bluuebunny/crossref_metadata_2025 huggingface dataset:
On the energy dependence of the very low energy reflectance from clean and manganese‐doped MgO(100) surfaces
Authors: He Jian‐Wei , Møller P. J.
Published: July 2007

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

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

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

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

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

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

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

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

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

An effect of guest concentration on various prompt and delayed emissions from a p‐terphenyl host‐pyrene guest mixed crystal is studied. The presence of an adventitious defect site is ascertained. The experimental results suggest that both the defects and the guests are distributed in a random manner in the host lattice and that the free singlet excitons are trapped at monomeric and nascent excimeric pyrene sites with equal efficiency. The defect phosphorescence decay is nonexponential at low guest concentration but becomes exponential at higher concentrations. Second order analysis of the decays at various concentrations confirm that at low guest concentration pyrene delayed fluorescence originates from homofusion of defect triplet excitons whereas heterofusion of defect–guest triplet excitons predominates at higher concentrations.
Capture of majority carriers by a semiconductor surface
Authors: Vardanyan R. A. , Khovakimyan L. B.
Published: July 2007

The coefficient of majority carrier capture by a semiconductor surface is calculated in quasi‐classical approximation. The carrier localization on the surface is accompanied by multiquantum fluctuational phonon emission. The temperature dependence of the capture coefficient is obtained.
The resistivity of liquid metals with finite mean‐free path
Authors: Khajil T. M. A. , Tomak M.
Published: July 2007

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

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

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

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

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

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

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

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

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

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

The general theory of multicomponent alloys is developed taking into account the multiparticle correlations between atoms with reducible and irreducible n‐body interaction potentials. The number of components and their interaction radius are not limited. The method described is not sensitive to the crystallographic structure of the alloy. The problem is being tackled using the idea of a multiparticle entropy. The internal energy is calculated by means of the high‐temperature expansion method employing an appropriately adapted diagram technique. The multiparticle correlation functions are assumed to be additional ordering parameters.
Generalized huggins‐mayer potentials and properties of alkali halides under the effect of high pressure
Authors: Shiromany A. , Srivastava G. M. S. , Shanker J.
Published: July 2007

Equation of state and dielectric properties of alkali halides under the effect of high pressure are investigated using the generalized Huggins‐Mayer potential form. Values of potential parameters derived from ultrasonic data and recent van der Waals interactions by taking proper account of thermodynamic contribution are used in the present calculations of the equation of state and the volume derivatives of dielectric constants. It is found that the inclusion of thermodynamic contribution based on recent data improves significantly the agreement between theoretical and experimental results. The present investigation is superior to other previous studies in respect of using a thermodynamically consistent set of potential parameters.
Optical study of the defect stability in Tl‐doped potassium iodide
Authors: Rzepka E. , Lefrant S. , Taurel L.
Published: July 2007

The thermal stability of the centres created in KI : Tl crystals submitted to an ionizing radiation at room temperature is studied. It is observed that F centres are transformed at room temperature, giving rise to more complex centres, unidentified either by EPR or Raman spectroscopy. The complementary hole centres consist of polyiodides in a low concentration and other complexes implying thallium and iodine interstitials. A recombination between some of these centres occurs at 127 °C. A quenching at LNT of a coloured sample annealed at 127 °C < T < 170 °C shows, by means of Raman experiments, that interstitials are still present since I−3 ions are observed in large concentration.
Coupled waves in conducting spin glasses
Authors: Sultanov K. M. , Kuliev Sh. A. , Abdullaev N. G.
Published: July 2007

The energy spectrum of semiconducting and metallic spin glasses is studied. It is shown that at resonance between the electromagnetic branches of the spectrum and spin waves there exist coupled spin‐spiral, spin‐Alfven, and other coupled spin‐magnetohydrodynamical waves. For all the cases a detuning of the resonances is found.
Heisenberg‐type and double exchange in mixed‐valence clusters
Authors: Belinskii M. I. , Ya. Gamurar V. , Tsukerblat B. S.
Published: July 2007

