bluuebunny/crossref_metadata_2025 huggingface dataset:
Piezoreflectance Spectra of the 1s Exciton in CuI
Authors: Certier M. , Erguig H. , Soltani M. , Sauder T.
Published: July 2007

The piezoreflectance spectra of CuI single crystals are studied in the 1s exciton line (Z12) with linearly polarized light at 95 K. The wave vector of the incident light is set perpendicular to the cleavage plane (110) and the alternating stresses are parallel to the crystallographic axes [001] and [110]. From the lineshape and sign of the structure it is concluded that the allowed transverse exciton Γ5 is split in two components. Small reflectivity peaks located on the low energy side of the Γ5 exciton are ascribed to the forbidden Γ3 + Γ4 exciton. The shear strain deformation potentials of the excitonic band are calculated from piezoreflectance measurements and assuming the cubic deformation potential c — a = −1 eV are estimated to: b = (−0.5 ± 0.1) eV and d = (−1.7 ± 0.3) eV.
Phase Transition of SBN: Ce Studied by Anisotropic Holographic Scattering
Authors: Rupp R. A. , Seglins J. , van Olfen U.
Published: July 2007

Light scattering phenomena in (Sr0.61Ba0.39) Nb2O6:Ce are investigated from room temperature to the phase transition temperature. Temperature dependence and sign of the birefringence are determined from anisotropic holographic scattering at a wavelength of 514.5 nm. The majority charge carriers are electrons.
Magnetic Susceptibility of Heavy Fermions and Mixed‐Valence Systems
Authors: Panwar S. , Singh I.
Published: July 2007

The electronic and magnetic properties of heavy fermions and mixed valence systems have been, in the recent past, described by the periodic Anderson model (PAM). In this paper the magnetic behaviour of the Anderson lattice is studied using the variational method, proposed recently. With this variational method, the variation is studied of average valence and spin susceptibility with temperature for different values of hybridization, different effective values of the f‐level, and for finite as well as infinite interaction U. When the f‐level lies deep below the Fermi level, the susceptibility diverges in most of the temperature region for infinite U, while for weak interaction the divergence is reduced to a small temperature region. When the f‐level lies near the Fermi level, the susceptibility is finite at T= 0 K, increases linearly at low temperature and shows Curie‐Weiss behaviour at higher temperatures.
The Magneto‐Optical Kerr Effect of Fe/Ag Bilayers
Authors: Zhou S. M. , Lu M. , Zhai H. R.
Published: July 2007

The magneto‐optical polar Kerr rotation spectra of Fe/Ag bilayers made by vacuum evaporation are measured. In the spectra an enhancement rotation peak occurs near 560 nm. For comparison, the calculated magneto‐optical Kerr rotation spectra are given. It is found that the necessary conditions for the Kerr rotation enhancement are low values of the optical constants n and k of the reflector, especially the value of n must be less than 1.0. The calculated off‐diagonal element of the effective dielectric constant tensor ϵxye is also given.
Dependence of Phase Shift on Chemical Environment in Extended X‐Ray Absorption Fine Structure
Authors: Murari A. , Chetal A. R.
Published: July 2007

The coefficient of the term linearly dependent on K, αj in the phase shift is calculated from the theoretical phase functions of Teo and Lee and from experimental extended X‐ray absorption fine structure (EXAFS) data for 3 d transition metal systems of manganese, cobalt, and copper. Attempts are made to explain the behaviour of the phase shift of a given atom pair in different chemical surroundings. The dependence of αj on chemical shift and partial charge is also studied.
IR‐Active Lattice Absorption of the α‐ and β‐Phases of Oxygen in the 45 to 150 cm−1 Region
Authors: Fugol I. Ya. , Pritula I. M. , Khashchina L. V.
Published: July 2007

New experimental date on the far IR absorption spectra of solid oxygen in its α‐, β‐, and γ‐phases are presented. The conditions of generating IR‐active optical modes in oxygen crystals is discussed. It is shown that the optical activity of the observed modes is due to the magneto‐elastic interaction with the short‐range magnetic order.
Recombination Processes in Short‐Period GaAsAlAs Superlattices of Type II
Authors: Maaref M. , Charfi F. F. , Scalbert D. , la Guillaume C. Benoità , Planel R.
Published: July 2007

An extensive study of time‐resolved photoluminescence is made in the temperature range 1.8 to 50 K on nine samples of type‐II GaAsAlAs (001) superlattices. In previous publications, the region is identified where the states derived from Xz valleys are lower than those derived from Xxy valleys, in agreement with a simple model involving the competition between X‐type valley anisotropy and X valleys splitting caused by the small lattice mismatch between AlAs and GaAs. An analysis is given of the luminescence processes and of their evolution with temperature, as a function of AlAs and GaAs layer thicknesses. Excitons localized by interface roughness are found to decay through radiative processes at low temperature. A model of thermal excitation from the tail of localized exciton states is introduced, which helps to identify the pertinent parameters of temperature behaviour in these samples.
Chemical Bonding and Lattice Relaxation in Substoichiometric Titanium Carbide and Nitride
Authors: Čapková P. , Skála L.
Published: July 2007

The rearrangement of interatomic interactions and redistribution ofelectron densities in titanium carbide and nitride resulting from the existence of the nonmetal vacancy are studied by the CNDO/2 method. The local lattice relaxation is taken into account and the differences and common features in the behaviour of the nonmetal vacancy in titanium carbide and nitride are discussed.
The Asymptotic Time Dependence of the Mean‐Square Displacement of Particles in a Multicomponent Two‐Sublattices One‐Dimensional Alloy
Authors: El‐Meshad N.
Published: July 2007

