bluuebunny/crossref_metadata_2025 huggingface dataset:
The Theory of Thermal Dechanneling from Axial Channels
Authors: Rozhkov V. V. , Rutkevich P. B.
Published: July 2007

On the basis of a thorough analysis of the stochastic proporties of particle motion in axial channels with thermally vibrating atoms the Fokker‐Planck equation for the distribution function of channeled particles over transverse motion energy is obtained. A random parametric effect of the thermal vibrations on the transverse motion of channeled particles is discovered. Analytical expressions for the distribution function over the transverse energy, the dechanneling function, and the integral penetration distribution foi particles injected into a crystal under channeling conditions are obtained.
EPR Investigations of a Structural Phase Change in Lead Phosphate
Authors: Razeghi M.
Published: July 2007

The temperature dependence of the EPR line width of the Mn2+ and Gd3+ in Pb3(PO4)2 is investigated from ‐270 to 500 °C. At the first‐order ferroelastic transition point (180 °C), an abrupt change in the fine‐structure splitting as well as in the resonance line width is observed. Various contributions to fine structure D and E parameters of Mn2+ and Gd3+ are computed, using a point‐multipole model. For temperatures near to Tc the correlation time of the fluctuations is estimated to be greater than 10−9.
On the Lifetime of Excited States in One‐Dimensional Disordered Systems
Authors: Esser B.
Published: July 2007

A macroscopic system realizing an ensemble of chain‐like recombination paths involving localized electron states is considered. The recombination process of a carrier occurring in a given chain is formulated in terms of a Markov process with absorbing state. An expression for the mean lifetime of a carrier in a chain taking account of the energy scatter of the electron states is derived. Using this expression and a density of states model the temperature dependence of the lifetime may be calculated explicitly.
Magnetic Phases of Strongly Correlated Electrons in a Nearly Half‐Filled Narrow Band
Authors: Spalek J. , Oleá A. M. , Chao K. A.
Published: July 2007

A canonical perturbation technique is used to investigate a nearly half‐filled nondegenerate generalized Hubbard Hamiltonian a t the strong intraatomic correlation limit. Quasiparticle state and the total energy for various magnetic orderings are obtained using the Green function method with the moment‐conserving decoupling scheme. A ground state magnetic phase diagram with antiferromagnetic, ferromagnetic, and mixed phases is derived and compared to existing results. The dependence of the critical W/U ratio for metal‐nonmetal transition on the interatomic exchange is estimated.
Nonlinear Energy Transport of Radiation in Dielectrics Due to Virtual Biexciton Formation
Authors: Schmitt‐Rink S. , Haug H.
Published: July 2007

The intensity‐dependent dielectric function is calculated from the photon self‐energy taking into account the two‐photon generation and autoionization (Fano effect) of biexcitons. The polariton density is determined self‐consistently as a function of the intensity, frequency, and space using the concept of the energy transport velocity. Two‐photon absorption spectra are calculated which are in good agreement with recent experiments.
On Elastic Properties of Dihedral Edge Dislocations
Authors: Bonnet R.
Published: July 2007

Firstly, the elastic displacement and stress fields of an edge dislocation located at the corner of a dihedral interface separating two media + and ‐ are discussed using isotropic elasticity. A fully analytical solution is derived and particular examples are calculated for the shear stress field when the ratio of both shear moduli μ+/μ− is 2 and 1/2, and the dihedral angle is 60°. Secondly, an example which illustrates the shear stress field of a dihedral edge dislocation at the corner of two {112}facets of a twin boundary in gold is presented using anisotropic elasticity.
Hole Effective Masses in CuInSe2
Authors: Neumann H. , Kissinger W. , Sobotta H. , Riede V. , Kühn G.
Published: July 2007

Optical transmittance and reflectance spectra are measured in p‐type CuInSeZ single crystals at 100 and 300 K in the photon energy range from 0.03 to about 1.0 eV. At hv = 0.07 to 0.70 eV the absorption is governed by intervalence band transitions. Assuming spherical parabolic energy dispersion relations for all the three valence bands, the effective masses of the heavy hole, light hole, and split‐off bands are determined to mhh = 0.71m0, m1h = 0.092m0, and msh = 0.085m0, respectively.
Vacancy‐Induced and Two‐Phonon Raman Scattering in ZrCx, NbCx, HfCx, and TaCx
Authors: Wipf H. , Klein M. V. , Williams W. S.
Published: July 2007

Raman scattering is studied in the substoichiometric transition metal carbides. The carbon vacancies in these cubic solids induce a one‐phonon spectrum that essentially mirrors the phonon density of states. The superconducting group Vb carbides NbC0.98 and TaC0.99 have one‐phonon Raman spectra that show characteristic phonon softening. Strong A + A and O + A two‐phonon peaks are also seen. In NbC0.76, which has a low Tc, the one‐phonon peaks harden, and the two‐phonon intensities weaken. Mechanical polishing of NbC0.98 and TaC0.99 reduces two‐phonon scattering and hardens and broadens the one‐phonon spectra. The non‐superconducting adjacent group IVb carbides ZrC0.96, ZrC0.87, and HfC0.99 have higher‐frequency one‐phonon peaks and no two‐phonon features.
Mössbauer Radiation Diffraction in Deformed Crystals
Authors: Baryshevskii V. G. , Skadorov V. V.
Published: July 2007

