bluuebunny/crossref_metadata_2025 huggingface dataset:
Hot Electron Magnetoconductivity in Polar Semiconductors
Authors: Licea I.
Published: July 2007

An investigation is made of the magneto‐conductivity of hot electrons in polar semiconductors with an arbitrary non‐parabolicity of the spherically symmetric conduction band in the electron temperature approximation. Basing on these results the Hall effect, the relative resistivity change, and the Hall velocity in a polar semiconductor with a non‐parabolic energy dispersion law like that of n‐type InSb are studied theoretically considering only optical polar scattering and treating comparatively both non‐parabolic and parabolic cases.
Bulk Negative Differential Conductivity in Stressed n‐Type Germanium
Authors: Chattopadhyay D. , Nag B. R.
Published: July 2007

The velocity‐field characteristics of stressed n‐type germanium at 300 °K have been theoretically obtained assuming a Maxwellian velocity distribution function. It is found that the experimentally observed bulk negative differential conductivity cannot be explained if the carriers are assumed to be confined to the 〈111〉 valleys only. The experimental results on the other hand may be fitted if carrier scattering to the 〈100〉 valleys is taken into account and the mobility of the carriers in these valleys is taken to be 690 cm2 V−1 s−1.
Induced Anisotropy in Coarse‐Grained and Single Crystalline Samples
Authors: Lesnik A. G.
Published: July 2007

The formation mechanism of induced anisotropy described in [1] is applied to coarse‐grained and single‐crystalline samples. In this case the anisotropy is explained by an inhomogeneous distribution of vacancies and foreign atoms along the block boundaries or by their piling up along the antiphase boundaries in an ordered alloy.
Theory of Optical Absorption by Diatomic Molecules Embedded in Rare Gas Crystals
Authors: Mannheim P. D. , Friedmann H.
Published: July 2007

A new mechanism for the study of lattice dynamics is presented, rotation‐translation coupling. With this mechanism the optically active rotational modes of diatomic molecules are coupled to the lattice vibrations of crystals in which the diatomic molecules are embedded. Using recently derived expressions which incorporate force constant changes in the vicinity of the defect we calculate and identify localized modes seen in the IR spectrum of HCl and HBr in rare gas crystals. Further we show how the lattice vibrations shift and broaden the rotational levels of the molecule. It is thought that this mechanism will prove useful in the study of the density of states of pure and impure crystals.
Magnetophonon Resonance in Tellurium
Authors: Bresler M. S. , Mashovets D. V.
Published: July 2007

An investigation was made of the magnetoresistance of tellurium single‐crystal samples in magnetic fields up to 400 kOe at liquid nitrogen temperatures for different relative orientations of magnetic field, sample current, and crystal threefold axis. For the interpretation of the observed magnetophonon resonance oscillations the system of Landau levels for the tellurium valence band is calculated in the quasi‐classical approximation taking into account the phenomena related to the presence of a saddle point in the band energy spectrum (intraband magnetic breakdown). The long‐wavelength optical vibrations in the tellurium lattice are discussed. It is suggested that from three optical phonon modes inducing the first‐order dipole moments one mode predominates in carrier scattering. The calculated positions of the magnetophonon oscillations are in good agreement with the experiment.
Theory of Electron Scattering from an Antiferromagnet. II. Itinerant Model
Authors: Falicov L. M. , De Wames R. E.
Published: July 2007

The theory of electron scattering by an antiferromagnet is extended to include the case of metallic targets, in which the periodicity of the magnetic arrangement is not necessarily commensurate with the lattice. In the Hartree‐Fock (random phase) approximation, it is shown that the elastic part of the differential scattering cross section has an expression equivalent to that derived for the Heisenberg model in an antiferromagnetic insulator, with the difference that the sublattice magnetization is replaced by the proper Fourier component of the oscillatory magnetization.
A Green's Function Approach to Intermediate‐ and Strong‐Coupling Polaron Theory in the Presence of External Fields
Authors: Porsch M.
Published: July 2007

A general method for studying the properties of free and bound polarons at arbitrary strength of electron‐phonon coupling is given from a unified point of view by first performing a suitable canonical transformation of the electron and phonon field operators and subsequently applying the double‐time Green's function formalism to the transformed polaron‐phonon Hamiltonian. Calculating the one‐polaron Green's function in the lowest approximation and employing the variational principle for the free energy, a pair of coupled differential equations is derived for the effective wave function and a trial function as introduced by the canonical transformation. From these equations, the weakly excited polaron energy levels at low temperatures can be found in the entire range of coupling constants when external fields are present. In this paper, detailed calculations are confined to the optical‐polaron problem in the zero‐field case. Intermediate‐ and strong‐coupling formulas for temperature‐dependent polaron self‐energy and effective mass are given. The strong‐coupling regime appears to increase with temperature.
Peierls Energies of Dislocations on {110} Glide Planes in the B.C.C. Lattice
Authors: Heinrich K. , Schellenberger W. , Pegel B.
Published: July 2007

The Peierls potentials of different types of crystallographic dislocations moving on {110} glide planes are computed by means of various models and a systematic study of a 35° dislocation is carried out for different model series. It is shown that the original Peierls model can lead to qualitative errors concerning the structure of the Peierls potential and yields too small dislocation widths.
Spin Wave Modifications in Ferromagnetic Alloys by Spherical Island Structures
Authors: Heiner E. , Elk K.
Published: July 2007

