bluuebunny/crossref_metadata_2025 huggingface dataset:
The Modulation of Light by Non‐Equilibrium Optical Phonons in n‐Ge
Authors: Vorobev L. E. , Stafeev V. I. , Shturbin A. V.
Published: July 2007

The modulation of light near the edge of the direct transitions in Ge in strong pulsed electric fields was investigated. High‐resistance n‐Ge (ϱ = 40 Ωcm) and low‐resistance p‐Ge (ϱ = 1.0 Ωcm) exhibit the Franz‐Keldysh effect only. The low‐resistance samples of n‐Ge (ϱ = 1.0 and 0.3 Ωcm) exhibit Franz‐Keldysh effect together with a variation of the absorption coefficient ΔαDE which is proportional to the free carrier concentration and has a threshold approximately at the photon energy deficit equal to the energy of the longwave optical phonon (0.037 eV). The additional absorption ΔαDE is caused by the indirect band‐to‐band transitions with participation of non‐equilibrium optical phonons, emitted by hot electrons. From a comparison of the experimental and calculated results for ΔαDE the lifetime is estimated to be 5 × 10−11 s at T0 = 90 °K for the long‐wave optical phonons. This value is in satisfactory agreement with Klemens' theory and also with the results of Raman scattering in Ge.
Free Carrier Absorption in CdSxSe1−x Single Crystals
Authors: Lisitsa M. P. , Demidenko Z. A. , Malinko V. N. , Pidlisnyi E. V. , Valakh M. Ya. , Vitrikhovskii N. I.
Published: July 2007

Free electron absorption in single crystals of CdS and CdSe, as well as in CdSxSe1−x solid solutions (x = 0.80, 0.58, 0.43, 0.36, 0.31, 0.21), has been investigated at Hall concentrations N = 2 × 1016 to 2 × 1019 cm−3 in the temperature interval T = 80 to 450 °K. For all the samples investigated, absorption coefficients K ∼ λn, where n = f(N, T), and a mixed mechanism of scattering by optical phonons and charged impurities is found, whereas for N of the order of 1016 cm−3 scattering occurs practically only by optical phonons. An interpretation of the wavelength and concentration dependences of the measured absorption coefficients is given.
Attractive Interactions between Flux Lines as Derived from a Generalized Neumann‐Tewordt Functional
Authors: Hubert A.
Published: July 2007

A generalized Ginzburg‐Landau (GL) functional is derived which is equivalent to the functional of Neumann and Tewordt (NT) if used to calculate first order corrections to known GL solutions, but which avoids the instabilities formerly encountered in a direct evaluation of the NT functional. Flux lines in superconductors near Tc are calculated using the circular cell approximation of Marcus. In contrast to the GL case we find a first order transition at Hc 1 which results from an attractive interaction between flux lines. The initial induction, the lower critical field, the magnetization curves and some flux line profiles are given as functions of the GL parameter ϰ, the temperature and the impurity parameter α. The results are consistent with former results of Jacobs and with experimental observations of Kumpf.
Neutron and Mössbauer Studies of the Planar Antiferromagnet KFeF4 in the Critical Region
Authors: Heger G. , Geller R.
Published: July 2007

Results obtained from diffuse neutron scattering investigations and those derived from magnetic Bragg peaks on single crystals of the planar antiferromagnet KFeF4 in the critical region reveal that the temperature dependence of the two‐dimensional short range magnetic order and of the sublattice magnetization follow (T – TN)−νv and (1 – T/TN)β laws, respectively, where v = 0.60, β = 0.20, and TN = 148 K. Mössbauer measurements at 300K showed that for KFeF4 the isomer shift is (+ 0.138 ± 0.002) mm/s

see page 232.

and the quadrupole splitting is (1.460 ± 0.005) mm/s

see page 232.

. In the temperature range 0.96 < T/TN < 0.998 the internal magnetic field follows the power law Hin(T) = H(0) D (1 – T/TN)β, where H(0) = H (4.2 K) = (540 ± 4) kOe, TN = (141.51 ± 0.05) K, D = 1.12 ± 0.03, and β = 0.209 ± 0.008.
Piezoemission Study of Free Excitons in CdS
Authors: Goede O. , Büchtemann A.
Published: July 2007

The free exciton emission bands in CdS are investigated with static uniaxial stress, σ, applied to the crystal. The measured peak shifts and intensity ratios as function of stress σ ∥ c and σ ⟂ c are interpreted on the basis of first order perturbation theory and lead to quantitative information on the stress‐induced mixing of exciton states. In particular, the A(Γ6)‐exciton, stress‐allowed for σ ⟂ c, is studied and the corresponding mixing matrix element is determined. For both stress directions a Boltzmann distribution of the exciton concentrations is found.
Evidence of Anomalous Z1‐Dependence of Interaction Potential by Multiple Scattering Experiments
Authors: Krygel P. , Lippold J. , Schwabe S.
Published: July 2007

On the basis of Meyer's theory of multiple scattering, the collision parameters are estimated, which substantially influence the scattering distribution. For reduced thicknesses of τ ≦ 2 higher values of collision parameters around p ≧ 2 a (a screening parameter) have an influence on the scattering distribution. Experiments with various ions at carbon targets seems to show deviations of the half‐width from the theoretical dependence on the atomic number. These deviations are interpreted by means of periodic fluctuations in the interaction potential, which have been shown experimentally by Nelson [2], and theoretically by Cheshire and Poate [1].
On Creep Strain Delay at the Superconducting Transition
Authors: Soldatov V. P. , Startsev V. I. , Vainblat T. I. , Danilenko L. A.
Published: July 2007