The dynamical theory of the diffraction of a spatially inhomogeneous field of Mössbauer γ‐quanta by perfect and deformed crystals is considered. The equations describing the dynamical diffraction of Mössbauer γ‐quanta is considered. It is shown that it is possible to enhance the Mössbauer radiation intensity by three orders as compared to the incident radiation intensity.
The Theory of the Superconductive Phase in Rare Earth Metal Compounds
Authors: Panina L. V. , Savchenko M. A. , Stephanovich A. V.
Published: July 2007

The microscopic theory of the superconductive phase in rare earth metal compounds is constructe by the contour averages mathematical method. It is shown that in the superconductive phase thd effect of exchange intensification occurs and increase of superconductive phase transition tempere ature is possible.
Random Bond Ising Model in Self‐Avoiding Walk Approximation
Authors: Medvedev M. V. , Sadovskii M. V.
Published: July 2007

Free energy of random bond Ising model is analysed by high‐temperature expansion in self‐avoiding walk approximation. Conditions of instability of the paramagnetic state are determined through the convergence criterion of the random high‐temperature series. Critical concentrations for the loss of the long‐range magnetic order are determined for different 2d and 3d lattices.
Crystal‐Field Analysis of Electronic Raman Scattering Data in YbAG and YbGG
Authors: Nekvasil V.
Published: July 2007

The relative Raman intensities associated with various electronic transitions are used simultaneously with the usually considered energy levels and g‐values for determination of the phenomenological crystal‐field parameters in YbAG and YbGG. The best‐fit parameters reproduce well all the available data but their values do not agree with the superposition model of the crystal field. It is concluded that the earlier crystal‐field calculation for Yb3+:YAG is impaired by the misinterpretation of the optical data.
Semiempirical Calculations of Defect Properties in LiF Crystal. II. Electron and Hole Centres and Their Recombination
Authors: Kotomin E. A. , Shluger A. L.
Published: July 2007

The electronic structure of F, H, Vk centres in a LiF crystal is calculated using the INDO semiempirical method. Use of both, the modified calculating scheme and earlier derived parameters permit to reproduce quite well optical absorption energies for hole centres and confirm their model of F molecular ion in a crystal. The locations of defect levels with respect to the band edges of a perfect LiF crystal are discussed. The calculations of pairs of nearest F–H, F–Vk centres predict energies of their radiative tunneling recombination of about 1 and 1.4 eV, respectively.
The Spectrum of an Intermediate Polaron and Its Bound States with Phonons at Strong Coupling
Authors: Gogolin A. A.
Published: July 2007

The spectrum of intermediate‐radius polaron at strong coupling g ≫ 1 is evaluated with the help of a new diagram technique. The bound states of the polaron with a large number of phonons N = 2g2 are found. In the case of an exciton, these states give rise to additional strong peaks in the absorption and luminescence spectra.
Effect of Fluctuations of the Coordination Number on Some Properties of Amorphous Ferromagnets
Authors: Bobák A.
Published: July 2007

The classical‐spin Bethe‐Peierls‐Weiss method is used to determine the Curie temperature and the paramagnetic susceptibility of amorphous ferromagnets. Due to the structure fluctuations of the coordination number a decrease of the Curie temperature and the paramagnetic susceptibility is found in comparison with the corresponding crystalline case. The same formalism is employed to study the chemical disorder introduced by substituting magnetic impurities in a non‐magnetic amorphous host. It is shown that the critical concentration of magnetic atoms for the appearance of long‐range magnetic order is increased by fluctuations of the coordination number.
Polarized Luminescence of Lead Ions Associated with Interstitial Radiation Defects in KCl:PbCl2 Crsytals
Authors: Egemberdiev Zh. , Zazubovich S. , Nagirnyi V.
Published: July 2007

Two types of lead ion and interstitial chlorine atom associates (Hz(Pb2+) centres) are found and investigated by optical methods in X‐irradiated KCl:PbCl2 crystals. The polarized 3.35 and 1.8 eV emission of these centres and the absorption bands near 5.55 and 4.4 eV are caused by electronic transitions in the Pb2+ ion perturbed by Cl molecular ion, which occupies a cation vacancy and an anion site. The broad absorption band at 3.7 eV is due to the transitions in the Cl ion perturbed by Pb2+. Using the ESR data obtained by Schoemaker et al. it is concluded that the centres with 3.35 and 1.8 eV emission are interstitial chlorine atoms trapped by Pb2+ ions associated with the nearest neighbouring (Hz[110](Pb2+) centres) and with the next nearest neighbouring (Hz[100](Pb2+) centres) cation vacancies, respectively. Lead ion and Cl molecular ion associates (V2z(Pb2+) centres) are found. The polarized 3.05 eV emission and the 4.3 eV absorption band of these centres are caused by the electronic transitions in Pb2+ ions perturbed by Cl. The broad 5.27 eV absorption band is due to the electronic transitions in the Cl ions perturbed by Pb2+. It is assumed that the V2z(Pb2+) centre may be represented as a Cl molecular ion which occupies two anion and one cation sites and is associated with a Pb2+ ion located (according to Itoh et al.) at a face‐centred position as well as with an interstitial chlorine ion and cation vacancy.
From Band to Hopping Diffusion of Excitons and Charge Carriers in Molecular Crystals. II. Results for Local and Weak Nonlocal Electron‐Phonon Coupling
Authors: Čápek V. , Munn R. W.
Published: July 2007

