bluuebunny/crossref_metadata_2025 huggingface dataset:
Band structure and X‐ray investigation of transition metal aluminides
Authors: Müller Ch. , Wonn H. , Blau W. , Ziesche P. , Krivitskii V. P.
Published: July 2007

Band structure and density of states of the transition metal aluminides VAl, CrAl, FeAl, CoAl, and NiAl are calculated using a modified KKR method and compared with various kinds of experimental soft X‐ray spectra. The trends of experimental observed physical properties through the 3d transition metal series are well reflected by the band structure data and turn out to be essentially due to the degree of filling up of the transition metal d‐band by Al p‐electrons. In particular the results are capable of explaining the instability of a homogeneous B2 phase in the case of CrAl.
Photostimulated low‐temperature recombination luminescence in γ‐irradiated KCl:Eu2+ crystals
Authors: Opyrchal H. , Nierzewski K. D.
Published: July 2007

A strong, few seconds lasting post‐luminescence due to Eu2+ emission is excited at 15 K by light flashes in KCl:Eu2+ crystals coloured by γ‐rays at RT. Within the range of 15 to 70 K the intensity of post‐luminescence varies in inverse proportion to the time and does not depend on temperature. Thermoluminescence glow curves photostimulated at 15 K point to the existence of several glow peaks most of which are due to the thermal activation of trapped‐hole centres of H‐ and Vk‐type. The plausible mechanisms responsible both for post‐luminescence and thermoluminescence as well as the role of Eu2+ ions are discussed.
Scattering of phonons by density fluctuations and thermal conductivity of amorphous solids
Authors: Joshi Y. P.
Published: July 2007

Phonon relaxation time due to the scattering by spatial density fluctuations is studied in amorphous solids. The relaxation time τf is found to vary with frequency ω as ω−4 in the case of phonons with frequencies small compared with ω0, which is a parameter characteristic of the density fluctuations. When ω is large in comparison with ω0, τf is found to vary as ω−2, and thus it undergoes a transition from a strong to a comparatively weak frequency dependence as ω increases and crosses ω0. It is discussed that the thermal conductivity of a typical amorphous solid can be explained in terms of the scattering by low lying tunnelling states and scattering by fluctuations in density and elasticity. The plateau in conductivity is interpreted as defining a temperature region where the former scattering mechanism with its comparatively weaker frequency dependence starts losing dominance over the latter. Finally the conductivity of vitreous silica is analysed, obtaining a good agreement between theoretical and experimental results.
Energy spectrum, magnetization, and critical field of the uniaxial two‐sublattice ferrimagnet by the Green function method
Authors: Kazakov A. A. , Litvinenko N. A.
Published: July 2007

The energy spectrum and the magnetization of an uniaxial two‐sublattice ferrimagnet in all intervals of fields and temperatures are calculated by the method of two times Green temperature functions both, in easy and hard directions. It is shown that the long‐wave spectrum of elementary excitations can “oscillate” depending on the value of the external magnetic field. It leads to the fact that in the region of critical fields and low temperatures the magnetization of system follows the simple power law T2 in the plane and T3/2 along the axis independently from the type of ferrimagnet anisotropy.
Excitation of magnetoacoustic surface waves by Bloch walls
Authors: Mitin A. V. , Tarasov V. A.
Published: July 2007

The possibility of excitation of magnetoacoustic surface waves by an rf field on Bloch domain walls is considered. The investigation indicates that dispersive and nondispersive surface waves may be generated. The resonance frequency of magnetic moment vibrations in a Bloch wall near specimen surface is received. When the rf field lies in the easy axis of magnetization the magnetoacoustic surface waves are excited actually in one side. The consideration is restricted to the case of dilute YIG for simplicity.
Electron–hole liquid in GaP the influence of the isoelectronic impurity nitrogen
Authors: Schwabe R. , Thuselt F. , Weinert H. , Bindemann R.
Published: July 2007

The radiative recombination of isoelectronic (nitrogen)‐doped GaP is investigated at high optical excitations and low temperatures (T = 2 K). Extensive experimental results on the luminescence in dependence on the excitation intensity, the nitrogen concentration (NN) and its decay behaviour are presented. At low nitrogen concentrations (NN ≦ 1017 cm−3) and highest excitation levels the radiative recombination from an electron–hole liquid (EHL) accompanied by momentum conserving phonons analogous to undoped GaP can be confirmed. At higher nitrogen concentrations in the range of NN = 3 8 × 1017 cm−3 a radiative no‐phonon transition appears from an EHL induced by the isoelectronic impurity nitrogen. The influence of the latter on the energy of the EHL is studied. The rising nitrogen density yields an increase in the EHL work function. At medium and high nitrogen concentrations (NN ≦ 1018 cm−3) a further up to now unknown luminescence band becomes visible arising from the radiative recombination of a “plasma‐like” phase which is strongl influenced by the isoelectronic impurity nitrogen and coexists with the EHL. Its density (np1) depends on the nitrogen concentration. The use of a simplified model basing on the polarization energy of the carriers caused by the short‐range isoelectronic potential enables the understanding of the experimental findings.
Experimental evidence for piezoelectrical free‐carrier scattering by screw dislocations in tellurium
Authors: Estienne J. P. , Farvacque J. L.
Published: July 2007

