bluuebunny/crossref_metadata_2025 huggingface dataset:
Comments on the Moss Formula
Authors: Gupta V. P. , Ravindra N. M.
Published: July 2007

An analysis is given of the well‐known Moss formula connecting the energy gap to the refractive index of semiconductors. Based on Penn's model for the dielectric function of semiconductors and the treatment of Wemple, the refractive index is related to the energy gap. For small gaps, the relation is shown to reduce to the earlier predicted linear form.
Paramagnetic Spin Relaxation Due to Spin–Spin Interaction Assisted by Spin–Phonon Interaction or Vice Versa (I)
Authors: Srivastava K. K. P. , Singh H. C.
Published: July 2007

The electronic spin–spin relaxation is induced by a dipolar magnetic coupling between the spin states of a pair of paramagnetic ions, whereas the interaction between spin states and lattice vibrations is responsible for the spin–lattice relaxation. Two‐phonon Raman and Orbach spin–lattice relaxation processes are well known where the spin relaxation between a ground doublet proceeds indirectly via transitions to and from an excited state at energy δ with the absorption and emission of two phonons having an energy difference equal to the energy separation between the two states of the ground doublet. In this paper a new type of “mixed spin relaxation process” is considered in which a spin–spin transition is needed to produce effectively an orbit–lattice (i.e. spin–phonon) transition and vice versa. In other words the spin–spin and spin–phonon interactions are simultaneously needed to produce a relaxation transition for this mixed process. A phenomenological approach indicates that the rate of this “mixed spin relaxation process” is proportional to T4[exp (δ/KT) + 1]−l (sin k Rjl/kRjl) and this process should proceed very efficiently at low temperatures when the phonon wavelengths are larger than the separation Rjl between the paramagnetic ions.
Correction to the Breakdown of the Long‐Wavelength Approximation and Characteristic Temperatures in Impurity Ion Paramagnetic Resonance
Authors: Singh A. , Shrivastava K. N.
Published: July 2007

In the temperature dependence of the zero field splitting of PbWO4:Mn2+, the long‐wavelength approximation breaks down near Tb ≈︁ 150 K. A correction factor is obtained using Bessel functions which explains the observation. In the temperature dependence of the zero field splitting of V2+ in NaCl, KCl, and RbCl a minimum is found and a zero at characteristic temperatures, Tc2 and T02, which are related by T02 = √2Tc2. This theoretical relation between the two temper‐atures is varified experimentally.
Piezoreflectance Study of E1 and E2 Transitions in Germanium from 107 to 326 K
Authors: Musilová J.
Published: July 2007

AC piezorefleetance spectra of pure germanium from 1.8 to 4.6 eV are studied within the temperature range 107 to 326 K using uniaxial stress and polarized light. The interpretation of corresponding spectra of the piezooptic tensor components is based on the critical point concept. The line shapes of the piezooptic tensor are well fitted by the 3DM1 model in the region of E1, E1+ Δ1 transitions. For the E2 structure the hydrostatic component of the piezooptic tensor agrees with the model of two degenerate saddle points, 3DM1 + 3DM2, however, the other two components are not found to correspond to single transitions. Values of transition parameters and their temperature coefficients are obtained. Also the deformation potentials are determined, using both, the experimental and calculated dielectric function data for numerical analysis.
On the Statistical Theory of Vacancies in Strongly Anharmonic Crystals
Authors: Zubov V. I.
Published: July 2007

The Gibbs free energy of the vacancy formationg(T, P) in a strongly anharmonic crystal is obtained, using the quasi‐chemical approximation for multi‐component systems and the improved self‐consistent field method for a crystal. The enthalpyh, entropys, and volumevof the vacancy formation are derived versus the temperature and pressure. The vacancy formation parameters, concentration of vacancies, and their effects on the thermodynamic properties of strongly an‐ harmonic crystals of Ar, Kr, and Xe are calculated along the solid–gas coexistence curves, computed in the preceding paper. The pair potential of Lennard‐Jones and those of Barker et al. are used. The three‐body interactions are included. All potentials givev= (1.02 to 1.04) atomic volume over the whole temperature range. The enthalpy and entropy of the vacancy formation decrease with increasing temperature, with the valuesh/kT= 9.48 to 9.60 ands/k= 3.14 to 3.40 at the triple points. The results are compared with experimental data for the thermodynamic properties and also with the available Monte Carlo computations and experimental estimations for the vacancy formation parameters.
The Lattice Structure Parameters and Configuration of Cu2+ Jahn‐Teller Centres in Tutton Salt Crystals
Authors: Petrashen V. E. , Yablokov Yu. V. , Davidovich R. L.
Published: July 2007

EPR of Cu(H2O) pseudo Jahn‐Teller (JT) centres in dia‐ and paramagnetic Tutton salt crystals A2M(SO4)2. 6H2O (I) with A = Rb, Cs, NH4 and M=Zn, Cu and K2M(ZrF6)2. 6H2O (II) crystals in the temperature range 300 to 4.2 K at frequencies 9.5 and 37 GHz is studied. The dynamic JT nature of Cu(H2O) centres in the whole class of Tutton salt crystals is established basing on the results of EPR study and X‐ray data. The influence of the lowering of lattice symmetry on the configuration and properties of centres and crystals is analyzed. The character of the distortion of Cu(H2O) complexes in “copper” crystals I and II is discussed in terms of cooperative JT effect. The main JT parameters of Cu(H2O) complexes for the crystals under consideration are estimated.
Symmetry Analysis of Some Properties of Ni2+ Ions in the Planar Antiferromagnet K2NiF4 and the Canted‐Spin Antiferromagnet NiF2
Authors: Joseua S. J. , Deonarine S.
Published: July 2007

