bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption interbande induite dans le silicium sous forte excitation picoseconde à 1,06 μm
Authors: Pugnet M. , Amand T. , Cornet A. , Brousseau M.
Published: July 2007

Nous présentons ici les résultats et la modélisation de nos expériences de transmission d'une impulsion laser YAG (Λ = 1,064 μm; durée 30 ps) à travers une plaquette de silicium cristallin, à T = 295 K et à fort niveau d'excitation (Ei ⪆ 0,05 J/cm2). Notre modèle est fondé sur les équations de continuité de la densité du plasma n(x, t) et de la puissance instantanée P(x, t). Nous montrons que la saturation de l'énergie transmise Et(Ei), observée expérimentalement, s'explique par une croissance dynamique du coefficient d'absorption interbande α(x, t); nous interprétons cette croissance comme la conséquence de la renormalisation de la bande interdite par un plasma électron‐trou non dégénéré sans échauffement du réseau. Nous prenons une expression phénoménologique: α(n(x, t)) = α0 + α1(n(x, t))r où r et α1 sont des paramètres ajustables tandis que α0 est le coefficient d'absorption interbande à faible excitation (α0 ≈ 9 cm−1); le meilleur ajustement est obtenu pour r ≈ 1; α1 ≈ 6 × 10−18 cm2; coefficient d'absorption par les porteurs libres: à = 4 × 10−18 cm2. Avec ces valeurs, le calcul montre qu'au‐dessus de 0,1 J/cm2, la distribution spatiale des porteurs est modifiée dans le sens d'une localisation vers la surface excitée d'autant plus grande que l'énergie incidente Ei croǐt; nous calculons l'énergie transférée au réseau par la relaxation du plasma et nous montrons qu'on ne peut pas expliquer la diminution dynamique de la bande interdite déduite de la modélisation de nos expériences par un échauffement du réseau. Une variation en n1/2 de la renormalisation de la bande interdite décrit mieux nos résultats que la variation en n1/3 prévue par la théorie.
Luminescence Properties of Rare‐Earth‐Activated Gadolinium Fluoride (GdF3) and Oxyfluorides (GdOF)
Authors: Lammers M. J. J. , Blasse G.
Published: July 2007

The luminescence properties of RE3+‐activated GdF3 and GdOF are studied for both uv and X‐ray excitation. GdOF exists in two modifications, an orthorhombic and a tetragonal one and both are used as a matrix. GdF3 is activated with Pr3+, Sm3+, Tb3+, Dy3+, Er3+, and Tm3+ and with a transition metal ion, viz. Mn2+. Both modifications of GdOF are activated with Tb3+, Er3+, and Tm3+. Especially the Tb3+‐activated samples are very efficient green phosphors under both uv and X‐ray excitation.
A Hole Center Causing Rose Coloration in AlPO4 and GaPO4
Authors: Requardt A. , Lehmann G.
Published: July 2007

In synthetic AlPO4 crystals and Sn‐doped GaPO4 hole centers localized on an oxygen showing an almost isotropic hyperfine splitting from a 31P nucleus are observed after X‐ray irradiation and analyzed by EPR. They are evidently caused by substitution of Al and Ga, respectively, by a divalent cation, most likely Mg2+. From the variation of the EPR linewidths with temperature an activation energy of 19 kJ/mole for thermal hopping of the hole is found in AlPO4. These centers cause a rose coloration and the first absorption band in AlPO4 has an oscillator strength near 0.3, in the same range as for the smoky quartz centers. It is interpreted as arising from optical transfer of the hole within the PO4 unit. Thermoluminescence emission is observed with destruction of the centers in AlPO4. From glow curves an activation energy of 136 kJ/mole is obtained which in this case represents the distance of the hole from the valence band. The properties of the center in AlPO4 are compared to those of a similar center in certain natural rose‐colored quartz crystals.
Radiative Optical Modes of Vibration in CeO2 Thin Films
Authors: Mochizuki S.
Published: July 2007

The radiative optical modes of vibration in a CeO2 thin film on a metal substrate are observed at various temperatures from room temperature to 1273 K by the reflection and thermal‐emission methods. A comparison of the experimental spectra and the theoretical ones shows good agreement. The upper branch of the polariton dispersion curve of CeO2 at room temperature is determined by analyzing the incident‐angle dependence of the virtual mode spectra centered near the longitudinal‐optical phonon frequency of CeO2 bulk crystal. The temperature dependence of the longitudinal‐optical phonon frequency of CeO2 is determined by the measurements of the virtual‐mode frequency shift with temperature under small incident (emission) angle condition and is discussed in terms of the thermal expansion and other anharmonic effects.
Thermoelastic Behaviour and Higher‐Order Elastic Constants of Ionic Crystals
Authors: Bhende W. N. , Bakhshi P. S. , Shanker J.
Published: July 2007

The relationships are given between thermoelastic properties and higher‐order (third and fourth order) elastic constants for twenty alkali halides with NaCl and CsCl structures, and for four alkaline earth oxides viz. MgO, CaO, SrO, and BaO crystals. Expressions for thermoelastic quantities such as the Grüneisen parameter and its volume derivative, Anderson parameters, and thermal expansion coefficient, are obtained in terms of higher‐order elastic constants. These expressions provide a new method for estimating thermoelastic quantities from higher‐order elastic constants. Calculations are performed using an interionic potential model which takes account of three body interactions, van der Waals interactions, and overlap repulsive interactions upto second neighbours. The results are discussed and compared with experimental data.
The Effect of Collective Electron‐Hole Plasma Modes on Charge Carrier Mobility in Polar Semiconductors
Authors: Kasiyan A. I. , Russu P. I.
Published: July 2007