The kinetics of the jump‐like process of creep strain increase during the crystal transition from the normal to the superconducting state was studied. It is shown that the crystal transition from the N (normal) to the S (superconducting) state in the creep process is characterized by a certain delay time during which the creep rate remains constant after the transition to the S state. The delay time dependence on temperature and stress was studied. The delay time decreased exponentially with the increase of stress on the crystal at the normal state and displays a sharp decrease with the lowering of temperature. The obtained results are accounted for on the basis of the model of a thermally activated unpinning of dislocations from obstacles taking into consideration the peculiarities of the model in the low‐temperature region.
Coherent Potential Approximation for Alloys with Random Off‐Diagonal Elements: Heisenberg Ferromagnet
Authors: Harris R. , Plischke M. , Zuckermann M. J.
Published: July 2007

We use ideas of Blackman et al. [4] for the coherent potential approximation (CPA) in alloys with configuration‐dependent hopping terms. In the locator formalism the inverse self energy obeys an equation of the same form as the standard CPA equation. We first apply the formalism to the magnon spectrum of the disordered anisotropic Heisenberg ferromagnet with equal exchange constants but different spins. We calculate densities of states, the magnon dispersion relation, and the lifetime of the states, and we find regions of localized states at both ends of the spectrum at low concentrations. The formalism is then applied to the case of different exchange constants but equal spins in order to discuss recent experimental results.
Selfconsistent Lattice‐Dynamical Theory of Structural Phase Transitions in Perovskite‐Type Crystals
Authors: Pietrass B.
Published: July 2007

The temperature dependence of the soft mode frequency and of the displacement parameter, associated with the structural phase transition in perovskite‐type crystals, is calculated from anharmonic lattice dynamics in a selfconsistent way. The theory describes the change‐over from the Landau‐type behaviour to critical behaviour near the phase transition. For SrTiO3 and LaAlO3 good agreement is obtained with the experimental data of Müller and Berlinger.
Spin Fluctuations of Fe3+ Ions in Cobalt–Zinc Ferrites Using the Mössbauer Effect
Authors: Bhargava S. C. , Iyengar P. K.
Published: July 2007

Resonance absorption spectra of Fe57 in CoxZn1−xFe2O4 (for x = 0.3, 0.5, and 0.75), in the temperature range from 80 to 700 °K, are discussed. The modulated lineshapes of the magnetic hyperfine spectra show features which are characteristic of relaxation effects. The theoretical line shapes, computed using the stochastic model, show good agreement with the experimental data at all temperatures for all the three compositions. For each composition, the relaxation time decreases very slowly with increase in the temperature, up to the magnetic transition temperature. Near the Néel temperature, the relaxation time starts decreasing rapidly, leading to the total collapse of the magnetic hyperfine splitting. The introduction of the Zn2+ ions in the ferrite increases the relaxation times at low temperatures (T < TN). The change is, however, negligible for the increase in the Zn2+ concentration from x = 0.5 to 0.3. This behaviour can be attributed to the role of Zn2+ ions in increasing the magnetic disorder in the ferrite. The increase in the concentration of Zn2+ ions causes the damping of the collective modes existing in an ordered ferrimagnetic system.
Hyperfine Magnetic Fields at As in Fe, Co, and Ni
Authors: Chopra R. C. , Tandon P. N.
Published: July 2007

The hyperfine magnetic fields acting at As nuclei in ferromagnetic Fe, Co, and Ni hosts were measured using perturbed angular correlation technique. Radioactive 75Se atoms were implanted into the ferromagnetic backings by means of a mass separator. The 121–280 keV γ–γ cascade in the decay of 75Se was used for the measurements. Values of the magnetic field at As in Fe, Co, and Ni of (318.8 ± 32.7), (248.9 ± 27.6), and (87.5 ± 9.9) kG, respectively, were obtained. The magnetic fields were found to be proportional to the host magnetic moments.
Electronic Structure of Binary Disordered Alloys (Al–Mg)
Authors: Grover P. S. , Chaudhry R.
Published: July 2007

The electronic structure of aluminium and concentrated aluminium‐magnesium alloys and the effect of alloying on the Fermi surface of aluminium were investigated. It is found that the energy bands are quite sensitive to concentration. Energy gaps at lower concentrations agree with optical absorption measurements.
On Electron Nuclear Double Resonance (ENDOR) of Electron Centres in Paraelectric Crystals in External Electric Field
Authors: Deigen M. F. , Vikhnin V. S. , Glinchuk M. D.
Published: July 2007

The spectrum of paraelectric resonance (PER) of an impurity noncentral ion in the vicinity of an electron centre (Li+ in the A‐centre in KCl) is calculated. The effect of an external electric field E on the ENDOR spectrum is studied. The applied field E causes new angular dependences of the spectrum. It is shown that from a comparison between the calculated PER spectra, the constants of hyperfine interaction (HFI) and quadrupole interaction (QI), and the experiment one can determine a number of characteristics of the A‐centre electron and the noncentral impurity ion.
The Debye Frequency Tensor of Single Crystals
Authors: Iosilevskii Ya. A.
Published: July 2007

The tensor of the Debye frequencies of single crystals of all the systems is obtained in terms of their elastic stiffness constants cpq and the density ϱ. The eigenvalues of this tensor are found numerically for materials with cpq and ϱ known at T = 4.2 °K. A large anisotropy of the Mössbauer effect for zinc is predicted.
Infrared Birefringence on Hot Electrons in n‐Type Ge
Authors: Vorobev L. E. , Stafeev V. I. , Ushakov A. U.
Published: July 2007