Using standard group theoretical techniques, earlier investigations on the symmetry properties of the Ni2+ ion in the 2D (planar) antiferrornagnet K2NiF4 and the 3D antiferrornagnet NiF2 are extended.
Soft X‐Ray Appearance Potential Spectra of Iron–Cobalt Alloys
Authors: Berndt K. , Engelmann J. , Storbeck F. , Blasek G.
Published: July 2007

The appearance potential spectra of ferromagnetic Fe–Co alloys in the b.c.c. and f.c.c. phases are measured under ultrahigh vacuum conditions. The results are discussed in terms of the electronic states above the Fermi energy of both the constituents Fe and Co and are compared with the APS spectra of the pure metals. The important role of the final state effects in the broadening of the APS spectra is taken into account. The width of the unoccupied local d states is only slightly changed upon alloying.
Thermally Stimulated Depolarization of NCS− Centres in Potassium and Rubidium Halide Crystals
Authors: Wilbrand A. , Fischer F.
Published: July 2007

TSD measurements are carried out on crystals of the chlorides, bromides, and iodides of potassium and rubidium doped with thiocyanate. In all systems a TSD band is found between 24 and 47 K which is attributed to isolated NCS− centres. With the aid of the measured bands the activation enthalpies and fundamental relaxation times for reorientation of the centres as well as their electric dipole moment are determined. The dependence of the activation enthalpies upon the host crystal is compared to that for NCO− centres.
Optical Gain and Induced Absorption in High‐Density Exciton System in CuCl
Authors: Hvam J. M. , Bivas A.
Published: July 2007

In CuCl at 1.7 K optical gain/absorption spectra are recorded in the spectral range of bound exciton and biexciton emission. Energy, effective temperature, and oscillator strength of biexcitons are determined. The observed kinetics indicates that the total pair density in the exciton‐biexciton system is pinned at about 1.6 × 1016 cm−3 for all excitation densities above 10 kW/cm2. At intense excitation, a Fano‐interference is observed between a discrete impurity absorption line and a continuum absorption tail. The latter is followed by a band of positive gain towards lower photon energies.
Atomic Pseudopotential of Polyvalent Crystals Such as Si
Authors: Ohkoshi I.
Published: July 2007

Pseudopotentials which were used in discussions of the cohesive mechanism of Si are reinvestigated on a view of atom energy. It is shown what kind of pseudopotential must be chosen in the calculation of the crystal energy of polyvalent crystals such as Si.
Random Torsional Oscillation Model for Rotational Molecular Motion. Cold Neutron Scattering Investigation of Cholesteryl Propionate, Nonanoate, and Myristate
Authors: Cvikl B. , Srebotnjak E. , Dušić M. , Dimic V. , Krištof E.
Published: July 2007

The incoherent scattering function for the case of simple translation of the centre of mass and the random torsional oscillations of the rigid planar body along the long molecular axes of an amplitude φ of perfectly ordered molecules in nematic phase, is calculated. The model of a random molecular torsional motion is built on the assumption that each proton in the molecule suffers a large number of equal angular displacements between two perfectly reflecting barriers at φ and −φ. The dynamics of molecular tail is described by uniaxial free rotational diffusion and freely rotating spherical top models, respectively. The predictions of the calculations are tested on measured time of flight spectra, obtained on three members of a homologous series of cholesteric liquid crystals and agreement is obtained.
On the Phase Separation of Carriers in Nitrogen‐Doped GaP
Authors: Thuselt F.
Published: July 2007

Unusual phase separation of excited carriers in isoelectronically doped GaP is discussed in terms of two models: (i) a perturbed neutral electron‐hole plasma, and (ii) electrons assumed to be strongly localized at the isoelectronic impurities embedded in a non‐neutral electron‐hole plasma. For low carrier densities where excitons are stable particles due to low screening, the special situation involving nitrogen pairs is taken into account.
On the Theory of Interstitial Alloys in the Coherent Potential Approximation (CPA)
Authors: Urbański M. K.
Published: July 2007

The single‐site CPA self‐consistent condition for the effective Hamiltonian of an interstitial disordered alloy is obtained. It is assumed that the Hamiltonian of the tight‐binding model is composed of the following terms: the Hamiltonian of the pure solvent and two disordered parts, the interstitial atom energy and the solvent‐interstitial interaction. In the model the interstitial atom interaction is neglected in order to use the single‐site approximation. The total density of electron states can be derived explicitly. A simple numerical example of the density‐of‐states calculation is presented.
Study of the Temperature Dependence of the Dielectric Properties of LiTaO3 by Raman Spectroscopy
Authors: Umarov B. S. , Vetelino J. F. , Abdullaev N. S. , Anikiev A. A.
Published: July 2007

The phonon spectra of the A1‐type symmetry vibrations in LiTaO3, are measured by Raman scattering techniques over a wide temperature range which include the phase transition point. These spectra are then analyzed using the Kramers‐Kronig relations to obtain the temperature and frequency dependence of the static dielectric constant, ϵzz, and the infrared (IR) reflection spectra. Good agreement is obtained with results of direct experimental measurement.
Simple Electronic Structure Calculation of Substitutional sp Impurities in α‐Iron
Authors: Demangeat C. , Parlebas J. C.
Published: July 2007