The dispersion relations for renormalized plasmon‐phonon modes for PbTe and GaAs are calculated. Electron μe and hole μh mobilities are computed for scattering by these modes. It is found that with the increase of the charge carrier concentration, n, mobilities decrease due to the effect of dynamic antiscreening. In the concentration range where the free‐carrier plasma frequency is comparable to the uncoupled LO‐phonon frequency the mobilities reach minimum values. With further increase of n the antiscreening effect decreases and the mobilities begin to grow tending in the limit of high n to values corresponding to the quasistatic screening.
Excitons in Periodic Structures with Homopolar Semiconductors and Anisotropic Dielectrics
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

The charge carrier interaction potential is determined and the exciton energy spectrum in periodic multi‐layer structures with anisotropic permeability constant is calculated. In contrast to three‐layer systems with semi‐bounded outer layers in periodic structures dielectric layers are found to affects the exciton spectrum not only by the mean geometric value of the dielectric permeability tensor component \documentclass{article}\pagestyle{empty}\begin{document}$ \bar \varepsilon _a = \sqrt {\varepsilon _a^\parallel \varepsilon _a^ \bot } $\end{document} but also by the anisotropy parameter εa = ε/ε. The absolute value of the exciton ground‐state energy in semiconductors with a dielectric constant εb > εa and low layer thickness la and lb in the two‐dimensional Coulomb interaction region is found to decrease as the thicknesses ratio λ = la/lb decreases. The effect of semiconductor layers thickness at different values of the anisotropy parameter εa as well as of thickness ratio λin the logarithmic law of interaction on the exciton ground‐state energy is studied.
The Energy Transport from One Molecular Chain to the Other which are in Mutual Contact by a Small Number of Crystal Sites
Authors: Gashi F. , Gashi R. , Shemsedini Z. , Žakula R. B.
Published: July 2007

By methods of non‐equilibrium statistical physics the kinetical coefficient is evaluated of energy transport from one molecular chain to the other which has the same contacting point. Especially the contributions of soliton waves in this energetic transport are studied. It is estimated that only the solitons found at the contact point give contributions compararable with those which in kinetical coefficient give collective exciting‐excitons.
On the Landau Phase Transition Theory for the Magnetic Itinerant Electron System Weakly Interacting with Localized Spins
Authors: Cong Bach Thanh
Published: July 2007

The Landau expansion form of the thermo‐dynamic potential of the magnetic itinerant electron system weakly interacting with localized spins is obtained from its functional representation. The dependence of the Landau parameters A, B (which appear in the expansion of the thermodynamic potential in powers of the magnetization) on temperature and microscopic parameters is presented. The phase transition type in some substituted ACo2 compounds (A is a heavy rare earth metal) is also discussed in the framework of the developed theory.
A Variational Method in Wannier Representation for an Electron Weakly Bound to an Isoelectronic Impurity
Authors: Geistlinger H. , Weller W.
Published: July 2007

The problem of a single electron bound to an isoelectronic impurity (solved in k‐space by Koster and Slater) is reconsidered in lattice space. An analytically tractable variational method for the ground state is developed and compared with the numerically evaluated solution in k‐space. For GaP:N taking into account the real band structure of the lowest conduction band the variational method yields a sufficient agreement with the k‐space eigenwert equation. The variational approach developed in the lattice space allows a straightforward generalization to the binding of an exciton (considered in I) which takes into account the electron‐hole correlation, crucially in the region of weak binding (e.g. GaP:N).
Effect of Irradiation with Intense Laser Light on Trans‐Polyacetylene
Authors: Iwahana K. , Knoll P. , Kuzmany H.
Published: July 2007

An effect of irradiation with intense laser light on trans‐polyacetylene is investigated. It is found that laser radiation gives rise to an enhancement of the background intensity in Raman scattering, which finally develops into a strong luminescence band. This new luminescence band consists of several bands with a peak‐energy spacing of 1200 to 1400 cm−1 and exhibits an unusual dependence on the excitation energy in that each band of luminescence shifts with the laser frequency like a fist‐order Raman spectrum. It is suggested from comparison with the other experimental results that the luminescence observed in the trans‐polyacetylene sample degraded by laser irradiation originates from the excited state with a decay time of 1200 ps.
Study of the Hall Coefficient in p‐Type Germanium Taking into Account Anisotropic Charge Carrier Scattering
Authors: Gutsul I. V. , Kirnas I. G. , Litovchenko P. G. , Marusyak V. I. , Nitsovich V. M. , Ostapov S. E. , Petrosyan E. E.
Published: July 2007

A quasi‐momentum distribution function of light and heavy holes in p‐type Ge is found by expanding the field and the collision terms of the kinetic equation in terms of the anisotropy parameter, indicating the degree of non‐sphericity of constant energy surfaces, in the zeroth, first, and second order approximation. The above method for solving the kinetic equation enables one to calculate the magnetic field dependence of the Hall coefficient taking into account the complex structure of the valence band of p‐Ge and the nature of carrier scattering. The calculated RH/R0 = f(B) dependence is compared with the experimental values obtained on samples with p = 1.7 × 1014 cm−3 having an orientation E ∥ 〈100〉, B ∥ 〈001〉 for T = 77 and 300 K.
Thermoelectric Power of Liquid Germanium and Tin
Authors: Bath A. , Hugel J. , Kleim R.
Published: July 2007

