bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Spin–Lattice Relaxation Measurements of the Jahn‐Teller Complex Cu(6H2 O)++
Authors: Lee K. P. , Walsh D.
Published: July 2007

Spin–lattice relaxation measurements at microwave frequencies in the liquid helium temperature range yield relaxation rates which are four orders of magnitude faster than the corresponding rate in a system without Jahn‐Teller complications. The results also emphasize that Jahn‐Teller parameters for this system cannot be determined to‐date with any certainty.
Investigating the Reliability of Local Empirical Pseudopotential for Band Structure of PbTe by Comparing Calculated Optical and Photoelectric Properties with Experimental Data
Authors: Ciucci G. , Nardelli G. F. , Antoci S.
Published: July 2007

A comparison is made between calculations based on the empirical pseudopotential method and optical and photoelectric data of PbTe. The agreement found with such experimental data is generally good concerning conduction and high energy valence bands. In regard of optical data, the results are comparable or better than the results obtained by previous authors on the basis of the APW‐k. p method. A critical point analysis of the main structures occurring in the calculated optical properties shows that volume effects in the BZ cannot be disregarded in the critical point assignment.
Uniaxial‐Stress‐Induced Splitting of Plasma Edge in Single Crystals of MB6 Compounds
Authors: Iller A.
Published: July 2007

Uniaxial‐stress‐induced shift and deformation of the plasma edge has been observed in polarized light for single crystals of LaB6, PrB6, NdB6, and CeB6. Stress modulation technique was used. It is shown, that the shift and deformation of the plasma edge is due to the change of the parameters which determine the reflectivity in this region, i.e. free electron concentration, effective mass, relaxation time, and lattice dielectric constant. The change of the parameters was estimated and the shift of the plasma edge determined, using experimental results for all investigated compounds.
Crystal Field Determination in Rare‐Earth Oxysulfides III. Specific Heat Measurements on Gd, Tb, Ho, and Dy Oxysulfides
Authors: Rossat‐Mignod J. , Quezel G. , Berton A. , Chaussy J.
Published: July 2007

The specific heat of gadolinium, terbium, dysprosium, and holmium oxysulfides has been measured between 1.4 and 80 K. The lattice contribution has been obtained by the analysis of the Gd2O2S results. For the terbium, dysprosium, and holmium oxysulfides, the specific heat studies allowed to remove the ambiguities of the optical measurements and to precise the crystal field splitting of the rare‐earth ground state multiplet.
Landau Levels in Uniaxially Stressed p‐Type Germanium
Authors: Jones J. C. , Wallace P. R. , Ulmet J. P. , Carrère G. , Askenazy S.
Published: July 2007

An effective mass stress Hamiltonian for the Γ8 ( j = 3/2) valence bands in germanium has been used to calculate the Landau levels in p‐germanium. The magnetic fields and the stress have both been taken in the [100] direction. The stress induced changes in the band structure are related to experimentally observed shifts in the de Haas‐Shubnikov longitudinal magneto‐resistance oscillations.
Evidence from Transport Measurements at High Pressures for Donor Ions Occupying Non‐Equivalent Lattice Positions in InSb
Authors: Porowski S. , Kończykowski M. , Chroboczek J.
Published: July 2007

Measurements of the Hall coefficient in undoped, extrinsic n‐type InSb were made as a function of temperature at hydrostatic pressures of several kbar. The results give evidence of the existence of donor states associated with ions which can occupy two non‐equivalent positions in the lattice.
Theory of the Order Parameter of NaNO2 in Molecular Field Approximation
Authors: Bonilla I. R. , Holz A. , Rutt H. N.
Published: July 2007

A static theory of the order‐disorder, ferro‐paraelectric phase transition in NaNO2 is developed. The Coulomb energy of the system is calculated in the point charge approximation. The model takes the relative displacement δ°(T) of the Na+ ions from their symmetrical position as a function of temperature into account; and the strong correlation of the Na+ ion position with the dipole orientation of the neighboring NO2− ion. The short range force potential acting on the Na+ ion is fitted by means of δ°(0) which can be taken from experiment. The long‐range order parameter Ω is calculated in the molecular field approximation. A first order phase transition is obtained for which the critical values Tc and Ωc are calculated as a function of the permanent electric dipole moment of NO2− and δ°(0). Only qualitative agreement with experimental results is obtained. The Made‐lung constant of the lattice is obtained as α = 1.93.
A Model of the Electrical Transport Phenomena in Imperfect Crystals of Copper Phthalocyanine. III. The Position of the Fermi Level in the Δ‐Model
Authors: Lehmann G. , Hamann C.
Published: July 2007