Hall effect and conductivity measurements are performed, in the case of tellurium, on samples containing a high density of a screw dislocations and on reference samples. Comparison of these measurements shows that a screw dislocations are responsible for a strong free carrier mobility decrease. A discussion of the results, based on theoretical calculations of the free carrier mobility permits to conclude to a piezoelectric scattering mechanism.
Resonance energy shift for mössbauer impurity atoms trapped by vacancies
Authors: Tewari A. K. , Roy S. K. , Bhattacharya D. L.
Published: July 2007

An estimation of the second order Doppler shift is made for a simple lattice model containing impurities and vacancies by employing the double time Green's function technique at the high temperature limit. The changes in the force constants due to the presence of substitutional impurities and the vacancies near to the impurity are evaluated from the Debye relation and are used to estimate the contributions. It is found that the two contributions (one due to the impurity and the other due to the vacancy near to the impurity) arising from the last two terms of the lattice Hamiltonian yield a negative shift of the absorption line, which is expected to be significant for high concentrations of point defects.
Scattering of phonons by spherical defect aggregates
Authors: Joshi Y. P.
Published: July 2007

The phonon relaxation time due to defect aggregates in solids is studied by the perturbation method. The defect aggregates are assumed to have spherical shape and to be formed by the clustering of a large number of atoms that differ from the host crystal atoms in mass alone. The relaxation time τ obtained is found to be of the Rayleigh type with a frequency dependence of ω−4 type for wavelengths λ large compared with the diameter d of the defects, ω being the phonon frequency. However, its dependence on ω is observed to be approximately of ω−2 type for λ smaller than, and yet comparable in order of magnitude with d. This result is at variance with the geometrical scattering that is generally taken to be valid for shorter‐wavelength phonons. The transition of τ from ω−4 to ω−2 is found to be rather smooth.
Electromagnetic waves in an inhomogeneous semiconductor under electric and magnetic fields
Authors: Dykman I. M. , Prokhnitskii L. A.
Published: July 2007

A theoretical investigation is made of the case of normal incidence of a linearly polarized electromagnetic wave upon a donor semiconductor to which a heating electric field F and a magnetic field H is applied. The impurity concentration is determined by the law N(z) = N0 + N1 cos qz. The wave inside the semiconductor splits into two waves with circular polarizations. Near the Bragg resonance, which can occur only for one of these two waves having the wave vector k ≈︁ q/2, some peculiarities appear in the reflection and the transmission of this wave. It is shown that the reflection and the transmission coefficients as well as polarization of the resulting wave can be considerably influenced by means of the magnetic and the electric fields.
The theory of magneto‐optical effects in shallow f‐centres
Authors: Gifeisman Sh. N. , Malkoch Yu. G.
Published: July 2007

In the framework of the path‐integral formalism of non‐relativistic quantum mechanics the Faraday effect theory of the local centres in ionic crystals with intermediate polaron coupling is developed. The dissipative properties of the phonon–impurity system are investigated by the non‐equilibrium density matrix method. In zeroth approximation the system is described by the trial Lagrangian, in which the Coulomb interactions of the impurity electrons with lattice defects and vibrations are imitated by elastic forces. In the first order on the difference between the exact and trial Lagrangians the expressions for the admittance tensor components of the system determining the Verdet and Cotton‐Mouton constants are obtained. At the absolute zero temperature the magnetic correction to the polaron effective mass is also calculated.
Impurity band in Hg1−xCdxTe alloys
Authors: Dugaev V. K. , Petrov P. P.
Published: July 2007

The problem of the acceptor impurity band formation in the Hg1−xCdxTe alloy with ϵg≦ 0.3 eV is discussed. Within the coherent potential approximation the density of states as a function of impurity concentration and alloy composition is calculated. The dependence of the impurity band centre location on alloy composition is obtained. The dependence of the acceptor impurity activation energy on the impurity concentration is determined.
Effects of controlled disorder on the phase transitions and charge density wave state in tetrathiofulvalene–tetracyanoquinodimethane
Authors: Gunning W. J. , Chiang C. K. , Heeger A. J. , Epstein A. J.
Published: July 2007

Transport and magnetic studies of the phase transitions and low temperature charge density wave state in TTFTCNQ as a function of irradiation induced defect concentration are presented. The transport and magnetic results imply a suppression and broadening of the phase transition and incomplete 3d ordering for T < 34 K. Although the TCNQ susceptibility decreases toward zero at T = 0 in irradiated samples, the contribution from the TTF chains remains finite.
Pre‐asymptotic charge oscillations around 3d transition metal impurities in aluminium
Authors: Lautenschläger G. , Mrosan E.
Published: July 2007

The charge density oscillations around 3d transition metal impurities in aluminium are discussed. Comparing the asymptotic and pre‐asymptotic results, in the pre‐asymptotic case a reduction of the amplitude of the charge density oscillations is found. This reduction reflects the energy dependence of the impurity scattering phase shifts.
On the susceptibility of strongly correlated electrons
Authors: Elk K.
Published: July 2007

The isothermal longitudinal susceptibility of the Hubbard model is calculated. It is shown that in strongly correlated electron systems the susceptibility is proportional to the reciprocal temperature in the low temperature region as well as at high temperatures. Therefore the particular experimental results of some quasi‐one‐dimensional systems cannot be explained by the influence of the electron correlation.
Flux redistribution of channeled fast electrons
Authors: Popov D. E. , Kaplin V. V. , Vorobiev S. A.
Published: July 2007

