bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Properties of Grain Boundaries in InSb Bicrystals
Authors: Herrmann R. , Kraak W. , Nachtwei G.
Published: July 2007

Electrical properties of n‐ and p‐type InSb bicrystals with tilt angles of θ = 6° and θ = 13° and carrier concentrations p = NA ‐ ND = 1.5 × 1015 to 3 × 1017 cm−3 are investigated in the temperature range from 77 to 293 K. From the experimental date it is found that in the vicinity of the grain boundary in p‐InSb bicrystals with p < 5 × 1015 cm−3 a pronounced potential barrier (ΦB ≈ 200 meV at T = 150 K) exists. For samples with p > 5 × 1015 cm−3 the height of the potential barrier strongly decreases with increasing carrier concentration p = NA ‐ ND. The dependence of the barrier height on the applied voltage is evaluated numerically, a remarkable decrease of the barrier height with increasing voltage occurs for U > 1 V. The conduction mechanisms through the potential barrier at various temperature intervals are discussed.
Neutron Spin Precession and Spin Dichroism of Nonmagnetic Unpolarized Single Crystals
Authors: Baryshevskii V. G. , Cherepitsa S. V.
Published: July 2007

It is shown that a spin‐orbit interaction of neutrons with crystal nuclei leads to a new phenomenon: neutron spin precession in nonmagnetic unpolarized single crystals and spin dichroism. The spin of the diffracted neutrons with a wavelength of 0.15 nm is rotated by 1.2 rad after transmission of neutrons through a tungsten carbide crystal (WC) of thickness 1 cm, set in exact symmetric Laue diffraction position (211). The neutron spin is rotated in the diffraction plane.
Pair‐Correlation Functions for Liquid Noble Metals through Percus‐Yevick Theory
Authors: Bhattacharya T. , Ghosh A. K.
Published: July 2007

Pair‐correlation functions for liquid Cu, Ag, and Au are calculated via a numerical iterative solution of the Percus‐Yevick equation using solid‐state pair potentials derived from first‐principles resonant model potential theory. Results are compared with experimental data and theoretical values obtained via the optimized random phase approximation.
Giant Moment, Superparamagnetic, Spin‐Glass, and Quasiferromagnetic Properties of Dilute PtFe Alloys
Authors: Kiymaç K.
Published: July 2007

Using an RF‐SQUID magnetometer, the temperature‐dependent magnetic susceptibility of dilute Pt‐Fe alloys containing 10.5 ppm Fe, and 0.87 and 0.99 at% Fe is measured in the temperature range 1 to 4.2 K and in de‐fields between 0.5 and 15 Oe. For 10.5 ppm Fe, the susceptibility obeys the Curie law, and the saturation magnetic moment μsat is ≈ 9.3μB/Fe. The Pt‐0.87 at% Fe and −0.99 at% Fe alloys are spin‐glass and quasiferromagnetic, respectively, and both show superparamagnetic properties at temperatures between transition temperature Tt and ≈ 2Tt. Furthermore, the susceptibility of these alloys, around and below Tt, is field dependent, decreasing in magnitude with increasing field.
Energy Spectrum and Light Absorption Coefficient for Bound Electron‐Exciton States in One‐ and Two‐Dimensional Molecular Crystals
Authors: Grigorchuk N. I.
Published: July 2007

The energies of bound charge carriers and Frenkel excitons in one‐ and two‐dimensional molecular crystals are calculated in the nearest‐neighbour approximation. The band width of the bound states is found to decrease with increasing interaction potential between these particles. The formulae for the light absorption coefficients with excitation of these states are derived. In the case of narrow electron bands and high temperatures it is shown, that the absorption edge is described by the Urbach rule, and the corresponding peaks in the band have van Hove nature.
Electron–Phonon Interaction as Effective Electron–Electron Interaction
Authors: Enders P.
Published: July 2007

After rederiving the tight‐binding electron–phonon Hamiltonian for a diatomic chain with special emphasis on the dispersion of the coupling constant the linear electron–phonon interaction is transformed into an effective Electron–Electron one following Chatterjee and Mitra (CM). The subsequent Hartree‐Fock decoupling with inclusion of anomalous averages allows for a simple treatment. Within the one‐mode approximation (keeping q = 0‐phonons only), Bogolyubov's method of approximating Hamiltonian leads to the mean‐field result, therefore, CM's “opposite Born‐Oppenheimer approximation” cuts off the kinetic phonon energy.
Electron Nuclear Double Resonance of the Trapped Hole Center in α‐Al2O3:Mg2+
Authors: Duvarney R. C. , Niklas J. R. , Spaeth J.‐M.
Published: July 2007