The mean‐square displacement, 〈r2(t)〉, of a labeled particle hopping on a linear two‐sublattice system through a multicomponent dynamic background with vacancy concentration v is calculated in the limit of very long times. Our result is 〈r2(t)〉 = (2va2/c1/2) (J∼t/π), where c is the background particle concentration and J∼ the collective hopping rate given by \documentclass{article}\pagestyle{empty}\begin{document}$ J^ \sim = \left[{\sum\limits_\lambda {x^\lambda \left({1/J^\lambda + 1/J_\lambda ^T } \right)} } \right]^{ - 1}. $\end{document} Here xλ represents the concentration of the A‐species of atoms whose hopping rate on each sublattice is Jλ and across the two sublattices is J.
The Function of an Impurity Contained in the Support of Catalysts
Authors: Xu G. D. , Wei S. Y. , Zhang T.
Published: July 2007

The function of a substitutional impurity in the support of catalysts is investigated by using the Green's function method and complex‐energy‐plane integration technique. The tight‐binding approximation is employed to model the H‐Ni/ZnO(contaminated) system, which is considered as an example. The Anderson‐Newns theory is used to compute the chemisorption energy and adatom charge transfer. It is found that (1) the impurities with the different effective levels, Δε, have different effects on the chemisorption process, (2) the effect of the impurity to be closest to the surface of the support is most significant, (3) the influences of an impurity in the support on the chemisorption are of secondary importance compared with those of an impurity in the active component of catalysts.
Electronic Structure of Vacancies in Transition Metals
Authors: Stepanyuk V. S. , Kozlov A. V. , Katsnelson A. A. , Szasz A. , Farberovich O. V.
Published: July 2007

The electronic structure of vacancies in Fe, Ni, and Cu transition metals and in Al is calculated by linear augmented plane waves (LAPW) in muffin‐tin Green function approach. The electronic structure of vacancies shows a strong hybridization with metallic d‐states, and a considerable effect is observed on the s‐ and p‐electrons in the density of states. The charge transfer differs from the similar case of the simple metals.
The Critical Conditions for Jerky Flow. Discussion and Application to CuMn Solid Solutions
Authors: Kubin L. P. , Estrin Y.
Published: July 2007

The transitions between smooth and jerky flow have recently attracted a significant interest. Considerations of critical strains and stresses for the onset or disappearance of jerky flow are presented in this paper aiming at explaining some recent observations by Kalk and Schwink on CuMn solid solutions. Three particular intervals of temperature, labelled α, β, and γ, are examined, which correspond to different patterns of behaviour of the critical strains. It is shown that the experimental observations at the lowest temperatures (interval α) are understood reasonably well. An explanation is proposed for the occurrence of an island of stability in the interval β which exploits the thermally activated nature of recovery processes. The particular type of behaviour observed at higher temperatures (interval γ) does not seem to be completely interpretable in terms of the current models.
Theory of Worksoftening in High‐Performance Alloys
Authors: Kuhlmann‐Wilsdorf Doris , Wilsdorf H. G. F.
Published: July 2007

In conventional alloys ductility decreases with rising flow stress to negligible values at some maximum yield stress (τm), whereas high‐performance alloys, e.g. formed throgh mechanical alloying or nano‐powders, can have much higher yield stresses with ductility, but tend to worksoften. A simple theory is presented, based on the LEDS concept, to account for both of these behaviors: The flow stress is τ = τ0 + const √ϱ with ϱ the dislocation density, and the workhardening coefficient is θ ≈ dτ0/dγ + Cβ with β the rate of glide dislocation trapping. β depends on the specific LEDS formed but always decreases with stress and can become negative for τ > τm at artificially high ϱ. Worksoftening results when unconventional manufacturing methodes have produced LEDS with metastable ϱ and/or τ0 values that are higher than conform to the LEDS generated through the conventional straining conditions in testing or use.
Anomalous Galvanomagnetic Properties of Quasi‐Two‐Dimensional η‐Mo4O11 Crystal under High Magnetic Fields
Authors: Inoue M. , Machel G. , Laue I. , von Ortenberg M. , Sasaki M.
Published: July 2007

Transverse magnetoresistance and Hall effect of the quasi‐two‐dimensional crystal of η‐Mo4O11, that undergoes twice in the charge density wave (CDW) transitions at Tc1 = 105 and Tc2 = 35 K, are measured in liquid He temperature ranges using a pulse magnet up to 33 T and a Bitter‐type magnet up to 17 T. In the CDW state below Tc2 there appear hysteresis phenomena in the magnetic field dependences of both quantities when a field sweep range exceeds a threshold value of about 10 T, and the Hall voltage has a unique magnetic field dependence. Computer simulations for these unique transport quantities are made using a possible band scheme consisting of multiple conduction carriers (electrons and holes).
Theory of Dephasing in Semiconductor Optics
Authors: Zimmermann R.
Published: July 2007

Inelastic scattering processes as well as static disorder lead to dephasing in semiconductors. The latter case is investigated here by calculating the signal in a degenerate four‐wave mixing experiment. Special emphasis is given to excitonic dephasing due to quantum well roughness.
Interband Quantum Kinetics with LO‐Phonon Scattering in a Laser‐Pulse‐Excited Semiconductor II. Numerical Studies
Authors: Bá;nyai L. , Thoai D. B. Tran , Remling C. , Haug H.
Published: July 2007

