bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy Levels of Z‐Centres in KCl:Ca2+. A Model Potential Perturbation Calculation
Authors: Kassim H. A. 
Published: July 2007

AbstractTheoretical calculations for the Zcentre in KCl:Ca2+ are presented. The spherically symmetric problem is solved numerically for the ground state and the effect of the Ca2+ impurity is taken as perturbation causing splitting of the excited state. The differences between the point ion potential and model potentials incorporating core repulsion, based on Bartram, Stoneham, and Gash's pseudopotential, are investigated and displacements of the impurity cation are also considered. The results agree fairly well with experiment and with recent variational calculations on the same system.
Decay Kinetics of Triplet Molecular Excitons in Alkali Cyanides
Authors: Henriques A. B. , von Der Weid J. P. 
Published: July 2007

AbstractThe decay kinetics and emission intensity of the UV luminescence associated to the a3II → Xi∑+ electronic transition of the CN− molecular ion in alkali cyanide crystals are investigated between 2 and 100 K. The results indicate that the a3II molecular excitons are coupled to lattice phonons, becoming self‐trapped at low temperatures. The splitting of the a3II state in the crystal field yields a single active state in NaCN and KCN whereas at least two active states are found in RbCN and CsCN. Thermal activation of migration of the molecular excitation leads to a quenching of the luminescence. These effects can be qualitatively understood by a charge transfer component in the excitation, already suggested from earlier studies.
Defect clusters and subcascades. II. The splitting threshold and subcascade dimensions
Authors: Melker A. I. , Romanov S. N. , Tarasenko N. L. 
Published: July 2007

AbstractEnergy thresholds of splitting displacement cascades into subcascades are calculated for some materials on the basis of the theory developed earlier. The group representation of splitting is constructed and the density of subcascade size distribution is obtained. A simple method for analytical estimation of temperature influence on splitting is suggested.
Cross‐relaxation of nuclear spins at low temperatures
Authors: Buishvili L. L. , Fokina N. P. , Chelidze L. T. 
Published: July 2007

AbstractA system of equations is derived for the evolution under action of cross‐relaxation (CR) of the dipole–dipole reservoir (DDR) and Zeeman subsystems, redefined taking into account the fact, that the part of secular dipole‐dipole interaction H′d, which causes homogeneous displacement of spin resonant frequencies at low temperatures (LT) must be subtracted from H′d. The case is considered, when the influence of the change of LT displacement due to CR on the energy, absorbed by DDR, is stronger, than that of the usual high‐temperature term. It is shown, that in this situation both, DDR and spins under CR acquire a negative temperature in spite of the fact, that the frequency of saturated spins is smaller than the frequency of spins under CR. This behaviour differs from the high‐temperature result.
Screening of the electron–phonon interaction in GaAs heterostructures
Authors: Xiaoguang Wu , Peetees F. M. , Devreese J. T. 
Published: July 2007

AbstractThe effect of the screening of the electron‐phonon interaction on the polaron binding energy (E) and the polaron mass (m*) is investigated within second order perturbation theory and for different approximations to the two‐dimensional electron gas response (random‐phase approximation, Hartree‐Fock and Thomas‐Fermi approximation). For the electron–phonon contribution to the polaron ground state energy a dynamical screening is applied which takes into account the dielectric response at all frequencies. The influence of the finite width of the electron layer on E and m* is investigated. The results are applied to GaAsAlxGa1–xAs heterostructures and it is found that the mass renormalization for ne = 1011 cm−2 is 0.4% which is a factor of eight smaller than the effective mass of the unscreened ideal 2D polaron.
Zero‐field splitting and g‐Values of d8 ions in a trigonal crystal field
Authors: Petrosyan A. K. , Mirzakhanyan A. A. 
Published: July 2007

AbstractOn the basis of the crystal‐field theory the zero‐field splitting and g‐values of the 3A2 ground state for d8 impurity ions in a trigonally distorted octahedral field are calculated. Using perturbation theory the analytical expressions connecting the spin‐Hamiltonian parameters with the crystal‐field parameters are obtained. It is shown that experimental data for Cu3+ in Al2O3, and Ni2+ in Al2O3, α‐LiIO3, LiNbO3 can be explained taking into account the influence of all the excited levels on the ground state.
Effect of pressure and temperature on the EPR spectrum of Cr3+ ions in α‐LiIO3 single crystals
Authors: Krupski M. , Mirzakhanyan A. A. , Hilczer W. , Sharoyan E. G. 
Published: July 2007