The change of the spin wave spectrum of a ferromagnetic alloy by spherical inclusions of a second ferromagnetic phase is determined in a semiclassical approximation. Inside the spherical islands resonances are obtained in dependence of the quotient of the stiffness constants and spin wave damping. The damping is described by the imaginary part of a complex potential.
Einfluß von Fremdmetalldotierung auf die galvanomagnetischen Effekte von Kadmiumeinkristallen bei tiefen Temperaturen
Authors: Schwarz H.
Published: July 2007

An Kadmium‐Einkristallen, die jeweils verschieden stark mit Zink, Quecksilber oder Silber dotiert waren, wurde die Orientierungsabhängigkeit des Halleffektes und des elektrischen Widerstandes im Magnetfeld bei 4,2 °K gemessen. Die Widerstandsverhältnisse der Proben lagen zwischen 13000 und 300. Bei beiden Effekten treten Abweichungen von der Kohlerregel auf, die beim Halleffekt besonders stark ausgeprägt sind. Die unterschiedlichen Ergebnisse des Hallkoeffizienten der einzelnen Kristalle lassen sich deuten, wenn man annimmt, daß durch die Verunreinigungen Kleinwinkelstreuung zwischen verschiedenen Teilen der Fermifäche hervorgerufen wird, wodurch der Charakter der Elektronenbahn geändert wird. Die Wahrscheinlichkeit für eine solche Streuung ist von der Art des Zusatz‐metalls abhängig.
Transmission Electron Microscopy of Point‐Defect Clusters in Neutron‐Irradiated Metals. II. Platinum
Authors: Rühle M. , Häussermann F. , Rapp M.
Published: July 2007

Annealed, neutron‐irradiated, and neutron‐irradiated and subsequently annealed platinum has been investigated by means of transmission electron microscopy. In as‐annealed platinum black dots were observed; these defects occurred only near the foil surfaces and were similar to the polishing defects in copper [8]. Frank dislocation loops, prismatic dislocation loops, and voids were found in neutron‐irradiated foils. After annealing for 1 h at 543 °K the same defect types were observed, however, the contrasts of the defects were stronger. The results are compared with those of other authors.
Isotope Effect for Self‐Diffusion in Single Crystals of Silver
Authors: Rothman S. J. , Peterson N. L. , Robinson J. T.
Published: July 2007

The simultaneous diffusion of 105Ag and 110Ag in silver single crystals has been measured over the temperature range 640 to 955 °C. The strength of the isotope effect has a constant value of 0.718 ± 0.011 for T < 750 °C, but decreases with further increases in temperature to a value of 0.639 ± 0.013 at 937 °C. This temperature dependence is significant at the 90% confidence level. Although one may explain the observed decrease by the temperature dependence of ΔK, we consider it more likely that the decrease results from the simultaneous operation of vacancy and divacancy mechanisms. At the melting point, the contribution of divacancies is from 11 to 45% of the total self‐diffusivity. The diffusion coefficient of 110Ag in Ag single crystals over the same temperature range was also obtained from these experiments. The least squares Arrhenius line is described by D0 = 0.67 cm2/s, Q = (45.2 ± 0.2) kcal/mol ( = (1.97 ± 0.1) eV). The values of D deviate systematically from this line in the manner expected for simultaneous vacancy and divacancy diffusion, although the statistical significance of these deviations is not certain.
Thermodynamic Equations for Dysprosium‐Aluminum Garnet near Its Néel Point
Authors: Bartis F. J.
Published: July 2007

A thermodynamic argument is used to derive equations describing a cubic antiferro‐magnet during a stress‐broadened first‐order phase transition. A linear relation between the heat capacity and the thermal expansivity of dysprosium‐aluminum garnet is found to be consistent with the present results as well as those obtained by Janovec for a lambda‐transition. It is concluded that agreement with an equation of this type is not sufficient to determine the nature of a phase transition.
Atomic Distribution and Magnetic Moment in Liquid Iron by Neutron Diffraction
Authors: Waseda Y. , Suzuki K.
Published: July 2007

The structure of iron in the liquid state has been studied by neutron diffraction at 1620 °C. After calculation of the interference function (Fourier analysis), the atomic radial distribution function was evaluated from which interatomic distance and coordination number were obtained. The value of the magnetic moment of iron in the liquid state which was derived from the correction for the paramagnetic neutron scattering was 1.2 μB.
Radiative Electron Capture by Gold and Platinum Impurities in Germanium
Authors: Slatter J. A. G.
Published: July 2007

It is shown that when negatively charged gold and platinum impurities in germanium capture thermal electrons into levels lying 0.2 eV below the conduction band, the probability of photon emission varies from 10−3 to 10−5 for the gold impurities, and from 10−5 to 10−7 for the platinum impurities, as the lattice temperature changes from 35 to 85 °K. These results were obtained with the use of an expression, derived from the principle of detailed balance, that relates the probability of a capture event being radiative to photoconductivity per unit intensity of illumination.
Theoretical Determination of the Second‐Order Elastic Constants of NaCl‐Type Alkali Halides
Authors: Szomor P. O.
Published: July 2007

The second‐order elastic constants of NaCl‐type alkali halides were calculated on the basis of Hooke's law. The calculations were performed using the Born‐Mayer model for the lattice energy of ionic solids. The results obtained show that contrary to expectations the Cauchy relations are not fulfilled.
On Elastic Properties of Covalent Crystals
Authors: Vukcevich M. R.
Published: July 2007