The superhyperfine (shf) interactions of trapped hole centers with 27Al neighbor nuclei in α‐Al2O3 doped with Mg2+ are resolved with electron nuclear double resonance (ENDOR). The hole is localized at an O2− ion. With electron nuclear triple resonance (double ENDOR) it could be established that one of the two next nearest neighbor Al3+ ions is missing, but no ENDOR lines due to a Mg2+ impurity can be identified. The isotropic shf interaction constants of the nearest and next nearest Al3+ neighbors are negative. The shf interactions are quantitatively interpreted by taking into account transferred exchange polarization at the oxygen ion. From this, it seems more likely than not that the Al3+ vacancy is filled by a Mg2+ ion.
Radiative Energy Transfer Process in the KCl:Pb++, Eu++ System
Authors: Capelletti R. , Cywiński R. , Manfredi M. , Solzi M.
Published: July 2007

Measurements of emission spectra, time decay scheme, and time resolved spectroscopy, in the temperature range 80 to 300 K, show the existence of an energy transfer process in the KCl:Pb++, Eu++ system at low concentrations of the dopants. The experimental results – in particular time decay scheme both of donors and acceptors – indicate that the transfer is of radiative type; the same conclusion is obtained from the comparison between the transfer probability estimated as if the transfer is supported by a non‐radiative mechanism and the intrinsic decay probability of the donors.
Nonlinear Thomas‐Fermi Dielectric Function in Semiconductors
Authors: Zhang Kaiyi , Chao K. A.
Published: July 2007

A nonlinear Thomas‐Fermi dielectric function is derived for semiconductors in the framework of the Resta model. When the concept of an effective impurity charge is introduced to ensure the correct screening radius, the analytical dielectric function agrees excellently with the exact numerical solution.
On the Frequency‐Dependent Small‐Polaron Conductivity in the High‐Temperature Regime
Authors: Loos J.
Published: July 2007

Using the Matsubara‐Green functions the small‐polaron conductivity problem at high temperatures is formulated without making any restricting assumptions concerning the dispersion of the lattice vibrations. All the results obtained within this framework remain finite and meaningful for arbitrary external‐field frequency and for arbitrary phonon bandwidth. Also the dc conductivity deduced as the zero‐frequency limit has no singularities.
Effective Masses in Monatomic and Diatomic Crystals and the Transfer‐Matrix Notion
Authors: Detcheva V. , Metodieva M. , Kandilarov B. D.
Published: July 2007

The effective masses of the electrons in monatomic and diatomic crystals are derived on the basis of one‐dimensional models and in the framework of a relevant transfer matrix. From the general expression thus obtained the particular cases corresponding to all possible band edges of models with δ‐function potentials are deduced. In certain special cases the effective mass characterizing the diatomic crystal may be expressed throughthe effective masses in the two monatomic crystals.
Photostimulated Electron Redistribution between F and F+ Centres in Corundum
Authors: Atabekyan R. R. , Ezoyan R. K. , Gevorkyan V. A. , Vinetskii V. L.
Published: July 2007

The kinetics of electron redistribution between F and F+ centres in corundum during photoexcitation with 6.05 and 4.86 eV quanta is investigated. Empty anion vacancies V(0) are found in corundum after irradiation with 50 MeV electrons at a dose ≈ 1018 electrons/cm2. The concentrations of these vacancies and of other (non‐vacancy) traps S are calculated. The ratio of the F and F+ centre oscillator strengths is defined to fF:f = 3.9:1.A phenomenological theory of the investigated processes is proposed under the assumption that the photoexcited electrons of F and F+ centres execute jumps between different charge states of the anion vacancies. Theory parameters as well as the values of ratios of the electron jumping probabilities are defined.
Optical Absorption of Cr4+ in GaAs:Cr
Authors: Ulrici W. , Kleinert P.
Published: July 2007

Optical absorption experiments are reported on p‐type GaAs:Cr:Zn from which the spectral shape of the absorption due to stable Cr4+ is derived in the energy region from the onset at about 0.4 eV up to the GaAs absorption edge. This absorption is interpreted as due to the photoionization transition Cr4+ + hv → Cr3+ + holevb superimposed by the simultaneously occurring transition Cr3+ + hv → Cr2+ + holevb. Its spectral shape at T = 78 K can well be fitted by a calculated one taking into account the uppermost three valence bands, the 4T1(F) ground state of Cr3+ and the 5T2 ground state of Cr2+ located within the gap, and the electron‐phonon coupling of these localized states. The fit yields the optical threshold energy E3 = 0.68 eV and the broadening parameter Γ3 = 0.24 eV of the Cr4+ → Cr3+ transition indicating a strong coupling of the 4T1(F) state of Cr3+ with TA phonons.
Electron Paramagnetic Resonance of Mn2+ in Double Nitrate Crystals
Authors: Seth V. P. , Jain V. K. , Bansal R. S.
Published: July 2007