The theory of double exchange is generalized to be applicable for the degenerate states of d‐electrons in crystal fields. A two‐centre three d‐electron system is studied, Coulomb and exchange intercentre interaction and resonance splitting of exchange multiplets are taken into account. A group‐theoretical classification method for resonance‐exchange multiplets in crystal fields is developed. A dependence of the electron transfer parameters on orbital symmetry and the spin of resonant states is found. The energy spectrum of a binuclear cluster is studied. The resonance interactions are assumed to be relatively weak in comparison with intera‐atomic interactions. It is shown that the Heisenberg‐type exchange parameter is redetermined as long as the transfer is taken into account. The results are compared to the conclusions of the Anderson‐Hasegawa theory.
Anomalous composition dependence of the dielectric constant in bismuth‐antimony alloys due to disorder‐induced indirect interband polarization processes
Authors: Enders P. , Schuchardt R.
Published: July 2007

The strong increase of the dielectric constant observed at a critical antimony concentration in semiconducting bismuth‐antimony alloys is interpreted in terms of a purely electronic interband polarization enhanced by vanishing of the thermal energy gap. The dominant indirect transitions between conduction and valence band states near the L‐ and T‐points of the Brillouin zone are considered as induced by the short‐range scattering potential due to the chemical disorder of the alloy. Reasonable system parameters allow for a satisfactory fit to the experimental data.
Nonlinear optical effects studied by time‐resolved reflection measurements
Authors: Hönerlage B. , Tomasini F. , Bigot J. Y. , Frindi M. , Grun J. B.
Published: July 2007

At very low temperature, the reflection properties of CuCl samples under high pulsed light excitation conditions are investigated by a “pump and probe” technique. The non‐linear reflection changes are shown to have different origins and different time scales: excitonic effects lead to a new reflection structure near the energy of the longitudinal exciton (∼ 1 ns time scale) and interaction with non‐thermalized acoustical phonons to a decrease of the exciton reflection maximum (∼ 60 ns).
On the force between high‐density jellium‐metal surfaces at small separation
Authors: Streitenberger P.
Published: July 2007

An exact relation is derived yielding the force constant of a bimetallic interface described by the jellium model in terms of the corresponding density‐potential response function. This relation in conjunction with a step‐density model and Thomas‐Fermi screening is used to derive an approximate formula for the force constant of a bimetallic interface in the limit of high densities. For identical metals it is demonstrated that the relation obtained satisfies the surface virial theorem and how the force between the two metal surfaces can be decomposed into the kinetic and interaction contributions. Starting from the small and large‐separation limiting behaviour of the force an attempt is made to estimate the adhesive energy of high density bimetallic contacts using the force constant calculated for the jellium model. For dissimilar metals it is found that the adhesive energy tend to a limit proportional to r1—5/4 r2—9/4 as r1, r2→ 0 and r1/r2→ 0, where r1 and r2 are the mean interelectronic distances of the two metals.
Flux spiral instability in a transport current carrying type‐II superconductor
Authors: Shehata L. N. , Saif A. G.
Published: July 2007

The nucleation of vortex flux lines on a surface of a transport current carrying type II superconductor in the presence of an external magnetic field He occurs at He ˜ Hcl, the first critical field. A spiral flux line in a long cylinder and in a long thin ellipsoid is investigated. In the latter case the force‐free magnetic fields are also considered.
Electric field gradient in Ag alloys
Authors: Ponnambalam M. J.
Published: July 2007

In Ag the impurity‐induced electric field gradient (efg) q and the asymmetry parameter η at the nearest neighbor due to Cd, In, Sn, and Sb are calculated. The valence efg qV is evaluated using an exact expression, valid at all distances from the impurity. The size effect efg qs is calculated using an oscillatory form for the displacement. Without the use of floating parameters, good agreement is obtained with the experiment for all the systems.
S‐Polarized nonlinear surface polaritons effects of a transition layer
Authors: Wendler L.
Published: July 2007