Measurements of the absolute thermopower S of liquid germanium and tin are reported. It is found that S = −0.5 μV/K for Ge at the melting temperature and S = −1.5 μV/K for Sn at 900 °C. Calculations of the resistivity and the thermoelectric power performed in the framework of Ziman's nearly‐free electron theory, using the pseudopotential approach and the t‐matrix formulation, lead to reasonable agreement with experiment.
Ferroelectric Liquid Crystals in External Fields
Authors: Talim S. L. , Pires A. S. T.
Published: July 2007

The phase transitions of ferroelectric liquid crystals in external fields (electric and magnetic) is studied, using the Landau free energy expansion for the order parameter. The commensurate‐incommensurate phase transition, in the presence of a magnetic field using solitons is analysed in detail. The normal‐incommensurate phase transition in the presence of electric and magnetic fields is also studied. The fluctuation spectrum of the soliton lattice is calculated, and a new branch of oscillation is found.
Atomic Scattering Factors of Solid Elements
Authors: Chakraborty S. , Manna A. , Ghosh A. K.
Published: July 2007

The atomic scattering factor fh for elastic scattering of electromagnetic radiation depends strongly on electronic charge distribution n(r) in the atom. Deviations of recently measured fh's from those calculated from free‐atom electron states confirm the notion that n(r) gets appreciably modified in a solid. Recently it has been shown that the superposition of free‐atom charge densities through Löwdin's α‐expansion can help to find n (r) in a solid, which, in turn, yields a reasonable metallic cohesive energy and binary alloy formation energy. Here it is shown that the same procedure produces fh values that compare well with experiments as well as with more sophisticated band calculations.
A Localized Orbital Description of Si Using Non‐Local Potentials
Authors: Sieh K. S. , Smith P. V.
Published: July 2007

In previous work it has been shown that the band structures and electronic charge density distributions of the homopolar covalent solids can be well described within a tight‐binding formalism employing just a small set of well localized atomic orbital basis functions. In this orginal work the crystal Hamiltonian was defined in terms of purely local pseudopotentials. Recent work on these tetrahedrally bonded systems has shown, however, that a better representation of their overall electronic structure results from the use of non‐local, energy‐dependent pseudopotentials. The aim of the present paper is thus to extend this localized atomic orbital method (LAOM) to enable the description of the crystal Hamiltonian in terms of non‐local, energy‐dependent pseudopotentials. This modified approach is then applied to the case of Si using the non‐local, energy‐dependent pseudopotential of Chelikowsky and Cohen, and it is shown that excellent representations of both the band structure and valence band charge densities are again obtained from a set of just ten atomic orbital basis functions. The fact that these basis functions are also extremely well localised about their respective lattice sites makes them ideal for subsequent defect calculations on these homopolar covalent solids.
On the Shape Dependence of the Kubo Effect and Polyhedral Shapes
Authors: Nara S.
Published: July 2007

The free electron energy levels of ultra fine metal particle are investigated on the basis of two‐dimensional jellium model with including size and shape dependences. Three shapes (a circle, square, and equilateral triangle), for which the energy levels are exactly solvable, are chosen and several electronic quantities are evaluated with the use of obtained eigenvalues, the number of which are ≈ 11000. The results show that when the size becomes smaller than ≈ 100 Å in diameter, some electronic quantities deviate considerably from the asymptotic form which is expected from the thermodynamic limit.
Electric Field Gradient in Dilute B.C.C. Alloys. Application to Vanadium Alloys
Authors: Pal B. , Singh J. , Raj S. D. , Prakash S.
Published: July 2007

The formalism for the size effect electric field gradient (EFG) in dilute alloys with b.c.c. crystal structure is developed. The components of the stress field tensor F̃ are evaluated assuming dressed point ions interacting through the screened Coulomb potential. The size effect EFG is found cylindrically symmetric at the first and second nearest neighbour sites. The valence effect EFG is calculated using the transition metal model potential theory. It is assumed that due to quasilocalized d‐electrons, the shielding and antishielding mutually cancel in host region, however antishielding is accounted for in the outer region. The square well potential for the impurity in conjunction with Friedel's sum rule is used to evaluate the scattering phase shifts. The EFGs for dilute alloys of vanadium (V) with transition metal impurities Ti, Cr, Mn, Fe, Nb, and Ta are calculated. It is found that at both, the first and second nearest neighbour sites, the valence effect EFG is significantly larger than the size effect EFG. The calculated results for VTi, VNb, and VTa are in reasonable agreement with the experimental values.
On Theoretical Modelling of Continuously Inhomogeneous Semiconductor Structures
Authors: Henniger U. , Keiper R.
Published: July 2007

Starting from Boltzmann's equation the system of particle and energy rate equations is derived und discussed for describing the carrier transport in continuously inhomogeneous semiconductors. Special attention is paid to the dependence of the Shockley‐Read‐Hall recombination rate on the carrier temperatures. The so obtained generalization of the net recombination rate takes into consideration the effects of local carrier heating or cooling. A comment to the standard conception of field dependent mobilities is given.
Effect of Magnetic Field on a Phase Slip Centre
Authors: Chashoo G. J. , Singh Y.
Published: July 2007

The effect of a magnetic field on nonequilibrium quasiparticle relaxation processes in superconductors is investigated. It is observed that the normal‐like length of the first phase slip centre (PSC) in an indium whisker gets much reduced under the influence of magnetic field for temperature near the critical temperature. This suggests that the number of PSC's in a whisker increases in presence of the magnetic field for I > Ic. The results are in agreement with the experimental data for tin.
Low‐Frequency Dielectric Response Function of Sodium Nitrate
Authors: Wyncke B. , Brehat F. , Kozlov G. V.
Published: July 2007