The position of the Fermi level is investigated in crystals and in thin films of copper phthalocyanine. Particularly the ideal crystal, the crystal with a discrete trap level, the crystal with a continuous trap distribution on the surface, and crystals and thin films with a combination of a discrete trap level in the bulk and with a continuous trap distribution on the surface were investigated. The position of the Fermi level has a great influence on the conductivity, on the existence of p‐type or n‐type conduction, and on the population of traps. Based on the ideas of the Δ‐model [1, 2] for each case considered the position of the Fermi level is given in dependence on the temperature and on the injection of additional charge carriers by the contacts.
Resistance Minimum in Dilute “Magnetic” Alloys in the Absence of Impurity Moments
Authors: Nandiniand R. , Kumar N.
Published: July 2007

A possible mechanism for the resistance minimum in dilute alloys in which the localized impurity states are non‐magnetic is suggested. The fact is considered that what is essential to the Kondo‐like behaviour is the interaction of the conduction electron spin s with the internal dynamical degrees of freedom of the impurity centre. The necessary internal dynamical degrees of freedom are provided by the dynamical Jahn‐Teller effect associated with the degenerate 3d‐orbitals of the transition‐metal impurities interacting with the surrounding (octahedral) complex of the nearest‐neighbour atoms. The fictitious spin I characterizing certain low‐lying vibronic states of the system is shown to couple with the conduction electron spin s via s‐d mixing and spin‐orbit coupling, giving rise to a singular temperature‐dependent exchange‐like interaction. The resistivity so calculated is in fair agreement with the experimental results of Cape and Hake for Ti containing 0.2 at% of Fe.
Toward a Dielectric Theory of Chemical Bonding
Authors: Bashenov V. K. , Alarashi R. A. , Foigel M. G.
Published: July 2007

The ionicities and effective energy gaps have been obtained for 32 A2B6 and A2B5 crystals using a new set of nonlocality factors. The calculation of these factors is performed by inserting the X1–X4 interband transition energies instead of the corresponding effective gaps for diamond‐type crystals. The most significant disagreement between these results and those of Van Vechten is observed at crystals composed of lighter elements belonging to the second row of the Periodic Table.
Optical Studies of Lattice Vibrations in Sphalerite Crystals — A Theoretical Approach
Authors: Talwar D. N. , Agrawal Bal K.
Published: July 2007

An approximate estimation of the zone‐boundary phonon frequencies has been made for the III–V compounds InP, InAs, and GaSb, using reliable theoretical approaches. Employing experimental data for the long wavelength phonons, the elastic constants, and the zone‐boundary phonon frequencies, the lattice dynamics of CuI and the III‐V compounds (crystallizing in sphalerite structure) has been discussed in a second‐neighbour‐ionic (SNI) model. The reliability of the calculated critical‐point phonons (CPP) for the III‐V compounds has been checked by Brout's sum rule and also by comparing them with the neutron or Raman data for various other compounds having similar crystal structure. The observed infrared and/or Raman spectrum peaks for the III‐V compounds have been assigned to calculated CPP at Γ, X, L, and W critical points. The assignments are consistent with Birman's selection rules and support the lattice dynamical results given here.
The Fermi Surface of White Tin from a RAPW Interpolation Compared with Experiment
Authors: Devillers M. A. C. , Matthey M. M. M. P. , De Vroomen A. R.
Published: July 2007

From extensive calculations it is shown, that the relativistic APW method, used as an interpolation scheme, can describe the Fermi surface of white tin consistent in detail with all available experimental data. At the same time this model of the Fermi surface of white tin is used to confirm and to complete the interpretation of radio frequency size effect measurements presented in a preceding paper.
Helicon Waves in Semiconducting Bi1−xSbx Alloys
Authors: Herrmann R. , Oelgart G. , Krüger H. , Haefner H.
Published: July 2007