The dispersion relation of s‐polarized nonlinear surface polaritons is derived for a system consisting of an optical nonlinear and an optical linear semi‐infinite medium which are seperated by a dipoleactive transition layer. With the power flow of the nonlinear surface polaritons an additional physical parameter enters the dispersion relation. The various resulting forms of the dispersion relation are analyzed and presented in graphical form. It is shown that in the case of resonance of the s‐polarized nonlinear surface polaritons with the transverse optical vibrations of the transition layer the dispersion curves are altered dramatically. The electromagnetic fields, the Poynting vector, and the energy flow of the s‐polarized nonlinear surface polaritons are calculated and presented in graphical form.
A Kinetic Theory for the Motion of Accelerated Particles in Solids. I. Derivation of Basic Equations
Authors: Urmanov A. R. , Linetskaya A. M. , Puzanov A. A.
Published: July 2007

An approach to the derivation of kinetic equations describing the motion of accelerated particles in solids is suggested based on the microscopic phase density method. It is shown that the unified approach suggested makes possible to obtain all the known kinetic equations used for describing the interaction of accelerated particles with condensed medium as well as more exact expressions for the coefficients of these equations. A new type of the kinetic equation is obtained describing, in particular, ion channeling in complex single crystals.
Investigation of Nonequilibrium Phonons in GaAs
Authors: Danilchenko B. A. , Poroshin V. N. , Slutskii M. I. , Asche M.
Published: July 2007

The propagation of acoustic phonons in GaAs generated by laser radiation of a clean as well as Au coated surface are experimentally investigated in dependence on the crystallographic orientation. The angular dependence of the T1 phonon spectrum is confirmed by comparison with the calculated focussing factor. Besides the ballistic motion of modes of L, T1, and T2 polarization a further peak is observed in the time of flight spectra, which is interpreted by repeated conversion and decay processes of high‐frequency phonons leading to T2 modes with a frequency low enough for ballistic motion.
Self‐Consistent CPA Treatment of the Conduction Electron Spin Polarization of Degenerate Ferromagnetic Semiconductors
Authors: Borgieł W. , Nolting W. , Borstel G.
Published: July 2007

A theory for the temperature and charge carrier concentration dependence of the electronic quasiparticle spectrum, of a ferromagnetic 4f‐system is presented. The s‐f model is used with inclusion of a Coulomb interaction between the conduction electrons. The full analytical solution of the zero band width limit of this model is used to formulate an alloy analogy, which is then treated by a conventional CPA‐procedure. In this way in a selfconsistent manner basic quantities are determined like the quasiparticle density of states and the conduction electron spin polarization. The quasiparticle spectrum is shown to be composed of three subbands which can be interpreted as consequence of different simple elementary processes. The spin polarization is almost complete for small band occupations n and temperatures below the Curie‐temperature. It drops down quite abruptly at a certain critical concentration, reaching again a relative maximum for the exactly half‐filled conduction band. The temperature dependence of the spin polarization for fixed n explains recently performed field emission experiments.
Exchange Potential and Plasma Modes in Coupled Two‐Dimensional Electron Systems
Authors: Sy H. K. , Song L. M.
Published: July 2007

An investigation is made of two 2‐D electron systems, separated spatially but coupled by Coulomb interaction. The acoustic and the optical plasma modes are studied using coupled Boltzmann equations. An effective exchange potential calculated using the formalism of Kohn and Sham is included. The dispersion relation is obtained numerically for different densities and separations.
Intermediate‐Valence and Kondo Impurities. Approximate Analytic Results within NCA
Authors: Kozabzewski B.
Published: July 2007

An approximate analytic solution of the NCA integral equations for an impurity system at arbitrary temperature is given. Renormalized energies of ionic states are found. It is also shown that the ion can change its state from magnetic to nonmagnetic one at some temperature corresponding roughly to the Kondo temperature.
Electron‐Phonon Interaction and Structure of Molecular Luminescence Center in Crystals
Authors: Reznik L. E.
Published: July 2007