Elastic properties of cubic, covalently bonded, monoatomic solids are discussed in terms of a new model based on a general expression for the potential energy which is a function of the interatomic distances and angles between the bonding electronic orbitals. Earlier theories are based on parametric expressions for forces, and cannot be used to obtain any additional information. The energy method used here is as general as the classical central force model. Hooke's law for high pressures is derived and effective elastic constants and their pressure derivatives are formulated. Diamond is used as a model solid, and encouraging agreement was found between the theoretical predictions and measured values of the second‐order elastic constants and the Grüneisen parameter.
Electric Field Dependent Plasma Oscillations in Semiconductors
Authors: Krishnamurthy B. S. , Paranjape V. V.
Published: July 2007

The direct effect of the electric field on the electron plasma dispersion relation in semiconductors with simple parabolic band model is investigated by the kinetic method. A model distribution function has been invoked which takes into account the fluctuation in density and momentum of electrons. The possibility of amplification or attenuation of the plasma excitation is discussed.
The Quasi‐Pauli Representation of Spin Operators
Authors: Žakula R. B. , Lalović D. I.
Published: July 2007

A quasi‐Pauli representation of the spin operators is given. Since there are exact methods of representing the quasi‐Pauli operators by either Bose or Fermi operators, this enables an analysis of the processes in ferromagnets and antiferromagnets to be performed in a more completed way.
Mössbauer Effect in Nickel‐Rich Nickel‐Manganese Alloys
Authors: Chandra G. , Radhakrishnan T. S.
Published: July 2007

Nickel‐rich nickel‐manganese alloys with manganese concentrations of 2, 4, and 16 at% have been investigated at various temperatures by Mössbauer spectroscopy using Fe57 as the probe nucleus. The sample with 16 at% Mn concentration has been studied in detail for various heat treatments. For all heat treatments it shows anomalous spectra consisting of an intense central line together with a magnetic structure, over a wide range of temperatures. Three different possibilities have been considered to account for the anomalous spectra. They are the co‐existence of two phases corresponding to long‐range order and short‐range order, the co‐existence of nickel‐rich and manganese‐rich regions, and relaxation effects.
Electromultipole Interaction and Paramagnetic Relaxation in Ruby I. EPR Line Shape in Ruby
Authors: Peskovatskii S. A.
Published: July 2007

The concentration dependence of the line shape in ruby is investigated experimentally at frequencies 9000, 11500 (crystal field splitting frequency), and 40000 MHz. Assumption of pure magnetic dipole‐dipole interaction does not explain the experimental results. The line broadening caused by quadrupole‐quadrupole interaction of spin states is calculated using a statistical theory. The calculated and experimental data are in good qualitative and quantitative agreement.
Exciton‐Induced Colour Centre Growth in KBr and KI Crystals at 5 °K
Authors: Kink R. , Liidja G.
Published: July 2007

Colour centres were created in KBr and KI at 5 °K by ultraviolet irradiation, corresponding to the exciton absorption region, and traced by luminescence methods. In KBr the Frenkel defects (I‐centres) are observed in thermal glow curves, created as a result of the fundamental reaction proposed by Pooley. In KI they are not observed, but F‐, F′‐, and VK‐centres were found in photostimulated luminescence. The dissociation of the migrating excitons at structure defects existing in crystals before irradiation seems to be important at the initial coloration stages in KI.
Comparison of the Small‐Polaron Theory with the Experimental Data of Current Transport in V2O5
Authors: Ioffe V. A. , Patrina I. B.
Published: July 2007

[Russian text Ignored].
Theory of the Shift of the F‐Band in Alkali Halide Mixed Crystals
Authors: Logatchov Yu. A. , Evarestov R. A.
Published: July 2007

A more accurate treatment of the F‐band shift in solid solutions of alkali halides is presented, based on other than usual approximations. The “local” concentration of the impurity cations near a F‐like centre is obtained as the function of the volume concentration from thermodynamical considerations. The transition energies of Fi‐centres (i = 0, 1, …, 6) are calculated in the model potential approximation. The models of F, F, etc. centres are discussed. The peak position of the F‐band in KCl(Na) with 50% concentration of NaCl is obtained. The results are in good agreement with experimental data. The main results have been published in [1].
On the Validity of Modified Wachtman's Equation for Nonoxide Solids
Authors: Rajagopalan S.
Published: July 2007

Modifications of the Mie‐Grüneisen equation of state have been used in the evaluation of bulk modulus at different temperatures for various nonoxide solids. It is shown that the estimation can be carried out correctly even when the constants γ and δ are not known. The agreement of the calculated values with the experimental ones are satisfactory enough that the modified Wachtman equation can be safely used to estimate the bulk moduli of solids even at temperatures inaccessible to direct experiment.
Investigation of the Lattice Dynamics of KBr: Sm2+ by Means of Vibronic Spectra
Authors: Kühner D. , Wagner M.
Published: July 2007

A low‐symmetry (C2 v) model of the lattice disturbance in the system KBr:Sm2+ is employed to calculate the vibronic sideband structure of a 4f‐4f transition. The result yields improvement over earlier calculations, which were based on higher‐symmetry models. The improvement is such that the chosen model may be considered to be realistic.
The Influence of Temperature on Slip Behaviour of Molybdenum Single Crystals Deformed in Tension in the Range from 293 to 573 °K
Authors: Richter J.
Published: July 2007