Investigations are made of helicon wave propagation in semiconducting BiSb alloys. The measurements were carried out at measuring frequencies between 28 and 40 GHz and at a temperature of 4.2 K in magnetic fields between 0 and 15 kG. The obtained charge‐carrier concentrations are in very good agreement with the values obtained from the direct‐current Hall effect. The influence of the displacement current on the dispersion of helicon waves is evidenced. Finally the angle dependence of the field values of the spatial resonances is discussed, which results when the magnetic field is rotated out of the direction of the surface normal.
On a Generalization of Weaire's Tight‐Binding Hamiltonian for Tetrahedrally Coordinated Networks
Authors: Streitwolf H. W.
Published: July 2007

Introducing two interaction operators for interactions at the atom and the bond Weaire's tight‐binding Hamiltonian is extended to take into account all interactions between nearest neighbour bonds. The integrated density of states is derived and the energy spectrum is discussed. It consists of two bands with δ‐peaks at the upper band edges. In a special representation the Green's function is calculated.
The Excitation Spectrum of a Rare‐Earth Alloy
Authors: Tucker J. W.
Published: July 2007

The dispersion relation for the coupled nuclear and electronic spin wave modes that exist in magnetically ordered materials due to the hyperfine interaction is derived for arbitrary concentrations of nuclear spins. The calculation makes use of a diagrammatic technique in which the evaluation of the thermal averages of products of spin operators is fa cilitated by the use of the drone‐fermion representation. Criteria are established for the validity of the proposal by Jones (1973) that the results of molecular field theory hold for arbitrary concentrations.
Magnetic Susceptibility and Electrical Properties of p‐Pb1−xSnxTe
Authors: Lasrkarev G. V. , Migley D. F. , Shevchenko A. D. , Tovstyuk K. D.
Published: July 2007

The results of the measurements of magnetic susceptibility, Hall effect, and resistivity of the p‐type Pb1−xSnxTe solid solutions with x = 0.11 to 0.31 and a hole concentration of (3.1 to 7.9) × 1019 cm−3 are given. The magnetic susceptibility was measured in a temperature range of 78 to 450 °K, the Hall effect and resistivity in a range 4.2 to 450 °K. At T > 80 °K the Hall coefficient essentially increases with temperature, which can be explained by the presence of light and heavy hole bands. The temperature dependence of the magnetic susceptibility shows that the latter is paramagnetic for the holes, and the Pb1−xSnxTe valence band has a complex structure. Near T = 300 °K the temperature dependence of the magnetic susceptibility has peculiarities. It is supposed that the latter are connected with the conversion of the g‐factor to zero when the hole effective mass increases with temperature.
Wave Scattering Processes on Fluctuations in Non‐Equilibrium Semiconductor Plasma with Isotropic Non‐Parabolic Dispersion Law of Carrier Energy
Authors: Tomchuk P. M. , Shenderovskii V. A.
Published: July 2007

An effect is investigated of the energy band non‐parabolicity on the scattering and transformation processes of electromagnetic waves on fluctuations in the non‐equilibrium semiconductor plasma. It is shown that the deviation of the carrier energy dispersion law from the isotropic parabolic one in a high electric field leads to a new mechanism of the light scattering which does not occur in the equilibrium plasma. The cross section of the electromagnetic waves scattering and transformation on fluctuations in the non‐equilibrium semiconductor plasma essentially depends on the energy band shape and magnitude and orientation of the external electric field.
Correlation Functions of Weakly Bonded Impurity Spin in a Ferromagnet at High Temperatures
Authors: Los V. F.
Published: July 2007

Correlation functions have been calculated for the operator of a weakly bonded impurity spin in the Heisenberg ferromagnet in the temperature range whose lower limit is determined by the condition kT ≫ S|ω0| (S is the magnitude of impurity atom spin, ω0 the excitation energy of a weakly bonded impurity spin) with the exception of a close neighbourhood around the point of the phase transition of second order. The calculation was performed by summation of the infinite series of the perturbation theory for correlation functions. No representation of the Holstein‐Primakoff type was used for the spin operators of the impurity atom and the spins of the matrix atoms.
Hot‐Electron Magnetic Susceptibility of Degenerate Semiconductors at Low Temperatures: Effect of Non‐Parabolicity
Authors: Pal B. P.
Published: July 2007