The far infrared and submillimeter dielectric function of sodium nitrate single crystal is determined by means of precise transmission spectroscopy (7 to 18 cm−1) and reflexion spectroscopy (10 to 600 cm−1) over the temperature range 20 to 275 °C, parallel and perpendicular to the crystallographic c‐axis. All the infrared active lattice modes are observed and their parameters calculated using the factorized form of the dielectric function. A relaxation is introduced to explain the low‐frequency dielectric function in the two directions.
Simple Dynamical Model for the Vibrations of the Halogen Crystals
Authors: Higgs J. F. , Anderson A.
Published: July 2007

A simple force constant model is developed and applied to the vibrations of the chorine, bromine, and iodine crystals. Fundamental frequencies are calculated and compared to those observed by infrared absorption, Raman scattering, and inelastic neutron scattering. The dependence of the halogen–halogen force constants on atomic separation is investigated.
On the Interaction between a Screw Dislocation and Point Defects in B.C.C. Transition Metals
Authors: Masuda‐Jindo K.
Published: July 2007

The interaction energy Eint between a (1/2) 〈111〉 screw dislocation and point defects (both substitutional solutes and vacancies) in b.c.c. transition metals is investigated using a tight‐binding electronic theory. It is shown that the “chemical interaction” (d‐band energy contribution) plays a central role in determining the Eint values for the substitutional solutes in b.c.c. transition metals. The solid solution hardening of the dilute b.c.c. transition metal alloys is also discussed in the light of the present electronic theory; particular attention is paid to the solid solution hardening due to the transition metal impurities and due to sp impurities (such as Al, Si, Ge and Be) in α‐Fe.
The Para‐ and Ortho‐Phases of High‐Density Excitons in Cu2O
Authors: Moskalenko S. A. , Russu S. S. , Baltaga V. V. , Bobrysheva A. I. , Lelyakov A. V.
Published: July 2007

The quasiequilibrium state of the two‐component non‐linear ortho‐para‐exciton gas is investigated. The concentration shifts of the exciton levels are taken into account at high densities of quasiparticles. Three exciton phases appear at any value of the chemical potential, due to the effect of optical bistability: the para‐phase, the ortho‐phase and the mixed ortho‐para‐phase. It is pointed out that Bose‐Einstein condensation (BEC) in the para‐ and ortho‐phase is possible, in spite of the fact that the initial level of ortho‐excitons is situated higher on the energetic scale than the initial level of para‐excitons.
Influence of Magnetic Quantization on the Effective Electron Mass in Ternary Semiconductors
Authors: Mondal M. , Ghatak K. P.
Published: July 2007

An attempt is made to study the effect of a quantizing magnetic field on the effective mass in degenerate n‐type ternary semiconductors at low temperatures taking n‐Hg1−xCdxTe as an example. It is found on the basis of the three‐band Kane model, which is the most valid model for n‐Hg1−xCdxTe, that the effective electron mass at the Fermi level shows an oscillatory magnetic field dependence as expected since the origin of the oscillations in the effective mass in small gap semiconductors is the same as that of the Shubnikov‐de Hass oscillations. Besides, the amplitude of oscillations is significantly influenced by the alloy composition whereas the period is found to be independent of the compositional parameters in ternary semiconductors. The corresponding results for parabolic energy bands are also obtained from the expressions derived.
Thermal Stability of Fe+ Centres and Thermoluminescence of ZnS Single Crystals
Authors: Tarkpea K. E. , Ots A.
Published: July 2007

ESR and TL methods are used to investigate cubic ZnS single crystals. At 104 to 153 K in iron containing ZnS crystals, the TL is shown to be connected with the thermal ionization of Fe+ centres according to the reaction Fe+ + kT → Fe2+ + e−. Various thermal stabilities of Fe+ centres are supposed to be conditioned by crystal field distortions around the Fe+ ion. The character of the distortion is determined by the high‐temperature heating conditions and the cooling rate of ZnS crystals. The activation energy of Fe+ centres is estimated.
Harmonics of Cyclotron and Cyclotron–Phonon Resonances in Semiconductors with Superlattice
Authors: Polyanovskii V. M.
Published: July 2007

The harmonics of cyclotron and cyclotron–phonon resonances (CR and CPR, respectively) in semiconductors with a superlattice (SL) in the presence of strong magnetic field, H, arranged parallel to SL axis, are studied based on quantum kinetic equation. The peculiarities of CR and CPR reveal themselves most explicitly with electromagnetic wave E(t) polarized transversely to H. This leads to sudden changes in the dependence of the absorption coefficient against incident light frequency. These sudden changes are shifted relative to the peaks of respective resonances by a miniband‐width. At E(t) ∥︁ H the sudden changes are absent and maxima of CR and CPR are defined less explicitly. In SL the maxima of CR and CPR as well as the sudden changes can manifest themselves in a rather weak magnetic field, while the respective quantum oscillations in conventional semiconductors are not revealed.
Raman and Infrared Spectra of Tellurium Sub‐Bromide (Te2Br)
Authors: Anastassakis E. , Raptis J. S. , Richter W.
Published: July 2007

Raman and IR spectroscopic techniques are used to study the long‐wavelength optical phonons of tellurium sub‐bromide (orthorhombic structure). By use of an appropriate range of temperatures and excitation wavelengths the frequencies for nearly all of the A2 and B2 q ≈︁ 0 optical phonons are determined. Oscillator strengths and LO‐frequencies for these phonons are obtained from the fitting of IR reflection spectra with a multiple oscillator model. The temperature dependence of the frequency and damping constant for some of these phonons is discussed. A remarkable feature of this material is the existence of low frequency (≈︁ 20 cm−1) phonons with extremely large polarizabilities (Δχ up to 150).
Energy Spectrum of the Cross Motion of Excitons in a Superlattice
Authors: Guseinov R. R.
Published: July 2007