Electron paramagnetic resonance of Mn2+ in Ce2Mg3(NO3)12 · 24 H2O and Ce2Co3(NO3)12 · 24 H2O single crystals is studied in X‐band at 290 and 90 K. Mn2+ substitutes for Mg and Co sites. The spin‐Hamiltonian parameters are evaluated. The broadening of the resolved spectra of Mn2+ doped in Ce2Co3(NO3)12 · 24 H2O on lowering the temperature is discussed in terms of host spin‐lattice relaxation narrowing. An analysis in terms of the superposition theory shows that the intrinsic second order parameter for the ligand oxygen atoms of the site I probably falls within the normal range of oxygen.
Point Defects in Sn‐Doped Sb2Te3 Single Crystals
Authors: Horák J. , Lošťák P. , Matyáš M.
Published: July 2007

Single crystals of Sb2Te3 doped with 0 to 1.0 at% Sn are prepared and the Hall coefficient, Seebeck coefficient, and plasma resonance frequency of the crystals are determined. The incorporation of one Sn atom into the Sb2Te3 crystal corresponds to an origin of ≈ 2.2 to 3.0 holes. Such an increase in the concentration of holes is explained in terms of a decrease in the bond polarization due to the incorporation of negatively‐charged Sn'Sb defects into the Sb sublattice which is favourable for the formation of antisite defects in Sb2Te3. The observed increase in the hole concentration corresponds to the sum of created Sn'Sb defects and newly formed Sb'Te antisite defects in Sn‐doped Sb2Te3.
On Binding and Superconductivity in Isotypes of Cr3Si
Authors: Schubert K.
Published: July 2007

An electron count which permitted previously the interpretation of Raub's rule, that certain homeotypes of Cr3Si occur at certain values of the average group number of the periodic system of elements, is applied to Matthias' rule for a high transformation temperature Tc to the superconductive state in dependence of the average group number. It is inferred that a b ensemble of electrons (b correlation) exists energetically above the ensemble of the d electrons of the first component (e correlation). A completely occupied b correlation with simple commensurability to e is favourable for a high Tc and this rule opens a useful method to find new alloys with high Tc. It appears that the present model for the dependence of superconductivity on binding is compatible with superconductivity theory, so that the confidence into the model for the binding is increased by the easy explanation of high Tc.
Directional Dispersion of the Phonon Modes in Optically Uniaxial Solids, Far‐Infrared Reflection Spectra, Dielectric and Optic Constants, Dynamic Effective Ionic Charges of the Defect Chalcopyrites CdGa2S4, CdGa2Se4, HgGa2S4, and HgGa2Se4
Authors: Haeuseler H. , Wäschenbach G. , Lutz H. D.
Published: July 2007

The polarized far‐infrared reflection spectra of single crystals of the defect chalcopyrites CdGa2S4 CdGa2Se4, HgGa2S4, and HgGa2Se4 grown by vapour phase transport technique are presented in the range from 40 to 600 cm−1. The spectra reveal ten reststrahlen bands as predicted by group theoretical treatment. The oscillator parameters εα∞, ωαf, ωαf, and γαf, the transverse and longitudinal optical phonon frequencies ωTO and ωLO, and the dynamic effective ionic charges are calculated using classical oscillator fit method. The high‐frequency dielectric constants εα∞ show an anisotropic behaviour, the effective charges (Szigeti charges), however, are approximately scalar quantities. From the obtained oscillator parameters the directional dispersion of the extraordinary polar phonons is determined. The reflection spectra calculated for oblique phonons are discussed and compared to those of pellets hot‐pressed from polycrystalline samples.
The Ionic Gyration and Electrogyration in Dielectric Crystals
Authors: Stasyuk I. V. , Kotsur S. S.
Published: July 2007

Explicit expressions are obtained for the ionic components of the gyration and electrogyration tensors for dielectric crystals. The symmetry aspects of the ionic gyrotropy and electrogyration are considered. The procedure of calculation of ionic contributions to the electrogyration is illustrated by the model example of the Pb2+ ion in crystalline fields of S4 and C3i symmetry.
Aggregation of Mn2+‐Cation Vacancy Dipoles in KCl Matrix
Authors: Cywiński R. , Mugeński E. , Nowy‐Wiechuła W. , Wiechuła J.
Published: July 2007

The aggregation kinetics of Mn2+‐cation‐vacancy dipoles in the KCl matrix is examined by ESR and photoluminescence methods at room temperature. In dependence on the annealing (aging) conditions four different Mn2+ aggregation products appear in the crystal: luminescent dipole aggregates (no ESR line) formed during short annealing at room temperature, two non‐luminescent Mn2+‐rich microprecipitates (K4MnCl6 and MnCl2) yielding distinct ESR lines, and a non‐luminescent aggregation product not yielding any detectable ESR line. The activation energies obtained for the Mn2+ emission decay at higher temperatures suggest that after a rapid, initial phasetransition like process (0.1 eV) the further slow decay is conditioned by Mn2+ diffusion (0.6 eV).
Resonance LO Phonon‐Assisted Luminescence and Light and Heavy Excitons in NaI Crystals
Authors: O'connel‐Bronin A. A.
Published: July 2007