Taking into account a Slater‐Koster fit to first‐principles band structures for the host, a simple tight‐binding model is proposed for the electronic structure of sp impurities (Al, Si, P … Sb) substituted in ferromagnetic metals. The aim of the model is to extend calculations to future more complicated problems like chemical binding energies between point defects. The method is applied to dilute σ‐iron based alloys: it is shown that a localized impurity model gives reasonable accord with experimental data on saturation magnetization, every time the dilution law is almost satisfied (FeAl, FeSi, FeP).
Mode Gammas and Thermal Expansion of CaF2
Authors: Govindarajan R. , Haridasan T. M. , Govindarajan J.
Published: July 2007

The perturbation approach of Achar and Barsch is employed to work out the mode gammas and the thermal expansion of CaF2. A shell model similar to that of Axe is employed. The pressure dependent dynamical matrix components are computed using the available data on the pressure derivatives of the elastic constants and the mode gammas of the long wavelength optical phonons. The modified moments of the phonon density of states are also computed. These results agree fairly well with the experimental values reported by Baily and Yates and the previously reported work of Ruppin.
Fourier Transform of Deuteron Modulation of Electron Spin Echo Envelope of Silver Atoms in Deuterated Ice and Solid Methanol
Authors: Aruna B. , Ramakrishna Y. V. S. , Narayana P. A.
Published: July 2007

The analysis of electron spin echo modulation in the frequency domain is studied by Fourier transforming the time domain data. Two systems are chosen for this purpose, silver atoms in deuterated ice and specifically deuterated solid methanol (CH3OD). It is shown that some of the artifacts introduced into the Fourier transform spectrum can be reduced by reprocessing the time domain data. The advantage of this type of analysis over the direct simulation of modulation is pointed out.
Effective Mass of Conduction Band Electrons in Hg1−xMnxTe from Shubnikov‐de Haas Oscillations
Authors: Jaczyński M. , Dobrowolski W.
Published: July 2007

The Shubnikov‐de Hass effect (SdH) is investigated in n‐type Hgl−xMnxTe mixed crystals for several compositions in the range 0.02 ≦ x ≦ 0.04. Experiments are carried out on samples with electron concentrations between 2 × 1015 and 8.5 × 1017 cm−3 at temperatures from 2 to 25 K. Prom the temperature dependence of the amplitude of the SdH oscillations the effective masses of electrons are determined. The data are analyzed in terms of a model developed to account for anomalies observed earlier in the SdH effect in this material.
Free Energies of Solid and Liquid Metals. Thermodynamic Calculation of Melting Temperature
Authors: Belan‐Gaiko L. V. , Bogdanov V. I. , Fuks D. L.
Published: July 2007

For the investigation of the free energy of metals and their temperature dependence in solid and liquid phases the pseudopotential method is used. In calculating the free energy in a solid phase the lattice vibration contribution is considered. In definition of the pair potential in liquid metals the volume change of the metal during its melting is taken into account. The structure of liquid metals is calculated with the help of the found potential using the integral equation method. The solutions of different equations for the radial distribution function are compared and the influence of the model form factor and the method of screening in the electron gas on the liquid metal structure is discussed.
Charge and Spin Density Waves in Jellium
Authors: Perdew J. P. , Datta T.
Published: July 2007

Is the uniform electron gas unstable against charge and spin density waves (CDW's and SDW's) of infinitesimal amplitude, which break the translational symmetry of the Hamiltonian? Within density functional theory, a CDW of wavevector Q ≈︁ 1.1(2kF) appears at low densities. The local spin density approximation (LSDA) for exchange and correlation places the transition at rs ≈︁ 30, while inclusion of the next term in the gradient expansion suppresses the transition to densities as low as rs ≈︁ 200. The best estimate is that the CDW appears out of the uniform ferromagnetic phase of jellium at rs ≈︁ 70; this is also where Ceperley, in a recent calculation with correlated wave functions, found Wigner crystallization. Although no SDW in jellium is predicted within LSDA, the calculated uniform spin susceptibility diverges at rs ≈︁ 35, close to the highest density (rs ≈︁ 26) at which Ceperley found ferromagnetism.
A Wide Concentration Range Induced Moment System: (TbcLa1−c)Be13
Authors: Clad R. , Bouton J. M. , Herr A.
Published: July 2007

Magnetic measurements are reported for polycrystalline compounds (TbcLa1−c)Be13, with 0.02 ≦ ≦ c ≦ 1, in the temperature range 1.6 to 300 K and for fields up to 70 kOe. The observed ordering temperature TN and the downward shift of the inverse susceptibility curve, in the paramagnetic regime (T ≧ TN), scales linearly versus c. The M(H, T) dependences are calculated by diagonalizing the Hamiltonian including crystal field, molecular, and Zeeman terms along the 〈100〉, 〈110〉, and 〈111〉 directions. The best fit to the magnetic data is obtained for the crystal field parameters x = −0.8 and W varying linearly from 0.05 K for c = 1 to 0.13 K for c = 0.16. A transition from Van Vleck paramagnetism to an ordered magnetic structure occurs at low temperature, as the terbium concentration exceeds the threshold value for ordering of about c = 0.06.
A Theory of a Diluted Ferromagnet
Authors: Kaneyoshi T. , Fittipaldi I. P. , Beyer H.
Published: July 2007

A new‐type effective field theory with correlations in a diluted ferromagnet is discussed. The critical percolation concentration is obtained analytically. The transition temperatures are given as a universal function of effective number of nearest‐neighbours. The specific heat and internal energy are also obtained.
Band Structures of Zirconium Hydrides
Authors: Darby M. I. , Read M. N. , Taylor K. N. R.
Published: July 2007