The anisotropy of the refractive index in n‐Ge caused by the redistribution of hot electrons in the presence of a strong electric field directed along [111] was measured on three samples with different carrier concentrations at T0 = 85 °K. Based on these data the dependence of the ratio of electron densities in the cold and hot valleys on the field strength was determined for three electron concentrations in the specimens. The experimentally determined dependences are compared with calculated ones.
Phenomenological Theory of Polar Systems
Authors: Dogonadze R. R. , Kornyshev A. A.
Published: July 2007

A dielectric formulation of the problem of a nonmagnetic condensed polar medium is developed on the basis of an effective Hamiltonian. The phenomenological theory obtained describes the systems with an arbitrary number of excitation branches and spatial dispersion and may be applied to both regular and irregular systems.
Atomic Coordination Valency of the Elements
Authors: Plendl J. N. , Gielisse P. J.
Published: July 2007

The atomic coordination valency, Za, recently derived from an atomistic formulation of compressibility, and a newly developed atomic parameter, Za/(n — 1), are investigated as a function of the atomic numbers. Here: Za = ϱ z, where z is the valence of the atoms in the elemental state, ϱ the ratio of the atomic coordination numbers of two successive polymorphs, and n the principal quantum number. The values of Za differentiate between the three fundamental states of solids: reduced, basic, and elevated. The parameter Za/(n — 1) represents an atomic determinant of bond strength. The three fundamental states of solids resolve the Periodic Table into a series of characteristic blocks of elements which manifest a distinct ordering, based on three categories of bond strength. Some new polymorphic forms may be predicted by this concept. An application to thermal expansion of metallic elements is given.
The Band Structure of Si III and Ge III
Authors: Alben R. , Goldstein S. , Thorpe M. F. , Weaire D.
Published: July 2007

Band structures have been calculated for the Si III and Ge III crystal structures, using a simple model Hamiltonian of tight binding type, with parameters appropriate to Ge. The calculations indicate, inter alia, that fivefold rings of bonds have important consequences for the band structure of a tetrahedrally bonded solid. Certain band edges are shifted if the structure contains many fivefold rings, as does Ge III. Current models for amorphous Si and Ge involve such rings, and it is suggested that if amorphous tetrahedrally bonded Sn could be made, the existence of a sufficient proportion of fivefold rings in the structure might result in this being a semiconductor with a sizeable gap (∼0.5 eV).
Electron Correlation in a Doubly Degenerate Band
Authors: Schneider J. , Heiner E. , Haubenreisser W.
Published: July 2007

The effect of intra‐atomic Hund's rule coupling on the occurrence of ferromagnetism is investigated by considering two orbitally degenerate narrow bands with arbitrary density of states and strength of intra‐atomic interaction. By using the Green's function technique the two orbitally degenerate narrow bands split due to intra‐atomic Coulomb and exchange interaction into Hubbard‐like spin‐polarized sub‐bands. To discuss the stability of the ferromagnetic and paramagnetic state the energy of the system is calculated for a square density of states. In the strongly correlated limit eight Hubbard‐like spin‐polarized sub‐bands appear and the ferromagnetic state is favoured if the difference between the Coulomb repulsion U and exchange energy J on an atom is finite. In the case where the ratio U1/Δ between U1 = U — J and bandwidth Δ of the unperturbed bands tend to infinity the ferromagnetic and paramagnetic state are degenerated energetically.
Pressure Dependence of Mg2Sn Electrical Resistance
Authors: Thrasher P. H. , Kearney R. J.
Published: July 2007

The electrical resistivity of single crystal Mg2Sn is measured as a function of hydrostatic pressure to 29 kbar at 298 °K and 27 kbar at 320 °K. Various p‐type samples with varying mobilities and mobility‐temperature dependences are analysed. The results for pressures less than seven kbar may be described by a simple valence‐conduction band model with an energy gap pressure dependence of +4.75 × 10−6 eV/bar. The results for pressures up to 29 kbar are described by a two conduction band model. The second conduction band is shown to be important even at low pressures; almost 10% of the conduction electrons are in the upper band at room pressure and temperature. The bottom and top conduction bands are identified as X3 and X1 bands respectively. The energy separation between these two bands is deduced to be 0.130 eV at zero pressure. The least‐squares pressure coefficients of the X3 and X1 bands with respect to the Γ15 valence band are measured to be +4.75 × 10−6 eV/bar and — 0.90 × 10−6 eV/bar, respectively. Estimated correction for a systematic error, introduced by the mobility dependence on pressure through the decrease of carrier lifetime when interband scattering increases, suggests that +4.75 × 10−6 eV/bar and—0.75 × 10−6 eV/bar may be better values for the Γ15— X1 energy derivatives with respect to pressure. The existence of a third conduction band, with an energy location of 0.075 eV above the X3 band at zero pressure and having a pressure rate of +6 × 10−6 eV/bar with respect to the Γ15 band, is suggested but not definitely established by a slight upward curvature of the resistance vs. pressure data below 7 kbar.
A New k · P Calculation for PbSe
Authors: Bangert E. , Kastner P.
Published: July 2007

The upper valence band and the lowest conduction band of lead selenide are extremely nonparabolic. This is usually explained by the influence of further bands. The experimental values, however, are not consistent with a simple two band model. Therefore a new k · P perturbation treatment is performed involving four bands. Formulae are derived for the energy dependence of cyclotron masses and of the products of cyclotron mass and g‐factor. By comparison with experimental data, all introduced free parameters can be determined. The calculation is restricted to the case that the magnetic field is oriented parallel to the major axis of the carrier ellipsoid.
Momentum Representation of Hydrogenic Ground States Derived from Complicated Band Extrema
Authors: Balslev I. , Christensen O.
Published: July 2007