Cylindrical molybdenum single crystals of middle orientation (Schmid factor ≈ 0.5 on the (101) [111] slip system) grown by electron zone melting were deformed in tension at various temperatures between 293 and 573 °K using a strain‐rate of 7 × 10−6s−1. Stress‐strain curves are parabolic in shape below 340 °K (T< 0.12Tm), whereas they are clearly divided into three work‐hardening stages above 400 °K (0.14Tm<T< 0.20Tm). The variation of yield stress and work‐hardening parameters in dependence on deformation temperature is systematically investigated and the results are discussed with respect to published work on other b.c.c. metals. Furthermore, molybdenum crystals with various specially chosen “single slip” orientations were deformed at 493 °K. It is found that under these conditions the orientation has no pronounced influence on the stress‐strain curve.
The Mobility of Grown‐in Dislocations in the Intermetallic Compound MgZn2 I. Stress Dependence for Edge Dislocations in Prism Slip at 390 °C
Authors: Paufler P. , Marschner J. , Schulze G. E. R.
Published: July 2007

The velocity x of grown‐in (1010) 〈1210〉 edge dislocations in the intermetallic compound MgZn2 has been measured as a function of stress τ at 390 °C. Dislocation displacements produced by compression pulses were detected by etch pitting (0001) surfaces. The fraction of mobile dislocations ϱm/ϱtot has also been determined as a function of stress. The results indicate that the velocity depends exponentially on τ−1. They are analyzed in terms of a modified kink model after Celli et al‥ Microscopic measurements are in good agreement with stationary creep data.
Experimental Determination of the Shielding of Crystal Fields at Rare‐Earth Ions
Authors: Pistor W.
Published: July 2007

By comparing crystal field parameters A〈rk〉 of Er3+ in ScCl3 and InCl3 with those of Cr3+ and V3+ in the same host lattices, experimental values for the shielding factors σ2and σ4 are determined. The values of σ2between 0.57 and 0.72 are in good agreement with the theory, whereas the values of σ4 between 0.71 and 0.79 do not agree at all with the theoretical values. Possible interpretations for the differences are given.
The Temperature Dependence of Slip Planes in Fe‐3.4% Si Single Crystals
Authors: Šesták B. , Blahovec J.
Published: July 2007

The temperature dependence of macroscopic slip planes observed on tension and compression sides of specimens deformed slowly by four‐point bending has been investigated over the temperature range 195 to 483 °K. The experimental results are presented in form of Ψ(χ)‐curves. The measured Ψ (χ)‐curves were compared with the curves calculated by Vítek using the theory of thermally activated motion of screw dislocations extended on {110} and {112} planes and good agreement was found above room temperature. Discrepancies between the calculated and measured Ψ(χ)‐curves exist at low temperatures, where slip along {110}‐planes becomes significant. Differences between the Ψ(χ)‐curves measured on the tension and the compression side cannot be satisfactorily explained by existing theories.
The Mean Free Path of Relativistic Electrons for Plasmon Excitation
Authors: Ashley J. C. , Ritchie R. H.
Published: July 2007

The interaction of a relativistic electron with a system of plasmons is considered. First‐order perturbation theory is used to calculate the rate of excitation of plasmons by the incident electron; the excitation rate leads to a mean free path. It is shown that relativistic corrections to the mean free path are generally quite negligible and thus the mean free path calculated using a nonrelativistic incident electron, when expressed in the “proper fashion”, is also correct relativistically.
Energy and Momentum Loss Rates for Hot Electrons in Silicon
Authors: Ahmad S. , Daga O. P. , Khokle W. S.
Published: July 2007

Using of time‐dependent perturbation and deformation potential theories for electronphonon interaction, the rate of change of crystal momentum and energy of electron along the field direction has been estimated in silicon. It is found that the loss rate of crystal momentum of the electron along field direction, due to acoustic phonon collisions is greater than that due to optical phonon collisions, whereas the rate of loss of energy of electrons due to optical phonon interactions is greater than that due to acoustic phonon collisions. The dominant acoustic phonon contribution to the loss rate of the crystal momentum of the electron along field direction is confirmed by the observed and estimated temperature dependence of the drift velocity of electrons in silicon for high electric fields.
Specific Heat and Superconductivity of Binary Alloys Containing V, Nb, and Ta
Authors: Corsan J. M. , Cook A. J.
Published: July 2007

The electronic specific heat coefficient, γ, the Debye temperature, θD, and the superconducting transition temperature, Tc, have been measured for binary b.c.c. alloys of V, Nb, and Ta. These results have been used to investigate the effects of compositional changes on parameters used in the theory of superconductivity to describe the transition temperature. It has been shown that changes in transition temperature with changes in composition cannot be explained simply on the basis of an alteration of the density of states except in the case of binary Nb‐Ta alloys. It is concluded that, for V‐Nb and V‐Ta alloys, changes in the interaction responsible for superconductivity also govern changes in the transition temperature.
Correlations of Electron Spin Resonance Parameters with Fe3+ Sites in Topaz
Authors: Howling D. H.
Published: July 2007

Correlations proposed between crystal field axes obtained from electron spin resonance measurements and specific Fe3+ sites in topaz (Al2[F, OH]2SiO4) have been investigated. The results of three studies have been examined and it is shown that none of the axes‐site correlations proposed is unique. Results obtained for Fe3+ by electron spin resonance are compared with the results of our earlier nuclear resonance studies of Al3+ in topaz. By comparison with the tensor‐site correlations previously advanced for Al3+ sites, preferred correlations between axes and sites for Fe3+ substituting for Al3+ in two groups of sites are given, each group consisting of four equivalent and congruent Al3+ sites.
Subdivision of the B.C.C. Transition Metals
Authors: Chen C. W.
Published: July 2007