The spectral features of the lowest 1B1 ⇄ 1A1 and 3B1 molecular ion in the crystalline state at low temperatures are studied, using single crystals of MNO2 nitrites and MN03 nitrates (M is a metal) in polarized light. It is shown that the electron‐phonon interaction broadens the spectra; the crystal microscopic characteristics mostly affecting the electron‐phonon coupling are deduced.
Phonon Dispersion of Disordered FeAl Alloy
Authors: Garg S. , Gupta H. C.
Published: July 2007

Phonon dispersion curves for disordered state (b.c.c.) of Fe‐22.5% Al alloy are calculated using the deLauney angular force model and compared with the experimental one. It is found that the force constants of the alloy system are related to the force constants of Fe and Al in the b.c.c. and f.c.c. phase, respectively. Good agreement is obtained between theory and experiment.
On the Temperature Variation of the Anderson‐Grüneisen Parameters of NaCl
Authors: Dhoble Aruna , Verma M. P.
Published: July 2007

It is shown thermodynamically that the isothermal Anderson parameter δTcan be expressed in terms of the constant pressure and constant volume temperature derivatives of the thermal expansion coefficient α. This result is used to study the temperature variation of δTfor NaCl. The study suggests that δT should present a sharply rising temperature variation particularly at temperatures approaching the melting temperature.
Vibrational Properties of Metastable Phases of Si and Ge
Authors: Winer K. , Wooten F.
Published: July 2007

The phonon dispersion curves and density of states for BC8 Si and ST12 Ge are calculated using a generalization of Weber's adiabatic bond charge model. Variations in the vibrational properties are related to the different types of structural distortions present in the two phases.
Surface Migration of Adsorbed Particles
Authors: Kozhushner M. A. , Prostnev A. S. , Rozovskii M. O. , Shub B. R.
Published: July 2007

A relaxation model of single adsorbed particle migration on the surface of solids is proposed. It is shown that the jump length is determined completely by an energy relaxation time for a continuous spectrum of the surface periodic potential. The jump length is found to exceed the lattice period appreciably if the adatom motion velocity is smaller than the sound speed in the solid. For this case an analytic expression for the diffusion coefficient is obtained which does not contain any fitting parameters.
L‐Holes and Their Effect on the Band Parameters of Bismuth
Authors: Mikhail I. F. I.
Published: July 2007

The effect of L‐holes on the temperature dependence of the band structure parameters of bismuth is investigated. Modified expressions for the weak‐field galvanomagnetic coefficients are derived. These expressions are used in a re‐evaluation of mobilities and band parameters in the temperature range 77 K ≦ T ≦ 300 K.
Exciton Absorption Spectrum in a ZnSiP2 Crystal
Authors: Madelon R. , Hurel J. , Paumier E.
Published: July 2007

Measurements of absorption coefficients near the pseudodirect band edge in ZnSiP2 are performed using polarized light from 4.2 K to room temperature. By fitting a theoretical curve with the measurements at several temperatures values of the band gaps As′, B′, and C′, as well as the binding energies of the first exciton level are obtained for the three transitions A′, B′, and C′. The last values are temperature independent. Other results are the broadening parameters of the three excitons. Also obtained are the spin‐orbit splitting, the crystal field splitting, and the exciton reduced mass.
Vibrational Spectra of TlInS2, TlIn0.95Ga0.05S2, and TlIn(S0.8Se0.2)2 Crystals in the Vicinity of Phase Transitions
Authors: Nurov Sh. , Burlakov V. M. , Vinogradov E. A. , Gasanly N. M. , Dzhavadov B. M.
Published: July 2007