The orientation dependence of planar channeling for 0.5 to 2.5 MeV electron transmission through crystalline Si is experimentally observed. The phenomenon of spatial flux redistribution results from the formation of bound states of the channeled electrons with atomic planes. Application of electron channeling is considered for the control of irradiation effects in crystals and the study of perfection of the crystalline structure.
Appearance potential spectroscopy in metals and alloys multiple scattering approach
Authors: Donato E. , Giuliano E. S. , Ruggeri R. , Stancanelli A.
Published: July 2007

A multiple scattering approach is used to deal with the excitation of core electrons in metals and alloys within the one‐electron approximation. By assuming the crystal potential in the nonoverlapping muffin‐tin form the excitation probability is written as a product of atomic‐like and the scattering path operator matrix elements. It is also shown how this way of factorizing the excitation probability is particularly suitable for describing the scattering process in random alloys, too.
Calculation of the interband absorption in lead chalcogenides using a multiband model
Authors: Ziep O. , Genzow D.
Published: July 2007

The optical absorption coefficient for direct allowed interband transitions between states L+6 and L−6 is calculated within the framework of a multiband model, including six bands in a k · p treatment. Due to the enhanced joint density of states at higher energies in the multiband model as compared with the Dimmock‐model a good agreement is obtained between calculated and experimental absorption spectrum also at photon energies hw > 2EG.
Mean component approximation for the correlation energy of multicomponent electron–hole plasmas
Authors: Wünsche H.‐J.
Published: July 2007

Electron–hole plasmas are considered that may involve several components, i.e., particles of different types with equal isotropic masses but different densities. The RPA energy of the correlation between any two components is expressed by that of an electron gas by finding an equivalent auxiliary plasma that is composed of an effective number of components equally occupied with a mean density (“mean components”) and that has the same correlation energy. Finally using an interpolation formula for the correlation energy of the electron gas, an analytic expression is obtained which is shown to yield good results for the spin‐polarized electron gas as well as for neutral and non‐neutral electro–hole plasmas.
Pade approximation method as renormalization group method
Authors: Praveczki E.
Published: July 2007

It is shown that the Pade approximation method applied to high temperature series can be regarded as a special renormalization group method. The renormalization group operator is constructured exactly and explicitly.
Theory of dechanneling
Authors: Rozhkov V. V.
Published: July 2007

A theory of dechanneling is proposed. It is based on the solution of the boundary problem for the kinetic equation describing the evolution of channeled particles. The expressions for the dechanneled fraction, distribution function of the lifetime of the channeled particles, and half‐thickness are obtained in a general form. The theory permits to take into account in a simple manner the simultaneous action of many dechanneling factors as well as incomplete dechanneling on channel borders and rechanneling.
Surface modes in piezoelectric crystals
Authors: Albuquerque E. L.
Published: July 2007

The dispersion relation and the surface mode are discussed for acoustic waves propagating in a piezoelectric semi‐infinite medium. The polarizations of these acoustic waves are chosen to allow coupling between the acoustic and electromagnetic field. Two physically different situations are considered for the piezoelectric medium and found that, while in one case propagation of surface waves occurs, there is no propagation of such waves in the other case.
Stimulated brillouin scattering in a magnetised semiconductor plasma
Authors: Guha S. , Apte N.
Published: July 2007

The stimulated Brillouin scattering of a large amplitude electromagnetic wave due to the excitation of an acoustic wave is investigated analytically in a piezoelectric semiconductor plasma in the presence of a homogeneous static magnetic field parallel to the direction of propagation. A dispersion relation is obtained using the coupled mode theory and the threshold electric field of the pump necessary for the onset of instability as well as the initial growth rate of the unstable mode are obtained analytically for the two cases: 1. without magnetic field, 2. with magnetic field. The threshold electric field and the growth rate for these two cases are compared and it is found that the magnetic field reduces the threshold electric field considerably but increases the growth rate.
Optical properties of charge carriers in magnetite and Mn ferrites
Authors: Šimša Z.
Published: July 2007

The optical properties of magnetite and iron‐excess manganese ferrites are analyzed in terms of both, the Drude‐Zener model of the free electron gas and the small polaron theory applied to the hopping of the electrons (Fe2+ ions) in the octahedral positions of the spinel lattice. Several characteristic parameters of the charge carriers are deduced and a comparison with the dc electrical conductivities is made.
Far‐infrared absorption of BaF2:Dy3+ versus concentration
Authors: Villermain‐lecolier G. , Morlot G. , Roger A. , Hadni A.
Published: July 2007

In the BaF2:Dy3+ FIR spectrum the vibration frequencies of the interstitial compensation ion Fi is observed and the presence of some cluster types is revealed for concentrations equal to or higher than 0.4 mol%. A 125 cm−1 line is observed for the first time with Dy3+. In BaF2:Nd3+ and BaF2:Ho3+ studies it is previously ascribed to the Fi vibration longitudinal mode in a trigonal site. Calculation gives 135 cm−1 for the transversal mode vibration; experimentally it is observed for the first time. The 42 cm−1 line previously ascribed to an electronic transition of Dy3+ in trigonal site disappears when the concentration increases.
Infrared spectra of calcium hydride fluoride CaF2−xHx
Authors: Vergnat‐grandjean D. , Vergnat P. , Brice J.‐F. , Leveque R.
Published: July 2007