A detailed numerical study of the coupled quantum kinetic equations with memory for electron–hole populations and interband polarization induced by an 100 fs short light pulse is performed. The considered model describes the interaction with a LO‐phonon bath. The equations are shown to have stable physical solutions, if collision broadening is self‐consistently taken into account. The completed collision approximation is found to be a good approximation of the fully retarded equations. Ignoring the collision broadening of the particle energy levels in the memory kernel, however, leads to instabilities of the solutions.
Size‐Dependent Thermodynamic Properties of Quasi‐One‐Dimensional Electron‐Hole Plasmas
Authors: Schweizer H. , Lehr G. , Prins F. E. , Lach E. , Fröhlich E. M. , Pilkuhn M. H. , Straka J. , Forchel A.
Published: July 2007

The thermodynamic properties of quasi‐one‐dimensional electron–hole plasmas (1D‐EHP) are studied in a wide range of wire widths. The wires are realized by implantation induced intermixing of GaAs/AlGaAs quantum wells. The 1D‐EHP is analyzed with respect to size‐dependent k‐space filling and band‐gap renormalization. The experimental results indicate strong filling effects depending on the lateral dimensions of the wires.
Many‐Body Effects and Multi‐Mode Behaviour in Semiconductor Lasers
Authors: Henneberger K. , Jahnke F. , Herzel F.
Published: July 2007

Within a quantum theory of semiconductor lasers based on a kinetic description of the interacting carrier–photon system the influence of many‐body and nonequilibrium effects is investigated. For the transient as well as steady state behaviour the lineshape of stimulated emission is discussed that reveals mode selection, mode pulling, and line narrowing, in connection with gain saturation and hole burning in the carrier kinetics.
Optical Properties and Switching Behaviour of Excited III–V Semiconductors near the Direct–Indirect Cross‐Over
Authors: Gürtler S. , Schwabe R. , Staehli J. L. , Unger K.
Published: July 2007

In highly excited semiconductor systems near the transition from direct to indirect band structure (cross‐over) the energetic distance between different conduction band minima is a nonmonotonous function of the excitation intensity. A theoretical explanation of this experimental result is given. Our investigations are based on the evaluation of photoluminescence spectra using a careful lineshape analysis. The nonmonotony is a consequence of the unequal shrinkage of the direct and indirect gaps as a function of the population of energetically comparable conduction band valleys in a material near cross‐over. In the situation discussed an optical switching mechanism is expected, which is explained with special respect to its switching time. Important problems of electron–electron interaction are discussed.
Optical Nonlinearities in n–i–p–i and Hetero‐n–i–p–i Structures
Authors: Malzer S. , Linder N. , Gulden K. H. , Höfler A. , Kiesel P. , Kneissl M. , Wu X. , Smith J. S. , Döhler G. H.
Published: July 2007

Optical nonlinearities in n–i–p–i and hetero‐n–i–p–i structures with selective ohmic contacts, are reported. In this kind of structures strong changes of the absorption and refractive index are achieved either by field effects and/or with phase space filling. The latter one is always present in normal n–i–p–i structures but relatively weak because of the 3D character of the DOS. In QW structures this effect is much more pronounced because of the reduced real and momentum space and the enhanced excitonic oscillator strength. First the dynamics of n–i–p–i structures is discussed and then first experimental results of electrically tuned absorption changes due to band filling are shown. Field effects are also known to give large transmission changes. In particular, results for the transmission modulation in a n–i–p–i structure with a contrast ratio 2 are shown. It is also demonstrated that fast switching can be achieved in n–i–p–i structures. In a device with a 32 μm distance between the n‐ and p‐contacts, a 3 dB frequency of 16 MHz is obtained in excellent agreement with the theoretical prediction. By scaling down the structure to 3 μm frequencies in the GHz range are expected to be feasible.
Hole Transport in Bis (4‐N,N‐Diethylamino‐2‐Methylphenyl)‐4‐Methylphenylmethane Doped Polymers
Authors: Borsenberger P. M.
Published: July 2007

Hole mobilities have been measured in bis (4‐N,N‐diethylamino‐2‐methylphenyl)‐4‐methylphenylmethane (MPMP) doped polystyrene (PS) and polycarbonate (PC) over a range of MPMP concentrations. The results are described by the disorder formalism. The formalism is based on the assumption that charge transport occurrs by hopping through a manifold of hopping sites subject to energetic and positional disorder. The results show that the width of the density of hoppiny states increases with increasing PC concentration and decreases with increasing PS concentration. The concentration dependencies are attributed to dipolar fields associated with dipole moments of MPMP or the host polymer. The positional disorder increases with increasing polymer concentration in both materials.
The cluster approach to ground state determination and statistical thermodynamics of lattice systems
Authors: Steinberg A. S. , Kats D. Ya. , Men B. A.
Published: July 2007

The generalization of the cluster approach to the ground state problem and the statistical thermodynamics of lattice systems is discussed. The special attention is paid to some analytical possibilities. For an illustration of the cluster approach achievements the set of available superstructures in the b.c.c. lattice is obtained and the problem of critical point determination in the f.c.c. lattice is solved in various approximations.
Broadening of the plasma resonance in metallic microcrystals
Authors: Yatom H. , Ruppin R.
Published: July 2007