Structural phase transitions in TlInS2, TlIn0.95Ga0.05S2, and TlIn(S0.8Se0.2)2 crystals are investigated by infrared reflection spectroscopy. An attempt is made to affect the parameters of the vibrational spectrum and the phase transition temperatures of a TlInS2 crystal by introducing impurities into it in order to obtain additional data necessary to construct phase transitions mechanisms.
EPR of Colour Centres in Alkaline Metal Doped Calcium and Strontium Fluorides
Authors: Hamaïdia A. , Hachimi A. , Margerie J. , Hémidy J. F.
Published: July 2007

The EPR is studied of FA centres in CaF2:Li+, CaF2:Na+, and SrF2:Na+. In CaF2:Li+, the spectrum is very similar to that of F centres in pure CaF2: the superhyperfine coupling constant with lithium is unmeasurable. On the other hand, in CaF2:Na+ and SrF2:Na+, the superhyperfine coupling with a sodium nucleus is noticeable. The EPR of so‐called “F2A” centres in the same matrices is also observed, which corroborates our previous magnetic circular dichroism conclusions: “F2A” centres do not correspond to two electrons in two adjoining anion vacancies with a substitutional sodium in close vicinity. From the observed superhyperfine structure, it is not yet found what is the genuine microscopic structure of these “F2A” centres.
Effect of the Temperature and Concentration of Magnetic Atoms on Phase Diagrams and Order Parameters in Ferromagnetic Superconductors
Authors: Kopeć T. K. , Klamut J.
Published: July 2007

Using the free energy functional with two coupled order parameters the model of a rare earth ferromagnetic superconductor is considered by means of the Landau theory of phase transitions. By making the experimentally verified assumption about the dependence of critical temperatures for magnetic and superconducting subsystems on the concentration of magnetic rare earth atoms, the resulting phase diagrams are analysed in the temperature—concentration plane. It turns out that for weak coupling between magnetic and superconducting subsystems the uniform coexistence state of ferromagnetism and superconductivity sets in, while for strong coupling a spatially non‐uniform metastable state exhibiting hysteretic behaviour for ferromagnetic and superconducting phase appears. The explicit dependence of the order parameters on the temperature and concentration of magnetic atoms is also given.
On the Influence of Noise on the Density of States and Absorption in Disordered Molecular Systems
Authors: Esser B. , Marlow F.
Published: July 2007

A disordered excitonic system perturbed by the Gauss‐Markov noise approximating the exciton‐phonon interaction is investigated. The structure of the mass operator is discussed in dependence on the eigenfunctions of the excitonic Hamiltonian. Explicit calculations of the density of states and the imaginary part of the DF ϵ2(ω) are performed for Lorentzian static disorder. The influence of the breaking of the k‐selection rule or of fluctuations in the transition matrix elements on the frequency dependence of ϵ2(ω) is considered.
The Multimode Jahn‐Teller Effect in the Luminescence Spectrum of the Tetrahedral 2T2 ⊗ (e + t2 + t2) Impurity System ZnS:Cu2+
Authors: Polinger V. Z. , Boldyrev S. I.
Published: July 2007

The multimode Jahn‐Teller effect for the tetrahedral 2T2 term coupled with E and T2 vibrations is discussed taking into account the spin—orbit interaction. Harmonic vibrations are considered at the trigonal minimum point of the lowest sheet of the adiabatic potential. Using the Green's function method the redetermined projected densities of states are obtained. The one‐phonon sideband of the 2E → 2T2 transition is expressed in terms of projected densities and compared with the experimental data for 2E → 2T2 luminescence of ZnS:Cu2+.
Lattice Thermal Conductivity of Gadolinium Monosulphide above Room Temperature in the Frame of Two‐Mode Conduction of Phonons
Authors: Al‐Edani M. C. , Dubey K. S.
Published: July 2007