On the basis of the exact expression for the potential of a point charge in a superlattice, consisting of alternating layers with two different values of the dielectric constant, the energy spectrum is found of an exciton motion, perpendicular to the superlattice layer direction, at small values of the exciton quasi‐momentum component parallel to the layer plane. An evident form of an exciton energy dependence on the cross component of quasi‐momentum which is perpendicular to the layer plane is adduced. The exciton energy has a maximum at the centre of the Brillouin zone of the cross quasi‐momentum and a minimum at its boundaries.
On the Microscopic Theory of the Resonant Nonlinear Refractive Index of InSb
Authors: Dat Nguyen Nhu , Van Hieu Nguyen
Published: July 2007

A new version is presented for the microscopic theory of the resonant intensity dependence of the refractive index of a narrow gap zincblende semiconductor InSb. A general formula is derived to include the contributions from both the mechanisms: the free charge carrier plasma and the dynamical Burstein‐Moss effect. The electron and hole distribution functions are determined by means of the transport equation.
Resonance Interaction between Impurities and Crystal Molecules. Application to Benzselenophene in Naphthalene
Authors: Brovchenko I. V. , Ostapenko N. I. , Shpak M. T. , Valkunas L. L. , Vektaris G. B.
Published: July 2007

Theoretical and experimental investigations of resonance hybridization between impurity and local states are carried out in naphthalene with benzselenophene impurities. The analysis of the vibron regions of the luminescence and absorption spectra of this system shows that the hybridization effect really takes part in such a system and there are two centres with impurities being in non‐equivalent positions. Moreover each impurity perturbs two molecules of the crystal. The matrix elements of the resonance interaction between impurity and crystal molecules are given.
The Energy of Shallow Donor States in CdTe, GaAs, AgCl, AgBr Semiconductors
Authors: Grinberg M. , Łęgowski S. , Męczyńska H.
Published: July 2007

The model given by Bajaj of the energy levels of shallow point defects is modified introducing wave‐vector dependent dielectric functions. In the coupled polaron Hamiltonian the electron–ion interaction is described by the potential screened by the dielectric function. The electron–phonon interaction depends on the dielectric function too. The obtained energies of the ground and first excited states reproduce quite well the experimental data.
Photostimulated Reorientation of AsO Centres in Alkali Halides
Authors: Lohmeier F. J.
Published: July 2007

Molecular arsenite centres in alkali halides show a photostimulated reorientation after excitation to different excited electronic states. The temperature dependence of the reorientation rates is determined. The rates are described in terms of phenomenological rate equations, considering thermal reorientation processes in the electronic ground and excited state. The reorientation parameters for thermally stimulated reorientations are determined for the ground and the excited 3B1 state. The low‐temperature photostimulated reorientation, which results from a strong linear electron‐phonon coupling, mainly takes place during the relaxation from the 3B1 triplet to the 1A1 ground state and is supposed to occur via the A2(τx) sublevel from which a radiative decay is symmetry‐forbidden.
Computing Method of Elastic Moduli and Young's Modulus for Texturized Materials with Hexagonal Crystal Symmetry
Authors: Kalvoda L.
Published: July 2007

A calculation method of the generalized Reuss, Voigt, and Hill approximation of the elastic moduli and the Young modulus for rolled polycrystalline materials with hexagonal crystal symmetry is presented. The method proposed is based on the knowledge of the coefficients of the generalized spherical harmonic expansion for the crystallite orientation distribution function. The method is demonstrated by computing a cold rolled Zr sheet.
Fast‐Ion Conduction in the Presence of a Lattice Phase Transition
Authors: Bingham D. , Burton C. H.
Published: July 2007

A mixed pseudo‐spin 1/2 model of ionic conduction in the presence of a displacive phase transition of the lattice is derived from a consideration of the microscopic forces acting on the migrating ions. A phonon‐mediated interaction between the ions entering into the lattice transition and the mobile ions results in a number of phase transitions in the mobile ion order parameter 〈Sz〉. These are correlated with the ionic conductivity. A search is suggested for a material displaying the required properties and some possible advantages of such a material are discussed.
A Theoretical Study of the Vibrational Properties of InxGa1‐xAs1‐ySby Quaternary Alloys
Authors: Kleinert P.
Published: July 2007

First theoretical investigations of the lattice dynamics of InxGa1‐xAs1‐ySby quaternary semiconducting alloys are presented. All corresponding pseudobinary end member components are known to exhibit mixed‐mode behaviour. Within a mean‐field approximation a Bethe lattice is self‐consistently constructed in such a way that for all mixed ternary end member compounds the results of the coherent‐potential approximation (CPA) are reproduced. Special results are obtained for the dielectric susceptibility spectra, which display only one dominant band for mixed quaternaries mainly containing GaAs but two bands for systems which contain mainly InSb. With respect to an interchange between x and y the lineshape of the spectra is nearly unchanged.
Phonon Scattering by Inhomogeneously Distributed Paramagnetic Ions
Authors: Toombs G. A. , Sheard F. W.
Published: July 2007

The Green function for phonons scattered by an inhomogeneous distribution of resonant centres is evaluated. A diagrammatic expansion in two parameters is used: the coupling strength and α, the ratio of the resonant phonon wavelengths to the characteristic wavelength for the distribution of scatterers. For small values of α, a spatially dependent local dispersion and damping of the phonons is obtained. The effect on the linewidth for inelastic neutron scattering is discussed.
Specific Heat and Magnetic Susceptibility Studies on Bromobis (1,4‐Diazacycloheptane) Copper(II) Perchlorate, [Cu(dach)2Br](C104), at Low Temperatures
Authors: Khattak G. D. , Hussain M. S. , Hunter G. W. , Wenger L. E.
Published: July 2007