The band structures of zirconium hydride compounds are calculated using the APW method. By comparing the bands for a hypothetical f.c.c. compound ZrH0 with f.c.c. ZrH2 (corresponding to the Δ‐phase), it is shown that the hydrogen atoms give rise to a new ‘bonding’ band below the Fermi level. The band structures of two interstitial hydrogen h.c.p. compounds ZrH0.5 and ZrH (α‐phase) are also calculated. For ZrH0.5 with two Zr atoms and one H atom in the primitive unit cell no new bands are formed. For ZrH with two Zr atoms and two H atoms in the primitive unit cell a new band is formed just below the Fermi level but the band structure is similar to that of ZrH0.5 and a‐Zr with the Fermi level raised. Estimates of the total energies show that the band structure contributions favour formation of f.c.c. ZrH2 (corresponding to the Δ‐phase) over an interstitial α‐phase, as is observed experimentally.
An Exactly Soluble Model Antiferromagnet
Authors: Bowers R. G.
Published: July 2007

An exact calculation of the complete equilibrium thermodynamics of a model antiferromagnet is presented. Attention is focussed on the Néel point and associated critical phenomena and many standard critical exponents are calculated. The model has infinite‐range interactions and classical exponent values result. The importance, for theory, of staggered magnetizations and magnetic fields is emphasized. Experimental results frequently refer to total magnetizations and uniform fields. The interrelation between these two descriptions is investigated within the context of the present model.
Pulsed Line Narrowing in Nitrogen‐14 NQR
Authors: Ya. Osokin D.
Published: July 2007

On the basis of the average Hamiltonian theory the possibility of the pulsed narrowing of nitrogen 14 NQR lines in powders is studied theoretically using the fictitious spin‐(l/2) operator formalism. The modified Carr‐Purcell pulse sequence t (τ–t–2τ–t–τ)n is shown to be more acceptable for this purpose. The average Hamiltonians of inhomogeneous broadening and heteronuclear dipole‐dipole interactions as well as the secular part of the Hamiltonian of the interaction of a spin system with torsional oscillations of molecules are proved to commute with the initial density matrix prepared by the first (preparatory) pulse t The experimental curves of the spin echo envelopes in the pulse sequence are given. The decay time constants obtained experimentally at 77 K range between 0.5 s in methylamine and 10 s in sodium nitrite and agree with the spin–lattice relaxation times in these compounds.
Modelling of the Radial Distribution Function of the Hard‐Sphere Liquid in a Quasi‐Crystalline Model
Authors: Medvedev N. N. , Naberukhin Yu. I.
Published: July 2007

The radial distribution function of the hard‐sphere system, which is obtained by the exact solution of the Percus‐Yevick equation, is described within the framework of the quasi‐crystalline model of liquids (crystal lattice with lattice sites smeared out by the Gaussian law). The b.c.c. lattice is found to allow to receive a practically ideal description of all oscillations of the RDF (with an exception of several first peaks) over a range of interparticle distances up to the 16 sphere diameters. Other lattices give a significantly worse description. It is proposed that the quasi‐crystalline model reflects the correlations in the particle positions connected with regularities of the random close packing.
Hyper‐Raman Scattering from Infrared Active Modes in Calcite Crystals
Authors: Polivanov Yu. N. , Sayakhov R. Sh.
Published: July 2007

The hyper‐Raman spectrum of calcite is measured with Nd–YAG laser excitation and single channel detection. The lines corresponding to the normal Raman inactive transverse and longitudinal optic phonons are observed. The hyper‐Raman cross‐section of these lines is measured.
X‐Ray Thermal Diffuse Scattering Measurements of the [110] Phonon Dispersion Curves of Indium Arsenide
Authors: Orlova N. S.
Published: July 2007

Investigation of the intensity of X‐ray diffuse scattering by lattice vibrations in indium arsenide along the second‐order symmetry axis is performed and the phonon frequencies of all modes in this direction are determined. An analysis of elliptic modes polarized in the (110) plane is given.
A Two‐Particle Model for the Ground State Energy of the Polaron
Authors: Bader G. , Lépine Y.
Published: July 2007

The dielectric polaron is treated using the Gross unitary transform and the Matz‐Green's function techniques. A two‐particle Feynman‐like model is introduced to compare these methods with the best Feynman path integral formulation. It is found that the introduction of a second particle in the model problem does not improve on the usual result for a one‐particle model. However, a comparison of these different approaches leads to the conclusion that no vertex contributions are contained in the Feynman approximation. It is also concluded that it is the very special treatment of the kinetic energy of the electron in Feynman's theory that makes his results lower in energy.
Interband Electronic Raman Scattering of Polarized Light in CuCl
Authors: An Nguyen Ba , Van Hieu Nguyen , Thang Nguyen Toan , Viet Nguyen Ai
Published: July 2007

The cross‐sections are calculated for the Raman scattering processes in CuCl with polarized photons in the initial and final states and with the transition of the electron from the highest valence band Γ7 to the conduction band Γ6. The exciton effect is taken into account.
Long‐Wavelength Phonons in the Different Phases of KNbO3
Authors: Fontana M. D. , Kugel G. E. , Metrat G. , Carabatos C.
Published: July 2007