The lattice thermal conductivity of gadolinium monosulphide is studied by estimating the separate contributions due to transverse and longitudinal phonons in the temperature range 300 to 800 K and a very good agreement is found between calculated and experimental values. It is also reported that the transverse phonons are mainly responsible for the heat conduction in the present sample. The temperature exponents for the three phonon scattering relaxation rates is also reported for gadolinium monosulphide for the first time.
Calculation of the Electronic Structure! of Disordered Hydrogen Adsorption on the Si(111) Surface
Authors: Gadiyak G. V. , Karpushin A. A. , Korolenko I. V. , Morokov Yu. N. , Yu. Semenova I. , Sorokin A. N. , Tomášek M.
Published: July 2007

The local density of electronic states is calculated on the (111) surface of silicon with disordered adsorption of atomary hydrogen with an arbitrary degree of coverage. Computations are made by the recursion method and the method of generalized Bethe lattices. The dependence of the electronic density of states on the degree of coverage of the surface by hydrogen is investigated. The results obtained by the recursion method and the generalized Bethe lattice method are compared with those obtained by other methods and by the experiment and a good overall agreement is found. The contribution of hydrogen and silicon atomic states into the electronic structure of the Si(111) surface is analyzed.
On the Theory of Crystal Excitation of Electric Discharges II. Ionization of Defects
Authors: Voitikov S. V. , Gribkovskii V. P.
Published: July 2007

The theory of point defect ionization in semiconductors by the streamer discharge electric field is developed. It is shown that due to high streamer velocity and sharp spatial inhomogeneity of the electric field defect ionization is a fast quantum process with a large energy uncertainty of particles involved and, hence, it does not occur by tunneling. Localized wavefunctions are described by the quantum defect method. The ionization probability is calculated and its straightforward dependence on the velocity and the form of the streamer field and the defect parameters is determined. As in the case of interband transitions considered in the first work the ionization probability is an increasing function of the streamer velocity and the field steepness.
Localization in Random Systems I. Effect of Off‐Diagonal Disorder
Authors: Brezini A. , Sebbani M. , Dahmani L.
Published: July 2007

The question of localization is examined in a tight‐binding Hamiltonian. The present method is based on a self‐consistent theory reproducing correctly the band edge in the limit of zero disorder. Randomness is introduced also in the off‐diagonal elements. In a purely diagonal disorder it is found that the results for the critical disorder Wc for which the Anderson transition appears are in good agreement with other theories while for a purely off‐diagonal disorder the states around the middle of the band remain extended.
Variable‐Range Hopping in Neutron‐Transmutation‐Doped Gallium Arsenide
Authors: Rentzsch R. , Friedland K. J. , Ionov A. N. , Matveev M. N. , Shlimak I. S. , Gladun C. , Vinzelberg H.
Published: July 2007

First investigations are reported on the temperature dependence of the dc‐conductivity at T = = 0.05 to 300 K on neutron‐transmutation‐doped (NTD) n‐GaAs in the vicinity of the metalinsulator transition (MIT). At a medium compensation of K = 0.60 to 0.77 the MIT takes place at the critical electron concentration of nc = 2.3 × 1016 cm−3. On the dielectric side of the MIT in the variable‐range hopping regime (VRH) at T ≦ 10 K the conductivity obeys the equation σ(T) = σ0exp [— (T1/T)1/2] and not the Mottlaw. The experimental values T1 are for all samples much smaller than T1theor for isolated shallow impurities and scale down according to T1 = = T1*(1 ‐ n/nc)1.6±0.4 with T1* = 5.9 K by approaching the MIT. This behaviour is due to the divergency of the localization radius a and the static DK ϵ0 at the MIT. The obtained linear In σ — T−1/2 law over about 2.5 orders of magnitude in temperature variation and more than three orders of magnitude of variation in conductivity is interpreted in the frame of a Coulomb gap with vanishing density of states at the Fermi level, as predicted by Efros and Shklovskii. The pre‐exponential factor ϱ0 = σ0−1 shows also scaling behaviour according to ϱ0 = ϱ0*(1 ‐ n/nc)1.2±0.4 and a value of ϱ0* = 2.2 Ω cm. At n → nc σ0 = ϱ0−1 reaches the value of Mott's minimal metallic conductivity of σMM = (Ce2/h) nc1/3, with 0.025 ≦ C ≦ 0.05.
Determination of Absorbate Geometries from Angle‐Resolved X‐Ray Photoelectron Spectra. Nitrogen on Fe(111)
Authors: Chassé A. , Rennert P.
Published: July 2007