The resonance multiphonon luminescence spectra of NaI single crystals are investigated experimentally in the temperature region of 4.2 to 65 K. A detailed investigation of the temperature dependence of peak energies and the intensities of the components of the luminescence spectrum as well as a theoretical analysis reveal an unknown component in the region of the zero‐phonon line. This component is interpreted in the framework of the model of light and heavy excitons. On the basis of experimental data for NaI the energy Δ, induced by the short‐range part of exchange interaction, is evaluated. Peculiarities of resonance luminescence in alkali iodides are analysed by using the theory of resonance multiphonon luminescence developed in the studies of semiconductor crystals.
Temperature Dependence of Optical Absorption near the Structural Phase Transition of Potassium Nitrate
Authors: Kawashima R. , Katsuki K. , Suzuki K.
Published: July 2007

The electronic properties of a KNO3 crystal near the structural phase transition are investigated by observing ultraviolet absorption spectra. The spectra show an anomalous temperature dependence near the transition point Ttr. The anomalous behaviour of the exponential absorption tails in the spectra is observed and discussed phenomenologically in relation to anomalies in the dielectric constant and lattice constant of KNO3.
On the Calculation of the Pressure Dependence of the Transverse Optical Mode Frequency of Cubic Ionic Crystals
Authors: Batana A. , Faour J.
Published: July 2007

Due to the lack of experimental measurements of the Grüneisen parameter γt = − ∂ In ωt/∂ In V at T → 0 K, this magnitude is calculated for salts of f.c.c. and b.c.c. structures using different models and low and room temperature experimental data. In general, for each salt, similar results are obtained with all models, being slightly larger those given by the shell model. When low temperature data are used the values are similar to the room temperature experimental data available.
Invariant Harmonic Oscillator Functions. I. The Cubic Groups Td, O, and Oh
Authors: Wöhlecke M. , Scherz U. , Maretis D. K.
Published: July 2007

Excited energy states of quantum mechanical two‐ or three‐dimensional harmonic oscillators are of interest in Jahn‐Teller calculations and rotational tunneling systems. A general method is given to calculate symmetry adapted oscillator functions by numerical computation. The basic relations are formulated in the occupation number representation. Therefore advantage of group properties like isomorphism and direct product groups can be taken easily. Results are given for two‐ and three‐dimensional irreducible representations of the cubic groups Oh, O, and Td.
The Nucleation Fields of Uniaxial Ferromagnetic Crystals
Authors: Kronmüller H.
Published: July 2007

It is shown that in crystals of hexagonal or tetragonal symmetry the nucleation field, HN depends on the orientation of the easy magnetic direction with respect to the c‐axis. For an easy direction parallel to the c‐axis HN depends for K1 > 4K2 exclusively on the first anisotropy constant, K1, whereas for oblique easy directions the maximum nucleation field for (−) (K1 + Kd) = K2 is governed exclusively by K2, and is given by 2K2/Ms.
Application of the Density‐Functional Formalism to Bound Polarons
Authors: Röseler J.
Published: July 2007

Calculating the ground state energy of the bound polaron with the help of the density‐functional formalism high quality results are found for all α and R. Beside the usual energy functional of the electronic density a new energy functional without self‐energy contributions can be constructed, allowing to prove quantitatively the applicability of the local density approximation if electron–phonon interaction is included. Furthermore, the validity of the parametrized formula for the exchange‐correlation energy of a homogeneous e–h plasma in polar semiconductors is confirmed by the calculations. Comparing both energy functionals new insights are obtained with respect to the structure of these functionals.
Self‐Consistent Theory of the Electronic Structure of Inversion Layers. I. A New Method Using the Modified Local Density Approximation
Authors: Übensee H. , Paasch G. , Zöllner J.‐P. , Fiedler Th. , Gobsch G.
Published: July 2007

A new method is described for the self‐consistent calculation of electron and hole densities, band bending, and subband energies in inversion layers of MIS‐structures. This method makes use of new and simple expressions for carrier densities which take into account the influence of the interface semiconductor–insulator within a modified local density approximation. The method allows for an accuracy which is comparable with that one of explicit self‐consistent solutions of the Schrödinger equation for inversion layers. But it is of the numerical simplicity of the classical method. This gives the possibility to calculate electronic properties of MIS‐structures in much more detail than before. In part I the method is described and its advantages for modelling MIS‐structures are shown. In part II the influence is discussed of quantum effects in inversion layers on electronic properties of MIS‐structures in comparison with classical calculations for various semiconductors in a wide range of temperature and doping concentration.
Mode Grüneisen Parameters for Long‐Wave Phonons of III‐V Covalent Compounds under Pressure
Authors: Soma T. , Kagaya H.‐Matsuo
Published: July 2007