Specific heat and magnetization investigations are made on polycrystalline [Cu(dach)2Br](Cl04) at low temperatures. For temperatures above 25 K, the dc magnetization shows a Curie‐Weiss behaviour with a Curie constant of 7.92 × 103 e.m.u. K/g and a paramagnetic Curie temperature of −60.8 K. The downturn of 1/X vs. T below 25 K is indicative of ferrimagnetic system. The specific heat between 4 and 8 K can be described by the relation: C = 11.9T3 mJ/mol K with the cubic term leading to a Debye temperature of θD = 194 K. Below 4 K the specific heat rises very rapidly suggesting magnetic ordering. This is substantiated by the presence of a sharp peak at 0.95 K in the ac susceptibility results which can be interpreted as being due to a ferrimagnetic phase transition. Both the magnetization and specific heat investigations indicate the behavior of the system to be ferromagnetic at low temperatures which is interpreted as the result of the canted antiferromagnetism of sublattice moments.
Wavelength‐Modulated Spectra of the Optical Properties of β′‐CuxZn1−x from 1.5 to 5.1 eV at the β′ ⇄ α + β′ Phase Transition
Authors: Stearns R. , Braunstein R. , Muldawer L.
Published: July 2007

Wavelength‐modulated derivative spectra of the reflectivity of β'‐CuxZn1‐x are determined between 1.5 and 5.1 eV for compositions near the β' ⇄ α + β' phase transition. The alloys are annealed and quenched in the β' phase on both sides of the phase transition. Both the intraband and interband properties show marked changes at the transition. The Drude effective mass increases dramatically for the β' phase in the region where α + β' composition is thermodynamically preferred, indicating flattening of the bands. Shifts in interband properties also show a marked change at the phase transition, with the conduction bands showing a much greater sensitivity than the d‐bands.
Simple Model Calculations for Optical Constants of Copper
Authors: Ahuja A. K. , Auluck S. , Joshi S. K.
Published: July 2007

A simple s‐d band model is used for the interpretation of ϵ2(ω) for copper. Fitting simple polynomials for transition matrix elments, analytical expressions are obtained for ϵ2(ω) in terms of band parameters of the s‐d model. This expression is used to study the effect of matrix elements and variation of relaxation energy on ϵ2(ω) of copper. It is shown that such a simple model is sufficient to study such a variation and, therefore, to study the qualitative variation of ϵ2(ω). The model is also used to explain the peaks in dϵ2(ω)/dω curve successfully. The effect of hybridization amongst s and d bands is also taken into account for the calculation of transition matrix elements and the resulting effects in ϵ2(ω) are calculated.
Relativistic Theory of Photoemission from Solids II. Fully Dynamical One‐Step Formulation
Authors: Braun J. , Thörner G. , Borstel G.
Published: July 2007

A fully dynamical one‐step theory of photoemission is presented. The present model is a relativistic generalization of the original non‐relativistic work due to Pendry and allows for calculating theoretical ultraviolet photoemission and bremsstrahlung isochromat spectra from non‐magnetic and ferromagnetic solids.
Many‐Body Theory for the Dense Exciton Gas of Direct Semiconductors II. Calculation of Exciton Level Shift and Damping in Dependence on Exciton Density
Authors: Boldt F. , Henneberger K. , May V.
Published: July 2007

The spectral properties of a dense exciton gas are discussed using an exciton self‐energy in second collisional Born approximation with respect to exciton‐exciton interaction. The shift of the continuum edge and the ls exciton as well as the increase of the damping of the ls exciton are calculated linearly in the exciton density. The distribution of ls excitons is assumed to be either a macro‐occupation of the k = 0 state or a Boltzmann distribution with effective temperature varying between 10 and 60 K. A weakening of the level shift is obtained with increasing temperature. In the case of macro‐occupation damping is calculated self‐consistently resulting in a sublinear dependence on density.
X‐Ray Photoelectron and Auger Electron Spectra of Cadmium Thin Films
Authors: Jewell B. E. , Halder N. C. , Swartz W. E.
Published: July 2007

The X‐ray photoelectron spectra (XPS) and Auger electron spectra (AES) are measured for Cd thin films deposited on microscopic glass slides by thermal evaporation at a pressure of 10−8 Torr. The deconvolution procedure is utilized with appropriate error control and response function for resolution and enhancement of the spectral lines. The binding energies of the valence and core orbitals are determined and the MNN transitions for the AES analyzed. The results of the present study indicate a reasonable agreement with the results for Cd metal.
Mössbauer Study of Two‐Electron Exchange between Neutral and Ionized Tin Donor Centers in Solid Solutions on the Basis of PbS and PbSe
Authors: Nasredinov F. S. , Prokofieva L. V. , Seregin P. P. , Zarubo S. V. , Ermolaev A. V. , Kurmantaev A. N.
Published: July 2007

The Mössbauer spectroscopy with 119Sn isotope shows that tin atoms in Pb1‐xSnxS and Pb1‐xSnxSe solid solutions are electrically active donor centers with charge states determined by acceptor impurities occuring in the solid solutions. The neutral state and the ionized one of the tin donor centers are identified as Sn2+ and Sn4+, respectively. A two‐electron exchange between the neutral and ionized tin centers is observed for the Pb1‐xSnxSe solid solutions doped with sodium and thallium acceptor impurities.
EPR of Mn2+ in Strontium Tartrate Trihydrate Single Crystals
Authors: Korkmaz M. , Aktaş B.
Published: July 2007