B.c.c. transition metals are shown to display large, systematic changes in many basic physical properties on going from vanadium to chromium or iron, from niobium to molybdenum, and from tantalum to tungsten. These property changes are attributed, at least partially, to the differences in the electronic structure and nuclear composition between the group V (V, Nb, and Ta) and group VI metals (Cr, Mo, and W) and iron. A subdivision of the b.c.c. transition metals is this proposed. Conditions under which the metals may sensibly be considered as an undivided group are discussed.
Initial Hardening in Quenched Gold
Authors: Bapna M. S. , Mori T. , Meshii M.
Published: July 2007

The effects of dispersed vacancies on mechanical properties were examined in as‐quenched gold single crystals. A quench from 1000 °C produced an initial hardening of ≈200 p/mm2 at 4.2 °K. This hardening increased with the quenching temperature and could be approximated by a linear or square root dependence on the vacancy concentration. The extent of the easy glide region was found to increase without a significant change in the work‐hardening coefficient. No yield drop was generated by the quench. The quenching effects were found to decrease with the test temperature below − 70 °C. The experimental results were interpreted in terms of hardening due to dispersed vacancies and/or small vacancy clusters. The kinetics of the aging at room temperature were also followed by yield stress measurements at liquid helium temperature.
Mössbauer Study of Dy161 in Gd2O3
Authors: Mørup S. , Trumpy G.
Published: July 2007

Mössbauer studies have been performed with a source consisting of Tb161 in Gd2O3 and with ferromagnetic metallic dysprosium as an absorber. – Hyperfine splitting of the Dy161, produced by the decay of Tb161 in the source has been obtained. The results are interpreted in terms of a pure quadrupole splitting of e2 q Q = 485 MHz at room temperature.
Compton profile anisotropies of Na2S and CaF2 in the LCAO approximation
Authors: Perkkiö S.
Published: July 2007

Three directional and the isotropic Compton profiles of the isoelectronic fluorite structure compounds Na2S and CaF2 are calculated using the linear combination of atomic orbitals (LCAO) approximation. The anisotropies between the directional Compton profiles are found to be confined below 2.0 at. units of momentum in the case of Na2S and below 3.0 at. units of momentum in the case of CaF2. The basic features of the anisotropies are found to be the same except near the zero momentum. Fourier transforms of the Compton profiles serve as a check of the reliability of the calculations. They reveal that orthogonalization up to the 21st order of neighbour atoms is sufficient for the [100] and [110] crystal directions but in the [111] crystal direction there are discrepancies for both compounds.
A theoretical study of the thermodynamic properties of diluted and random‐bond ising systems
Authors: Agudelo A. C. , Ospina O. E. , Zamora Ligia E. , Bohórquez A. , Alcázar G. A. Pérez
Published: July 2007

The thermodynamic properties of a diluted and random‐bond Ising model are calculated using the variational method based on the Bogolyubov's inequality for the ferro‐ and antiferromagnetic regions. It is assumed that the system is composed of clusters of one and two spins. Previously reported results for Ising models pure and diluted by site and by bond are recovered with this model.
Fluctuations and susceptibility dispersion in ferroelectrics on the basis of the fokker‐planck equation
Authors: Kaupužs J.
Published: July 2007

The Fokker‐Planck equation is used to calculate the dielectric susceptibility dispersion in ferroelectrics within the Landau‐Ginzburg model. Equations for both the time‐space correlation function and the susceptibility are obtained on the basis of subsequent consideration of fluctuations. Two cases are considered: 1. a small‐size single domain ferroelectric and 2. an infinite‐size ferroelectric. Effects related to a switching of the system between two stable states due to fluctuations are investigated. The obtained results explain in general (without specific effects) both frequency and temperature dependence of the susceptibility (or permittivity) and the dielectric loss angle observed experimentally in ceramics and polycrystals (case 1) as well as in single ferroelectric crystals (case 2).
The Influence of Ultrasound on the Rayleigh Scattering of Mössbauer Radiation in Deformed Crystals. New Integral of Motion of the Takagi Equations
Authors: Iolin E. M. , Muromtsev A. V. , Rusevich L. L.
Published: July 2007

The exact integral of motion Q is found for the Takagi equations. These are transformed from the system of two differential equations with partial derivatives to the system of two ordinary differential equations. The RSMR spectra are analyzed for the cases of perfect and deformed single crystals excited by ultrasound. An oscillatory Q‐dependence of the total intensity of diffracted beam and RSMR spectrum is found.
Spin Wave Theory of Anisotropie Two‐Sublattice Heisenberg Ferrimagnets
Authors: Xue Desheng , Gao Meizhen , Kong Yong , Li Fashen
Published: July 2007

In random‐phase approximation, the method of matrix Green functions is applied to investigate the spin wave spectra and mean values of the spin vector in the z‐direction of anisotropic two‐sublattice Heisenberg ferrimagnets both for NaCl and CsCl structures. The results show that each of the spin wave spectra breaks into acoustic and optical branches and changes closely with the structure type, temperature, and the anisotropic exchange interaction coefficient A. The ground state at T = 0 K has all negative energy states filled by magnons. The thermal expectation values, 〈Sz〉, of the spin in z‐direction are calculated numerically for each sublattice at T = 0 K. It is seen that 〈SZ〉 with A > 1 is larger than that without anisotropy. In the special case of an isotropic antiferromagnet, our results agree completely with the existing works.
ESR Study of the Phase Transition Sequence in Betaine Phosphite
Authors: Pöppl A. , Völkel G. , Metz H. , Klöpperpieper A.
Published: July 2007