The existence of solid solutions of composition CaF2−xHx (0 < x ≦ 1.24) is recently established, crystallizing in CaF2‐type cubic symmetry. There is a statistical substitution of fluoride ions by hydride ions in the CaF2 unit cell. The infrared spectroscopy gives informations on these compounds by the behaviour of the H− ions localized vibration and that of the fundamental TO mode when x varies.
A boltzmann equation for phonons and electrons
Authors: Nonnenmacher Th. F. , Zweifel P. F.
Published: July 2007

A Boltzmann equation is derived for electron and phonon transport in bounded systems, which is more reliable than the relaxation time approximate version of the transport equation. The corresponding cros‐sections have non‐azimuthal symmetry. This requires basically new solution techniques which will also be discussed.
Disorder effects on free excitons in CdS1−xSex mixed crystals
Authors: Goede O. , Hennig D. , John L.
Published: July 2007

In CdS1−xSex mixed crystals the energies and transition probability ratios for free A, B, and C excitons are obtained as a function of the composition x by reflection and emission measurements at 77 K. The observation of the disorder‐allowed A(Γ6) exciton transition, the bowing of the energy difference between A and C exciton, and the broadening of the exciton lines clearly demonstrate the importance of disorder effects in these mixed crystals. This conclusion is further supported by the stress‐induced enhancement of the A(Γ6) exciton transition probability by uniaxial stress parallel to the c‐axis of the mixed crystals. The experimental results are discussed on the basis of an effective exciton Hamiltonian consisting of a quasi‐cubic VCA‐Hamiltonian for wurtzite‐type mixed crystals and an additional lower‐symmetric term which describes the disorder effects phenomenologically by two fluctuating crystal fields parallel and perpendicular to the c‐axis, respectively.
Pseudoharmonic effect of phonons in electrical resistivity
Authors: Malinowska‐Adamska C. , Wojtczak L.
Published: July 2007

The temperature behaviour of the electrical resistivity of metals is investigated in the case when the anharmonic properties of phonons are taken into account in the pseudoharmonic approximation. The results show that the pseudoharmonic effects of lattice vibrations are not negligible in calculations concerning the high temperature region. A comparison with experimental data is given for copper, silver, and gold theoretically described by various potential functions.
EPR of Mn2+ doped single crystals of Ni(NH42) (SO4)2 · 6 H2O and NiSO4 · 7 H2O
Authors: Misra S. K. , Mikolajczak B.
Published: July 2007

X‐band EPR measurements are reported on single crystals of Ni(NH4)2(SO4)2 · 6 H2O and NiSO4n · 7 H2O doped by Mn2+ ions at various temperatures. The absolute sings of the spin Hamiltonia parameters are determined from liquid helium temperature data. A study is made of the variation of linewidths with temperature, as well as with the magnitude of the static magnetic field. The spin Hamiltonian parameters are evaluated rigorously from a simultaneous fitting of the data obtained for several orientations of the static magnetic field, using the method of least‐squares fitting and numerical diagonalization of the spin Hamiltonian matrix appropriate to electronnuclear spin coupled systems.
On high energy particle channeling
Authors: Darbinian S. M. , Ispirian K. A.
Published: July 2007

The motion of relativistic positive and negative particles in the cases of axial and planar channeling is considered theoretically. The channeling fraction and the distribution in the transversal plane for high energy particles passing through crystals are calculated.
Spectral density of phonons in alkali metals
Authors: Vaks V. G. , Zein N. E. , Kravchuk S. P. , Trefilov A. V.
Published: July 2007

The phonon spectral density g(ω) is calculated for five alkali metals. The previously described pseudopotential model giving a rather accurate description of these metal dynamics is used for the phonon frequencies computation. The calculated g(ω) are similar for all the alkali metals and have the characteristic three‐peak form with a high and narrow high‐frequency peak. The volume dependence of g(ω) is investigated and is found to be rather sharp. It is shown that g(ω) is very sensitive to the choice of the theoretical model so that its measurements can provide an effective method for the proof of the accuracy of these models.
Band‐edge discontinuities and interface potential step in the two‐band narrow‐gap approach
Authors: Detcheva V. , Kandilarov B. D.
Published: July 2007

Relations between band‐edge discontinuities and interface potential step are derived in the framework of the two‐band narrow‐gap approach and in terms of gap‐attached bulk properties of the two semiconductors in epitaxial contact. The expressions, initially deduced for energy gaps opened at the centre or edges of the corresponding Brillouin zones, are subsequently extended to include the more general case of gaps opened in other points inside the Brillouin zones.
Coherent States and Bloch Functions
Authors: Krivoshlykov S. G. , Manko V. I.
Published: July 2007

A quasi‐momentum representation for an arbitrary quantum system is constructed by the integrals‐of‐motion method and by the coherent‐states method. Explicit expressions in terms of the Jakobi ϑ‐functions are obtained for the Bloch functions of the following dynamical systems: a relativistic and non‐relativistic electron moving in a constant uniform magnetic field and a non‐relativistic electron moving in both, time‐dependent and constant crossed electromagnetic fields. New mathematical relations between the Jacobi ϑ‐functions are obtained.
A Percolative Approach to the Theory of the AC Hopping Conductivity
Authors: Zvyagin I. P.
Published: July 2007