Electron paramagnetic resonance spectra of Mn2+ doped in single crystals of strontium tartrate trihydrate grown by a diffusion technique are investigated at room temperature. Mn2+ enters the lattice substitutionally and is trapped at two magnetically equivalent but crystallographically inequivalent sites. The EPR measurements give the following values for the spin‐Hamiltonian parameters: gx(= gy) = 1.9781, gz = 2.0012, Ax(= Ay) = −94.9 × 10−4 T, Az = −96.2 × × 10−4 T, BB(= D/3) = +160.9 × 10−4 T, B(= E) = +101.2 × 10−4 T, B = +0.013 × 10−4 T, and B = +0.088 × 10−4 T.
Dielectric Screening, Polar Phonons, and Longitudinal Electronic Excitations of Quantum Well Double Heterostructures Application to Light Scattering from Charge Density Fluctuations
Authors: Bechstedt F. , Enderlein R.
Published: July 2007

The screened Coulomb potential and the inverse dielectric function of a quantum‐well double heterostructure (DHS) are expressed in terms of bulk dielectric functions. The spatial dispersion is fully taken into account. The polar LO‐phonon modes, longitudinal electronic modes, and coupled electron‐phonon modes of a DHS are derived from the poles of the inverse dielectric function. Emphasis is paid to phonon slab modes. The close relation between the screened Coulomb potential and the density‐density correlation function is used to calculate the Raman cross section for scattering from charge density fluctuations. In typical cases slab modes are essential for Raman spectra of DHSs and superlattices.
Reality Condition on Order Parameter in Magnetic Phase Transitions
Authors: Damnjanović M. , Vujičić M.
Published: July 2007

A relationship between the symmetry group of the low‐temperature phase and an order parameter of the phase transition is discussed, both, for ordinary and for magnetic groups. It is shown that when magnetic groups are considered, an extra condition, the “reality” of the order parameter appears.
On the Relation between Stopping Power and Proton‐Induced Electron Emission for Intermediate Energies: Aluminum
Authors: Brauer W. , Rösler M.
Published: July 2007

A theory is given of proton stopping power −dEo/dx in polycrystalline aluminum for energies between Eo = 40 and 800 keV corresponding to the calculation of proton‐induced electron emission. In good agreement with the experimental data a maximum of the stopping power (− dEo/dx)max = 13.9 eV/10−10m is obtained at E = 55 keV. The main contribution to the stopping power in the whole energy range is found to be the individual interaction between protons and valence electrons. Calculated and measured values of the 2p ionization cross section are proved to be in accordance. The averaged energy loss per collision ΔE0 turns out to be weakly dependent on the proton energy Eo. The validity of the empirical relationship between electron yield γ and stopping power is investigated.
Piezo‐Optical Activity of Cubic Crystals
Authors: Tsitsishvili E. G.
Published: July 2007

Effects of the piezo‐optical activity in the cubic crystals are studied. It is shown that if the stress and the wave vector are directed along different fourfold axes polarisation plane rotation takes place, but only a “weak” gyrotropy may be observed at P ‖ [001] and q ‖ [110] (as well as at P ‖ {111} ⟂ q). It is shown that in the case of the crystals with a sufficiently high exchange splitting Δst the stress‐induced k‐linear term plays a dominant role. In the case of interband effect the bilinear term leads to the splitting of the spin‐degenerate subbands. In the exciton case there is a mixing of exciton states and it is shown that the optical activity is better observable for the Z3‐exciton.
Critical Currents in Type‐II Superconductors
Authors: Przełoźny Z.
Published: July 2007

The flux‐line lattice stability in the longitudinal and transverse problems is investigated within the Ginzburg‐Landau theory. In the longitudinal problem the vortex lattice is unstable against growth of helical deformation, if the current along the flux‐lines exceeds a critical value. If the current is perpendicular to the vortex direction, the homogeneous flux‐line lattice is always unstable if there is no volume pinning. The long range of an effective interaction leads to increase of both, the pinning and critical current value. The peak effect in longitudinal current versus magnetic field dependence is interpreted by a collective behaviour of bent flux‐lines close to upper critical magnetic field.
Volume Dependence of Dielectric Properties of Some Fluorite‐Structure Crystals
Authors: Dutt N. , Agrawal G. G. , Shanker J.
Published: July 2007

The experimental data on first and second order pressure derivatives of dielectric constants are used to make an analysis of the volume dependence of polarizabilities, effective charge parameter, and transverse‐optic‐mode frequencies in CaF2, SrF2, BaF2, PbF2, Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. The short range force constant responsible for the ion‐displacement polarizability is calculated with the help of Lundqvist's three‐body interaction model. Reasonable values for the volume derivatives of various dielectric parameters are obtained. Values of optic‐mode‐Grüneisen parameters and their volume derivatives are also calculated and compared with available experimental data.
The Random Field Ising Model in 2 + ϵ Dimensions Critical Behaviour from Interface Fluctuations
Authors: Nattermann Th.
Published: July 2007

The critical behaviour of the random field Ising model is calculated in d = 2 + ϵ dimensions, using previous findings on interface roughening of Grinstein and Ma. The critical exponents are expressed in terms of ϵ and the dimension β/v of the order parameter field. ϵ and β/v are assumed to be small. For nonvanishing random field strength HR the critical behaviour is governed by a zero temperature fixed point HRc ˜ (ϵ + 2β/v)3/4. In particular μ = 3/4 and v−1 = ⅔ (ϵ + 2β/v) are obtained for the interface tension and the correlation length exponent, respectively. The relation between interface and bulk correlation lengths in the various regions is discussed.
The Narrow Bands of Exciton Absorption in Polar Solids and Alloys
Authors: Kusmartsev F. V.
Published: July 2007