Raman scattering measurements are made in the tetragonal phase of KNbO3. The results are in good agreement with the previous corresponding neutron data, and can be assigned to symmetry species. Comparison of these results to those available in the other phases enables to observe the effect of the phase transformations on the LO–TO splitting and on the degeneracy lifting of each initial cubic mode. It is shown that the lowest frequency F1u cubic mode is the most affected one by the different cell distortions; at each transition, it splits into “soft” and “stiffened” components; it is the so‐called “ferroelectric” mode which drives the successive phase changes. On the other hand, it is remarkable that the phonons derived from the F2u “silent” mode are nearly degenerate, even in the ferroelectric phases; it is also the case for the phonons issued from the second F1u cubic mode. Finally, it is emphasized that the phase transitions in KNbO3 can be described by a displacive‐type mechanism.
Laser Induced Fluorescence and Raman Studies with KI:NO
Authors: Jodl H. J. , Bolduan F.
Published: July 2007

Raman, absorption, and fluorescence spectra associated with NO substitutional impurity in KI as a function of different parameters are reported. This model substance is chosen of special interest to concentrate on the following topics like laser induced fluorescence (3B1 → 1A1), like the structure of zero‐phonon line and sideband in absorption and fluorescence spectra, like Raman scattering of intramolecular vibrations, and like Fermi resonance of matrix isolated species. The data are discussed in the light of various anharmonic effects, of different coupling models, and of special spectroscopic aspects.
Studies on Anharmonic Damping Mechanisms of High‐Frequency Phonons in Crystals Containing Complex Ions
Authors: Tsyashchenko Yu. P. , Krasnyanskii G. E.
Published: July 2007

Polarized infrared reflection spectra of CaWO4, KReO4, K2SO4, K2CrO4 single crystals are investigated in the region of 600 to 900 cm−1 within the temperature range of 170 to 625 K. The temperature dependences of the damping constants of the high‐frequency phonons v3(Eu), v3(Au) in CaWO4 and KReO4, v3(B1u) in K2CrO4, v4(B1u) in K2SO4 are obtained by means of the dispersion analysis. The observed temperature behaviour of the damping constants is mainly explained by the anharmonic mechanisms of double‐quantum phonon scattering on other high‐frequency phonons involving low‐frequency ones, as it is demonstrated by a satisfactory agreement between experimental and theoretical curves.
Reflectivity Spectra and Band Structure of the ZnTe–CdTe System
Authors: Sobolev V. V. , Maksimova O. G. , Kroitoru S. G.
Published: July 2007

The reflectivity spectra of ZnTe:Cs, ZnTe:Ga2O3, and ZnxCd1−xTe crystals (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0) in the range 1 to 12.5 eV at 293 K and in the range 1 to 5.5 eV at 90 K are investigated. The experimental results obtained prove the hypothesis about a great detailed similarity of ZnxCd1−xTe band structures in a wide energy range and the existence of a straightline dependences of interband transition energies on molar composition. A strong effect of fresh cleavage and impurities on the reflectivity spectra is detected.
Anisotropic Ferromagnet with Two Spins per Site
Authors: Piasecki R.
Published: July 2007

Green's function diagrammatic technique is used to investigate a model ferromagnet in the case, when the effective spin at a site is compounded of two anisotropically interacting electron spins. The results are: the transverse magnetic excitations (Δm = ± 1) and the longitudinal ones (Δm = 0) are obtained in the random phase approximation, magnetization and critical temperature are calculated to the first order in 1/z (the reciprocal of the effective number of ions interacting with a given ion), and exchange interaction at the same site gives the contrary effects in changes of the magnitudes mentioned above in dependence on the exchange‐isotropy parameter I and the exchange‐anisotropy parameter D.
Semiempirical Calculations of the Impurity Level Positions with Respect to the Perfect Crystal Bands
Authors: Ermoshkin A. N. , Kotomin E. A. , Evarestov R. A.
Published: July 2007

The semiempirical calculation of the electronic structure of a perfect KCl crystal carried out within the framework of the large unit cell model is compared with those performed using the cluster model. The positions of the ground state one‐electron levels of the impurity ions Tl+ and Tl2+ within the gap are also calculated using molecular clusters of different sizes. The procedure which takes into account the crystal lattice polarization produced by a charged impurity is proposed for the calculation of Tl2+ levels. The electronic density distributions and the impurity level positions with respect to the KCl bands are compared for both the defects.
Extended Deformable Shell Model Calculation of the Lattice Statics and Dynamics of LiH and LiD Crystals
Authors: Ghosh S. , Basu A. N. , Sengupta S.
Published: July 2007

An attempt is made for a unified study of the lattice statics and dynamics of the LiH–LiD crystals. So far it has not been possible to obtain a unified description of the different properties of this crystal on the basis of a single model and with a single set of model parameters. All the previous calculations suffer from this inadequacy and their results show that there are certain problems regarding the stability of the static lattice structure, the cohesive energy, and a consistent description of the dielectric properties and the dispersion of phonons for this crystal. The present calculation based on the extended deformable shell model removes the earlier difficulties and provides a good overall description of the lattice mechanical properties of this crystal with a single set of parameters. Another salient feature of the present investigation is to indicate the effect of the quadrupolar distortion of the charge cloud, in particular, of the hydrogen ion which has been neglected in all the earlier investigations of the lattice mechanics of these crystals. The calculation clearly points out that without including this effect it is not possible to obtain a coherent description of the lattice mechanics in all its totality. Some qualitative justification for this effect being significant for the crystals under consideration is also presented.
Infrared Absorption Studies on Crystalline α‐P4S3
Authors: Chattopadhyay T. , Von Schnering H. G.
Published: July 2007

Infrared absorption spectra of crystalline α‐P4S3 are obtained at 80 and 275 K. Absorption features due to low frequency external modes are observed and are identified for the first time. Absorption due to internal molecular modes is compared with the data reported in previous works. α‐P4S3 belongs to the centrosymmetric space group Pmnb(D2h16) and therefore none of the infrared active modes are Raman active. The present work is complementary to a recent Raman study on α‐P4S3.
The Change of Electronic Structure and Optical Properties in FeCo at the Order‐Disorder Transition
Authors: Kulkova S. E. , Egorushkin V. E. , Anokhina I. N. , Fadin V. P.
Published: July 2007

