bluuebunny/crossref_metadata_2025_split huggingface dataset:
Coherent Potential Approximation for Alloys with Random Off‐Diagonal Elements: Heisenberg Ferromagnet
Authors: Harris R. , Plischke M. , Zuckermann M. J. 
Published: July 2007

AbstractWe 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

AbstractThe 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

AbstractResonance 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractBand 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractAn 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

AbstractThe 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

AbstractA 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractInterference 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

AbstractAn 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

AbstractThe 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

AbstractThe 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.
Renormalization of the Bandwidth by Interatomic Interaction
Authors: Heiner E. , Schneider J. 
Published: July 2007

AbstractThe 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

AbstractA 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

AbstractCalculations 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.