A simple structure of an approximate ground state wave function is suggested in the case of a particle with complicated effective mass bound by a Coulomb potential. The state is expressed in the momentum representation and contains only the binding energy and the explicit band structure near the band extremum. The theory is checked by investigating separately two cases: ellipsoidal, parabolic bands and spherical, non‐parabolic bands.
The Influence of Elastic Anisotropy on Extended Dislocation Nodes
Authors: Pettersson B.
Published: July 2007

The interaction forces between the partial dislocations comprising an extended dislocation node are calculated using elasticity theory for anisotropic media. The calculations are carried out for nodes of screw, edge, and mixed character in Ag which has an anisotropy ratio A equal to three, and in a hypothetical material with A = 1 and with the same shear modulus as Ag. The results are compared with those of three previous theories using isotropic elasticity theory and developed by Brown and Thölén, Siems, and Jøssang et al. As expected, the effect of anisotropy in Ag is of the same order of magnitude as the uncertainty due to the dislocation core energy.
Angular Dispersion of Polar Phonons in Biaxial Crystals
Authors: Graf L. , Schaacx G. , Unger B.
Published: July 2007

The angular dispersion (kph ↫ ω/c) of polar phonons in the region of the ν3,‐states of the SO42−‐ions in biaxial MgSO4, · 7 H2O has been observed with Raman‐scattering experiments at T = 300 K and T = 1.9 K. The agreement of the experimental data with the existing theories of angular dispersion is very satisfying. Some accidental degeneracies of transverse with longitudinal states of different symmetry could be observed. This degeneracy, where the two branches cross with equal slopes of opposite signs, can be lifted due to the temperature dependence of the frequencies. This behaviour is also interpreted theoretically and some simple relations are derived for the slopes and the curvature of dispersion branches, which relate these data to the effective charges and the dielectric function ε(ω), and which characterize the critical points occuring on the dispersion branches.
Nuclear Magnetic Resonance of 9Be in (Mn1‐x Fex) Be2 Alloys
Authors: Domngang S.
Published: July 2007

The nuclear spin‐lattice (T1) and spin‐spin (T2) relaxation times of 'Be nuclei have been studied by means of spin‐echo techniques in (Mn1‐x Fex) Be2, alloys. T1 T is a function of temperature and composition. The spin‐spin relaxation processes as measured by the decay of the spin echo envelope exhibit Gaussian time dependences. The relaxation rate 1/T2 is found to increase when the temperature decreases. The Knight shift of 9Be is negative and is a function of temperature and composition of the alloys. An interaction A I 〈S〉 between nuclear spin (9Be) and impurity spin (iron) allows to explain the behaviour of the relaxation rate and Knight shift of 9Be for x ⩽ 0.16.
A Kinetic Approach to Fracture of Solids III. The Statistical Features of the Time to Break (Longevity). Homogeneous Material
Authors: Petrov V. A.
Published: July 2007

The statistical features of longevity are determined by two factors: by the random character of the thermally activated nucleation and propagation of the main crack and by the inhomogeneities of the structure of the sample. The shape of the distribution function of the longevities of homogeneous samples and its dependence on the size of the sample are calculated. This distribution and the experimentally observed distribution of brittle strengths are both described by Weibull's function. On the basis of this coincidence the analogy between fracture under prolonged load and brittle fracture is discussed.
Kinetics of Plastic Deformation at Superconducting Transitions
Authors: Bobrov V. S. , Gutmanas E. Yu.
Published: July 2007

An analysis has been carried out of the influence of the superconducting (SN and NS)transitions on the kinetics and parameters of plastic deformation for the models of viscous and thermally activated dislocation motion. The experimental investigations have been performed on Pb single crystals at 4.2 °K. The experimental ratio of plastic strain rates in normal and superconducting states in the point of SN transition corresponds to the change of electronic drag coefficient according to the BCS theory. The experimental results are in a better agreement with the analytical expressions obtained for the model of viscous dislocation motion.
Analysis of Point Defect States in Copper V. Temperature Dependence of the Defect Production by Electron Irradiation
Authors: Becker D. E. , Dworschak F. , Wollenberger H.
Published: July 2007

The temperature dependence of the defect production has been investigated by irradiating high purity copper samples with 3 MeV electrons in the temperature range 50 to 207 K. From the initial damage rate the capture radius of the vacancy for migrating interstitials is derived. The observed temperature dependence of the capture radius can be accounted for by a drift diffusion of the interstitials in the elastic strain field of their native vacancy.
On the Numerical Calculation of the Density of States and Related Properties
Authors: Lehmann G. , Taut M.
Published: July 2007

A method is given for the numerical calculation of energy surface integrals within the Brillouin zone like density of states, conductivity, susceptibility, dielectric function etc. The Brillouin zone is divided into tetrahedrons in which the integrand is interpolated linearly. In this way the integration can be done analytically avoiding the histogram method. Several similar methods are discussed with regard to the quotient of accuracy and effort.
A Transition Matrix Approach to Electron Scattering from Amorphous Materials
Authors: Boyce J. F. , Burge R. E. , McCloughrey J. K.
Published: July 2007

A formalism is developed to permit the calculation of scattering by high energy electrons. It is shown to reproduce the usual results of the dynamical theory for scattering from a perfect crystal. For a particular model of a distorted crystal an explicit closed form is obtained for the correction due to amorphism.
Interpretation of the Edge Emission of II–VI Compounds
Authors: Hagston W. E.
Published: July 2007