The ac hopping current in the multiple‐hopping regime is evaluated using the time‐dependent cluster construction. The dependence of the ac conductivity on frequency, temperature, and electronic structure of the material is discussed.
Generation of Free Excitons and Exciton—Impurity Complexes by Fast Electrons in Extremely Anisotropic Crystals
Authors: Kozitskii S. V. , Rack M.
Published: July 2007

The generation of free and bound excitons by fast electrons is calculated within the framework of an extremely anisotropic model of a layer crystal. Selection rules are discussed. The oscillator strength of the exciton—impurity complex is two or three orders of magnitude higher than that for free excitons in the case of direct transition and greatly decreases with increasing distance between the extrema in the case of indirect transition. For small scattering angles θ ≈ 10−3 rad the probability of generation of excitons as well as exciton—impurity complexes by fast electrons does not depend on θ.
Non‐Linear Transport Properties of Band Electrons at Arbitrary Electron—Phonon Coupling
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

The equations of motion for the electronic density matrix represented by functional integrals are obtained at arbitrary electron—phonon coupling strengths. The method of a self‐consistent calculation of the functional integrals is generalized to the varying strong electric fields. This approach is applied to the problem of a stationary non‐linear response of band electrons to a constant electric field and strong electromagnetic fields. The relations are found which determine an immediate dependence of these fields on the given average response. The effects connected with the polarization due to a non‐linear electron response are described.
Calculation of the Electron Momentum Distribution in Zinc in Connection with Positron Annihilation
Authors: Pawłowska Z.
Published: July 2007

The electron momentum distribution in zine is computed on the base of band structure calculations in three directions: [001], [100], [110]. The group‐theoretical treatment resulting in a selection rule for the h.c.p. structure shows that some bands cannot contribute to the photon‐pair momentum density along symmetry lines in momentum space. Results of the application of the selection rule to the h.c.p. structure are given in the paper.
Many‐Body Effects on the Third‐Order Elastic Constants of Alkaline Earth Fluorides
Authors: Bedi S. S. , Verma M. P.
Published: July 2007

Expressions for the third‐order elastic (TOE) constants are derived from the Lundqvist potential for the non‐centrosymmetric solids crystallizing in the fluorite structure. The values of the TOE constants determined from these expressions agree fairly well with the measured values for CaF2, SrF2, and BaF2 and show significant improvements over the values obtained in other theoretical studies.
Electronic Structure of the Ferromagnetic Semiconductor s—d Model from the Lowest Six Momenta
Authors: Čápek V. , Chvosta P.
Published: July 2007

The lowest six momenta of the spectral function of the ferromagnetic semiconductor s—d model are expressed in terms of the spin correlation functions. This information is used to fit the unknown spectral function by a physically acceptable model function preserving the lowest six momenta. Using that, a model electronic quasiparticle energy structure is calculated and discussed for all relevant areas of parameters involved. Throughout the work, the rotational symmetry of the original Hamiltonian is preserved.
Magnetoplasma Effects in Bi and Semimetallic BiSb Alloys at 300 GHz
Authors: Braune W. , Lebech J. , Saermark K.
Published: July 2007

Experimental results on magnetoplasma phenomena in Bi and semimetallic BiSb alloys with Sb concentrations up to 6.8 at% are presented. The Voigt configuration and trigonal sample planes are used. The field configurations B ‖ C2 ⟂ Erf and B ‖ C1 ⟂ Erf are considered. The field positions for the dielectric anomaly and cyclotron masses are determined as functions of the Sb concentration.
On the Composition Dependence of the Lattice Dielectric Constant in Bismuth—Antimony Alloy Systems
Authors: Schuchardt R. , Rudolph R. , Keiper R. , Krüger2 H.
Published: July 2007

The strong increase of the lattice dielectric constant observed at a critical antimony concentration in semiconducting bismuth—antimony alloy systems can be interpreted in terms of an electronic interband polarization enhanced by vanishing thermal energy gap. In particular, the indirect transitions between the T‐point valence and the L‐point conduction bands assisted by elastic short‐range scattering mechanisms play a dominant role. The direct transitions within the quasidegenerate L‐point band complex lead only to a nearly composition independent “background” dielectric constant when the available information on the composition dependence of the relevant band structure parameters is taken into account.
Calculations of the Properties of Superconducting Alloys via the Average T‐Matrix Approximation. An Application to the Binary Substitutional NbMo Alloy System
Authors: Chatterjee P.
Published: July 2007

The theoretical formula of McMillan, modified via the multiple‐scattering theory by Gomersall and Gyorffy, has been very successful in computing the electron—phonon coupling constant (λ) and the transition temperature (Tc) of many superconducting elements and compounds. For disordered solids, such as substitutional alloys, however, this theory fails because of the breakdown of the translational symmetry used in the multiple‐scattering theory. Under these conditions the problem can still be solved if the t‐matrix is averaged in the random phase approximation (average T‐matrix approximation). Gomersall and Gyorffy's expression is reformulated for λ in the random phase approximation. This theory is applied to calculate λ and Tc of the binary substitutional NbMo alloy system at different concentrations. The results appear to be in fair agreement with experiments.
Structural Studies of Thermal Vibrations in Ferroelectric SbSI
Authors: Itoh K. , Matsunaga H. , Nakamura E.
Published: July 2007