An analytical theory of the Urbach‐Martienssen tail for the Wannier‐Mott excitons in polar crystals and alloys is developed on the basis of the expansion of all quantities in the small parameter δ ≪ Ei(μ/M), where M and μ are the translation and the reduced masses of the exciton; Ei is the exciton Rydberg, and δ is the photon deficit. The exciton absorption coefficient and the length of the absorption tail are calculated. The latter characterizes the width of the exciton absorption band. The results obtained are compared with experiments on the exciton absorption in layered polar crystals.
Shake‐Effect in Radiative Auger Recombination of Carriers on Two‐Electron Impurity Centres in Semiconductors
Authors: Kovarskii V. A. , Chernysh L. V. , Sheinkman M. K.
Published: July 2007

A calculation of the Auger‐capture cross‐section of a hole on two‐electron impurity centres is performed. For the first time in a semiconductor (and probably in solid state, in general) the “shake‐effect” accompanying this process is taken into account. Consideration of this effect gives the possibility of truthfull explanation of selection rules and magnitude of “two‐electron” luminescence in crystals.
Optical and Electrical Properties of Vanadium‐Doped GaAs
Authors: Ulrici W. , Friedland K. , Eaves L. , Halliday D. P.
Published: July 2007

The results of optical absorption, luminescence, and temperature‐dependent Hall‐effect measurements on GaAs: V, p‐type GaAs: V: Zn, and n‐type GaAs: V:Si grown by LEC‐technique are reported. It is found that the V ground state is located very close to or within the valence band and that the V acceptor level is at Ec − 0.14 eV. The near mid‐gap level in GaAs: V responsible for its semi‐insulating behaviour is proposed to be due to the complex (V‐ X), where the correlation between the photoionization absorption connected with this centre and the EL2 concentration suggests that X is an arsenic vacancy. From this level scheme conditions for growing GaAs: V which is semi‐insulating at room temperature are derived. The optical experiments on GaAs: V with different Fermi‐level positions confirm the assignment of the triple‐peaked absorption band at 1.1 eV to the 3A2 → 3T1(F) transition of V. The weak absorption due to the 3A2 → 3T2(F) transition is found at 0.79 eV. The analysis of the temperature dependence of the line shape of the → 3T1(F) absorption band as due to strong dynamical T ⊗ τ2 Jahn‐Teller effect provides the JT‐parameters EJT = 780 cm−1, hωτ = 75 cm−1, and c = 4.7 eV/nm. The temperature dependent asymmetry of the line shape is explained on the basis of earlier theoretical predictions of the simultaneous action of strong linear and quadratic JT‐coupling.
Transient Absorption of Xe and Kr Guest Atoms in Ar Crystals
Authors: Schröder T. , Dössel O. , Nahme H. , Schwentner N.
Published: July 2007

Time resolved absorption spectra for transitions starting from the lowest excited electronic states of Kr(5s,3P2) and Xe(6s,3P2) guest atoms in Ar crystals show broad bands at 1.81 and 1.72 eV, respectively. The bands are assigned to ns → np multiplets of the guest atoms. The blue shift of 0.3 eV with respect to the gas phase and the large band width indicate a strong coupling of the transitions to the matrix. Transitions to higher states are not observed. The initial electronic states are metastable with lifetimes of 10−5s (Kr) and 3 × 10−6s (Xe).
The Spontaneous Resistivity Anisotropy of Iron‐Based Alloys
Authors: Muth P. , Christoph V.
Published: July 2007

The spontaneous resistivity anisotropy of Fe‐based alloys measured at low temperatures and the quotient of the residual sub‐band resistivities of the majority and minority spin‐band electrons are obviously strongly related. It can be shown that a theoretical expression derived for the spontaneous resistivity anisotropy of Ni‐based alloys represents the data of Fe‐based alloys, too. Other existing theoretical relations for the spontaneous resistivity anisotropy are analysed.
Acoustic Phenomena in Tunnel Junctions
Authors: Urushadze G.
Published: July 2007

A change of the electron energy in a deformed crystal is described through strain potential tensor λik(p), its magnitude in real crystals still remaining obscure; no detailed calculations or systematic measurements in different metals have so far been attempted. Tunnel current (tc) response to an external sound field propagating parallel to the junction plane of a tunnel diode in the normal state is determined. Amplitudes of a sound‐induced high‐frequency t c with a nonzero potential difference at the junction are established, and the possibility of measuring metal strain potential in an electron tunelling experiment is discussed.
Facilitated Electron Tunneling through Polymer Molecules
Authors: Davydov A. S. , Gaididei Yu. B.
Published: July 2007

Electron transfer from donor to acceptor molecules through finite length polymer chains is investigated. It is shown that the probability of the electron transfer through short chains may differ essentially from the exponential dependence inherent in tunneling through a simple potential barrier.
Solution of the Spin‐Hamiltonian with Orthorhombic HF and g‐Tensors (S = 1/2). I. Theory
Authors: Seth V. P. , Yadav A. , Bansal R. S.
Published: July 2007

Using perturbation methods a theory is developed to analyse the ESR spectra characterized by ℋ︁ = βH · g·S + S·A·I for S = 1/2. It is found that the perturbation is zero for isotropic g and A. For axially symmetric g and A, the perturbation term is zero when H is along z axis and for H along any other direction perturbation terms should be added which may be very small. For the general orientation of H an expression for the line position of the allowed transition is given.