The broadening of the plasma resonance absorption peak in metallic microcrystals is calculated for spherical and cylindrical geometries. A high, but finite surface barrier is employed in the quantum mechanical model. The finiteness of the barrier is found to lead to a larger broadening than predicted by previous calculations, in which an infinite surface barrier was assumed.
Effect of an electric field on size‐quantized photoemission from nanometer films of semiconductors
Authors: Das N. R. , Ghosh S. , Chakravarti A. N.
Published: July 2007

An attempt is made to investigate the electric field‐aided photoemission from quantum wells in ultrathin films of degenerate wide‐gap semiconductors, taking n‐type GaAs as an example. It is found that the photoemission varies in a step‐like manner with photon energies close to the electron affinity and exhibits oscillatory character in its dependence on the film thickness, both in presence and absence of an electric field. For photon energies significantly lower than the electron affinity, a quantized nature of the emission is observed in certain ranges of film thicknesses. Besides, the rate of increase of the photoemission with electric field is seen to increase with increasing film thickness over the range of thicknesses of interest in ultrathin films.
Three resonant energy levels in the presence of a asymmetric damping. A quantum non‐perturbative approach
Authors: Giry M. , Henri‐Rousseau O.
Published: July 2007

The irreversible evolution at zero temperature of a linear chain of three resonant excited states, the last of them being damped, is studied at zero temperature, using the resolvent method. The matrix elements of the time‐evolution operator and of the reduced density operator are obtained in closed form. The decays contain always three exponentials. Relaxation is always underdamped. Comparison is made with the corresponding classical kinetic model.
Density of states and localization length in weakly disordered peierls chains
Authors: Mertsching J.
Published: July 2007

The electron density of states and localization length are calculated for weakly disordered dimerized tight‐binding chains. Pure bond disorder causes a singular density of states for small bond gaps, and a singular localization length for zero bond gap, whereas larger bond gaps are transformed into pseudogaps. A site gap for diatomic chains is preserved under bond disorder. Site disorder destroys any gap and does not create singularities in the density of states or localization length. These results are compared with those obtained from the continuum approximation with Gaussian disorder, which is not strictly justified for short‐ranged disorder potentials. However, the continuum results are valid for weakly disordered chains provided the bond and site disorders in the chain correspond to bond‐gap and combined site and site‐gap disorders in the continuum model, respectively.
Electronic excitations localized in KBr and KI crystals near vacancy defects of different sizes
Authors: Vasilchenko E. , Sarmukhanov E. , Shunkeev K. , Elango A.
Published: July 2007

The luminescence and excitation spectra of electronic excitations localized near a single anion vacancy (eα), a divacancy (ed), and a vacancy quartet (eq) are investigated. The shifts of spectral characteristics of these excitations with respect to those of the excitations in regular lattice sites (er) have regular behavior. It turned out that the charge of vacancy defects is the main perturbing factor for eα, ed, and eq creation and the dimension of vacancy defects is the main perturbing factor for the radiative decay eα, ed, and eq. The peculiarities of eα, ed, and eq relaxation are discussed in terms of strong and weak off‐centers for er in KBr and KI, respectively.
Ab initio calculation of magnetic moments and hyperfine fields in Y2Fe17Z3 (Z = H, C, N)
Authors: Beuerle T. , Fähnle M.
Published: July 2007

The local magnetic moments and hyperfine fields on the crystallographically different Fe and Y sites and on the interstitial atoms Z are calculated for the technologically important hard magnetic compounds Y2Fe17 and Y2Fe17Z3 with Z = H, C, N by means of the local‐spin‐density approximation and the linear‐muffin‐tin‐orbital theory in atomic‐sphere approximation. The competing effects of volume expansion and hybridization between the interstitial atoms and the neighbouring Fe atoms are discussed, and the limitations of the atomic‐sphere approximation are elucidated.
Rows in two‐dimensional quasilattices
Authors: Hoffmann S. , Trebin H.‐R.
Published: July 2007

Pentagonal Penrose tilings and their octagonal and dodecagonal counterparts represent simple models for two‐dimensional quasicrystals. We investigate the properties of rows, which are the analogues to lattice lines of periodic lattices, by a method based on the projection formalism. It enables us to derive the vertex pattern and the mean vertex density of an arbitrary row as well as the mean vertex density of the corresponding family and its dependence on the row direction. The row separation and their quasiperiodic sequences are also being studied.
Change of exciton energy in ultrafine particles with a charge on its surface
Authors: Lai Zu‐You , Gu Shi‐Wei , Zhou Bingsuo , Xiao Liangzhi , Lee Tiejing
Published: July 2007

The Hamiltonian of the exciton in an ultrafine particle (UFP) with a charge on and a dipole at its surface is given. The exciton energy is calculated as a function of the charge number, the dielectric constant of the medium outside the UFP, and the strength of the dipole. The results for ZnO and CdS UFP indicate that the changes of the exciton energy due to the charge and the dielectric constant outside the UFP are of the same order.
Magnetic properties and structure of (FexAl1–x)2O3 solid solutions
Authors: Viktorov V. V.
Published: July 2007

On basic of the magnetic properties the statistical homogeneity of the Al2O3–Fe2O3 solid solution system is estimated. Summarizing the data obtained the conclusion is made that the distribution of Fe3+ ions in the (FexAl1–x)2O3 lattice for concentrations 0.005 ≦ x ≦ 0.07 does not obey statistical regularities.
Thermodynamics of the one‐dimensional Blume‐Capel model with an arbitrary spin
Authors: Jaščur M. , Kaneyoshi T.
Published: July 2007