The structure of the PO radical is examined by ESR spectroscopy in γ‐irradiated betaine phosphite. The ESR spectra of the PO radical indicate two phase transitions of betaine phosphite at Tc1 = 355 K and Tc2 = 216 K. The symmetry of the ESR spectra of the PO center in the paraelastic, antiferrodistortive, and ferroelectric phases of betaine phosphite is in accordance with the space groups P21/m, P21/c, and P21.
On the Conditions of the Magneto‐Optical Kerr Effect Enhancement in Metallic Bilayers
Authors: Zhou Shi‐Ming , Zhai Hong‐Ru , Lu Mu , Miao Yong‐Zhi , Song Jing‐Tao , Zhang Hong‐Yan
Published: July 2007

The magneto‐optical Kerr rotation spectra of Fe/Ag, Fe/Au, and Ni/Cu bilayers are measured and calculated based on Fresnel's equations. The theoretical spectra agree with the measured ones. A magneto‐optical Kerr effect enhancement peak is observed near 600 and 500 nm for the bilayers Fe/Ag and Fe/Au, respectively, with the Fe layer thickness less than the light penetration depth. No Kerr effect enhancement occurs for Ni/Cu bilayers for any thickness of the Ni layer. It is concluded that the plasma or the absorption edge of the reflector in bilayers is neither a necessary nor a satisfactory condition but provides an important opportunity for the enhancement. Whether the enhancement occurs depends not only on the optical constants of the reflector but also on the optical and magneto‐optical parameters of the top ferromagnetic layer.
Electron Energy Structure of Boron Nitride and Diamond. Similarities and Distinctions
Authors: Ilyasov V. V. , Nikiforov I. Ya. , Safontseva N. Yu.
Published: July 2007

With the coherent potential method of the multiple scattering theory the electronic structure of boron nitride (in sphalerite modification) and diamond is calculated. The cluster approximation with muffin‐tin crystal potential is used. The electron structure and chemical bonding of these substances are investigated for the first time with this modern theoretical method. It is shown that the chemical bonding has mainly covalent character, the ionicity being about 20%. A fine structure of the upper part of the valence band of boron nitride is revealed and its nature is discussed. The similarity of the total electron state densities of the diamond and boron nitride in sphalerite modification is proved and the distinctions are discussed. The calculated local partial electron densities of states are used to explain the experimental X‐ray emission and X‐ray photoemission spectra of diamond and boron nitride.
NMR Investigations of Molecular Motions and Critical Dynamics at the Ferroelectric Phase Transition of Tris‐Sarcosine Calcium Chloride
Authors: Michel D. , Hacker U. , Erge Th. , Petersson J.
Published: July 2007

35Cl and81Br quadrupolar perturbed NMR measurements in single crystals of tris‐sarcosine calcium bromide (TSCB) and tris‐sarcosine calcium chloride (TSCC) are used to study molecular motions and the dynamical critical behaviour near the ferroelectric phase transition of TSCC. At temperatures higher than about 130 K the spin‐lattice relaxation timeT1of81Br nuclei in TSCB is dominated by an activated process with an activation energy of 28 kJ/mol which is attributed to the reorientation of parts of the sarcosine molecule. The results are compared with those obtained from35Cl measurements of TSCC. Near the ferroelectric phase transition of TSCC at 131.5 K a critical contribution to the35Cl spin‐lattice relaxation rate is observed which can be explained on the basis of the standard mean‐field model in accordance with the static critical behaviour belowTc.
Pressure Dependence of Co Diffusion in Amorphous Fe40Ni40B20
Authors: Rummel G. , Mehrer H.
Published: July 2007

The pressure dependence of the diffusion coefficients of Co is studied in structurally relaxed meltspun amorphous Fe40Ni40B20. The non‐crystallinity of diffusion samples before and after the high‐pressure diffusion anneals is checked by X‐ray diffractometry. The radiotracer 60Co is used and ion beam sputtering is employed for serial sectioning of diffusion samples. The activation volume for diffusion is Δ V = −(0.07 ± 0.02) atomic volumes of Co. Within the experimental errors no significant pressure dependence is observed. This observation indicates that Co diffusion in Fe40Ni40B20 is likely to be mediated by a collective mechanism.
The Crystalline Structure as a Basis for a Reversed Access to the Special Theory of Relativity
Authors: Günther H.
Published: July 2007

Lorentz symmetries in solids become evident if the soliton properties of kinks on dislocations for the measurement of length and time are used. This procedure can be extended for deriving the complete theory of special relativity, thus reversing the traditional way. The continuum approximation of the crystalline lattice with dislocations appears as a model for a relativistic space‐time.
Mechanical Behaviour of Linear Viscoelastic Liquids
Authors: Povolo F. , Hermida Élida B.
Published: July 2007

The mechanical properties of some amorphous materials can be described in terms of a modified anelastic element (MAE). The characteristics of this element, developed in a previous publication, are extended in order to describe the mechanical behaviour of linear viscoelastic liquids. This treatment leads to the modified standard linear liquid (MSLL). Through this phenomenological model the quasi‐static and dynamic moduli or compliances of viscoelastic liquids can be analytically expressed as a function of time or frequency. The parameters involved in the equations – γ, t0 or ω0, and η0 – are related to the width and the mean characteristic time of the relaxation or retardation spectrum and to the zero‐shear‐rate viscosity of the specimen. A procedure to determine the parameters of the MSLL from dynamical properties is proposed. It is applied to measurements in molten polypropylene (PP) given in the literature, and the results are discussed. Moreover, since the shape of the spectrum associated to the MSLL is quite similar to the well‐known log‐normal distribution function, also a comparison is done.
On the Hartree Interaction in Quasi‐2D Systems
Authors: Combescot Monique
Published: July 2007