Previous results giving the diffusion coefficient for an exciton or charge carrier in a perfect molecular crystal in both the band and the hopping regime, and interpolating between regimes, are evaluated with and without nonlocal electron‐phonon coupling to second order. Nonlocal coupling produces an extra contribution to the hopping rate; it may increase or decrease the usual scattering rate, depending on the relative signs of local and nonlocal coupling, but changes the transfer rate in the opposite direction through the phonon displacements in the polaron states produced by local coupling. Increasing the temperature decreases the importance of nonlocal coupling in the band regime but increases it in the hopping regime. The significance of these effects depends on the detailed coupling model. The enhanced hopping rate agrees with previous published work.
The CrK Series X‐Ray Emission Spectra
Authors: Sørum H. , Weng O. M. , Bremer J.
Published: July 2007

The electron‐excited CrK series X‐ray spectra are investigated by means of curved crystal spectrometry, permitting at the same time high intensity and resolution. No directly observable fine structures are found for α1,2 and β1, 3. Low‐energy satellites as well as weak high‐energy satellites are, however, found both for α1,2 and β1,3. A low‐energy satellite may be present also for β5. The high‐energy satellites for the α‐doublet are most probably due to a spectator vacancy in the M‐level and/or in the valence band V, corresponding to the transitions KM → LM, KV → LV, respectively. Similarly the β1,3‐structure may contain contributions from the transitions KM → LM and/or KV → MV. In addition to the structure caused by interaction between the final‐state hole and the unfilled 3d‐level the radiative Auger effects K → LV, K → MV, and K → VV are probably present at the low‐energy side of the lines. The FWHM value for α1 is (1.90 ± 0.05), for α2 (2.60 ± 0.05), for β1,3 (2.68 ± 0.05), and for β5 (4.9 ± 0.1) eV. The spin‐orbit split between the 3p3/2 and 3p1/2 levels is measured for the first time to be 0.74 eV for Cr, a value which is thus considerably less than the theoretical value of 1.06 eV.
An Expansion Technique for Evaluating Densities of States and Related Integrals
Authors: Pisani C.
Published: July 2007

A calculation technique for the densities of states is presented which is based on the expansion of ϱ(ϵ) in a series of orthogonal functions in the band interval. Formulae are proposed for a simple evaluation of the expansion coefficients. When the eigenvalue and eigenvector dependence on κ is expressed as a linear combination of lattice waves and spherical Legendre polynomials are used as orthogonal set, the expansion coefficients are calculated exactly. The properties of convergence and some numerical aspects are discussed with reference to cubic lattices in the tight binding approximation.
Lindeman's Melting Law and the Effect of Pressure on the Melting Point of Si and Ge
Authors: Soma T. , Matsuo H.
Published: July 2007

Using the volume dependence of the mean‐square displacement, the pressure effect on the melting temperature of Si and Ge is studied by Lindeman's melting law. The melting curve obtained decreases as function of the compressed volume, and is qualitatively in agreement with the observed tendency for Si and Ge.
Transitions in Fast Ionic Conductors
Authors: Gluck P.
Published: July 2007

A two state model, based on different degrees of ionicity in the β and α phase, is used as a basis for discussion of the phase transition and dc conductivity in AgI‐type ionic conductors. A decrease (softening) in the frequency of anion oscillations is capable of accounting both for the transition from low to high ionicity (β → α) states, and for the increase in dc conductivity by several orders of magnitude.
Thermodynamic Properties of Liquid Alkali Metals and Their Equiatomic Alloys
Authors: Mishra S. C. , Srivastava P. L.
Published: July 2007

Gibbs‐Bogolyubov variational technique is used to study ‘internal energy’ and ‘entropy’ of liquid alkali metals as well as ‘heat of mixing’ and ‘entropy of mixing’ of their equiatomic alloys. Shaw and Ashcroft forms of model potentials with Vashistha, Singwi and Shaw forms of exchange and correlation functions are used for the calculations of the said properties.
On the Role of the Coulomb Interaction of Charge Carriers in the Electronic Raman Scattering on Donor Levels in Direct Band Gap Semiconductors
Authors: An Nguyen Ba , Van Hieu Nguyen , Thang Nguyen Toan , Viet Nguyen Ai
Published: July 2007

The electronic Raman scattering of light on the donor levels in the semiconductors with direct band gap is revised, taking into account approximately the Coulomb interaction between the charge carriers (conduction electron and hole). Except for the scattering in the resonance energy domain, where the polariton effect is essential, the influence of the Coulomb interaction of the charge carriers on the scattering cross section is not significant.
Towards a Molecular Theory of the Solid–Liquid Phase Boundary
Authors: Klupsch Th.
Published: July 2007

A rigorous statistical description of the interface formed by the coexisting liquid and f.c.c. crystal‐line phase of an one‐component Lennard‐Jones system at thermal equilibrium is presented taking into account the aspect of structural change. Based on correlation functions resulting from the coupled Ornstein‐Zernike and Percus‐Yevick equations for inhomogeneous systems, the one particle distribution function is found from minimization a free energy functional. For temperatures kT/ε = 0.752 slight above the triple point, the calculated mean free interface energy (continuum approximation) is ΔJ σ2ε−1 = 0.96 and the interface width l = 3.6σ which is comparable with computer simulation results, but not with lattice models.
Non‐Condon Approximations and the Static Approach in the Theory of Non‐Radiative Multiphonon Transitions
Authors: Gutsche E.
Published: July 2007