The dependence of hot‐electron magnetic susceptibility (χ) in a non‐parabolic conduction band on external dc electric field (E) has been investigated theoretically. The non‐parabolicity is accounted for by Kane's model for the conduction band and the “effective‐electron‐temperature model” has been used to describe the electron distribution function. Ionized impurity scattering has been assumed to be the dominant momentum relaxation process while energy loss was assumed to be accounted for by scattering due to acoustic phonons via the screened deformation potential, the screened piezoelectric interaction and scattering by polar optical phonons. Individual energy loss rates have been calculated using the corresponding analytical expressions which take account of the non‐parabolic nature of the energy bands both in the density of states and electron wave functions. Numerical results are presented for the typical case of an n‐InSb sample for fields up to ≈ 2 V/cm. The plot of susceptibility (χ) as a function of the dc electric field (E) leads to a kink at a field strength of ≈ 370 mV/cm corresponding to an electron temperature ≈︁ 18.5 °K. This is attributed to the onset of the spontaneous emission of polar optical phonons. Results are compared with those obtained on assuming a parabolic model for the energy bands. Non‐parabolicity, in general, leads to a lower value of the hot‐carrier magnetic susceptibility (absolute magnitude) as compared to its value in the parabolic band model for the same value of the electric field.
Energy Transfer between Heated Carrier Systems in a Semiconductor by Inter‐Carrier Scattering
Authors: Hearn C. J.
Published: July 2007

An analysis is made of the rate of transfer of energy between two heated carrier systems in a semiconductor via the inter‐electronic interaction. The systems can be either two electron systems, in different valleys of a semiconductor, or an electron system and a hole system. The systems are taken to have different mean energies and drift velocities and their momentum distributions are approximated by displaced Maxwellian functions. It is shown that this process can be of importance in certain types of hot carrier calculations.
The Mechanism of Luminescence of Crystals of the Complex Formed by Manganese (II) Chloride with 3‐Ethyl‐4‐Methylpyridine Hydrochloride
Authors: Burić I. , Nikolić K. , Aleksić A.
Published: July 2007

The luminescent properties of crystals of the complex formed by manganese(II)chloride with 3‐ethyl‐4‐methylpyridine hydrochloride were studied at different temperatures by measurement of absorption, excitation, reflectance, and luminescence spectra. From the reflectance and excitation spectra in this compound 4T1g, 4T2g and 4A1g, 4Eg energy levels are found. It was established that there is a tetrahedral coordination of the manganese (II) ion, which brings about a green luminescence. The mechanism of the process of absorption and emission is explained in terms of the theory of the configurational model. The parameters required for plotting a configurational coordinate diagram were computed. Theoretically determined values for the maxima of excitation and luminescence are in good agreement with those determined experimentally. It is established that the phenomenon of luminescence is due to the 4T2g → 6A1 transition.
The Van Hove Diagonality Conditions and the Ionic Configuration Average in the Theory of Liquid Metals
Authors: Schaich W. L.
Published: July 2007

The justification of the ionic configuration average within the limitation of infinite order perturbation theory is considered. The diagonality conditions of Van Hove are shown to result from this average if the momentum matrix elements are appropriately coarse grained. An omission in a previous proof of these conditions is thereby rectified. Finally a comment is made on the generality of the results.
Pseudopotential Spherical Functions in Metals
Authors: Srivastava S. K.
Published: July 2007

A theory of the new defined spherical function known as pseudopotential spherical function s1(yr) has been presented. This spherical function is similar to other well known spherical functions: Bessel spherical function ji(qr) Neuman spherical function n1(qr), and Hankel spherical function h1(qr). The spherical function is used in description of the electron‐ion potential and the effective density of metals. The results of the effective density for Na, Mg, and Al are presented. The obtained values are similar to the values obtained from the atomic orbital method.
Crystal Field Theory Interpretation of the Magnetic Susceptibility of CdYb2Se4 Spinel
Authors: Pokrzywnicki S. , Czopnik A. , Wróbel B. , Pawlak L.
Published: July 2007

The paramagnetic susceptibility of polycrystalline CdYb2Se4 spinel has been measured between 4 and 300 K. The data are interpreted within the crystal field approximation and a correction is made taking into account exchange interactions. The energy levels and crystal field parameters b4 and b6 have been calculated.
Parametric Excitation of Spin Waves in Ferromagnets with Domain Structure
Authors: Filippov B. N. , Lukomskii V. P. , Lebedev Yu. G.
Published: July 2007