AbstractResults concerning the influence of hydrostatic pressure, uniaxial stress, and temperature on the spin‐Hamiltonian parameters of Cr3+ in α‐LiIO3, single crystals are reported. Assuming that the local compressibility and thermal expansion of the nearest surroundings of the Cr3+ ion is the same as of α‐LiIO3 crystal lattice it is shown that the temperature and pressure variations on the D‐parameter are mainly due to thermal expansion and compressibility of the lattice, whereas the contribution from spin‐phonon interactions is several times smaller. The deformation of the oxygen octahedron close to the Cr3+ ion is determined.
X‐ray absorption K‐edge spectra of titanium in (Ti, Nb)O2 rutile solid solutions
Authors: Poumellec B. , Lagnel F. , Marucco J. F. , Touzelin B. 
Published: July 2007

AbstractThe X‐ray absorption K‐edge spectrum of titanium in (Ti, Nb)O2, rutile or pseudo‐rutile solid solutions is recorded. The second derivative spectra are used to measure the energy shift of the transitions. A partial transfer of Nb 4d electron onto Ti 3d orbitals is found implying a similar energy of these orbitals. On the other hand a decrease of the crystal field is observed when Nb content increases. This may arise from the increase of the mean number of conduction electrons or the increase of the mean size of the metallic octahedron. Lastly, because the electronic structure of the titanium evolves continuously with the niobium content, niobium is certainly randomly distributed around the titanium.
Computer simulation of proton channeling catch‐up in bent crystals
Authors: Taratin A. M. , Vorobiev S. A. 
Published: July 2007

AbstractA detailed computer simulation of the catch‐up effect for a 1 GeV proton beam in a uniformly bent silicon crystal is performed. The dependences of the efficiency of proton catch‐up in the channeling regime on bending radius of the crystal and on incidence angle of the beam are reported.
APW research of Europium monochalcogenides. II. Magnetic form factor and insulator‐metal transition in euo under pressure
Authors: Vlasov S. V. , Farberovich O. V. 
Published: July 2007

AbstractThe augmented plane wave (APW) method is used for theoretical study of the electronic structure of europium monoxide under hydorstatic pressure. A magnetic state of europium ion at the atmospheric pressure is analysed by comparison of theoretical magnetic form‐factor with the experimental one of Cable et al. The energy gap between the filled majority spin f‐states and the bottom of the conduction band is found to be closed at the pressure 140 kbar in good agreement with recent experiments of Zimmer et al.
Linear density response at the metal surface in the hydrodynamic approximation
Authors: Bao Tran Thoai Duy 
Published: July 2007

AbstractThe density response of a jellium surface to external charge and potential perturbations is derived within the hydrodynamic approximation. Retardation effects, damping, and spatial dispersion are included. Results agree with those of Schaich, and in the non‐retarded limit with correct results of Das Sarma and Quinn. The density–potential reponse function, which gives information on inelastic scattering experiment, is explicitly given in the two‐step density model. Dispersion of the surface modes are also discussed in this model.
On the energy dependence of the very low energy reflectance from clean and manganese‐doped MgO(100) surfaces
Authors: He Jian‐Wei , Møller P. J. 
Published: July 2007

AbstractExperimental results of the energy dependence of very low incident energy (0 to 20 eV above the vacuum level) electron reflectance from undoped and from Mn‐doped MgO(100) surfaces are obtained. The fine features of the spectrum are discussed in terms of bulk and surface electronic interband transitions, defect center absorption, and impurity level resonance.
The influence of an electric field on the mobility in semiconductors
Authors: Burghardt H. , Frauenheim T. , Hamann C. 
Published: July 2007

AbstractAnalytical expressions for the electric field dependence of the mobility μ and the mean drift velocity (〈vD〉) are derived in stationary equilibrium taking into account acoustical phonon, nonpolar optical phonon, and polar optical phonon scattering. Using these expressions an analytic relation for the field dependence of the total mean drift velocity for GaAs is found, which besides the field dependence well accounts for the temperature dependence of the experimental data and results obtained from Monte‐Carlo calculations.
Bond charges and electronic charge transfer in ternary semiconductors
Authors: Pietsch U. 
Published: July 2007