The anharmonic atomic potential at 1.3 °C and the temperature dependence of thermal amplitudes in SbSI are calculated from crystal structures previously reported. The contribution of the soft mode to the thermal amplitude is found along the c‐axis for the Sb atom over a wide temperature range around the transition temperature. The temperature dependence of the frequency of the soft transverse optic mode is estimated in the paraelectric phase up to 100 °C.
Raman Scattering from Tunneling of Substitutional Molecules in Cubic Crystals
Authors: Dick B. G.
Published: July 2007

Raman scattering cross‐sections for tunneling transitions of diatomic molecular or off‐center defects in cubically symmetric environments are derived for 〈100〉, 〈111〉, and 〈110〉 defects for arbitrary scattering geometry. These cross‐sections are expressed in terms of the anisotropic electronic polarizability components of the defect.
ESR and Annealing of Defect Centers Trapped in Irradiated Cadmium Chlorate
Authors: da Silva E. C. , Torriani I. C. L. , Leite N. F. , Hennies C. E. , Évora C. , Vargas H.
Published: July 2007

By ESR and optical absorption experiments the paramagnetic centers O, ClO2 and (ClO2Cl)− are identified in irradiated single crystal Cd(ClO3)2. The ESR spectrum and its angular dependence show that the centers have one magnetically equivalent site per unit cell. The thermal annealing of the centers show that they follow a first‐order kinetic activation process. Crystallographic data are obtained for Cd(ClO3)2 not available in the literature.
Low‐Temperature Specific Heat of VN0.74 and VN0.89
Authors: Lovtchinov V. , Mädge H. , Christensen A. N.
Published: July 2007

The specific heat and superconducting critical temperature of VN0.74 and VN0.89 are measured in the temperature range 1.8 < T < 20 K. The density of states at the Fermi level is determined on the basis of these results. The dependence of the density of states on the parameter of the electron—phonon interaction is discussed as well. It may be expected that increasing the N content of the mentioned compounds would lead to the intensification of the electron—phonon interaction and the increase of the density of states at the Fermi level and the critical temperature, respectively.
Charge Carrier Scattering on Neutral Centres in n‐Si
Authors: Dmitrenko N. N. , Kurilo P. M. , Litovchenko P. G. , Lutsyak V. S. , Nitsovich V. M.
Published: July 2007

A discussion of the basic models for the calculation of the relaxation time for scattering on neutral impurities is given. A comparison is made between the experimental results of scattering of electrons on neutral oxygen atoms in n‐Si and the theoretical data obtained on the basis of three models. It is shown that an S‐partial wave approximation with a phase shift linear in the momentum, taking into account Friedel's sum rules and the many‐channel scattering in many‐valley semi‐conductors, gives a good agreement with experimental results for impurity concentrations from 1013 to 1017 cm−3 in the temperature interval 77 to 300 K.
Ionization Loss Spectra of FeS2
Authors: Kätzel W. , Berndt K.
Published: July 2007

The technique of energy loss spectroscopy is applied to the semiconducting compound FeS2. A comparison is made of the ionization losses with the density of unoccupied states. Taking into account other spectroscopic data the full density of states of the FeS2 valence and conduction electrons is confirmed.
Study of Lattice Instability by mm‐Wave Magnetoplasma Reflection in PbTeSnTe Compound Semiconductors
Authors: Nishi S. , Kawamura H. , Murase K.
Published: July 2007

The static dielectric constant ϵs in Pb1−xSnxTe (x = 0 to 0.4) single crystals is determined from a magnetoplasma reflection measurement between 4.2 and 100 K. At high temperatures ϵs increases with increasing temperature obeying a Curie law. The Curie constant is practically unchanged for different alloy composition, and the inverse static dielectric constant at 4.2 K decreases lineaarly with x, going to zero at alloy composition x = 0.35. At x = 0.40, a phase transition is observed at 20 K. The temperature dependence of ϵs is explained by anharmonic terms of lattice vibrations.
Molecular Liquid Model and Electronic Structure of Amorphous Silicon and Germanium
Authors: Halder N. C. , Shenoy S. R.
Published: July 2007

The atomic and electronic structure of amorphous Si and Ge are investigated by the molecular liquid model (MLM). The theoretical structure factor is calculated using simple parameters — nearest neighbor distance, packing density, and coordination number. The electronic density of states obtained with the MLM structure factor, and nonlocal energy dependent pseudopotential in second order perturbation theory, show good agreement with experimental and other theoretical results.
Theory of Absorption by Excition Polaritons in a Spatially Dispersive Medium
Authors: Nkoma J. S.
Published: July 2007

Absorption by normal modes of excition polaritons under normal incidence excitation is considered and an effective absorption coefficient is derived by considering the energy fluxes of the individual modes. The frequency integrated absorption in the presence of spatial dispersion is found to be a constant for large damping and approaches zero for zero damping. It is also found to oscillate for small depths due to interference of the polariton modes. Excition mass variations are also considered. The derived expressions are illustrated by parameters modelling the Is exciton of PbI2.
Discontinuous Phase Transitions Induced by Magnetic Field in a Ferromagnet such as U3X4. Application to U3As4
Authors: Kowalczyk B.
Published: July 2007