The present state of the interpretation of the broad band edge emission in II–VI compound semiconductors is investigated and a possible reformulation given which assigns both the “high” and “low” energy series emission to transitions between donor‐acceptor pairs. It is shown that the resulting model, although similar to one proposed recently by Gutsche and Goede [1], is different in the nature of the interpretation of the phenomena and furthermore is free from various conceptual difficulties to which the model of these latter authors is subject. A means of forming an experimental investigation of the validity of these newproposals is described.
The Band Structure of Silver and Optical Interband Transitions
Authors: Christensen N. E.
Published: July 2007

The band structure of silver has been calculated using the relativistic augmented plane wave method. Relativistic effects must be included if a quantitative interpretation of optical experiments is made. The L′2 → L1 gap is 1 eV smaller in the relativistic calculation than it is in a non‐relativistic band structure obtained from the same potential. This reduction of the L‐gap is in the present case sufficient to lead to an extra interband edge below the main edge at 3.98 eV. The optical function ε2(ω) and the energy distributions of photoemitted electrons have been calculated.
Antiresonance (Fano Effect) with Phonon Contribution
Authors: Cho Kikuo
Published: July 2007

Interference effects between a discrete and continuous electronic states have been studied in the presence of the effect of phonons on the discrete state. With the use of a Langevin type equation for the motion of the discrete state, the contribution of phonons is expressed as a frequency dependent life time for the discrete state. By noting the presence of an extra continuum interacting with the discrete state through phonon participation, the quant um mechanical Langevin equation can be approximately derived. This result gives a theoretical foundation for the recent, successful analysis of the antiresonance in an exciton spectrum which made use of a phenomenological life time parameter.
Pair and n‐Tuple Effects in Imperfect Crystals
Authors: Bscruno R.
Published: July 2007

An algebraic procedure is formulated to consider the normal modes of a crystal with substitutional impurities. An explicit expression is derived for n‐tuple effects (the interaction of n defects) for a lattice with random defects. When specialized to pairs (n = 2), the deviations from randomicity are also considered.
The Orientational Disordering and Thermodynamic Properties of Simple Molecular Crystals
Authors: Slusarev V. A. , Freiman Yu. A. , Krupskii I. N. , Burakhovich I. A.
Published: July 2007

The Bogolyubov variational method has been applied to the investigation of the ordered phase thermodynamics for N2, CO, N2O, and CO2 crystals in the molecular field approximation. The libron heat capacity, the ordering parameter, and the libration excitation energy have been calculated for the whole range of the ordered phase. The orientational disordering temperature has also been estimated. The theoretical results have been compared with experimental data.
Mössbauer Studies of the Magnetic Behaviour of the Cr2O3Fe2O3 System
Authors: Srivastava J. K. , Prasad K. G.
Published: July 2007

The magnetic structure of the (1 − x) Cr2O3–x Fe2O3 system ≦x ≦ 4.95 wt %), was investigated by Mössbauer measurements in the temperature range 80 to 300 °K. The temperature dependence of the hyperfine field clearly supports the cone spiral structure, originally proposed by neutron diffraction work. A linear decrease in the cone half‐angle θ has been observed with increasing temperature and also to some extent with increasing x. The variation of the quadrupole coupling constant, |1/2 e2 q Q|, with temperature and x follows qualitatively the temperature and composition dependence of the trigonality (c/a) of the rhombohedral Cr2O3–Fe2O3 lattice. It is also suggested that the anomalous Mössbauer spectra observed earlier in the 0.965 Cr2O3–0.035 Fe2O3 system [8] were probably due to the superparamagnetic effect.
The Effective Lattice Theory
Authors: Beuno R.
Published: July 2007

Some aspects of the random impurity problem in crystals are studied. A method of dealing with substitutional impurities having large defect spaces is given. The expressions found are derived within the single‐site scattering approach and are exact to c. The effect of “local” force constant changes is also considered.
Renormalization of the Bandwidth by Interatomic Interaction
Authors: Heiner E. , Schneider J.
Published: July 2007

The renormalization of the bandwidth by the interatomic interactions between the electrons is investigated. It is found, that the interatomic Coulomb interaction causes a broadening of the band, whereas the interatomic exchange interaction gives a narrowing of the band. The influence of these changes in the bandwidth on the Stoner criterion is discussed.
Impurity Diffusion in F.C.C. Metals
Authors: Neumann G. , Hirschwald W.
Published: July 2007

A new model of the impurity diffusion in f.c.c. metals is described. A relation for the difference of the vacancy migration energies ΔHM□ of the impurity atom and the matrix atom is developed:

Tm and T are the melting temperatures of the matrix and the foreign metal and HM□ is the vacancy migration energy in the matrix metal. – ΔHM□, the binding energy between impurity and vacancy is calculated from the different jump frequencies of a matrix atom in the neighbourhood of the impurity atom: ΔHM□ = 0.17 ΔHM□ The temperature dependence of the correlation factor is determined from the calculated jump frequencies using Le Claire's approximation method.
Anharmonic Effects on Surface Mode Absorption in NaCl and KBr Slabs
Authors: Placido F. , Hisano K.
Published: July 2007

Calculations of surface mode dispersion curves for semi‐infinite slabs of NaCl and KBr have been made. Anharmonic effects have been taken into account using previously derived dielectric constants and good agreement with experiment has been obtained where measurements e available.
Higher Energy Levels of Cr3+ Ion Pairs in Ruby and Lanthanum Aluminate
Authors: Heber J. , Platz W. , Leutloff S.
Published: July 2007