The elastic shear constants C44 and C′ = (C11 ‐ C12)/2 of GaAs, GaSb, InAs, and InSb under pressure are studied theoretically using the presented binding force and the local Heine‐Abarenkov model potential. The pressure derivatives dC44/dP and dC′/dP decrease under compression from the experimental data at low pressures. Then, the Grüneisen parameters of the individual modes of long‐wave phonons under pressure are obtained in the [100], [110], and [111] directions.
T = 0 Magnetic Polaron in f‐Systems with Antiferromagnetic s‐f Exchange
Authors: Nolting W. , Dubil U.
Published: July 2007

The s‐f model is used for investigating ferromagnetic f‐systems with an antiferromagnetic s‐f exchange interaction between conduction electrons and localized magnetic f‐moments. Particularily the electronic excitation spectrum is discussed for the exactly solvable limiting case of an empty conduction band at T = 0, and that via physical quantities such as the one‐electron spectral density, the quasiparticle density of states, and the electronic selfenergy. The ↑‐spectrum is simple, consisting of one single, only rigidly shifted band. The ↓‐spectrum, however, is built up by two quasiparticle subbands of quite different physical meaning. The gap between them closes if the Bloch band width exceeds a critical value. The ↓‐spectral density consists of one low‐energy δ‐peak and a relatively broad, high‐energy scattering part, which results from magnon emission processes. The dependence on model parameters and the distinction from f‐systems with ferromagnetic s‐f exchange is discussed in detail.
Critical Behaviour of the Ising Model with a Transverse Field
Authors: Wesselinowa J. M.
Published: July 2007

A Green's function technique is developed to study the Ising model with a transverse field. The order parameter and the longitudinal susceptibility are determined. The critical exponents β and γ are numerical calculated for different dimensional lattices. Using the scaling laws, α, δ, η, and v are obtained too. The critical exponents are in very good agreement with other theoretical results and with the experimental data.
Two‐Dimensional Properties of n‐Inversion Layers in InSb Grain Boundaries under High Hydrostatic Pressure
Authors: Kraak W. , Herrmann R. , Nachtwei G.
Published: July 2007

Magnetotransport properties of n‐inversion layers in grain boundaries of p‐InSb bicrystals are investigated under high hydrostatic pressure up to 103 MPa. A rapid decrease of the carrier concentration in the inversion layer is observed when hydrostatic pressure is applied. A simple model taking into account the pressure dependence of the energy band structure of pure InSb is proposed to describe this behaviour.
Free Carrier Absorption in n‐Type Indium Selenide
Authors: Mari B. , Segura A. , Chevy A.
Published: July 2007

Infrared absorption of n‐type indium selenide is studied in the 2.5 to 50 μm spectral range at 300 °K. Lattice absorption peaks are observed between 25 and 50 μm. Tin‐doped samples with an electron concentration higher than 1016 cm−3 exhibit free carrier absorption for λ larger than 10 μm. The experimental results are interpreted by the quantum theory of free carrier absorption. Quantitative account of absorption coefficients is given by introducing homopolar optic phonon and ionized impurity scattering.
The Coulomb Correlations and the Electric Field Local Heterogeneities on the Proton Conductivity of Hydrogen Bonded Chains
Authors: Krasnogolovets V. V. , Tomchuk P. M.
Published: July 2007

The hydrogen bonded chain conductivity is considered without the assumption of small concentration of the charge‐carriers (excess proton) in the small polaron model. The current density expression is derived considering Coulomb correlations and electric field local heterogeneities. The case of a space‐charge‐limited proton current is considered.
Resonatorless Optical Bistability of CdS Reconsideration of Transient Effects Due to Lattice Heating
Authors: Rossmann H. , Henneberger F.
Published: July 2007

Results of laser pulse transmission measurements are presented. Introducing a variable optical delay between exciting and probing pulse the relaxation behaviour of induced optical nonlinearities in the spectral region around the An = 1‐exciton resonance is investigated. Optical bistability reported earlier is supported. Crystal lattice heating and nonstationarity due to long‐living extrinsic effects are observed, too, but are of minor importance.
Valence Band Investigations of GaS by Means of Polarized X‐Ray Emission Spectra
Authors: Dräger G. , Czolbe W. , Schulz E. , Simunek A. , Drahokoupil J. , Kucherenko Yu. N. , Nemoshkalenko V. V.
Published: July 2007