The one‐dimensional Blume‐Capel model with an arbitrary spin S is studied by the use of a recently developed effective‐field theory. The internal energy, susceptibility, specific heat, and all the two‐spin correlations are obtained analytically and for spin‐1/2 the exact results are reproduced.
Experimental determination of the nucleation field of isotropic polycrystalline hard magnets
Authors: Hermann R.
Published: July 2007

The single particle nucleation field of polycrystalline hard magnets is investigated using the perpendicular susceptibility. From the measurement of the perpendicular component of the magnetization and the analysis of its first derivative the nucleation field of Ba‐ferrite samples is determined.
Free carrier contribution to dynamic dielectric function of heavily doped semiconductors. Application to n‐type silicon
Authors: Auslender M. , Hava S.
Published: July 2007

A new approximation to calculate the free carrier contribution to the infrared dielectric function in n‐type silicon at room temperature that interpolates the quantum and classical absorption regimes is proposed. This approximation generalizes the Drude‐Lorentz approximation by introducing a frequency dependence of the relaxation parameter which is calculated via Mori formalism. Calculations of carrier concentration and wavelength dependences of the relaxation parameter, dielectric constant and optical constants for concentration intervals from 1.3 × 1015 to about 1.3 × 1020 cm−3 and in the wavelengths range from 1 to 30 μm are performed. The deviations from the Drude‐Lorentz approximation are discussed.
Energy Levels in Quantum Wells of Nonparabolic Semiconductors
Authors: Nag B. R. , Mukhopadhyay S.
Published: July 2007

Energy eigenvalues are given for quantum wells of the GaAs/GaAlAs, GaInAs/AlInAs, and InAs/GaAlSb systems, for widths ranging between 0.5 and 20 nm. The energy band nonparabolicity is shown to affect the values significantly particularly for wells with widths of about 2 nm. The InAs/GaAlSb wells are shown to have lowest energy levels with odd parity because of the negative effective mass in the barrier layer for energies below the mid‐gap energy. Experiments are suggested for the detection of these levels.
EPR Linewidth Studies of Gd3 +‐Doped LaxNd1−xF3 Single Crystals
Authors: Paradowski M. L. , Korczak W. , Pierre J. , Subotowicz M.
Published: July 2007

The dependence of the EPR linewidths of Gd3+ ion in LaxNd1−xF3 single crystals on temperature (77 K ≦ T ≦ 510 K) and composition (0 ≦ x ≦ 1) is analyzed to study the interactions between the paramagnetic Gd3+ and Nd3+ ions. The effect of the dilution of the paramagnetic Nd3+ ions with the diamagnetic La3+ ions is also discussed.
Thermodynamic Characteristics of a High‐Tc Two‐Band Superconductor
Authors: Konsin P. , Örd T.
Published: July 2007

Thermodynamic characteristics are calculated for a two‐band HTSC where superconducting pairing is caused by the dominating interband Coulomb interaction. From the expansion of free energy near Tc in powers of two gap order parameters the expressions for entropy, specific heat with its jump at T = Tc and Hc are derived. A comparison of the calculated values of ΔC/Tc, ΔC/Cn for La2−xSrxCuO4+δ and Y1Ba2Cu3O7, with the experimental ones is made. The theoretical dependence of ΔC on the hole concentration is obtained.
Polarized Neutron Experiments on REBa2Cu3O7—δ with Polarization and Time of Flight Energy Analysis
Authors: Schärpf O. , Chattopadhyay T. , Weber H. W. , Hyun O. B. , Finnemore D. K.
Published: July 2007

Experimental data on HoBa2Cu3O7—δ (with δ = 0 and 1), obtained with a diffuse neutron scattering instrument and employing full polarization analysis, are used to evaluate the magnetic scattering cross sections. The validity of the evaluation is confirmed by a comparison with calculations based on published crystal field parameters for this compound. Similar experiments on Tm‐ and Yb‐substituted materials and the corresponding evaluations show, that both ions are in a 3 + state (Tm3+, Yb3+) at low temperature, whereas a change of valence occurs somewhere below 300 K.
Electron—Electron Scattering in a Hot Electron Gas Interacting with Optical Phonons
Authors: Dedulevich S. , Kancleris Ž.
Published: July 2007

The electron—electron scattering influence on the hot electron gas is analyzed. The strongest influence is observed in the slightly heated electron gas (in the case of so‐called warm electrons) at the lattice temperature T ∼ T0/5 (T0 being the characteristic temperature of the optical phonon). The interparticle collisions are shown to increase the electron energy loss via optical phonons and to decrease by the same amount the energy loss conditioned by acoustic phonon scattering.
Contribution to the Theory of the Spin 5/2 Blume‐Capel Model
Authors: Kaneyoshi T. , Jaščur M.
Published: July 2007

The exact formulation of the spin‐5/2 Blume‐Capel (BC) model is given on the basis of both the exact Ising spin identities and the differential operator technique. The general expressions for evaluating the phase diagram and paramagnetic susceptibility of the spin‐5/2 BC model are obtained within the statistical accuracy equivalent to the Zernike approximation of the spin‐1/2 Ising model. Some numerical results for the phase diagram and paramagnetic susceptibility of the spin‐5/2 BC model on the honeycomb lattice are given.
The Role of Impurity Scattering in the Quantum Waveguide
Authors: Mao J. M. , Huang Y. , Zhou J. M.
Published: July 2007