The Hartree interaction in quasi‐2D systems (i.e. the Coulomb excitations with zero momentum transfer) is rederived without introducing any particular cut‐off to withdraw the divergences arising from the long‐range character of the Coulomb potential. It is shown that the remaining finite part is indeed independent of the cut‐off as physically expected.
Tight‐Binding Approach for the Band Structure of GaAs‐Type Semiconductors
Authors: Merian M. , Lampel G. , Coqblin B.
Published: July 2007

The band structure of GaAs‐type semiconductors is studied within a tight‐binding calculation, taking into account the s and p atomic energies including spin‐orbit couplings for both anions and cations. The 16 × 16 matrix is solved analytically in perturbation theory for the conduction band near the Γ‐point, which yields an analytical derivation of the k3‐term. The computed values of the coefficient of the k3‐term agree with experimental ones in III–V semiconductors.
Atomic Short‐Range Order in Cu17 at% Mn
Authors: Roelofs H. , Schönfeld B. , Kostorz G. , Büher W.
Published: July 2007

The elastic diffuse neutron scattering from a Cu–17.2 at% Mn single crystal aged at 483 K to produce an equilibrium state of atomic short‐range order is measured at room temperature in a minimum separation volume of reciprocal space. Warren‐Cowley short‐range order parameters and coefficients of linear static displacement scattering are determined by least‐squares fitting. Effective pair potentials obtained from the short‐range order parameters by applying the inverse Monte‐Carlo method are used to calculate the diffuse antiphase boundary energy γAPB. For copper‐base alloys the concentration dependence of γAPB is one of the suggested reasons for the transition of wavy to planar slip. Based on the values of γAPB, the concentration ranges for the transition in CuMn, CuAl, and CuZn may be understood.
Lattice Dynamical Investigations on DX Center Localised Vibrational Modes in AlxGa1−xAs and AlxGa1−xAs:Si
Authors: Pothiraj R. , Ramachandran K.
Published: July 2007

Localised vibrational modes (LVMs) associated with DX centers in AlxGa1−xAs and n‐type AlxGa1−xAs are worked out from a molecular model, introducing the relaxation. Preliminary studies indicate that some of the basic features observed from deep level transient spectroscopy (DLTS) can be viewed from the present calculations.
Compound‐Source MBE for ZnSe‐Based Lasers
Authors: Ohkawa K. , Takeishi H. , Hayashi S. , Yoshii S. , Tsujimura A. , Karasawa T. , Mitsuyu T.
Published: July 2007

Compound‐source molecular beam epitaxy (CSMBE) for ZnSe‐based lasers is developed. This method employs compound sources instead of elemental sources. The ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructure is grown by this method. Continuous‐wave operation of the device by CSMBE at room temperature is demonstrated.
Second Harmonic Generation in Thin Film II–VI Optical Waveguides
Authors: Wagner H. P.
Published: July 2007

The absolute values and dispersion of the second harmonic generation (SHG) coefficient of the cubic wide‐gap II–VI semiconductors Zn Te, ZnSe, and ZnS are investigated within the spectral range from 1047 to 1321 nm. The fundamental beam is generated by a Nd:YLF laser and by a tunable KTP optical‐parametric oscillator (OPO) pumped by a ps‐Ti:sapphire laser system. The measured values of the SHG coefficient are significantly higher than in commercially used birefringent crystals as KDP or ADP. In a further step symmetric single crystalline ZnSe/ZnTe/ZnSe/GaAs(001) waveguides are grown by metalorganic vapour phase epitaxy (MOVPE). Using end‐fire coupling a strong phasematched signal of the 1170.5 nm fundamental wave is observed. Phase matching is achieved by coupling the TE0 fundamental mode to the TM2 second harmonic mode.
Electrical Contacts to p‐ZnSe Based on HgSe and ZnTe
Authors: Einfeldt S. , Behringer M. , Nürnberger J. , Heinke H. , Behr T. , Becker C. R. , Hommel D. , Landwehr G.
Published: July 2007

Low resistance ohmic contacts to p‐ZnSe involving the use of HgSe are studied in detail. HgSe does not form an ohmic contact with p‐ZnSe because of a residual valence band offset at the heterojunction of 0.5 to 0.6 eV. Annealing of HgSe/ZnSe:N does not result in an ohmic contact because a large miscibility gap in Hg1−xZnxSe prevents a HgZn interdiffusion and therefore the formation of a graded heterojunction. The molecular beam epitaxial (MBE) growth of Hg1−xZnxSe does not circumvent this problem because these epitaxial layers suffer from a compositional phase separation due to spinodal demixing during MBE growth. A significant improvement of HgSe based contacts could be achieved by the use of ZnSe1−xTex:N transition layers between HgSe and ZnSe:N. Either by abrupt, graded, or multilayer heterojunctions, very low contact resistivities are obtained. The growth of ZnSe1−xTex:N layers on top of ZnSe:N is shown to reduce the carrier concentration in ZnSe:N by several orders of magnitude. A nitrogen diffusion process is believed to be responsible for this effect.
(Dark Line Defects, Bright Line Lasers)‐Microscopic Studies of Single‐Shot Lasing in CdSe Quantum Wells
Authors: O'Donnell K. P. , Bagnall D. M. , Wright P. J. , Cockayne B.
Published: July 2007