The off‐diagonal matrix element of the non‐adiabaticity operator, on which usually the theory of non‐radiative multiphonon transitions is based, is recast into a form much more convenient for the calculation of the transition probability W in non‐Condon approximation. This is demonstrated first for a two‐level system coupled to a single vibrational mode and then extended to the more general case of a complete set of non‐degenerate electronic states coupled linearly to N normal modes. The strong anharmonicity of the adiabatic potentials due to the mixture of states caused by the electron–phonon interaction operator is stressed, which limits the applicability of the theory to situations where only transitions well below “level crossing” are important, and, consequently, imposes serious restrictions on the parameters of the system and the temperature. Keeping to these restraints and to two electronic levels, the transformed matrix element of the non‐adiabaticity operator is identical with that of the perturbation operator used in the static coupling scheme of Haug and Pässler, which turns out in this way to be equivalent to a closed‐form of non‐Condon approximation based on a well defined set of adiabatic wave functions. Explicit expressions for W are also given, and some aspects of the theory in general are discussed.
The Influence of Compositional Disorder on Electrical and Optical Properties of ZnSxSe1−x Single Crystals
Authors: Mach R. , Flögel P. , Suslina L. G. , Areshkin A. G. , Maege J. , Voigt G.
Published: July 2007

The influence of compositional disorder in mixed crystals on the electrical and luminescence properties of ZnSxSe1−x single crystals grown by iodine transport are investigated by measuring the spectral distribution of photoluminescence, the excitonic reflection spectra, and the temperature dependence of free‐carrier concentration and mobility. Obviously there are effects describable in a virtual crystal approximation (VCA) as the dependence upon composition x of donor (iodine) depth and a net shift of the self‐activated (SA) luminescence. Beyond VCA the fluctuations of composition result in a broadening of excitonic spectra as expected for extended wave functions or resolvable spectral shifts (subbands) of the SA luminescence as expected for highly localized states, corresponding to the well‐defined varieties of nearest‐neighbour compositional configurations. There are indications that the abundances of the latter are not statistical.
Effect of Heavy Doping on the Electronic Heat Capacity in Semiconductors
Authors: Chakravarti A. N. , Ghatak K. P. , Ghosh S. , Chowdhury A. K.
Published: July 2007

An attempt is made to derive an expression for the electronic heat capacity in heavily‐doped semiconductors having Gaussian band‐tails. With the help of this expression, the electronic heat capacity is studied as a function of the electron concentration corresponding to different temperatures, taking heavily‐doped n‐GaAs having Gaussian band‐tails as an example. It is concluded that the nature of the dependence is determined exclusively by the movement of the Fermi level with changing electron concentration through the allowed energy states.
Collective Motion in Silver Chalcogenides
Authors: Sukegawa F. , Kobayashi M. , Yokota I.
Published: July 2007

A collective motion in silver chalcogenides is investigated in a hydrodynamic description. The frequency‐ and wave vector‐dependent conductivity and the dispersion relations of collective modes are given. The plasmon–LO phonon coupling is discussed in detail.
Electrical Properties of InSb Irradiated with Fast Neutrons
Authors: Baramidze N. V. , Bonch‐Bruevich V. L. , Giorgadze M. P. , Kurdiani N. I.
Published: July 2007

Variations are studied of the electrical properties of n‐ and p‐InSb induced by fast neutron irradiation (fluences from 1 × 1017 to 8 × 1019 n/m2). Temperature dependences of the concentration and mobility of the charge carriers is studied by the Hall effect method in the temperature range from 77 to 300 K — both before and after irradiation. Parameters of a random field produced by disordered regions are calculated. Charge carrier scattering by a smooth random field created by disordered regions seems to be the dominant scattering mechanism after irradiation.
Fermi Surfaces in NbcMo1−c Random Alloys
Authors: Donato E. , Ginatempo B. , Giuliano E. S. , Ruggeri E. , Stancanelli A.
Published: July 2007

Within the KKR‐CPA framework the Bloch spectral function of some NbcMo1−c alloys are computed. By looking at the results for E = EF the topology of the Fermi surface is studied as function of concentration. The results are in agreement with experimental indications given by the measurements on elastic constants in such alloys. The calculations confirm that a rigid band model for these systems can explain the observed properties at the Fermi level.
Luminescence Spectra of Anthracene Crystals at 0.4 K
Authors: Aaviksoo J. , Liidja G. , Saari P.
Published: July 2007

For the first time the luminescence of a molecular crystal is studied at sub‐Kelvin temperatures. Despite the fact that at ultralow temperatures the number of phonons in the crystal is negligible the bandshapes in spectra do not exhibit any pronounced “bottle‐neck” features that follow from the polariton picture. In order to explain the experimental results strong coupling of excitons with intramolecular vibrations in anthracene is taken into account.
Force Constant Change in Sn–Impurity Systems: 107Pd:119 Sn, 197Au:119Sn, and 63.5Cu:119Sn
Authors: Paul P. , Puri S. P. , Raj D. , Puri R. K.
Published: July 2007

The recoilless fraction f and the SOD data for the Sn as impurity in 107Pd, 197Au, and 63.5Cu are analysed for any force constant change in the framework of Mannheim's model, using the experimentally determined phonon frequency distribution function for the hosts. Furthermore, correction for anharmonicity is applied, wherever necessary. On the basis of f‐values, the force constant ratio λ′/λ turns out to be 0.46 ± 0.02, 0.86 ± 0.04, and 0.58 ± 0.03, respectively, for the systems 107Pd:119Sn, 197Au:119Sn, and 63.5Cu:119Sn. All these cases are of weak binding. This conclusion is at variance with an earlier report of strong binding for the system 107Pd:119Sn. Expectedly no localized modes are found to exist for either of the systems.
Excitons Perturbed by a Sodium Ion in CsI:Na Crystal
Authors: Kayal A.‐H. , Mori Y. , Rossel J.
Published: July 2007