The effect of elastic scattering due to impurities in the quantum waveguide is investigated with an exact calculation. It is treated as a one‐dimensional scattering problem and any width effect is neglected. The method is applied to the quantum modulated transistor and the Aharonov‐Bohm ring. The overall transmission of the structure is found to be distorted owing to the scattering, the extent of the distortion being not only dependent on the impurity potential strength, but also on the impurity positions. The cutoff frequency of the transistor affected by the scattering is also discussed.
Binding Energy of the Excitonic Ions X− and X in a Weak Electric Field
Authors: Katih M. , Diouri J. , Feddi E.
Published: July 2007

The effect of a uniform and weak electric field on the ground state of the excitonic ions X− and X is calculated using a variational method. The calculation is given for Γ direct gap materials in the framework of an isotropic effective mass approximation with no account of spin and band degeneracy effects. The binding energy of X− and X complexes, as a function of the mass ratio parameter m*e/m*h and the field strength F, is presented.
Electronic Structure of Ordered and Disordered HfRh Alloys
Authors: Jezierski A. , Podgorniak J.
Published: July 2007

The electronic structure of HfRh ordered and disordered alloys is studied by the LMTO and TBLMTO‐CPA methods. The superconducting phase transition temperature is estimated from the band structure calculations. The influence of the short‐range ordering on the electronic structure of HfRh alloys is examined.
Free Magnetic Polarons in Three, Quasi‐Two, and Quasi‐One Dimensions
Authors: Benoit À La Guillaume C.
Published: July 2007

The stability of free magnetic polarons (FMP) is studied as a function of dimensionality in the mean‐field approximation. The non‐linear Schrödinger equation is solved numerically in a self‐consistent manner. Realistic properties of the magnetic medium (here diluted magnetic semiconductors) are included. Hole FMP stability in quantum wells or quantum wires with finite barrier is treated, the magnetic medium being either in the barrier or in the well (or wire). Qualitative extension to exciton FMP is given: the stability of exciton FMP in CdMnTe/CdTe/CdMnTe quantum wells seems unlikely.
Vibrational Mechanics and Binding Energies of Some Rare Earth H. C. P. Metals
Authors: Saxena K. C. , Rathore R. P. S.
Published: July 2007

A theoretical study comprising the ion–ion and electron‐ion interactions to predict the phonon dispersion and binding energies in h. c. p. metals like holmium (Ho) and terbium (Tb) is developed. The former interactions are taken care of by the modified generalized exponential potential bearing only two parameters and capable of expressing interactions among any number of ions. The Krebs scheme, combined with the adequate dielectric screening, is employed to describe the latter interactions. The reliable and precise model so developed predicts the binding energies and phonon dispersion in the said metals in a direct and simple manner.
Hopping Mechanism of Hole Transport in SiO2 at Cryogenic Temperatures
Authors: Bolotov V. V. , Vishnyakov A. V.
Published: July 2007

On the basis of the small polaron conductivity conception the hole transport between localized states at cryogenic temperatures in SiO2 is analyzed. The phonon‐hole coupling constant is determined. It is shown that the hopping probability increases exponentially with increasing energy difference ΔE between the initial and final states up to ΔE = 2.5 eV and then rapidly drops. It is shown that both field and time dependencies of the hole transport can be explained by the proposed model.
New Spin‐Reorientation Transition in Irradiated Single Crystal Hematite
Authors: Donbaev K. M. , Zhetbaev A. K. , Mukusheva M. K.
Published: July 2007

It is experimentally observed that irradiation of single crystal hematite Fe2O3 leads to a decrease of the Morin spin‐reorientation transition temperature and to the apperance of a reverse transition, i. e. to recovery of a weakly ferromagnetic phase at very low temperatures. Parameters of the new transition depend on the type and energy of particles. The possible reasons for the new transition and the formation mechanism of the disordered zones produced by atom‐atom collision cascades are discussed. The splitting of the Mössbauer spectra in the temperature interval of the new transition undoubtedly shows that it is a first‐order transition.
The Observation of a New D‐Absorption Band in KBr–In Crystals
Authors: Plavina I. , Popov A. I.
Published: July 2007

Excitation spectra of the activator luminescence directly excited by UV light and creation spectra of the photostimulated luminescence are investigated for KBr‐In crystals with different In+ concentrations. It is shown that the electronic states responsible for these two kinds of luminescence differ. Careful analysis of the excitation spectra allow us to find a new D‐absorption band having Gaussian shape with the following parameters: peak position at E0 = 6.375 eV and halfwidth ΔE = 0.14 eV. The oscillator strength f = 0.12 of the D‐band is also determined.
Quantum Beat Signals and Mössbauer Spectra with Recoil‐Free γ‐Rays Scattered by an Oscillating Quartz Crystal
Authors: Kuhn D. , Jex H.
Published: July 2007

The elastic and inclastic scattering from piezoelectrically excited X‐cut quartz crystals is investigated with γ‐diffractometry from the (1120) Bragg planes using a 57Co Mössbauer source and a resonance absorber. The investigation is performed in the energy coordinate by means of Doppler motion of the source and in the time domain by means of the quantum beat technique. Examples of γ‐Brillouin spectra from the energy analysis and quantum beat signals of the quartz vibrations with frequencies even below the radiation width are presented.
Some Correlation between Exciton–Phonon Coupling Function and Phonon Damping
Authors: Grigorchuk N. I.
Published: July 2007

For isotropic systems the direct correlation between the exciton or polariton damping function Γ (v) and the coupling function ϕ (q) in a general form is obtained for the cases of their interaction both with longitudinal optical (LO) and longitudinal acoustic (LA) phonons. On the basis of such calculations it is shown how from the known Γ(v) dependence one can judge on the explicit ϕ(q) form.
Spin Waves in Superlattices with Exchange Coupling between Nearest and Next‐Nearest Neighbours
Authors: Barnaś J. , Hillebrands B.
Published: July 2007