The change of the electronic structure and the optical properties of the metallic alloy FeCo is studied at the order–disorder transition. Energy‐band calculations for ordered and disordered alloys FeCo are performed by the KKR and the model Hamiltonian methods. In the model of direct and indirect transitions the optical conductivity is calculated. The calculations are in satisfactory agreement with the experimental data.
On the Dynamics of the Plasma Phase Transition in Highly Excited Direct Gap Semiconductors
Authors: Koch S. W.
Published: July 2007

The temporal behaviour of the system of electronic excitations in highly excited direct gap semiconductors is treated within a hydrodynamic model. A functional Fokker‐Planck equation for the probability distribution of the local density of electron–hole (e–h) pairs is established taking into account the diffusion of e–h pairs due to the local chemical potential as well as recombination losses due to the finite e–h pair lifetime and generation of e–h pairs by external laser excitation. Several time dependent moments of the probability distribution functional are calculated together with the structure factor, i.e. the Fourier transform of the two‐point density correlation function. The results show that for e‐h pair densities inside the instability region of the thermodynamic chemical potential no true phase separation into an e–h pair gas phase and an e–h liquid phase takes place.
Electronic States of CdIn2Se4 and ZnIn2Se4: Role of the Cation Pseudovacancies
Authors: Margaritondo G. , Katnani A. D. , Lévy F.
Published: July 2007

The two defect‐zincblende semiconductors of the II–III2–VI4 family, CdIn2Se4 and ZnIn2Se4, contain a regular network of vacant cation zincblende sites with several properties similar to those of true vacancies. Electronic state calculations predict dramatic effects of these pseudovacancies in the upper valence band. These effects are searched for by photoemission spectroscopy with synchrotron radiation. The spectra do reveal the states related to the pseudovacancies. However, they rule out a theoretically predicted pseudovacancy gap in the valence band density of states. These results agree with the recent hypothesis of a partial cancellation of the pseudovacancy effects by lattice relaxation around each pseudovacancy.
Hund's‐Rule Effect in Degenerate Anderson Model
Authors: Oleś A. M. , Chao K. A.
Published: July 2007

A charge‐transfer operator method is proposed to investigate the Hund's‐rule effect on the formation of local moments in the doubly degenerate Anderson model. The results are compared to the Hartree‐Fock solution.
Temperature Investigation of Infrared Absorption in DMB–TCNQ Salt
Authors: Graja A. , Świetlik R. , Sekretarczyk G.
Published: July 2007

IR spectra of the simple salt TCNQ with dimethylbenzimidazole vs. temperature are studied. The temperature dependence of the absorption coefficient for bands due to activation of the ag mode is explained in terms of changes in energy level population, geometry, and electron interactions.
Low‐Temperature Heat Conduction of γ‐Irradiated MgO Crystals
Authors: Abramishvili M. G. , Altukhov V. I. , Kalabegishvili T. L. , Kvachadze V. G.
Published: July 2007

The effect of γ‐irradiation on heat conduction χ(T) and the spectra of optical and paramagnetic absorption of MgO crystals are studied. For the first time two stages of thermal destruction of optical absorption bands are found and their relation with the process of thermal recovery χ(T) is established. Contributions to the heat conduction due to both, resonance scattering of phonons on paramagnetic ions Cr2+ (generated in the process of irradiation on account of the change of the impurity valence of Cr2+ and Fe2+) and Rayleigh scattering on radiation (including proper) defects of the MgO lattice are analysed. From the comparison of the calculated data of heat conduction χ(T) with pertinent experimental data on Cr2+ ion concentration and effectiveness of Rayleigh and anharmonic phonon scattering in MgO crystals are obtained.
On the Theory of Double γ‐Nuclear Magnetic Resonance
Authors: Dzyublik A. Ya.
Published: July 2007

The amplitude for resonant scattering of γ‐quanta by Mössbauer nucleus in a crystal is calculated when there are NMR transitions between sublevels of the excited nucleus. It is shown that under the influence of the NMR transitions the scattering amplitude splits into some resonant terms. The cross‐sections for absorption and scattering of γ‐quanta are expressed over the amplitude. The calculations agree with the experiment of Heiman et al.
Interstitial and Vacancy Distributions in a Simple Model of Radiation Damage
Authors: Pázsit I.
Published: July 2007

The spatial distribution of vacancies and interstitials in a displacement cascade is investigated in the Kinchin‐Pease model of radiation damage. The one‐dimensional defect equations are solved in an infinite medium and a half space. The numerical results illustrate the different spatial behaviour of the defect types, and the effect of the free boundary can be assessed.
Phonon Frequencies and Dynamics of the OH− Molecule in Sodium Hydroxide
Authors: Giessler A. , Schaack G. , Bleif H.
Published: July 2007

Raman and infrared reflection spectra of the fundamental lattice modes of NaOH are observed in a broad temperature range in the low‐temperature orthorhombic and in the monoclinic phase (T ≧ 514 K). The temperature dependent wave numbers and line shapes especially of the (OH) libration and of the (OH) valence mode are determined. The results are discussed using data from X‐ray and neutron scattering experiments and are compared with the conclusions of recent theories on molecular dynamics in crystals.
Electron Paramagnetic Resonance Studies of Ferroelectric Fe3+‐Doped Tris‐Sarcosine Calcium Chloride Monocrystals
Authors: Kröber T. , Wartewig S. , Windsch W.
Published: July 2007