The emissions around 420 and 370 nm of CsI:Na excited by the UV absorption associated with the Na+ ion are studied by measuring the temperature dependence of decay times, the magnetic circular polarization, and ESR by optical detection. The results suggest that the 420 nm band is due to a localized exciton adjacent to a substitutional Na+ ion. The basic properties of the electronic structure and kinetics of the exciton are obtained. These results are discussed in relation with the self‐trapped excitons in CsI.
Local Density Approximation Applied to Bound Excitons in Polar Semiconductors
Authors: Haufe A. , Henneberger K. , Röseler J. , Wünsche H.‐J.
Published: July 2007

Binding energies of excitons at donors in polar semiconductors are calculated in the frame of the Fröhlich model for the interaction of the particles with LO phonons. Beside the exchange‐correlation parts of the direct particle‐particle interactions, the dynamical effects of the particle‐phonon interactions are also treated within the LDA. For this purpose, a single parametrized formula for the dependence of the corresponding energies of a homogeneous e‐h plasma on the particle densities is used. The numerical results for both, the bound polaron D° and the bound exciton D°X agree with the best available theoretical data as well as with experimental values. From these facts it is concluded that LDA is an appropriate method for treating not only electron–electron interactions but also the electron–phonon interaction in inhomogeneous e‐h systems.
EPR and ENDOR Investigations of an Oxygen Centre in CaF2
Authors: Fuchs H. J. , Karthe W. , Böttcher R.
Published: July 2007

A paramagnetic centre is produced in CaF2 by oxygen diffusion and subsequent X‐irradiation. From EPR and ENDOR results it is possible to ascribe C1 symmetry to this centre. The experimental data at measuring temperatures T ≦ 77 K can be fitted well to an oxygen‐fluorine‐vacancy complex. The anisotropy is averaged due to inner motions at room temperature and therefore in the spectra C2v symmetry results.
Energy Bands of Krypton and Xenon with the GOPW Method
Authors: Baroni S. , Grosso G. , Parravicini G. Pastori
Published: July 2007

The Hartree‐Fock band structures of krypton and xenon are calculated from first principles, using the Gaussian orthogonalized plane wave method. This method, which exploits the Gaussian representation of occupied orbitals, allows to include the exchange potential in its nonlocal from. As a side‐product, it is shown that some drawbacks, erroneously attributed to the OPW method, disappear if appropriate care is done in describing core states and the consequent orthogonalization effects.
Electronic Structure of Fe—45Al Films
Authors: Nemoshkalenko V. V. , Lesnik A. G. , Kudryavtsev Yu. V. , Lezhnenko I. V. , Nikolaev L. I. , Voskrekasenko L. S.
Published: July 2007

Fe−45 Al films of the ordered (OS) and disordered (DS) states are examined with a RSM‐500 spectrometer. Fe Lα1,2, Fe M2,3, and Al L2,3 spectra are obtained. The Al L2,3 spectrum structure is less pronounced in DS than in OS, which may be accounted for by a different degree of symmetry in aluminium atom neighbourhood in DS. The slopes of high‐energy edges for Al L2,3 spectra differ, which indicates higher density of s‐states at Fermi level in OS, as compared to DS. While comparing the Fe Lα1,2 spectra in DS and OS, an integral intensity increase is observed, narrowing and shift towards low energies. This confirms the previously proposed hypothesis for the electron transition from Al to the iron 3d subband. The X‐ray spectral data correlate with previously obtained data on high‐frequency conductivity.
Cohesive Energies and Bulk Modulus for Alkali Halide Mixed Crystals
Authors: Shanker J. , Jain G. D.
Published: July 2007

An interionic potential model is developed for evaluating the cohesive energies and bulk modulus of alkali halides mixed crystals. The short range overlap repulsive interactions up to second neighbours and the van der Waals interactions are taken into account. The model is applied to study the mixed crystals NaCl‐NaBr, KCl‐KBr, and KBr‐KI for the entire range of composition. The results are discussed and compared with available experimental data.
Induced Magnetism in a System Consisting of van Vleck Ions Coupled to an Electron Gas in the Narrow Band Limit
Authors: Palermo L. , da Silva X. A.
Published: July 2007

The magnetic properties of a model consisting of an electron gas interacting by exchange with van Vleck ions, is studied under the action of a crystal field in the narrow band limit. Iso‐Tc curves (in the plane of the interaction parameters), ionic and electronic magnetizations and susceptibilities versus temperature, as well as the magnetic specific heat, are obtained for several values of exchange and crystal field parameters.
Static Atomic Polarizabilities from Norm‐Conserving Pseudopotentials
Authors: Bachelet G. B. , Tanner C. , Schluter M.
Published: July 2007

The transferability of norm‐conserving pseudopotentials, recently developed by Hamann, Chiang and Schluter is examined by calculating static atomic polarizabilities using Sternheimers procedure. Excellent agreement between full‐core atoms and pseudo‐atoms confirm the high quality of norm‐conserving pseudopotentials.
Theory of Donor–Bound Exciton Complexes in Semiconductors
Authors: Clark T. D. , Bajaj K. K. , Theis W. M. , Phelps D. E.
Published: July 2007

The ground‐state energy of a system consisting of an exciton in the presence of a neutral effective mass donor in semiconductors is calculated for an arbitrary value of the electron to the hole mass ratio using a variational approach. Both, the conduction band and the valence band are assumed to be simple and parabolic. The variational wave function used is a product of a generalized Hylleras‐Ore type wave function for the two electrons and an envelope function which depends on the hole—positive ion distance only. It is shown that the exciton—neutral donor complex is bound for all values of the electron to the hole mass ratio.
Spectra of Rare‐Earth Ions in the Crystal Fields of Double Tungstates and Molybdates
Authors: Malkin B. Z. , Kaminskii A. A. , Agamalyan N. R. , Bumagina L. A. , Butaeva T. I.
Published: July 2007