AbstractBy means of a simple molecule‐theoretic model of “linear superposition of two‐electron molecules” the bond charges between nearest neighbours and the effective charges of ions are calculated for ternary zinc‐blende structure alloys as well as chalcopyrite semiconductors. Taking into account both, the charge transfer among the ions caused by the differences of electronegativities of atoms used and between the bonds created by the internal stress of the lattice a nearly unvaried averaged bond charge amount of the alloy is found, but rather dramatically changed local bond charge parameters in comparison with the respective values of binary compounds used. This fact should influence the noncentral force interaction in such semiconductors.
Quantum‐mechanical calculations for hydrogen in niobium and tantalum. I. Model potential and properties of self‐trapped states
Authors: Klamt A. , Teichler H. 
Published: July 2007

AbstractFor interstitial hydrogen (and μ+) in Nb and Ta an empirical short‐ranged host‐lattice–particle interaction potential is presented, which well reproduces the available experimental data. By use of this potential various quantum‐mechanical states of the light‐particle–lattice system are investigated and their properties (such as excitation energies and transfer integrals) calculated. In particular, for H and μ+ in Nb and Ta the relative stability of the tetrahedral (1T‐) and octahedral (4T‐) configurations is investigated. For these cases in the present model no potential barrier is found between the two configurations which indicates an instability of the energetically higher state against symmetry‐breaking fluctuations.
Fokker‐planck equation in the spin‐glass dynamics
Authors: Kolley E. , Kolley W. 
Published: July 2007

AbstractThe dynamics of a ζ4‐model for an itinerant‐electron spin glass is described by the Fokker‐Planck equation and the underlying Langevin equation. The situations before and after performing the configuration average are opposed in terms of Markovian and non‐Markovian behaviour, white and coloured noise, single‐ and two‐time actions, respectively. The resulting generalized Fokker‐Planck equation involves a memory function and has hierarchical structure.
Time‐dependent solitary solutions of the quantum‐mechanical anisotropic Heisenberg chain
Authors: Freudenhammer A. 
Published: July 2007

AbstractA solitary time‐dependent solution for a linear chain of quantum‐mechanical Heisenberg spins is given. The interaction is of the anisotropic exchange type. An approximation to a one‐spin‐flip of the Hamiltonian on the basis of local spin quantization directions leads to the result that kink solutions only exist for a static kink.
Model calculations of the LUC‐CNDO modulating function
Authors: Smith P. V. , Szymanski J. E. 
Published: July 2007

AbstractAttempts to calculate the properties of the homopolar oovalent solids using the periodic large unit cell, complete neglect of differential overlap (LUC‐CNDO) formalism have invoked the so‐called k = O approximation. In response to controversies over the application of this approach we have shown that, for LUC's of intermediate (practical) size, it is necessary to introduce into the k = O LUC‐CNDO formalism an appropriate distance‐dependent modulating function. By analysing a variety of one‐dimensional tight‐binding models we have further suggested that this modulating function may be usefully approximated by the face‐centred cubic sine2 function. The main objective of this present paper is to investigate further the case for this sine2 functional form by using the model Hamiltonian representation of the covalent solids of Chadi and Cohen to compute the modulating function directly. The possibility of employing alternative, higher‐quality, LUC sampling schemes within the periodic LUC‐CNDO method is also critically examined using this model Hamiltonian approach.
Valence band splittings in wurtzite‐type semimagnetic semiconductors
Authors: Gubarev S. I. 
Published: July 2007

AbstractValence band splittings induced by exchange interaction with manganese 3d‐electrons are calculated for wurtzite‐type semimagnetic semiconductors. Variations of the wave functions, selection rules, and probabilities for optical transitions between the conduction band and the A, B, C valence subbands are obtained for H ∥ C6 and H ⟂ C6 geometries.
The incommensurate CDW phase of a quasi‐one‐dimensional peierls‐fröhlich system with a nearly quarter‐filled band. II. Numerical results
Authors: Fischbeck H. J. 
Published: July 2007