Ferroelectric tris‐sarcosine calcium chloride (TSCC) single crystals are investigated using the EPR method in X‐ and Q‐band in a temperature range from 3.7 to 300 K. The spectra of the Fe3+ ions substituted for Ca2+ ions are well described by a spin Hamiltonian of orthorhombic symmetry with the following parameters at room temperature: g = 2.005 ± 0.002, D = ( −1250 ± 10) × 10−4 cm−1, E = (−70 ± 5) × 10−4 cm−1, F = (30 ± 6) × 10−4 cm−1, a = (81 ± 7) × x 10−4 cm−1. The structural ferroelectric phase transition is evidenced in a splitting of the EPR lines, which is proportional to the spontaneous polarization of the crystal. This proportionality allows to determine the coefficients A, B, and C of the Landau expansion of the free energy G. The results are compared with dielectric measurements and EPR measurements of TSCC:Mn2+.
Theory of Multi‐Phonon Transitions beyond the Condon Approximation
Authors: Teichler H.
Published: July 2007

For light interstitials in deformable host‐materials (“small‐polaron”‐like systems) the theory of multi‐phonon transitions is developed beyond the so‐called Condon approximation, i.e., the effect of phonon fluctuations on the defect transfer‐integrals J is explicitly taken into account. In particular the rate of incoherent defect transitions is evaluated up to second order in J and the temperature dependence of the multi‐phonon processes is discussed in the regime above half of the Debye temperature.
The Temperature Dependence and Dynamical Properties of the Bond Charge in GaAs and InSb
Authors: Pietsch U. , Unger K.
Published: July 2007

Using a bond charge model for the interpretation of X‐ray structure factors the temperature dependence of parameters describing the bond charge (amount, position, and spatial extend) is studied for GaAs and InSb. Four structure factors each at eight temperature values are taken from Bilderback, three factors showing a detectable influence of bond charges. The mean squared displacements of the bond charges are found to be larger than those of the matrix atoms. This is an analogous finding to that in silicon and can be explained by the dynamical change of the bond charge amount for changing bond lengths. The constant of dynamical charge transfer are determined to be 0.185 electrons/Å and 0.165 electrons/Å for GaAs and InSb, respectively.
Magnetic Contribution to the Phonon Spectrum of Fe
Authors: Selyutin G. Ya.
Published: July 2007

The magnetic contribution to the phonon spectrum in a crystal is discussed. The temperature dependence of the intensity and the lineshift of the Mössbauer spectrum of Fe in the region of the magnetic transition is well explained.
Self‐Interstitials in Normal Metals
Authors: Augst G. R.
Published: July 2007

General expressions describing the formation energy of interstitials of different configurations in simple metals are obtained. The electronic contribution is considered within the framework of pseudopotentials theory and is based on the linear response approximation. To obtain the lattice relaxation energy two terms of the displacement expansion iteration series are calculated. Numerical calculations for aluminium show that the minimum energy corresponds to a 100 dumb‐bell, with the distance of each ion from the vacancy being 0.52a/2. For interstitials of other configurations the disturbance of the lattice is so strong that the iteration series for the relaxation energy is divergent.
Band Structure and Electron–Electron Interaction in Samarium Monosulphide
Authors: Farberovich O. V.
Published: July 2007

The method of augmented plane wave (APW) is used to obtain the band structure of the SmS compound in the semiconductor and metal phases. The noncentral part of the Coulomb electron‐electron interaction is taken into account in the first order perturbation theory. In this case the radial part of the wave APW‐function is taken as a zero approximation function. A multiplet structure of the excited configuration f5d, which provides a good description of the X‐ray photoelectron spectrum and optical spectrum ϵ2(ω), is obtained. The configuration fd is calculated for the interpretation of the optical absorption spectrum of the samarium monosulfide metal phase.
Unsymmetrized Self‐Consistent Field Approximation for Strongly Anharmonic Crystals with Taking into Account the Polarizability of the Ions
Authors: Zubov V. I. , Sh. Soulayman S.
Published: July 2007

The unsymmetrized self‐consistent field method for strongly anharmonic (nonmetallic) crystals is improved by taking into account the ionic polarization caused by the displacement of the outer electron shell relative to the ionic core during the oscillations of the ions. The self‐consistent potential and the Helmholtz free energy of a strongly anharmonic crystal are found using the adiabatic approximation. In the case of the strong anharmonicity of fourth order, the equations of state of a diatomic crystal are obtained. In the zeroth approximation, the inclusion of the ionic polarizability leads to an added renormalization of the force coefficients of second order. The corrections to the Helmholtz free energy and to the equations of state are considered.
A Comparison between the Theoretical and Experimental Electronic Density of States of Liquid Metals
Authors: Halder N. C.
Published: July 2007

The electronic structure and electronic density of states (EDS) are investigated for eleven liquid metals. The theoretical results are obtained from the nonlocal, energy dependent pseudopotentials which reveal some new features not observed previously with the local pseudopotential theory and on Fermi surface approximation. However, a good agreement is found when these theoretical results are compared with the recently available experimental measurements, especially for liquid Cu, Ga, In, Tl, Sn, and Pb.
Valence Band Structure of (Bi1–xSbx)2Te3 Single Crystals
Authors: Stordeur M. , Langhammer H. T. , Sobotta H. , Riede V.
Published: July 2007