The polarized K‐emission valence band spectra of the layer compound GaS are measured for both the components, Ga and S. For Ga spectra Johanm‐type and double crystal spectrometers are used. All spectra are calculated by the self‐consistent pseudopotential band structure method and also by the Xα‐scattered‐wave cluster calculation. The calculated and measured spectra are compared and discussed.
Continuity Chords of Bands and Band Structure of Semiconducting Compounds with TlSe‐Type Structure
Authors: Gashimzade F. M. , Guliev D. G.
Published: July 2007

All the simple band representations of the space group D are constructed. Using the concept of band representation and continuity chord, developed by Zak, it is shown that for semiconducting compounds with TlSe‐type structure it is possible to extract some useful information about the character of localization of basic functions, belonging to the same band, i.e. in fact about the nature of the chemical bond responsible for the observed band structure. In particular, from the comparison with a calculated band structure of the ternary semiconducting compounds with TlSe‐type structure, it is shown that the lower four valence bands correspond to s‐states of chalcogen atoms. Two following valence bands correspond to s‐states of three‐valent cations. Then, a large group of ten valence bands is produced by pz‐states of chalcogen atoms and px−, py−, Pz−states of three‐valent cations.
Hybrid Pairing and Heavy Fermions
Authors: Schuh B.
Published: July 2007

If the d–f exchange interaction is taken as the driving attractive interaction for a non‐phonon hybrid pairing mechanism one gets results of surprising significance for heavy Fermion superconductors: a transition temperature one order of magnitude smaller than the single ion Kondo temperature, a large critical field initial slope, and a huge specific heat jump, whereas the ratio of the gap at zero temperature to kTc has the BCS value 3.53. Except for the relation between Tc and the Kondo temperature these statements remain to be true for any other hybrid pairing interaction of the same form.
Dielectric Properties of Trans‐Polyacetylene. I. The Macroscopic Longitudinal Dielectric Function of a Peierls‐Fröhlich Semiconductor of Commensurability 2 within RPA
Authors: Drechsler S. L. , Bobeth M.
Published: July 2007

The macroscopic longitudinal wave‐vector‐ and frequency‐dependent dielectric function (MLDF) of a Peierls‐Fröhlich semiconductor with one electron per site is calculated within the 3D‐approach of Lee and Kivelson based on the Su‐Schrieffer‐Heeger model. Local field and exchange effects as well as overlap terms in the calculation of the matrix elements of the charge density are neglected. Conditions for the occurrence of an interband plasmon are presented, which are fulfilled in trans‐(CH)x. Special attention is paid to the q‐dependence of the MLDF and the influence of the interchain hopping.
Multiple‐Quantum Transitions and Optical Excitation Processes in the EPR of Mn2+ in ZnS
Authors: Backs I. , Stösser R. , Lieberenz M.
Published: July 2007

Microwave induced resonances of multiple‐quantum transitions, observed by the EPR method, are described for the system ZnS:Mn. From the anisotropy of the resonance lines, the EPR signal intensity‐microwave power dependence and the theoretical and practical intensity ratio of one‐and multiple‐quantum transitions it is concluded that the resonances are due to double‐and triple‐quantum transitions. Under certain experimental conditions a phase inverted EPR line is observed. Some irregularities are observed in the manganese EPR spectrum for the samples used. This is an optical excitation following microwave saturation, a different saturation dependence of the various fine structure transitions, a deviation of the occupation numbers from thermal equilibrium and of the fine structure line intensity from the theoretical ratio.
Mössbauer Absorption Spectra under Continuous Localized Diffusion
Authors: Afanasev A. M. , Sedov V. E.
Published: July 2007

The problem of the Mössbauer lineshape broadening due to the diffusion of the Mössbauer atom in an arbitrary drift potential is reduced to a solution of the Schrödinger equation of the quantum mechanics. The spectral shape can be obtained in explicit form provided an exact solution of the quantum problem is known. By means of this method the diffusion analogues of the quantum oscillator and the Coulomb atom are considered and analytical expressions for the respective spectra are derived. The temperature dependence of the intensity of the unbroadened spectral component is shown to be related with the form of the arbitrary “diffusional” potential by a simple law. The temperature curves of the mean‐square Mössbauer atom displacement resulting from this relation show the nontrivial behaviour at each value of the wave vector of absorbed γ‐quanta even in the simplest case of anharmonical oscillator. Under certain choice of the parameters the temperature trend of these curves may simulate the phase transitions.
EPR of Cu2+ in Barium Formate Single Crystal
Authors: Chand Prem , Upreti G. C. , Umar M. , Singh R. J.
Published: July 2007