The excitation and absorption spectra were investigated of the doubly excited states |2E × 2E〉, |2E × 2T1〉, |2T1 × 2T1〉 × 2T1, and |2E×2T2 of the first nearest neighbour Cr3+ ion pairs in LaAlO3, and of the first, second, third, and fourth nearest neighbour Cr3+ pairs in ruby. The spectra were analyzed for electronic and vibronic transitions.
Fluorine Superhyperfine Structure in the EPR of CdF2:Gd3+
Authors: Naehring F. K.
Published: July 2007

The fluorine superhyperfine structure (SHFS) in the EPR spectrum of Gd3+ in cubic environment has been investigated in CdF2 crystals. The measured 9 GHz EPR spectra of the SHFS are explained using the formalism of Ranon and Hyde and the principal values of the SHFS tensor from the ENDOR results of Borcherts, Cole, and Horn. A formula for the intensities of the SHFS lines is presented.
Self‐Consistent Normal Mode Analysis of the One‐Mode Polaron
Authors: Trubatch S. L.
Published: July 2007

The self‐energy of the one‐mode polaron model is calculated in both the weak and strong coupling regimes by using a variational principle which removes all linear Bose coupling terms and diagonalizes all quadratic Bose terms in the Hamiltonian. It is clearly demonstrated that this approximation scheme is not powerful enough to yield the correct strong coupling behavior.
Mössbauer Lineshape in the Presence of Radio‐Frequency Fields
Authors: Krishnamurtht B. , Sinha K. P.
Published: July 2007

A general expression for the Mössbauer lineshape in the presence of a radio frequency field is derived. As an example the effect of the rf field on Fe57 nuclei is discussed for a situation where the 3/2 sublevel of 14.4 keV state of Fe57 is selectively populated. At resonance, both the diagonal and non‐diagonal matrix elements contribute to the correlation function. As a result, in addition to a slight rf induced distortion of the main Mössbauer line. additional transition lines are obtained. Thus the present calculation supports the experimental observations of Heiman et al. [5].
EPR of Mn2+ Ions in Mixed Crystals Znw, Hg1−wSe and ZnmHg1−mTe
Authors: Leibler K. , Giriat W. , Cęllciński K. , Wilamowski Z. , Iwanowski R.
Published: July 2007

The results of EPR measurements are presented of mixed crystals ZnmHg1−mTe and ZnwHg1−wSe doped with Mn2+. The spectroscopic splitting factor g and the hyperfine structure constant A were determined as a function of the composition of the crystals investigated and measurements are made of electrical conductivity and electron concentration.
Phonon Assisted Interband Optical Transitions in Disordered Semiconductors
Authors: Esser B.
Published: July 2007

The absorption coefficient of a disordered semiconductor, α(ω), is calculated in the case of indirect transitions. The absorption coefficient is influenced by a random force field present in disordered semiconductors. The case of a smooth random field is considered. The same exponential optical tail is obtained as in the case of direct transitions: In α(ω) ∼S (ℏω – ℰg,i), where S is the inverse of some characteristic energy and ℰg, 1 is the indirect band gap. Par enough in the allowed band the absorption coefficient takes on the usual quadratic frequency dependence.
Order States and Magnetic States of CrPt3
Authors: Besnus M. J. , Meyer A. J. P.
Published: July 2007

Ordered CrPt3 which has the AuCu3 type structure is known to be ferrimagnetic. The f.c.c. disordered CrPt, phase has been obtained by severe cold work and is found to behave as a paramagnet. Through thermomagnetic analysis and consideration of neighbours. this paramagnetism is tentatively ascribed to a kind of antiferrornagnetism. The forced disordered atomic arrangement results in a incoherent distribution of localized moments without apparent external moment. The magnetic properties of partially ordered samples have also been studied.
Intervalence Band Transitions in Tellurium. I. Polarization E ‖ c
Authors: Fischer D. , Bangert E. , Grosse P.
Published: July 2007

High resolution measurements of the intervalence band transitions of electrons in tellurium (hole concentration p = 1014 to 1018 cm−3) were performed for polarization E ∥ c in the spectral range 60 to 200 meV a t low temperatures. The absorption spectra were calculated on the basis of a k · p perturbation method. The transition probabilities show a considerable variation ∼(ℏω)−2. A procedure is described for the determination of the valence band parameters from the optically measured data. With these parameters it is possible to determine the hole concentration of the samples from their absorption spectra.
On Different Perturbation Theoretical Approaches to the Heisenberg Ferromagnet with Spin 1/2
Authors: Kühnel A.
Published: July 2007

The results of the drone‐fermion representation (DFR) in application to the Heisenberg ferromagnet with spin 1/2 are compared with the corresponding results obtained by means of Pauli operators. The expressions calculated in DFR by Spencer for the perturbational expansion of the one‐particle Green function are shown to coincide with the corresponding expressions arising in the calculations using Pauli operators. In spite of that, different ways of summation of diagrams lead to different results for physically interesting quantities; these differences can be explained explicitely as resulting from certain neglections made in the use of DFR.
Defect‐Induced Optical Absorption near the Electronic Band Edge of Tellurium
Authors: von Alpen U. , Doukhan J. C. , Escaig B. , Grosse P.
Published: July 2007