The Hamiltonian of neodymium and erbium ions in low‐symmetric crystal fields of KLn(MeO4)2 (Me = Mo, W and Ln = Y, Gd, Er, and Lu) crystals is constructed with fifteen crystal field parameters calculated within the framework of the exchange charge model. The theory using only one empirical scaling factor gives a satisfactorial description of Nd3+ and Er3+ ion spectra Stark structure and of its changing in the homologous series of tungstates investigated by absorption and luminescence.
The Luminescence of Broad‐Band Emitters in LiLaP4O12
Authors: Blasse G. , Dirksen G. J.
Published: July 2007

The luminescence of LiLa1−xMxP4O12 (M = Ce3+, Eu3+, Tb3+, Bi3+) is measured. Only for the Ce3+ ion pronounced concentration quenching is observed. The spectra of pairs of activators differ slightly from those of isolated activators. The Bi3+ emission is strongly Stokes shifted and is ascribed to AX emission. The Sb3+ ion shows a very efficient luminescence in this host lattice with an unusually small Stokes shift.
Lattice Mode Anharmonicity in Sodium Nitrite
Authors: Brehat F. , Wyncke B. , El Sherif M.
Published: July 2007

The anharmonicity of the infrared active lattice modes of sodium nitrite is studied from measurements of the infrared reflectivity from 60 to 400 cm−1, in the temperature range 80 to 460 K. The infrared reflectivity spectra are analysed with the four‐parameter dispersion model based on the factorized form of the dielectric function. The comparison between the results of the quantum theory and the dependence observed of damping shows that cubic anharmonicity drives the lattice dynamics of sodium nitrite when the crystal structure is ordered, and that disorder occurs below the phase transition temperature. The results confirm the breakdown of the selection rules in the paraelectric phase and suggest the existence of dielectric dispersion in the low frequency region.
The Temperature Dependence of the Spin Resonance in Boron Doped a‐Si:H
Authors: Overhof H.
Published: July 2007

The temperature dependence of the g = 2.01 ESR line in boron doped a‐Si has recently been studied by the Marburg group. For this line the g‐value, linewidth, and lineshape factor depend both on temperature and doping level. The temperature shift of the g‐value is explained by a model in which the g‐factor depends explicitely on energy and the temperature dependence is a result of the temperature shift of the mean energy of the spins. This model is combined with a density of states distribution recently reported to interpret the de transport data. From this the energy dependence of the g‐factor is obtained and the linewidth ΔH due to g‐value broadening as a function of temperature and doping level is calculated. At elevated temperatures an additional contribution to the linewidth, δHpp(T), is observed and is interpreted by the Marburg group as due to spin relaxation via exchange interaction between the localized spins and rapidly relaxing carriers in extended states. It is shown that long‐ranged potential fluctuations combined with this relaxation mechanism can easily explain both the relation between the dc conductivity σ and δHpp(T) and the rapid rise of the lineshape factor l as δHpp(T) begins to dominate the total linewidth.
Phase Transition of Fröhlich Polarons, in the Feynman Approximation, Subjected to a Magnetic Field
Authors: Peeters F. M. , Devreese J. T.
Published: July 2007

It is shown that the Fröhlich polaron in a magnetic field (calculated in the Feynman approximation) exhibits a first‐order phase transition if the electron‐phonon coupling constant α is larger than 4.2. For α = 4.2 the transition is of second order. The phase transition manifests itself as a discontinuity in the magnetization, the susceptibility, and the polaron mass for a critical value of the magnetic field. It is found that at the transition point the phonon cloud surrounding the polaron cannot follow the electron in its motion in the magnetic field.
Interactions in Non‐Simple F.C.C. Metals
Authors: Agrawal R. M. , Rathore R. P. S.
Published: July 2007

Five schemes characterised by the nature of the interactions are developed on the lines of a recent model due to Fielek, which includes the interaction among all the constituents of a non‐simple metal. Equilibrium conditions are derived for each of the schemes. Volume interactions are described on the lines of a simple scheme which explains realistically phonon dispersion in non‐simple metals like palladium and nickel. These schemes are employed to explain cohesion, the Cauchy deviation, anisotropy, and phonon dispersion in the said metals. Overall standard errors of the mean percentage deviation are calculated to draw conclusions regarding the nature of the relevant interactions.
EPR Line Critical Broadening Asymmetry and Shape Peculiarities Due to Dipole‐Dipole Electric Interaction in Ferroelectrics
Authors: Vikhnin V. S. , Borkovskaya Yu. B.
Published: July 2007

The effect of the electric dipole‐dipole interaction by the soft optical phonon field on the EPR line shape in ferroelectrics is studied by means of the line shape statistical theory. The critical dependence of the EPR line broadening and asymmetry is examined. The EPR line shape critical broadening and asymmetry are shown to depend on the soft optical phonon damping. Two mechanisms of the EPR line broadening are considered: i) the dipole electric fields and ii) the spin‐spin cross‐relaxation caused by the electric dipole‐dipole interaction in “soft” matrices. New critical dependences are discovered.
Correction of the Knight Shift for Demagnetizing Effects
Authors: Bakonyi I. , Panissod P. , Tompa K.
Published: July 2007