The spin‐wave spectrum of infinite ferromagnetic superlattices composed of two b. c. c. materials with exchange coupling between nearest and next‐nearest neighbours is analysed theoretically in the exchange‐dominated region. The appropriate dispersion equations are derived by the transfer matrix method. Some numerical examples are presented for superlattices with (100) interfaces.
The Influence of Local Anharmonism on the Low‐Temperature Heat Conductivity in Disordered Systems
Authors: Salamatov E. I.
Published: July 2007

A model of a disordered system containing anharmonic quasi‐local modes is considered. The influence of such local anharmonism on the low‐temperature heat coefficient is analyzed. It is shown that taking into account the local anharmonism results in qualitatively differentT‐dependences of the heat conductivity coefficient for different anharmonic parameters. The results obtained can be used in interpreting some experimental data.
Temperature Dependence of the Vibrational Properties of PbI2
Authors: Julien C. , Mavi H. S. , Chévy A.
Published: July 2007

Infrared reflectivity measurements are made from 20 to 600 cm−1 at temperatures ranging from 5 to 575 K for the layered semiconductor PbI2. The reflection spectra are analysed by means of a damped Lorentzian‐oscillator model and the calculated optical properties are presented. The infrared reflectivity of PbI2 shows a broad reststrahlen band, pointing to the significantly ionic character of this compound. Optical properties are compared with those of other layered semiconductors. The optical constants, long‐wavelength optical‐phonon frequencies (both TO and LO), and their half‐widths are obtained as functions of temperature. The temperature dependence is discussed using an anharmonic coupling model with a two‐phonon decay. The general behaviour of anharmonic effects in PbI2 agrees qualitatively with the theory of Maradudin and Fein.
Thermal Expansion Coefficient and Bond Strength in Hg1−xCdxTe and Hg1−xZnxTe
Authors: Bagot D. , Granger R. , Rolland S.
Published: July 2007

The thermal expansion coefficient variations in Hg1−xCdxTe (MCT) and Hg1−xZnxTe (MZT) are analyzed between 4 and 280 K. The phonon density of states (DOS) is modelled with seven Einstein peaks whose weight is adjusted to fit the specific heat variations. The mean Grüneisen coefficient associated with each peak is then deduced from a fit of the experimental linear thermal expansion coefficients. The variation with x of these Grüneisen coefficients allows to compare the bond strengths in MCT and MZT. A better stability of MZT as compared to MCT is deduced.
On the Concentration Fluctuation and Ordering Potential of LiPb in the Bhatia‐Young Approach
Authors: Bari A. , Joarder R. N. , Bhuiyan L. B.
Published: July 2007

An ordering potential out of the Thomas‐Fermi type interaction between dissimilar species based on the concept of charge transfer and hence concentration dependent, is constructed to study the thermodynamic properties of the LiPb alloy in the Bhatia‐Young formalism. It is seen to be fairly successful in predicting the experimental behaviour of concentration fluctuation and the excess stability in the system.
Presurface Exciton on the Media‐Parting Border
Authors: Paziuk V. V. , Tkach M. V.
Published: July 2007

The spectrum of the presurface exciton localized on the media‐parting border is found with the help of a self‐consistent method. The dependence of the spectrum on the effective mass component of the electron and hole correlation is obtained for arbitrary correlations between the permittivity values of the contacting crystals. The exciton spectrum can be described by three models depending on the static and dynamic characteristics of the contacting crystals: 1. the three‐dimensional exciton model, 2. the presurface exciton model, and 3. the model of electron and hole bound to their electrostatic images.
Dissociation of Electronic Excitations Localized near Vacancy Defects in KI Crystal
Authors: Vasilchenko E. , Sarmukiianov E. , Elango A.
Published: July 2007

On the example of a KI crystal it is shown that, together with the radiative decay of electronic excitaions localized near anion vacancies (cα) and vacancy quartets (cq), their nonradiative decay also occurs at 4.2 K. VK and F centers are created at eα decay and VK centers and electron vacancy complexes are created at cq decay. These defects arise as a result of eα and eq dissociation that occurs in the electric field of an anion vacancy or of a vacancy quartet. Some details of such a process are discussed.
Magnétorésistance de n‐CuInSe2 en régime métallique
Authors: Essaleh L. , Galibert J. , Wasim S. M. , Leotin J.
Published: July 2007

Une étude du comportement électrique d'échantillons de n‐CuInSe2 de concentration électronique supéricure à la densité critique de Mott a été faite à bassc température et en champ magnétique jusqu'à 35 T. La variation de la magnétorésistance relative, à température fixée, en fonction du champ magnétique, dans la limite des champs faibles, est analysée dans le cadre des modéles tenant compte des effets d'interaction électron‐phonon et électron‐électron. L'étude de la conductivité totale, dans la limite des basses températures et des forts champs, montre que le terme de correction à la conductivité électrique, en régime métallique, est conforme au modéle de Kaveh et Mott.
Raman Scattering in nZnSIn2S3 Layered Compounds
Authors: Razzetti C. , Lottici P. P. , Bini S. , Curti M.
Published: July 2007