Because of the plasticity of tellurium at room temperature many physical properties are strongly influenced by the preparation technique. Systematical inquiries about the influence of dislocations on the optical constants near the electronic band edge of tellurium are reported. Comparison of the absorption spectra of samples in which defined dislocations had been introduced by plastic deformation clearly shows that these defects cause sharp absorption lines near the electronic band edge. Models for the interpretation of the absorption mechanism are discussed.
Motional Averaging of Electric Field Gradients. Temperature Dependence of Nqr in Sodium Chlorate and Copper Chlorate
Authors: Vijaya M. S. , Ramakrishna J.
Published: July 2007

The temperature dependence of CI35 nuclear quadrupole resonance in sodium chlorate and copper chlorate is studied in the temperature range 77 to 300 °K. It is shown that the assumptions made in Bayer's theory, are valid for the case of chlorates and hence the frequency of the torsional oscillations of the ClO3 group is calculated using Bayer's theory. The calculated value of the torsional frequency compares well with available Raman spectroscopy data.
Impurity‐Induced First‐Order Raman Spectrum in Cesium Halides
Authors: Ram P. N. , Agrawal Bal K.
Published: July 2007

A theoretical study of impurity‐induced first‐order Raman scattering has been made in cesium halides. The defect model parameters obtained earlier by the analysis of infrared absorption experiments have been used in the calculation. The frequency dependence of the Raman intensity is found to be quite insensitive to variations in the force constants describing the defects. The Raman technique may therefore be employed for detailed study of the perfect phonon without having a detailed knowledge of the defect perturbation.
Some Problems of the g‐Shift of S‐State Ions in AII BVI Semiconductors
Authors: Leibler K. , Wilamowski Z.
Published: July 2007

The negative values of Δg of Mn2+ in mercury compounds are discussed from the point of view of the electron charge transfer theorie. A simple model including the second shell of Mn neighbours is proposed.
Pinning of Single Crystals of Niobium Irradiated at Helium Temperature
Authors: Andronikashvili E. L. , Ashimov S. M. , Tsakadze J. S.
Published: July 2007

The influence of neutron irradiation at 4.2 °K on the pinning of Abrikosov flux lines in single crystals of niobium was studied. It is shown that the irradiation with fast neutrons of 3.5 × 1012 cm−2 increases the mean value of the pinning force. Exposition of the specimen during 80 h at T = 100 °K and the recooling do not eliminate the irradiation effect. It is assumed that the observed increase in the pinning force is caused by additional pinning of dislocations available in the specimen (pinning‐centres) by point defects, formed during irradiation.
Variation de x⟂ avec une tension uniaxiale dans FeCl2 et FeBr2 à 4,2 ° K
Authors: Hammann J. , Nasser J. A.
Published: July 2007

Dans la structure hexagonale de FeCI2 et FeBr2 les plans réticulaires perpendiculaires à l'axe ternaire ne contiennent qu'un seul type d'atomes. Les propriétés de ces deux métamagnétiques ont été analysées en introduisant l'anisotropie cristalline, un échange ferromagnétique entre ions Fe2+ d'un měme plan, et un échange antiferromagnétique entre ions Fe2+ de deux plans successifs. Nous rapportons ici les mesures de la variation de x⟂ à 4,2 °K (phase antiferromagnétique) lorsqu'on rapproche les plans contenant les ions Fe2+ par une contrainte uniaxiale. Nous en déduisons les variations au cours de cette deformation des paramètres microscopiques précédents.
On the Theory of Thermal Conductivity in Disordered Solids
Authors: Böttger H.
Published: July 2007

Consistent equations for the averaged Green's functions are derived for a disordered‐ harmonic solid The lattice thermal conductivity is related to the solution of a Bethe‐Salpeter equation.
Mössbauer Study of a Stressed Single Crystal of KFeF3
Authors: Varret F. , Imbert P.
Published: July 2007

A single crystal 〈111〉 platelet of the cubic perovskite KFeF3 has been submitted to a stress parallel to its plane. By this means, a single magnetic domain may be obtained in the magnetic region having a trigonal symmetry. By measuring the relative intensities of the Mossbauer lines, the orientation of the hyperfine field and of the electric field gradient tensor with respect to the plane of the platelet have been determined throughout the temperature range 4.2 ≦ T ≦ 110°K.
Variational Estimate of the Energy Spectrum of a Bound Polaron
Authors: Clark T. D. , Bajaj K. K.
Published: July 2007

Variational estimates of the energies of the ground state and of a few low lying excited states of a system consisting of a single electron coupled simultaneously to a point defect via the Coulomb potential and to a longitudinal optical phonon field are obtained. The trial functions used are linear combinations of intermediate coupling free polaron functions and thus the results are felt to be valid into the intermediate coupling region. The numerical results obtained from the above are compared wit.11 a closed form, analytic approximation and a good agreement is found between the two.
Hydrostatic Pressure Dependence of First‐Order Raman Frequencies in Se and Te
Authors: Richter W. , Renucci J. B. , Cardona M.
Published: July 2007

Measurements of the effect of hydrostatic pressure on the first order Raman frequencies of Se and Te are reported. All the frequencies of the Raman active modes decrease with pressure. The valence force field model proposed by Nakayama and Odajima is used to explain this behaviour.
Transport of Heat between Electrons and Phonons in Copper below 0.2 K
Authors: Peterson R. E. , Anderson A. C.
Published: July 2007

The thermal resistance between phonons and electrons has been measured in thin copper samples at temperatures from 0.03 to 0.2 K. The measurements are in qualitative agreement with a calculation based on the free‐electron model, except the temperature dependence changes to T‐3 from the expected T‐4 as the electron mean free path becomes limited by sample boundaries.
Zero‐Field NMR and Electronic Structure of MnSb and Related Compounds
Authors: Bouwma J. , Haas C.
Published: July 2007