For the first time the anisotropy of reflectivity of polarized infrared light of p‐(Bi1–xSbx)2Te3 single crystals (0.5 ≦ x ≦ 1) is given in the wave number range of 300 to 4000 cm−1. In addition to the excitation of free carriers in the reflection spectra of the mixed crystals also interband excitations are observed. For the mixed crystals the optical results are consistently described in terms of a six‐valley band model and scattering on acoustical phonons together with the data for the dc conductivity, the Hall coefficients, and the thermopower. The main axis masses mii, the ellipsoid tilt angle θ, the effective density of states mass m, the cyclotron mass and the susceptibility masses are given in dependence on composition, also the Fermi level EF, the carrier density p, the mobility u⊥c, the high frequency dielectric constant ϵ′∞, and the relaxation time τ. The interband influence is interpreted by a simple band model and the energy gap Eg calculated.
On the Time Dependence of the Capture of Non‐Equilibrium Hopping Charge Carriers
Authors: Esser B. , Zvyagin I. P.
Published: July 2007

The capture of non‐equilibrium carriers hopping between randomly distributed localized states and being trapped by same recombination centres or traps is considered. Exponentially varying transition probabilities between the localized states are assumed. Using a time dependent cluster construction based on the large scatter of the transition probabilities in the system the capture rate R(t) is calculated. R(t) is shown to follow a power‐law dependence, R(t) ∼ t−ξ, where ξ is equal or close to 1.
Electron–Phonon Interaction in Mixed Valence Systems
Authors: Brouers F. , de Menezes O. L. T.
Published: July 2007

A Hamiltonian describing mixed valence compounds and including d and f electron–phonon coupling is considered. It is shown that the renormalization of the electronic energy parameters induced by the electron–phonon interaction depends strongly on the nature (static or adiabatic) of the approximation used. The importance of the d electron‐phonon coupling on the narrowing of the f level and the softening of the phonon modes is discussed.
Theory of Secondary Electron Emission. II. Application to Aluminum
Authors: Rösler M. , Brauer W.
Published: July 2007

The theory of secondary electron emission developed in Part I is applied to polycrystalline aluminum and the results are compared with experimental data. The angular distribution of secondary electrons (SE) obtained proves to be a cosine distribution in accordance with measurements. Caused by the V220 model potential coefficient one gets an unrealistic peak at E ≈ 10 eV in the energy distribution of SE. Yield values of SE calculated for primary energies E0 = 1 to 2 keV differ by a factor 1.5 to 2 from the measurements. SE created by plasmon decay contribute nearly 60% to the theoretical yield.
Studies of the Oscillator Strengths of Infrared Vibrational Modes in Glow‐Discharge Hydrogenated Amorphous Silicon
Authors: John P. , Odeh I. M. , Thomas M. J. K. , Tricker M. J. , Wilson J. I. B.
Published: July 2007

The infrared oscillator strengths of the stretching and wagging/rocking modes in glow‐discharge hydrogenated amorphous silicon, a‐Si, is compared to similar data reported recently for sputtered material. For both materials the integrated band intensity of the wagging/rocking mode at 640 cm−1 is proportional to the total hydrogen concentration, NH and is independent of preparation conditions. The derived oscillator strength for the latter mode is identical in glow‐discharge and sputtered a‐Si films. In contrast, the integrated band intensity of the composite silicon‐hydrogen stretching mode is film dependent; the derived average oscillator strength, Γs (2000 + 2100), is significantly smaller in glow‐discharge, compared to sputtered a‐Si. A reanalysis of previous isothermal dehydrogenation data is presented in which Γs (2000 + 2100) is shown to be a function of NH and increases markedly at hydrogen contents < 10 at%. No conclusions can be drawn, however, concerning the oscillator strengths of the deconvoluted bands at 2000 and 2100 cm−1 occurring within the stretching region. A previously proposed model for the mechanism of the dehydrogenation of a‐Si is unaffected by the present results.
Some Remarks on Deviations from Matthiessen's Rule in Dilute Alloy Systems
Authors: Barnard R. D.
Published: July 2007

An examination is made of the main features of deviations from Matthiessen's rule in alloy systems from a general standpoint based on anisotropy in the electron–phonon and electron–impurity scattering. It is shown that in virtually all metals at low temperatures, a T3 or near T3 contribution to the electron–phonon inverse relaxation time exists due to the geometry of the Fermi surface or from U processes which produce large angle scattering. As a result, such metals at low temperatures will possess considerable anisotropy in the electron–phonon mean free paths associated with the transport properties, which arises purely from the geometry of the transitions. It is further shown that the usually small high curvature regions of the Fermi surface which produce the anisotropy, while not making a large contribution to the resistivity of the intrinsically pure metal, however are of paramount importance when impurity scattering dominates and results in near T3 variations of the temperature dependent resistivity, ϱT. Such variations appear to be obeyed almost universally in alloys for which ϱ0≫ϱID. The role of anisotropy in the mean free paths due to gold and dislocations as scattering centres in the silver system is discussed in connection with their failure to adhere to the universal curve of ϱT versus log ϱ0 for silver.
Optical Studies of Er3+ Ions in Er2Ge2O7 Single Crystals
Authors: Yaeger I. , Shuker R. , Wanklyn B. M.
Published: July 2007

Optical absorption spectra of Er3+ in Er2Ge2O7 single crystals are studied in the spectral range 14500 to 25000 cm−1 as a function of temperature from ambient down to 12.5 K. The transitions associated with the Er3+ absorption lines are identified. Kramers' doublets of the Er3+ ground term 4I15/2 are found at 13, 88, 125, 185, 230, 392, and 436 cm−1 relative to the ground level.