A preliminary report on single quantum well (SQW) lasers of CdSe grown by metalorganic vapour phase epitaxy (MOVPE) in a matrix of ZnSe is presented. The total device thickness is 3 μm. The quantum wells are „ultrathin”︁ in the range from 0.5 to 2 monolayers (ML). The time‐integrated spectra of spontaneous emission from the top surface of as‐grown samples are compared with spontaneous and stimulated edge emission from cleaved laser cavities. Such temporally and spatially integrated spectra are contrasted with single‐shot laser spectra obtained by adapting a 1 m single monochromator to operate as a videospectrograph. In addition to „hidden”︁ mode structure, the spectrograms reveal the strongly heterogeneous character of laser emission from these samples: bright lines that indicate materials of good quality are found adjacent to dark lines where the emission is weak or absent.
High‐Brightness Blue‐Green Light‐Emitting Diodes on ZnSe Substrates
Authors: Harsch W. C. , Cantwell G. , Schetzina J. F.
Published: July 2007

High‐brightness blue and green light‐emitting diodes (LEDs) are produced based on homoepitaxial films grown by molecular beam epitaxy (MBE) on high quality ZnSe substrates. The substrates are grown by the Eagle‐Picher, seeded physical vapor transport (SPVTTM) technique. The substrates are twin‐free and contain no low‐angle grain boundaries. Double‐crystal X‐ray diffraction rocking curve studies yield a FWHM 〈400〉 = 11″ to 16″, indicating a crystal quality comparable to that of GaAs substrates. The blue LEDs produce 327 μW (10 mA, 3.2 V), with the light output sharply peaked at 489 nm, and exhibit an external quantum efficiency of 1.3%. The green LEDs produce 1.3 mW (10 mA, 3.2 V) peaked at 512 nm, corresponding to an external quantum efficiency of 5.3%.
Compton Profile of Ag2O
Authors: Bandyopadhyay S. , Saha S. K. , Chatterjee A. , Chatterjee A. K.
Published: July 2007

The Compton profile (CP) of polycrystalline silver oxide is measured with the use of 59.54 keV γ‐radiation from a 241Am source. The experimental data are compared with the renormalized free atom (RFA) model calculation using Watson's +1 well solution for the oxygen 2p state. Oxygen 2p6 and hybridization of different 4d–5s configurations of Ag are considered in the theoretical calculations. The theoretical values from such calculations agree well with experimental results.
Self‐Consistent Study of Lattice Relaxation around a Magnesium Impurity in Aluminium
Authors: Thiagarajan M. , Iyakutti K. , Palaniyandi E.
Published: July 2007

The lattice relaxation around a Mg impurity in an Al cluster is studied self‐consistently, using the MSXα method. The atoms surrounding the Mg impurity in an Al cluster are allowed to relax to various extents and the correponding total energies of the cluster are computed. It is found that for a particular relaxed position, the cluster total energy is minimum. For the Mg impurity in Al, the corresponding relaxation is found to be 4% of the lattice constant.
Lamb shift for an excitonic molecule
Authors: Ivanov A. L. , Haug H.
Published: July 2007

A theory of the optical decay and of the resonant two‐photon creation of an excitonic molecule (xx) in direct band gap semiconductors is given within the polariton picture. An xx Schrödinger equation which includes both the Coulombic exciton‐exciton attraction and the polariton coupling is derived and analyzed. The solution of this equation describes an xx as the quasibound two‐polariton complex. The real and imaginary parts of the xx energy give self‐consistently the inverse xx radiative lifetime Γxx and the radiative renormalization Δxx of the xx energy (the xx Lamb shift), respectively. For 3D (CuCl) the polariton effects result in a non‐parabolic xx dispersion with M → ∞ at zero momentum; for quasi‐2D (GaAs) they lead to a “Mexican hat” structure in the xx dispersion at small momenta together with a considerable increase of both the xx effective binding energy ϵxx and lifetime Γxx. For quasi‐ID the influence of the polariton effects is so strong that an xx can exist only as a broad resonance (ϵxx ∼ Γxx).
Capture of carriers into a GaAs/AlGaAs quantum well relevance to laser performance
Authors: Haverkort J. E. M. , Blom P. W. M. , van Hall P. J. , Claes Hall J. , Claes J. , Wolter J. H.
Published: July 2007

An experimental and theoretical study of the carrier capture time into a semiconductor quantum well is presented. Oscillations of the phonon emission induced capture time are experimentally observed and good agreement with theory is found. The calculations show that not only the LO‐phonon emission induced capture time (ph‐capture) oscillates as a function of well width, but also the carrier–carrier scattering induced capture time (c‐c capture) oscillates by more than one order of magnitude as a function of the active layer design. Recently it has been shown that the carrier capture time is directly related to the modulation band width in a quantum well laser. As a result, it might be possible to tailor the modulation band width by optimizing the capture efficiency using a proper design of the active layer in a quantum well laser.
Dynamics of screening and field fluctuations on ultrashort time scales
Authors: Manzke G. , Henneberger K. , Heeg J. , El Sayed K. , Schuster S. , Haug H.
Published: July 2007

The short‐time dynamics of screening in semiconductors is considered after excitation with an ultrashort laser pulse of several 10 fs. While the kinetics of carriers is considered within standard approximations (semiconductor Bloch equations) the plasmon kinetics includes particularly memory effects due to non‐Markovian behaviour and retardation as well as interaction between plasmons and the coherent interband polarization induced by the laser pulse. For pulses shorter than the relaxation time of carriers pronounced oscillations of the plasmon density (intensity of plasma oscillations) are found which are strongly enhanced by the coherent interband polarization. It is concluded that the notion of plasmons as quasiparticles fails on an ultrashort time scale, because it is essentially an adiabatic concept and experiments in the fs‐domain should be reinterpreted in this respect.