Results of an X‐band (≈ 9.3 GHz) EPR study of Cu2+ doped barium formate single crystal at RT and at LNT show that Cu2+ enters the host lattice at substitutional Ba2+ sites as well as at an interstitial site. Both the copper (II) complexes show a low symmetry and low site symmetry effects are evident in EPR at RT. The g and A tensors are non‐coaxial at RT for both the copper complexes, however, they become coaxial at LNT. From experimental EPR data the ground state orbitals are obtained and discussed. The ground states are found to be mainly d with some admixture of d. From the observed g‐value the energy splitting of the d‐orbitals of the metal ion are also obtained. The angular behaviour of EPR spectra is in conformity with the symmetry class of the host crystal as determined by X‐rays.
The Magnetic Hyperfine Field for 119Sn Atoms in GdMnSi Intermetallic Compound Zero Contribution from 3d Moments
Authors: Delyagin N. N. , Krylov V. I. , Mujiri G. T. , Nesterov V. I. , Reiman S. I.
Published: July 2007

The magnetic hyperfine interaction for 119Sn impurity atoms in the GdMnSi host has been studied by Mössbauer spectroscopy. The hyperfine field for Sn atoms at 5 K is Bhf = (−27.6 ± 0.3) T. The temperature dependences of Bhf and magnetization are practically identical. As follows from the comparison of the results to data for GdCoSi host and an analysis of the satellite structure of the spectrum in Gd0.95Y0.05MnSi host, the contribution to the transferred hyperfine field from 3d moments in GdMnSi vanishes. Both experimental results and obtained empirical function of the radial dependence of the gadolinium contribution to Bhf emphasize the short‐range nature of the hyperfine interaction for Sn impurity atoms in Gd‐based magnetic alloys.
Many‐Body Theory for the Dense Exciton Gas of Direct Semiconductors. III. Response of the Many‐Exciton System on an Externally Driven Light Field
Authors: May V. , Boldt F. , Henneberger K.
Published: July 2007

The nonlinear response of a dense exciton gas in a platelet on an externally driven field is discussed using non equilibrium Green's function technique. A generalized Boltzmann‐equation of excitons containing many‐particle corrections in the diffusion‐, drift‐ and collision‐term is derived. Using a relaxed distribution (Boltzmann‐distribution) and an unrelaxed distribution (macro‐occupation) of ls‐excitons a bistability in exciton density as well as in the transmitted light intensity is obtained for a nearly resonant and stationary applied pump‐field of intensity 0.1 and 10kW/cm2, respectively. The bistability is obtained as a pure excitonic effect below the Mott‐transition.
Influence of Real Structure in the Interfacial Region on the Maxwell‐Wagner Effect Spherical Void in a Ionic Crystal
Authors: Poźniak J.
Published: July 2007

Theoretical calculations of the Maxwell‐Wagner polarization are performed by considering thermodynamic equilibrium conditions. The model applied consists of a spherical void inside an ionic matrix of NaCl type containing charge carriers (cation and anion vacancies) subjected to the action of an external dc electric field. Accurate analytical solutions obtained for the stationary state describes i) the distribution of the charge carriers in the vicinity of void and ii) the distribution of potential and electric field both, inside the void and in the crystal matrix.
New Aspects in the Theory of Impact Ionization
Authors: Ziep O. , Mocker M. , Keiper R.
Published: July 2007

The field induced change of the electron states causes avalanche processes already below the usual ionization threshold. For the calculation of the impact ionization coefficient αI the field influence of the corresponding microscopical rates has to be considered especially below the characteristic threshold energy due to higher occupation numbers. A comparison of the calculated values αI with experimental data in GaAs raises essential questions concerning the validity of the used stationary transport models.
Electron‐Spin‐Resonance Study of Co2+ and Ni+ Centers in AgCl(Cu, Co, Ni)
Authors: van Robbroeck L. , Goovaerts E. , Schoemaker D.
Published: July 2007

Single crystals of AgCI doped with Co2+, Ni2+, and Cu2+ are studied by means of electron spin resonance. As‐grown AgCI crystals contain three different Co2+ centers: called Co2+(cub), Co2+‐ (ortho) and Co2+(tetra) corresponding to centers in which a substitutional Co2+ ion is either unperturbed, or perturbed by a nearest or by a next nearest cation vacancy. Ultra violet illumination at 50 K produces Ni+ and Cu2+ centers and converts the Co2+(cub) centers to Co+ centers which are stable up to 80 K. The thermal decay of the latter enhances the Ni+ concentration. Above 160 K the Ni+ centers are thermally unstable and contribute to the decay of the Cu2+ centers. It is concluded that the presence of Co2+ ions supresses the darken ability of AgCI which is enhanced by the Ni2+ impurity.
Impurity Induced Energy Transfer in a Linear Polymer Chain
Authors: Čabrić B. , Ristovski L. J. , Davidović G.
Published: July 2007