In the framework of a microscopic spin model which contains the fourth‐order crystal‐field anisotropy terms it is shown that the first‐order phase transitions induced by magnetic field are possible in ferromagnets such as U3X4. Using the variational method the behaviour of the system in external fields of various orientations at zero temperature is discussed. The critical field as a function of Hamiltonian parameters and the tricritical point line are found. The dependence of the magnetization on the magnetic field value is also determined. The results obtained are qualitatively consistent with the experimental data for U3As4.
Cluster MO Calculations of Electronic Structure and X‐Ray Emission Spectra of Carbide Alloys TixV1−xC
Authors: Ivanovskii A. L. , Gubanov V. A. , Zhukov V. P. , Shveikin G. P.
Published: July 2007

Results are given of calculations of the electronic structure of TixV1−xC solid solutions within a wide range of different compositions by means of the semi‐empirical molecular orbital method in the cluster model. The calculated X‐ray emission spectra are in reasonably good agreement with experimental data.
Thermodynamical Values of a Uniaxial Two‐Sublattice Ferromagnet in a Magnetic Field
Authors: Kazakov A. A. , Litvinenko N. A.
Published: July 2007

The magnetization of a uniaxial two‐sublattice ferromagnet with anisotropy of the pair and oneion type in longitudinal and transverse magnetic fields is investigated using the method of parametrical two‐times Green temperature functions. The methods of calculation of the thermodynamical characteristics of the system in low‐ and high‐temperature regions are discussed.
A First Principles Model of Energetical Trap Distributions in Solids. I. The Linear Finite Defect Chain
Authors: Hamann C.
Published: July 2007

Based on the real structure of solids a model of the finite defect chain is developed. Preserving a general validity the model allows the treatment of arbitrary potential functions containing the Coulomb potential as a case of special interest. For this potential the model leads to an analytical derivation of the exponential trap distribution, this way being physically testified for the first time.
Resonant Behaviour of X‐Ray Intensities in Metallic Nickel Near the 2p Threshold
Authors: Riehle F.
Published: July 2007

The Lα and Lβ lines of metallic nickel and the X‐ray isochromats recorded at quantum energies in the vicinity of these lines are reported. An X‐ray spectrum with constant energy of the final state (XCEF) is defined and yields resonance‐like intensity variations of the bremsstrahlung similar to the rare earth metals. The energy dependence of the resonances is in qualitative agreement with the model of Wendin and Nuroh showing the importance of transitions involving normally empty localised d‐states.
Theory of Absorption Profiles of Molecular Transitions in Dense Media
Authors: Paerschke H. , Süsse K.‐E. , Welsch D.‐G.
Published: July 2007

Within the framework of quantum theory the radiationless phase relaxation of a molecular transition in a dense medium is described by an integro‐differential equation. From the Fourier transform of this equation formulas are derived for calculating the corresponding absorption profiles. For the case that the acoustic phonons of the medium are responsible for the phase relaxation a detailed discussion of the resulting absorption profiles is given.
Comparison of Experimentally and Theoretically Determined Dipole Concentrations of NaCl:Ca++ Considering Aggregation Phenomena
Authors: Grau P. , Dubiel M. , Berg G.
Published: July 2007

The equilibrium state of impurity–vacancy dipoles at RT is calculated by means of experimental determined association energies of the different dipole aggregates and of the point charge approximation, respectively. A comparison with experimental results originating in dielectric loss measurements shows a good agreement, especially with the results computed by the experimentally determined energies. It is possible to quench‐in dipole concentrations higher than the equilibrium value by cooling down the samples from 600 °C. But at higher impurity content partially aggregates or precipitates are formed during this cooling process, as a comparison with the Stasiw‐Teltow association theory shows.
Electronic Excitations in Some Transition Metals and Their Oxides. Characteristic Energy Loss Measurements up to 50 eV
Authors: Frandon J. , Brousseau B. , Pradal F.
Published: July 2007

The characteristic energy losses suffered by a 20 keV electron beam through thin films of some transition metals (Sc, Y, Zr, Hf) and of their oxides are measured using a magnetic spectrograph up to 50 eV. The energy loss function — Im 1/ε and the complex dielectric constant are calculated. It is shown that the electron excitation spectra of metals contain two main contributions in this energy range: one due to collective excitations of the conduction electrons, the other to the excitation of inner p‐electrons. In the case of oxides the spectra display three strong structures: there are probably two collective excitations of valence electrons followed by the excitation of inner p‐electrons of the metal.
Transient Light‐Induced Spatial Gratings by Successive Optical Coherent Pulses
Authors: Shtyrkov E. I. , Nevelskaya N. L. , Lobkov V. S. , Yarmukhametov N. G.
Published: July 2007

A new optical quantum phenomenon by interference of atomic states is discussed. The possibility of formation of induced transient gratings in a resonant medium is shown in the case when the medium is excited by two laser pulses arriving at experimental sample noncoincident in time but from different direction. The theory of such gratings is presented for detuning of the incident light from resonance. A technique is developed for observing such gratings for the 4A2–2E(Ē) transition of ruby.
Crystal‐Independent Repulsive Hardness Parameters for Alkali Halides
Authors: Shanker J. , Agrawal D. P.
Published: July 2007