AbstractAt T = 0 the thermodynamic potential of the incommensurate state of a quasi‐one‐dimensional Peierls‐Fröhlich system with a nearly quarter‐filled band is numerically calculated in the one‐kink‐band approximation which is analytically derived in the meanfield, continuum, and low‐energy approximation in the first part or this work. For special electron densities the one‐kink‐band approximation is exact in the frame of the remaining approximations. At T = 0 the commensurate‐incommensurate transition is mostly discontinuous. Only for surplus of electrons there is a small range of parameters where it is continuous. At the transition the chemical potential, the density deviation, the electronic energy spectrum, and geometric characteristics of the lattice distortion are computed. In addition the dependence of the energy spectrum on the chemical potential away from the transition is calculated.
Delayed luminescence of organic mixed crystals. Guest concentration effect in the p‐terphenyl host‐pyrene guest system
Authors: Pal A. J. , Misra T. N. 
Published: July 2007

AbstractAn effect of guest concentration on various prompt and delayed emissions from a p‐terphenyl host‐pyrene guest mixed crystal is studied. The presence of an adventitious defect site is ascertained. The experimental results suggest that both the defects and the guests are distributed in a random manner in the host lattice and that the free singlet excitons are trapped at monomeric and nascent excimeric pyrene sites with equal efficiency. The defect phosphorescence decay is nonexponential at low guest concentration but becomes exponential at higher concentrations. Second order analysis of the decays at various concentrations confirm that at low guest concentration pyrene delayed fluorescence originates from homofusion of defect triplet excitons whereas heterofusion of defect–guest triplet excitons predominates at higher concentrations.
The resistivity of liquid metals with finite mean‐free path
Authors: Khajil T. M. A. , Tomak M. 
Published: July 2007

AbstractThe electrical resistivity of Sb, Bi, Sn, and Pb are calculated using Ziman's formula modified for finite mean‐free path. Square‐shouldered hard‐sphere model potentials are used to find the structure factor. The effect of structure factor on the calculation of the resistivity is studied.
Electrical resistivity of amorphous Fe1–xZrx alloys
Authors: Paja A. , Stobiecki T. 
Published: July 2007

AbstractThe electrical resistivity ϱ of amorphous Fe1–xZrx films is measured as a function of concentration x in the range 0.11 ≦ x ≦ 0.52 at the temperatures 4.2 and 80 K. The results are analyzed in terms of the Faber‐Ziman diffraction model. It is found that the effective numbers of conduction electrons ZFe = 2.0 and Zzr = 2.4 give the best fit to the experimental data. The results of other authors for amorphous FeZr films and ribbons are also discussed. It is suggested that the effective valencies in Zr‐rich alloys (x ≧ 0.60) may differ from those in Fe‐rich ones.
Photovoltaic effect in semiconductors stimulated by an additional electromagnetic wave
Authors: Shon Nguyen Hong , Anh Vo Hong 
Published: July 2007

AbstractA new mechanism of the interband photovoltaic effect in semiconductors is considered when the direct current in a sample is excited by the simultaneous action of two linearly polarized electromagnetic waves. By solving the kinetic equation in relaxation time approximation for the photo‐electrons and using perturbation technique for calculating the transition probabilities the formula for the photostimulated current is obtained that shows an explicit dependence on the polarization direction, mutual orientation, and intensity of the two driving fields. In the resonant condition concerning the external field frequencies, rΩ1 = sΩ2 (r, s are odd integers) there appears a jump in the current value due to the optical rectification effect. Numerical estimations are given which show that the effect is strong enough to be measurable in available experiments.
A comparison of differential equations that control transmission of strained, non‐centrosymmetric crystals by fast electrons
Authors: Kästner G. 
Published: July 2007

AbstractElectron transmission of non‐centrosymmetric crystals is treated by matrix notation of the dynamical many‐beam theory consistently applied to conventional and modified Bloch waves, correlated plane waves, and column models. Attributed known and supplementary differential equations are shown to be derivable and transformable into each other in a systematic way. Also two integration procedures are generalised. Besides, an explicite expression is derived that describes the physics of scattering of modified Bloch waves in the many‐beam case. It is also shown how those Bloch wave transitions forbidden under Bragg orientation at centrosymmetry, arise due to non‐centrosymmetry.
Far‐infrared reflectivity spectra of ammonium dihydrogen phosphate in the para‐ and antiferroelectric phases
Authors: Wyncke B. , Brehat F. , Arbouz H. 
Published: July 2007