The NMR line shift contributions originating from demagnetizing fields due to classical macroscopic magnetic effects are discussed for ellipsoidal samples. It is shown that by measuring the line shifts along two main axes of the ellipsoid, the measured difference is sufficient to correct for demagnetizing effects in materials with homogeneous magnetization. It is also found that for special sample geometries even the demagnetizing factors are unnecessary for calculating the demagnetization correction term. Based on line shift and magnetization measurements, the measured and calculated differences of the NMR line positions are compared for sandwich‐type samples. Alloys with inhomogeneous magnetization are also briefly discussed.
Tunneling into Proximity Electrodes with Magnetic Impurities
Authors: Meyer H.‐G. , Seidel P.
Published: July 2007

Starting from a model of classical spins a proximity electrode with magnetic impurities in the barrier‐near layer is investigated theoretically. The rising differences to the common McMillan tunneling model (MTM), especially, the impurity band in the former gap region of the density of states provide significant changes in the tunneling properties of such a electrode, for example, in all current amplitudes of the Werthamer theory. Detailed investigations of the current‐voltage characteristic for tunnel junctions with proximity electrodes containing magnetic impurities show enhanced excess currents below the sum jump which are qualitative similar to experimental results on tunnel structures with electrodes of Nb or V.
Resonance Spectrum of Elliptically Polarized Spin Waves in Thin Magnetic Films
Authors: Duda A.
Published: July 2007

The resonance consequences of elliptic spin‐wave polarization are analysed in the general case where both, internal and surface anisotropy contribute independently to the ellipticity. The surface source of ellipticity is found to modify essentially the quantization rule of wave numbers, leading to new effects in the frequency and surface anisotropy dependence of the resonance modes.
The Coherence Description of the Dynamical X‐Ray Diffraction from Randomly Disordered Crystals I. General Formalism
Authors: Holý V.
Published: July 2007

The problem of the incoherent diffraction of X‐ray from randomly disordered crystals is solved by means of the mutual coherence formalism. On the basis of rigorous solving of the Takagi equations by the Green method a formula is found which determines the mutual coherence functions of waves emitted from disordered crystals. The coherence formalism enables to compute easily diffracted intensities and the angular distribution of intensities. The connection is shown between the theory presented and other theories of incoherent diffraction.
Electronic Energy Bands of Nearly Free Electron Metallic Glasses An Application to Mg0.7Zn0.3 Amorphous Alloy
Authors: Sen M. , Chatterjee S.
Published: July 2007

A theory of electronic energy bands of nearly free electron like metallic glasses is presented by extending the l‐dependent pseudopotential method of Ziman. First an averaged Green's function is obtained for the substitutional alloy assuming average T‐matrix approximation (ATA). Then this Green's function is again averaged for the topological disorder by a Gaussian distribution of the lattice points in a manner similar to Kramer. The secular determinant obtained from the singularity condition of this doubly averaged Green's function is now complex and hence the energy‐eigenvalues obtained from this secular determinant are also complex. This theory is applied to the nearly free electron like metallic glass Mg0.7Zn0.3. From the complex energy bands the density of states and joint density of states for the optical absorption spectra are obtained as a function of the disorder parameter α. The results show that the Fermi energy falls at the minimum of the density of states for α = 0.05 confirming the observation of Nagel and Tauc. From this density of states the Pauli spin paramagnetic susceptibility is also calculated and the results show a temperature independence of this quantity completely in agreement with the experiment.
Analysis of Thermal Capture of the Acceptor Level of Gold in Silicon
Authors: Morante J. R. , Carceller J. E. , Cartujo P. , Barbolla J. J.
Published: July 2007

On the basis of the quantum‐defect wave function and assuming a linear coupling, a calculus of the Huang‐Rhys factor S and the other matrix elements is made using the theory of non‐radiative multiphonon assisted carrier capture processes at deep levels in semiconductors. A fitting of the temperature dependence of the capture cross‐section of the gold acceptor level in silicon is made also, while pointing out as well the existence of high sensitivity in the absolute values of the capture rate to the S parameter. The number of phonons used is p = 8, whereas the values of the associated Huang‐Rhys factor varies accordingly to the sample.
Numerical Approach to the Band Structure in TCNQ Stack
Authors: Komorowski L.
Published: July 2007

Parameters of the band structure as given by the Hubbard model are explored to characterize the stack composed of (TCNQ) dimers with −4 ≦ n ≦ 4. The energy gap is found to be drastically modified by the value of the charge n. The semiconducting state of the chain is realized only, with the minimum energy gap EH ≈ 0.20 eV.
Analysis of Temperature Variation of EPR Linewidths for Gd3+ Ion in CeF3 Single Crystals
Authors: Korczak W. , Subotowicz M.
Published: July 2007

The 9.4 GHz electron paramagnetic resonance (EPR) of Gd3+ doped in CeF3 and LaF3 single crystals is studied at temperatures of 120 to 293 K. The g‐values and linewidths of the Gd3+ spectra in the kramers host Ce3+ ions are compared with those for the isostructural diamagnetic La3+ lattice. The observation of resolved Gd3+ spectra in the presence of paramagnetic Ce3+ ions is interpreted in terms of a random modulation of the interaction between the Gd3+ impurity and Ce3+ host ions by rapid spin‐lattice relaxation of Ce3+ ions using the generalized theory of magnetic resonance by Kubo and Tomita. The Debye temperature θD is estimated of the CeF3 single crystal at 193 to 220 K. It is found that the effective host spin‐lattice relaxation time can be written in the following form τ1 ∼ T−n where n ε (1.7 to 2.2) (H ‖ z) for T > θD. Two‐phonon Raman scattering dominates in CeF3 single crystals at temperatures larger than θD.
Sternheimer Quadrupole Factors in Ferric Compounds
Authors: Liverts E. Z. , Zhetbaev A. K.
Published: July 2007