The angular dependence of the X‐ray photoelectron intensity of excited deep core levels is used to investigate the structural properties of several molecular and atomic states for chemisorbed nitrogen on the Fe(111) surface. The N1s photoelectron intensity is calculated within the small scattering center approximation and the single scattering model for several angular variations and geometrical arrangements. It is shown that the angular anisotropies can be used to explain the surface geometries of the nitrogen–iron system.
Crystal Instability Due to Defect–Phonon Interactions
Authors: Wautelet M. , Legrand P. B.
Published: July 2007

The effects of defect–phonon, defect–defect, and phonon–temperature couplings are introduced phenomenologically in the total energy of crystals. Crystal instability appears to be induced mainly by defect–phonon and defect–defect interactions.
Correlation between X‐Ray Chemical Shift and Partial Charge in Some Minerals
Authors: Hemachandran K. , Chetal A. R.
Published: July 2007

The K‐absorption spectra of arsenic in the minerals orpiment, realger, smaltite, and arsenopyrite are investigated using a Cauchois‐type bent‐crystal X‐ray spectrograph. The chemical shift of the edges is determined and correlated with the partial charge, calculated using Sanderson's method. A theoretical justification is given for the relation observed between the shift and partial charge.
2kF Distortion of the Impurity Distribution in the Peierls‐CDW State
Authors: Bâldea I.
Published: July 2007

A self‐consistent approach is developed to investigate the possibility that the impurity distribution embedded in a one‐dimensional system becomes distorted with the wavevector 2kF (kF Fermi wave vector) as a result of the Peierls‐CDW (charge density wave) distortion. The properties of the double distorted state are obtained for both, T = 0 and T ≠ 0 (e.g. enhancement of both, the gap in the electron spectrum and the CDW effective mass). The conditions to have a state with 2kF‐double distortion are derived. The interplay between the 2kF‐regularity in the impurity distribution and the disorder effect usually associated with the latest is also discussed.
Lattice Thermal Conductivity of Deformed Lithium Fluoride Crystals at Low Temperatures
Authors: Kumar A. , Srivastava B. K. , Ansari M. A.
Published: July 2007

Static dislocation scattering processes result in an additional phonon scattering term because of the simultaneous presence of the impurity–phonon scattering processes. The additional term of the inverse of relaxation time is calculated by using the perturbation and is found to be proportional to the square of the phonon frequency. Excellent agreement is obtained between theory and experimental results of the phonon conductivity of the plastically deformed LiF crystals.
Magnetoelastic Model of the Temperature‐Induced Metamagnetism for Lattices with Axial Symmetry
Authors: Pawlicki P.
Published: July 2007

A new model of the temperature‐induced metamagnetic phase transitions is considered for the class of the two‐sublattice magnets with axial symmetry. The model Hamiltonian contains the usual Ising term as well as the coupling of the uniform strain with both exchange interaction and single ion anisotropy. Within the MFA all these couplings are found to be necessary to produce the metamagnetic phase change. The results are compared with experiment.
Plasmon Excitations in Thin Metal Films. Transition from Three‐Dimensional to Two‐Dimensional Behaviour
Authors: Teich W. G. , Mahler G.
Published: July 2007

Density oscillations are studied within the infinite‐barrier model. Intersubband and intrasubband modes are characterized with respect to their frequency, spatial amplitude field, and collective nature. Special attention is paid to particular limiting cases in terms of the length scales defined by slab thickness and excitation wavelengths.
Comparison between Monte Carlo and Davydov Approaches to Hot‐Carrier Transport in Semiconductors
Authors: Reggiani L. , Brunetti Rossella , Normantas E.
Published: July 2007