Measurements of zero‐field NMR of MnSb and related compounds are reported. The spin‐echo signals of Mn1.00Sb show quadrupole splittings. Broad unresolved signals have been observed in Mn2Sb. Non‐stoichiometric B8‐type compounds Mn1 +xSb1 ‐ySny (Mn, V)1+xSb and (Mn, Cr)1+xSb show broad unresolved signals or no signals at all. In all compounds the observed frequencies of the Mn55 and Sb123 resonances at 4.2 °K arc about 200 and 210 to 220 MHz, respectively. A comparison of the properties of MnSb with the properties of related compounds leads to a metallic band model for MnSb with about six d‐electrons per Mn atom.
One‐Phonon Far‐Infrared Absorption of NaI Crystalline Powder
Authors: Novák L.
Published: July 2007

Far‐infrared transmission measurements of crystalline NaI powder samples at 300 and 100 K have been performed. Except for the strongest absorption band at the frequency of the pseudo‐surface polariton mode, all the lattice bands were correlated with energies at some critical points of the Brillouin zone of the perfect crystal. The measured spectra are assumed to be a manifestation of the grain‐boundary activated band‐mode absorption. The interpretations of other authors are discussed.
Evidence for Strain Broadening in the ESR Absorption Spectrum of Tetragonally Distorted Cr3+ Ions in MgO
Authors: Serway R. A. , Marshall S. A. , Robinson R. B.
Published: July 2007

An anisotropic line broadening has been observed in the ESR absorption spectrum of Cr3+ ions located at tetragonally distorted sites of single‐crystal MgO. For an impurity ion whose tetragonal distortion is along a crystal [100] direction. a sin2φ dependence is observed in the linewidth when the Zeeman field is rotated in the plane perpendicular to this direction. The broadening is interpreted by introducing strain induced terms in the conventional spin‐Hamiltonian, where it is assumed that the strain‐coupled coefficients are the same as those measured for Cr3+ at the cubic sites of MgO. This formalism is consistent with both the angular dependence that is observed in the linewidth and the order of magnitude of the line broadening after correcting for contributions from other mechanisms such as variations in the crystalline electric field parameter D. Analysis of the linewidth data gives C44 α/g β = (2.13 ± 0.2) Oe and ΔD/g β = (1.3 ± 0.3) Oe, where a is the parameter that characterizes the distribution of strains within the crystal.
Tl0, Ga0, and In0 Centres in KCl Crystals
Authors: Jaek I. , Kink M.
Published: July 2007

Absorption bands of In0 and Ga0 centres are observed in KCl : In and KCI : Ga which are related, by analogy with the corresponding bands of Tl0 centres in KCI: Tl, to the electronic transitions 62P1/2 → 72S1/2 (the 1.80 eV band in KC1:In and 1.95 eV band in KC1:Ga) and 62P1/2 → 6 2P3/2 (the 0.75 eV band in KCl: In) in free In and Ga atoms. The luminescence of Tl0 and In0 centres at about 0.82 and 0.70 eV, respectively which may be excited in the absorption bands of Tl0 and In0 centres, as well as in the F band, is found and investigated. Luminescence of {In0 + Vk} pairs is detected in KCl: In and is analogous to that observed earlier in KCl: Tl.
Theory of Paraelectric Resonance (PER) Lineshape
Authors: Vikhnin V. S. , Glinchuk M. D. , Gordon A. Ya. , Deigen M. F.
Published: July 2007

A study is made of the shape of paraelectric resonance (PER) lines broadened by internal electric fields and stresses. The calculation was made using the first‐ and second‐order statistical theory. The low‐field PER lines of the allowed transitions (E ‖ Eshf ‖ [001]) are shown to be asymmetric. An essential dependence of the lineshape of some transitions on the direction of the electric illumination is obtained. The contribution of the cross‐relaxation in the ion‐ion interaction reservoir into the broadening of PER lines is estimated. The agreement between the theory and the experiment for KCI:Li+ is obtained.
Positron Annihilation in Potassium Chloride and Potassium Iodide
Authors: Hautojärvi P. , Nieminen R.
Published: July 2007

Positron lifetime spectra have been measured in several single crystals of KCI and KI. Angular correlation measurements of 2 γ annihilation radiation have been made in polycrystalline materials. The positron wave function calculated by the spherical symmetric Wigner‐Seitz approximation in the neighbourhood of the anion is similar to that in metals and indicates that positrons are free in alkali halides. The experimental angular correlation curves are much narrower than those calculated with positrons in crystalline states, when Wigner‐Seitz wave functions of the positron were used and only annihilations with anion shells were taken into account. The assumption that positrons are trapped by crystal defects explains the observed differences in lifetime measurements. The angular correlation curve caused by trapped positrons has been constructed by subtracting the calculated momentum distribution of the ground state positrons in the crystal bulk from the total experimental curve.
An Extended Breathing Shell Model for Alkali Halides and Its Test at Experimental Results
Authors: Bluthardt W. , Schneider W. , Wanger M.
Published: July 2007

Group‐theoretical as well as quantum‐mechanical arguments require an extension of Schröder's breathing shell model (BSM) to include axial deformations. An extended model (EBSM) is presented and numerical results are given for KBr and KI. The dispersion curves do not differ appreciably from those of the BSM and the agreement with the experimental curves is equally good. In contrast, however, the eigenvectors, as tested indirectly via a calculation of the U‐center sideband, evidently are improved by the EBSM.