Expressions for the quadrupole shielding (antishielding) factors corresponding to various types of electronic charge distributions in a solid complex are obtained by means of Sternheimer's perturbation‐numerical method generalized for the two‐centre case. According to the formulae obtained Sternheimer quadrupole factors in isostructural CuFeO2 and α‐NaFeO2 compounds are calculated which differ significantly from those used earlier. The importance of considering the exchange effects which give rise to anomalous shielding effects for some electronic charge distributions is revealed.
Ultrasonic Investigation of the Acceptor Ground State of Si(B) II. The Distribution of Splittings
Authors: Zeile H. , Lassmann K.
Published: July 2007

The splitting distribution of the acceptor ground state of Si(B) is determined from the frequency dependence of the ultrasonic resonance attenuation and from acoustic paramagnetic resonance. In the purest crystals a residual distribution is found of unknown origin. In crystals containing carbon or oxygen at concentrations above about 1022 m−3 the distribution is broadened in rough accordance with statistical estimates.
Elastic‐Stiffness Constants under Pressure of CsK, CsRb, and RbK Solid Solutions
Authors: Soma T. , Kimura Y. , Kagaya H.‐Matsuo
Published: July 2007

The shear elastic constants C44 and C′ = (C11 ‐ C12)/2 under pressure of the Cs1‐xKx, Cs1‐xRbx, and Rb1‐xKx systems are investigated theoretically using the pseudo‐alloy atom model and the local Heine‐Abarenkov model potential. The pressure‐derivatives dB/dP, dC44/dP, and dC′/dP have a maximum as function of the atomic fraction x, and decrease under compression from the data at low pressures, especially for shear coefficients.
Polaron Lattices
Authors: Pecheur P. , Selme M. O.
Published: July 2007

The Holstein polaron model in a linear chain is extended to several electrons. A filled polaron band results, which is stable for any number of electrons (in the continuum limit).
Effect of Hydrostatic Pressure on the Band Structure of InSb A Pseudopotential Calculation
Authors: Gorczyca I.
Published: July 2007

The band structure of InSb is computed by the empirical pseudopotential method (EPM) including a nonlocal correction. The pressure dependence of energy gaps and wave functions for InSb is then calculated. The pressure coefficient of the momentum matrix element between valence and conduction band is found to be very small as is usually assumed. The pressure coefficients of the parameters of electron scattering on short‐range potential centres associated with ionized impurities in InSb are also given in comparison with experimental estimations.
Soliton Magnetoelastic Excitation in the Heisenberg Ferromagnetic Chain
Authors: Izyumov Yu. A. , Laptev V. M.
Published: July 2007

The Heisenberg ferromagnetic chain of spins interacting with the atomic displacement by exchange interaction modulation is considered. The soliton solution of the nonlinear equation for the amplitude of the spin deviations is obtained in the limit of small velocities (which is equivalent to the limit of large soliton localization size). The nonlinear term of the equation is caused by the spin‐lattice interaction and is formed by the spatial derivatives of the amplitude. The soliton energy is higher than the linear spin wave one.
Recombination Processes at Temperatures Lower than 20 K in Hg0.8Cd0.2Te
Authors: Calas J. , Allèggre J.
Published: July 2007

Measurements of photoconductivity lifetime τ in the range 2 to 20 K are reported for Hg0.8Cd0.2Te. A strong increase of this parameter (from 10 to 800 μs) is observed. This effect is attributed to Auger band‐to‐band recombination according to Gerhardts' theory. Another sample of the same ingot, after a long time bromine etching, shows a rapid decrease of the photoconductivity signal (τ ≈ 30 μs) at 2 K which agrees well with the three bands of Casselman's theory of Auger recombination processes. These results prove the existence of an inversion n‐type layer around the p‐type bulk in the annealed samples, according to Scott and Hager's hypothesis.
Ligand Field Theory of the Magnetic Behaviours of Neodymium Ethylsulphate Nonahydrate
Authors: Nath A. , Ghosh U. S.
Published: July 2007

An attempt is made to examine to what extent the set of parameters deduced by Gruber and Satten from their optical absorption experiment is consistent with the g‐values and magnetic susceptibility data of neodymium ethylsulphate nonahydrate. The intermediate coupling scheme and different J‐mixing under the crystal field of D3h symmetry is used in the calculation. It is found that in order to interpret the g‐values and magnetic behaviour Gruber and Satten's parameters are more or less satisfactory needing slight changes in some of their values and a very small covalency reduction is introduced of the orbital angular momentum which is assumed to partake of the symmetry of the complex.
Estimation of Fermi Surface Parameters from the One‐Dimensional Fourier Transform of the Positron Annihilation Angular Correlation Curves
Authors: Gerber W. , Hoppe W. , Brümmer O. , Dlubek G.
Published: July 2007

The advantage of the use of the Fourier transformation in analysing angular correlation curves of 2γ‐positron annihilation radiation is studied on the simple metals Al, Mg, and Li. It is observed that the values for the Fermi momentum estimated from the first and from the second zero of the one‐dimensional Fourier transform of angular correlation curve show systematic differences. This discrepancy may be resolved by taking into account the electron‐positron correlation using a momentum dependent enhancement factor. The estimated values for the Fermi momentum have an accuracy of 0.5 to 1% and agree excellently with the calculated free electron gas values.