AbstractFor the first time complete data are given on far‐infrared reflectivity spectra of an ammonium dihydrogen phosphate single crystal in the temperature range 7 to 300 K, over the spectral region 15 to 400 cm−1, and for all crystallographic orientations of the crystal, in the para‐ and antiferroelectric phase. This study shows that although the crystal shatters when cooling through the phase transition, it preserves its crystallographic properties.
Colour centres and luminescence in KCl:Ni crystals
Authors: Bhuniya R. C. , Rao S. E. 
Published: July 2007

AbstractOptical absorption, thermoluminescence (TL), and monochromatic thermoluminescence (MTL) glow curves, TL and X‐ray luminescence spectra of KCl:Ni crystal are measured. Strong absorption bands appear at 218 and 275 nm due to the transition of an electron from adjacent Cl− ion to Ni++ ion. The reduction of these bands upon X‐irradiation shows the trapping of electrons by the divalent impurity ions. Only the first peak at 90 °C appears in the TL and MTL glow curve. F bleaching neither produces Z centres nor reduces the V band appreciably but decreases the TL glow curve heavily. These results are also interpreted in terms of electron trapping by impurity ions. Emissions due to decay of STE (Cl ion trapping an electron in the excited state) near free cation vacancy (445 nm) or unperturbed lattice sites (545 nm) are very weak while those near Ni++ ions, free (585 nm) or associated with cation vacancies (380 nm), are very strong. This is due to the high mobility of STE and Cl centres towards Ni++ ions.
The functional integral method in the theory of ferromagnetism for a magnetic system containing itinerant electrons interacting with localized spins
Authors: Cong Bach Thanh 
Published: July 2007

AbstractThe functional integral method is developed for a special model of ferromagnetism, which can be considered as the generalization of the Heisenberg, Stoner, modified Zener models. The functional representation for the thermodynamic potential and the Green function is obtained by using the Wick theorems for Fermions and arbitrary spins. The relation between this model and other ones is discussed for the expression of the Curie temperature as an example.
Excitation spectrum and virial coefficients of superfluid He4
Authors: Bahadur R. , Bhardwaj N. , Kachhava C. M. 
Published: July 2007

AbstractA correlation between the equation of state and the excitation spectrum, of superfluid He4 is established in terms of the t‐matrix formalism by using the Lennard‐Jones interaction potential between the He atoms. The potential parameters are determined from the second virials in such a way that the errors are distributed in the entire temperature range. It is found that the excitation spectrum leads to better values in higher k‐region, but the reproduction deteriorates in the range of low k‐values.
Theory of the carrier mobility in quantum wells including multi‐subband coupling
Authors: Ziep O. , Suhrke M. , Keiper R. 
Published: July 2007

AbstractThe mobility of a quasi‐two‐dimensional (Q2D) electron system with acoustic deformation potential interaction is calculated within a non‐perturbative treatment of intersubband coupling. In dependence on carrier concentration the mobility shows oscillations at low temperatures which are explained by the Q2D‐character of the density of states. At high temperatures the ordinary bulk behaviour is obtained.
Subband energies at semiconductor heterojunctions. An approximate quantization formula
Authors: Paasch G. , Fiedler T. , Bartoš I. 
Published: July 2007

AbstractIn extension of a previous paper a quasiclassical method is developed for the calculation of eigenvalues in a potential with a finite step. The method is based on the use of a suitable and simple approximation for the density of states and on counting the number of states. In this way one obtains a quantization formula of the Bohr‐Sommerfeld type with a defined ground‐state contribution. The latter contains contrary to the usual quasiclassical method the influence of the interference of incoming and reflected waves at the potential step as well as the tunneling into the classically forbidden region. Several tests for model systems show the accuracy of the method. Since steps of the band edges are characteristic for semiconductor heterojunctions the method is well suited for the calculation of subband energies in such systems.
Multiparticle Correlations in Alloys
Authors: Dabrowski L. 
Published: July 2007

AbstractThe general theory of multicomponent alloys is developed taking into account the multiparticle correlations between atoms with reducible and irreducible n‐body interaction potentials. The number of components and their interaction radius are not limited. The method described is not sensitive to the crystallographic structure of the alloy. The problem is being tackled using the idea of a multiparticle entropy. The internal energy is calculated by means of the high‐temperature expansion method employing an appropriately adapted diagram technique. The multiparticle correlation functions are assumed to be additional ordering parameters.