectra of all ferrites show three reststrahlen bands ω1, ω2, and ω3, one of which ω2 splits into two components. On the basis of experimental spectra optical, dielectric, and oscillator parameters and also effective dynamic ion charges are determined by Kramers‐Kronig (K‐K) analysis, dispersion analysis (DA), and successive analysis (DA–K‐K). The position and intensities of the bands, ω1 – ω3, are changed with the change in ferrites' composition and show one‐mode behaviour. Conclusions are drawn about the preferable use of the DA–K‐K method for spectra analysis of the ferrites as well as about considerable covalency of the chemical bonds in the ferrites under investigation.
The Site‐Bond Diluted Transverse Ising Model with S = 1
Authors: Yang Chuan‐Zhang , Zhong Jia‐Lin 
Published: July 2007

AbstractWithin the framework of an effective‐field theory, the longitudinal and transverse magnetization, critical temperature, and critical transverse field of a site‐bond diluted transverse Ising ferromagnet on square lattice with S = 1 are calculated in detail. The dependence of these thermodynamic quantities on the concentration of the sites and of the bonds is investigated.
Nucleation Fields in Periodic Multilayers
Authors: Nieber S. , Kronmüller H. 
Published: July 2007

AbstractThe linearized micromagnetic equation for inhomogeneous nucleation in planar soft magnetic regions is derived and interpreted as a one‐dimensional Schrödinger equation. Based on the Kronig‐Penney model of quantum mechanics, the nucleation field for a periodic multilayer composed of two alternating layers with different crystalline anisotropies is determined.
Laser Induced Absorption of CuCl Microcrystallites in a Glass Matrix
Authors: Gilliot P. , Merle J. C. , Levy R. , Robino M. , Hönerlage B. 
Published: July 2007

AbstractUsing a “pump‐and‐probe” technique, the absorption changes of CuCl microcrystallites embedded in a glass matrix are studied. A blue shift of the resonance and a bleaching of the absorption at photon energies lower than the photon energy of the pump, is observed accompanied by an increase of absorption at higher photon energies.
EPR and Optical‐Absorption Studies of Cu2+‐Doped Mg(CH3COO)2 · 4H2O Single Crystal
Authors: Misra Sushil K. , Wang Chunzheng 
Published: July 2007

AbstractX‐band EPR measurements on a single crystal of Cu2+‐doped magnesium acetate tetrahydrate, Mg(CH3COO)2 · 4H2O, are made at 295, 77, and 4.2 K. The Cu2+ spin‐Hamiltonian parameters are evaluated, at all these temperatures, from the EPR line positions by means of a rigorous leastsquares fitting method. The principal values of the g2 and A2 tensors indicate that the local symmetry of the Cu2+ ion, in the host lattice of Mg(CH3COO)2 · 4H2O crystal, is tetragonal. The room‐temperature optical‐absorption spectrum, recorded in the 190 to 820 nm wavelength range, is exploited to study the crystal‐field and charge‐transfer transitions of the Cu2+ ion. On the other hand, both the room‐temperature optical‐absorption and EPR data are used to estimate the core polarization contribution (K) and the molecular‐orbital coefficients for the Cu2+ ion doping Mg(CH3COO)2 · 4H2O.
Optical Effects Due to Antiferromagnetic Spin‐Ordering in (Cd, Mn)S at Highest Mn Concentrations
Authors: Heimbrodt W. , Bencke C. , Goede O. , Gumlich H.‐E. 
Published: July 2007

AbstractThe antiferromagnetic phase transition in (Cd, Mn)S single crystals and thin films with highest Mn concetrations 0.8 ≦ xMn ≦ 1 is studied by luminescence emission and excition measurements. Luminescence bands near 1.5 and 1.8 eV are observed in the samples with rocksalt and zincblende/wurtzite structure, respectively, being excited by the corresponding Mn2+ d–d transitions. Below the spin‐freezing temperature TN(xMn) the Mn2+ d–d excitation bands shift to higher energies nearly proprotional to the square of the ordering parameter, due to the different spinordering induced energy relaxatio in ground and excited states. The exchange interaction parameters of the excited Mn2+ states are determined in a mean‐field approximation. Using a ligand field model the p‐d mixing and crystal field parameters for the paramagnetic and antiferromagnetic phase are obtained. The observed finestructure of the 6A1g → 4Eg, excitation band in MnSRS is ascribed to covalent bondig effects and LO phonon and magnon interaction. The measured luminescence decy parameters increase significantly below TN(xMn), indication the energy transfer rates to be influenced by the antiferromagnetic spin ordering.
The Magnetic Contribution to the Electrical Resistivity in the Ising Ferromagnet
Authors: Urbaniak‐Kucharczyk A. , Balcerzak T. , Mielnicki J. , Wiatrowski G. 
Published: July 2007

AbstractOn the basis of the model proposed by de Gennes and Friedel the magnetic contribution to the electrical resistivity ρm in an Ising ferromagnet is calculated using the Matsudaira approximation method for estimating the two‐spin correlation function. As the main result the dependence of ρm on the temperature T and the orientation of the Fermi wave vector kF with respect to the crystallographic axes is obtained. Especially, ρm(T, kF) for Ising ferromagnets with plane square and s.c. lattice structure is estimated and discussed in more detail.
Hydrogen Diffusion and Radiationless Electron Transfer in Metal Hydrides
Authors: Lupu D. 
Published: July 2007

AbstractThe activation energies of the radiationless electron transfer are calculated from the optical spectra of some Sc, Y, La, Lu, Nb, Pd, and Mg2Ni hydrides. These are practically the same with the values of the activation energies for hydrogen diffusion, pointing out to the importance of electron transfer from localized states to the conduction band in the mechanism of hydrogen diffusion. The exponential temperature dependence of the radiationless electron transfer probability together with vibrational tunneling at low temperatures can account for the temperature dependence of H diffusion in the entire temperature range. A comparison between the electronic structures of NiH and PdH qualitatively explains the higher activation energy of H diffusion in Ni. Isotopic effects in Pd hydrides and a treatment based on Frenkel defects also support the proposed model.
Electronic Structure and Optical Properties of Mgo Band Structure Calculation and Cluster Model
Authors: Stepanyuk V. S. , Szász A. , Grigorenko B. L. , Farberovich O. V. , Katsnelson A. A. 
Published: July 2007

AbstractThe electronic structure of MgO is calculated in the framework of local‐density theory using the LAPW resp. cluster Xα‐SW method. The results of an improved permittivity calculation is compared with optical measurements, yielding good agreement. The effect of pressure on the electronic structure is studied for NaCl symmetry. It is found that the direct gap becomes indirect already for a compression of V/V0 = 0.9.