Hot‐electron transport in a simple model semiconductor is investigated in the presence of acoustic phonon scattering. Analytical calculations, based on the Davydov distribution function, are compared with Monte Carlo results to demonstrate the limitations of the analytical approach. By displaying the results in a universal form, it is found that the quasi elastic approximation required to fully justify the Davydov approach, is controlled by the condition τ/τ∈ < 10−3, τ and τ∈ being the momentum and the energy relaxation times, respectively.
Lattice Theory of Fourth‐Order Elastic Constants of Primitive Lattices. Application
Authors: Kumar K. , Goyal S. C.
Published: July 2007

The lattice theory of fourth‐order elastic constants for primitive lattices recently derived by Goyal and Kumar is applied to obtain the relations for the FOE constants of CsCl structure type solids within the framework of the Lundqvist many‐body potential. The derived relations of FOEC are used to compute the numerical values of the FOE constants and the second‐order pressure derivatives of the SOE constants of cesium halide solids.
The Burstein Absorption Edge Structure in Lead Chalcogenides
Authors: Masłowska A.
Published: July 2007

The indirect interband absorption below the Burstein edge is calculated for lead chalcogenide mixed crystals using the model of nonparabolic, nonspherical, and multivalley conduction and valence bands. It is found that the absorption due to the chemical disorder is important at low temperatures.
Theoretical Analysis of the Effective Electron Mass in n‐Channel Inversion Layers on Ternary Chalcopyrite Semiconductors
Authors: Mondal M. , Ghatak K. P.
Published: July 2007

An attempt is made to derive expressions of the effective electron masses in n‐channel inversion layers on ternary chalcopyrite semiconductors, taking n‐channel inversion layers on CdGeAs2 as examples, under both the weak and strong electric field limits. It is found, on the basis of newly derived 2D electron energy spectra for both the limits by considering the various types of band‐anisotropies, that the effective electron masses at the Fermi level depend on the electric sub‐band index due to the combined influence of the crystal field splitting, the anisotropic spin—orbit splitting parameter, and the band non‐parabolicity. The anisotropy parameters enhances the numerical values of the effective Fermi level masses for both the limits for relatively low values of the surface fields. Besides, the corresponding results for the isotropic three band Kane model are also obtained from the expressions derived.
3d‐Like Electron Localization and Charge Transfer in Hydrogenated V‐Fe Alloys
Authors: Ostrasz A. , Szuszkiewicz M.
Published: July 2007

The influence of hydrogen absorption on the electronic structure of V1−yFey, alloys is studied. The results of the Mössbauer effect and X‐ray diffraction measurements as well as the positron annihilation data allows to determine the mean number of 3d‐like electrons localized at Fe atoms in V1−yFeyHx alloys. The problem of hydrogen participation in charge transfer in these alloys is also discussed.
Optical Absorption Spectrum of Cr3+ Ions Doped in NH4Cl Single Crystals
Authors: Rao J. Lakshmana
Published: July 2007

Optical absorption spectrum of Cr3+ ions doped in NH4Cl single crystals is studied in the temperature range from 300 to 143 K. From the observed spectrum it is concluded that Cr3+ ions enter interstitially into NH4Cl lattice and the site symmetry of the Cr3+ ions are lowered from octahedral to tetragonal. The crystal field and the tetragonal parameters are evaluated.
Lattice Dynamics Study of Polycrystalline 57CoO by Mössbauer Spectroscopy
Authors: Fontcuberta J.
Published: July 2007

The Mössbauer emission spectra of 57Co1−xO recorded in the temperature range from 293 to 1273 K for samples synthesized under various oxygen‐pressure conditions, are used to determine the Debye temperature θD of these oxides. It is found that θD decreases when the non‐stoichiom‐etric parameter x increases. In the highest temperature range, anharmonic contributions to the vibrational spectrum of the solid are observed.