In the present paper an analysis is performed of interionic forces in alkali halides by adopting Smith's distortion model for repulsive interactions. This model represents the repulsive potential in terms of parameters which depend only on the ions. By treating the repulsive hardness parameters ϱ to be crystal independent the cohesive energies, bulk modulus and its pressure dependence are calculated for 16 alkali halides with NaCl structure. A distinguished feature of treating the parameters as ion dependent and crystal independent is that the values of ϱ do not remain the same for cation–cation, cation–anion, and anion–anion interactions. This is a modification over the traditional Born model where a common value of ϱ is taken for all the ion pairs. To make a simultaneous fit of the cohesive energy, bulk modulus, and its pressure dependence, it is essential to consider the van der Waals interactions as well as the three‐body interactions.
Study of Excitons in SiO2. Luminescent Centres as Exciton Detectors
Authors: Trukhin A. N.
Published: July 2007

The spectral and temperature dependences of the quantum yield of energy transfer to a luminescent detector by an exciton are studied in SiO2. The results obtained show, that in addition to the “hot” exciton migration there is a diffusion of the relaxed exciton. The exciton structure of SiO2, consists of three states, a self‐trapped exciton, a relaxed exciton, and a resonant exciton.
Energy Gap Determination in a‐As2S3 by Means of Faraday Rotation
Authors: Van Keybus P. Den , Vanhuyse B. , Grevendonk W.
Published: July 2007

Non‐resonant interband Faraday rotation (FR) is measured in a‐As2S3 between 480 and 850 nm in a magnetic field up to 4 T at temperatures between 4 and 300 K. Analysis of the results by means of theoretical dispersion formulae reveals an energy gap EgFR of about 3.1 eV. This value is higher than the optical gap (2.36 eV) but is in agreement with the gap determined with dispersion measurements; from literature similar FR data are found for e‐Si where FR reveals a direct gap. The temperature influence is small. From 50 to 300 K, EgFR slowly diminishes, and has a constant value below 50 K.
Gain Spectrum of an e–h Liquid in Direct Gap Semiconductors
Authors: Haug H. , Thoai D. B. Tran
Published: July 2007

The gain spectrum of a degenerate electron–hole plasma for direct gap semiconductors is calculated as a function of the density and the temperature of the electronic excitations. The calculation contains the collision broadening in the single‐particle self‐energies and the excitonic enhancement due to the attractive electron–hole interaction in the plasma. For the example of GaAs the numerically calculated spectrum is shown to fit the experimentally determined spectrum.
Experimental Evidence of Asymmetry of the Bound Exciton Absorption Line in GaP:N
Authors: Sokolov V. I. , Chernyaev V. V.
Published: July 2007

The absorption and electroabsorption spectra in GaP:N at 77, 20.4, and 4.2 K are investigated around the A‐line. The change of the electroabsorption lineshape with decreasing temperature can be understood by assuming that the A‐line is asymmetric.
Stimulated Bremsstrahlung of Electrons in a Crystalline Lattice
Authors: Jivanyan H. A.
Published: July 2007

On the basis of classical and quantum theories the dynamics of electron scattering on a crystalline lattice in the presence of a strong electromagnetic wave is investigated. A condition is found under which coherent energy exchange takes place between the electron and external radiation along the lattice length. It makes possible the electron acceleration in crystals and leads to the effect of an electron beam space bunching.
The Effect of Partial Charge Transfer in NMP‐TCNQ
Authors: Papatriantafillou C.
Published: July 2007

There appears to exist clear experimental evidence that the charge transfer in the organic salt NMP‐TCNQ is only partial, but there is ambiguity with respect to the exact value of it. In the present paper the material is considered as a realization of the one‐dimensional disordered Hubbard model, and a study is made of its electronic properties using as a parameter the degree of charge transfer in it. For small deviations from the complete charge transfer the results reduce to previously presented ones. For moderate and large deviations there appears a strong intrinsic static disorder, while other band parameters remain unchanged.
On Functional Fourier Transformation for the Hubbard Model and Similar Models
Authors: Behn U. , Weller W.
Published: July 2007

Describing spin ↑ ‐electrons the particle number operators ni↓ are replaced under the functional integral directly by c‐numbers vi↓(t) using a δ‐functional instead of the Hubbard‐Stratonovich transformation. The Green function of the Hubbard model is thus exactly represented as an averaged Green function of electrons moving in a stochastic potential Uvi↓(t). The probability distribution for the stochastic process v ↓ containing the many body aspect and the information about the ↓‐electrons is determined. The vi↓(t) are only allowed to jump between 0 and 1 reflecting the property of Fermion particle number operators. This exact scheme can be interpreted as a dynamically generalized alloy analogy.
The Sublattice Displacement in Zincblende‐Type Crystals. Are the Internal Atomic Displacements Identical if the Same Deformation Is Caused Mechanically and Electrically?
Authors: Suisky D. , Enderlein R. , Lange H.
Published: July 2007

Relations are derived for the Sublattice displacements in zincblende‐type crystals if the deformation is caused purely mechanically or electrically. The ratio of the two internal displacements induced by field (d) and strain (d) is expressed in terms of characteristic crystal constants. A plot of d/d versus the internal strain parameter ζ for the various zincblende‐type materials shows a strong increase of the Sublattice displacement ratio for ζ → 1 and a change of sign at ζ = 1.