Polarized Raman spectra obtained from carefully characterized single crystals of ZnIn2S4, Zn2In2S5, and Zn3In2S6 are reported. The low‐frequency Raman spectra clearly differentiate between the compounds, whereas at high frequencies the vibrational features are less sensitive to the actual crystal structure. The results are interpreted on the basis of a molecular‐like picture based on the intrinsic cationic disorder.
On a Generic Equation Modelling the Persistent Photoconductive Response of Semiconductor Devices
Authors: Anagnostakis E. A.
Published: July 2007

A generic equation is proposed and discussed concerning the persistent photoconductive response of semiconductor devices, both of homostructure and heterostructure type, in terms of the sheet electron density evolution against the incoming cumulative photon dose. The equation models the persistent photoconductivity (PP) development up to saturation of the capacity of the fundamental conduction sub‐level accessible to the photoelectrons. The solution of the generic equation is effected through the respective Green's function, materialising the PP descriptor of the illuminated semiconductor structure, as the superposition of the eigenfunctions of the adjoint homogeneous Fredholm integral transformation.
Dynamical Theory of X‐Ray Diffraction by Elastically Bent Crystals with Microdefects I. Basic Equations
Authors: Kostyuk A. N. , Molodkin V. B. , Olikhovskii S. I.
Published: July 2007

Basic equations of dynamical diffraction theory for elastically deformed single crystals with defects are obtained in the momentum space. The solutions for strong Bragg and diffuse waves are obtained and investigated in the two‐beam approximation. The corrections to Fourier components of the polarizability describing the influence of the diffuse and „quasi‐diffuse”︁ scattering on the attenuation of the amplitudes of strong Bragg waves are obtained in a general form.
A 35Cl NQR Study of [(CH3)2NH2]2SeCl6 Crystals
Authors: Horiuchi K.
Published: July 2007

The temperature dependence of the 35Cl NQR frequencies in [(CH3)2NH2]2SeCl6 is measured between 78 and 300 K. Two NQR lines with intensity ratio of 2:1 are observed at 196 K. The low‐frequency line fades out below about 80 K, while the other line is still clearly observed. This arises from a second‐order structural phase transition in which the librational modes of [SeCl6]2− anions become soft. Both lines fade out at higher temperature, however, the low‐frequency one at 26 K lower than the other one. This is explained by the C4 reorientional motions of the octahedral anions having a relative low activation energy of 18 kJ mol−1.
Effective Mass Theory for Abrupt Heterostructures
Authors: Brezini A. , Sebbani M.
Published: July 2007

For an abrupt heterojunction between two otherwise homogeneous semiconductors in one dimension the effective mass Hamiltonian

is studied with 2α + β = −1, where m(z) is the position‐dependent effective mass and Ec(z) the position‐dependent conduction band edge. Through exact model calculations on heterostructures, when the effective‐mass theory is valid the parameters α and β are found to be 0 and −1, respectively. This conclusion is obtained in the asymptotic limit of the energy near the band edge and holds for equal lattice parameters. Results are also compared to other theories.
Luminescence of KI: Pb Crystals
Authors: Nagirnyi V. , Nikl M. , Polak K. , Zazubovich S. , Jaanson N.
Published: July 2007

Spectral, polarization, and kinetic characteristics of triplet luminescence of Pb2+vc− centres in KI:Pb are studied in the temperature range from 4.2 to 250 K. It is suggested that thermally stimulated T → X transitions as well as the transitions between T or X minima of various orientations can explain the temperature dependences of the intensities and the polarization degrees of luminescence (near 70, 30, and 130 K, respectively). It is concluded that in spite of a strong spin‐orbit coupling in the KI:Pb crystal, the Jahn‐Teller effect can be considered as the main interaction in the relaxed excited state (RES) of Pb2+vc− centres. A considerable influence of spin‐orbit interaction on the structure of RES was also detected.
Electronic and Electrochemical Properties of Nonstoichiometric Tungsten Diselenide
Authors: Molenda J. , Bąk T.
Published: July 2007

Results of investigations on electronic (electrical conductivity and thermoelectric power) and electrochemical properties of non‐stoichiometric tungsten diselenide are presented in the paper. It is proved that holes connected with non‐stoichiometry (W1−xSe2) are the dominating current carries within the temperature range 77 to 300 K. A qualitative scheme of the electronic structure of W1−x‐Se2 is proposed. A strict correlation between the electronic and electrochemical properties of tungsten diselenide is proved for the cell of Li/Li+/Liy W1−xSe2 type.
Long‐Wavelength Absorption in Small Metallic Spheres
Authors: Olszewski S.
Published: July 2007

The absorption of electromagnetic waves by the free‐electron gas in a metal can be calculated if the standing‐wave‐like character of the electron wave functions is taken into account. This approach differs from the zero light absorption obtained from the electron wave functions having the cyclic boundary conditions given by Bloch. For long wavelengths the theory is applied to the case of small metallic spheres. For sludges of copper and lead the calculated absorption coefficient at frequencies above 1012 Hz is found to be quite close to the observed data.
Dielectric Screening and Phonon Dispersion in Metallic Glasses
Authors: Agarwal P. C. , Azez K. A. , Kachhava C. M.
Published: July 2007

Expressions for the phonon frequencies of the binary and ternary metallic glasses are derived on the basis of various dielectric screenings. A model which assumes a central force, effective between nearest neighbours, and a volume dependent force is adopted. The phonon frequencies of longitudinal and transverse phonons for the binary metallic glasses Ca70Mg30, Mg70Zn30, and Cu57Zr43 and for the ternary metallic glass Pd77.5 Si16.5Cu6 are computed. The theoretical results compared with experimental data underline the importance of dielectric screening by conduction electrons.