A molecular polymer chain with a substitutional isotopic impurity is considered and the process of the energy transfer from the impurity to the host molecule on account of the exciton‐phonon interaction is investigated. The probability of this energy migration process is determined and numerically analysed.
A Theoretical Model of Ferroelectrics Exhibiting One, Two, or Three Curie Points
Authors: Piekara A. H.
Published: July 2007

A theory is presented for a ferroelectric crystal which is composed of two subsystems. In each of the subsystems there exist different interactions between the own elements as with the foreign ones. Even for simple ferroelectrics with a suitable nonlinearity of the argument of tgh there exists a thermal hysteresis for Ps.
On the Effects of Localized States and Tunnel Charge in Thin Insulating Films
Authors: Campabadal F. , Aymerich‐Humet X.
Published: July 2007

Tunneling barrier modifications produced by the charge of localized states in thin SiO2 films and by the charge due to the electron tunnel penetration into the barrier are analysed by means of the Green's function formalism. The charge density distribution at each energy, the modification of the electric field at the insulator, and the barrier height are obtained at equilibrium and under biasing conditions. It is shown that there exists a thickness d0 such that for d > d0 the charge inside the insulator is due to the localized states whereas ford d < d0 the tunnel charge predominates. Moreover, in the latter case, it is shown that in SiO2 films the electric field enhancement can be so large as to make the external electric field needed for breakdown lower than expected, in accordance with the anomalous breakdown values experimentally observed in the literature. An analysis of the transient charge localization in the oxide is also performed.
Polycrystalline Simple Average of Mechanical Properties in the General (Triclinic) Case
Authors: Ganster J. , Gems D.
Published: July 2007

For the general (triclinic) case the complete numerical values of the decomposition coefficients (d.c.) are given, necessary to calculate averaged mechanical properties (fourth rank tensors) from the single crystal compliance S (Reuss model) or stiffness C (Voigt model) and the generalized Fourier coefficients Cmmt of the orientation distribution function. With the help of a group theoretical approach many symmetries of the dc are found. This leads to a considerable reduction of numerical data. It is shown that only CTmmiwith l ‐ 0, 2, 4 are needed, i.e. that the simple mechanical average is not affected by ghost phenomena and depends only on certain features of the texture.
Solution Enthalpies of Aliovalent Defects in Alkali Halides
Authors: Bandyopadhyay S. , Deb S. K.
Published: July 2007

The solution enthalpies of some monovalent, divalent, and trivalent defects in some alkali halides are calculated following the standard procedure of minimising the total lattice energy change numerically, as a function of the relaxation of the nearest neighbours, when the defect cation occupies a substitutional position. The energies considered are electrostatic, dipole, dipole–dipole, and repulsive. It is found that inclusion of dipole–dipole energy helps to avoid the polarisation catastrophe problem for divalent and trivalent defects. The Born‐Mayer repulsive parameters of the aliovalent defects are obtained from the charge density overlap method. Thus it is shown that a single method is capable of yielding positive values of solution enthalpies for all the aliovalent defects considered.
Hartree‐Fock Approximation in the Scattering Wave Method. K‐Absorption of Ionic Crystals in the Single‐Scattering Formalism (XANES and EXAFS)
Authors: Bugaev L. A. , Vedrinskii R. V.
Published: July 2007

The scattering wave (SW) method is generalized for the case of Hartree‐Foek atomic potentials in ionic crystals. An earlier developed scheme of these crystal potential construction is used to obtain photoelectron scattering phase shifts. X‐ray absorption near edge structure (XANES) and extended fine structure (EXAFS) of crystals NaCl, KCl, NaF are then calculated in the single‐ scattering approximation of the SW‐method. The approach used permits to obtain EXAFS of ionic crystals with high accuracy and also the main features of their XANES.
Energy Levels of Z‐Centres in KCl:Ca2+. A Model Potential Perturbation Calculation
Authors: Kassim H. A.
Published: July 2007

Theoretical 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

The 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

Energy 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

A 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.
Selection rules for matrix elements of electron–phonon scattering processes in crystals with TlSe structure
Authors: Gashimzade F. M. , Tagirov E. V.
Published: July 2007

Selection rules for intervalley transitions in TlSe structural crystals are obtained. The results are presented both, without and with regard to the electron spin.
Screening of the electron–phonon interaction in GaAs heterostructures
Authors: Xiaoguang Wu , Peetees F. M. , Devreese J. T.
Published: July 2007

The 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

On 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

Results 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.
Mössbauer lineshape in presence of relaxation and diffusion. A Stochastic Approach
Authors: Moretti P. , Cianchi L.
Published: July 2007

The shape of the Mössbauer relaxation pattern is derived for a Mössbauer ion performing an arbitrary diffusive motion; the formulae used in the literature follow as particular cases.
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

The 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

A 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

The 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

The 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.