An investigation is made of the mechanism of the influence of an rf field on ferromagnets with different domain structures leading to a modulation of the internal effective magnetic field, in which the magnetization vector precesses. The calculations are made for the case when external polarizing magnetic field H0 = 0. It is shown that the mechanism of the parametric excitation investigated is strongly sensitive to the type of the domain structure. It turns out that the parametric excitation of the spin waves in a ferromagnet with a Shirobokov domain structure is impossible. However, in a ferromagnet with a striped domain structure the investigated mechanism leads to the excitation of wall and intra‐domain modes and to the simultaneous excitation of both the modes as well. The excitation thresholds are found of all the modes under consideration and analysed.
Unpinning of Dislocations within the Framework of the Vibrating‐String Model
Authors: Frank W. , Frydman R.
Published: July 2007

Within the framework of the vibrating‐string model the frequency of thermally activated unpinning of dislocations from point obstacles is calculated with the aid of a dynamical theory. It is found that the unpinning frequency Γ has the form of an Arrhenius equation, Γ = Γ0e−ΔG/kT, where ΔG is the portion of the free enthalpy of activation which may be obtained by means of elasticity theory using isothermal elastic constants. The pre‐exponential factor Γ0 is shown to decrease with increasing length of the pinned dislocation segments, contrary to the result obtained on the basis of the absolute rate theory.
Inelastic Neutron Scattering by Cerium Hydrides. II. Lattice Dynamical Approach
Authors: Vorderwisch P. , Hautecler S. , Deckers H.
Published: July 2007

A central force lattice dynamical model, previously applied by Slaggie to the CaF2 structure of ZrH2, is extended to the BiF3 structure. It is used for the analysis of inelastic neutron scattering data on cerium hydrides CeH1.98 and CeH2.72. For both samples the force constants are derived. The values of the corresponding force constants in both hydrides are different. The additional hydrogen in the CeH2 structure is found to be weakly bound.
Optical Absorption and Photoconductivity in Liquid Selenium
Authors: Rabit J. , Perron J. C.
Published: July 2007

The optical absorption and photoconductivity is measured in liquid selenium. Urbach's spectral rule is satisfied and explained with the Dow and Redfield model. The optical gap is close to 1.8 eV at 230 °C and its temperature dependence is 8 × 10−4 eV/K. Optical absorption and quantum efficiency curves are not energy shifted in contrast to amorphous selenium. The low energy photogeneration is due to a “dynamically”‐assisted separation of electron–hole pairs. Two lifetimes are found and the shorter is related to the structural relaxation. The values of photoconduction and conduction gaps are about 1.8 eV in agreement with the optical result but the Fermi level must be shifted towards the valence band.
Longitudinal Photon Drag in p‐Type Tellurium
Authors: Auth J. , Genzow D. , Herrmann K. H. , Wendt M.
Published: July 2007

The longitudinal photon drag has been studied for free holes in tellurium using a Q‐switched CO2 laser. A great effect is expected due to the great interband matrix element for allowed direct hole transitions between the sub‐valence bands H4 and H5. Due to the low crystal symmetry interesting anisotropy phenomena take place. The measurements have been performed for a set of samples, oriented either parallel or perpendicular to the c‐axis. The drag effect connected with sub‐band transitions is dominating as predicted by the theory (tensor coefficient σ1133 = 5 × 10−12 cm/V at T = 77 K). The coefficient σ3311 connected with the absorption of radiation polarized Fω ⊥ c was found to be −1 × 10−12cm/V at 77 K.
A New Exchange Interaction Operator for Magnetic Materials
Authors: Popović‐Božić M.
Published: July 2007

Girardeau's second quantization representation for compound particles is used to derive, starting from first principles, a new approximate Hamiltonian for the exchange interaction of two atoms in their ground states. As distinct from the conventionally used Dirac‐Heisenberg Hamiltonian, the new operator comprises the effects of exchange of one, two, three etc. pairs of electrons. Assuming that the exchange interaction in a many‐atom system can be taken as a sum over the operators for interaction of pairs of atoms, a new approximate Hamiltonian for magnetic crystals is derived.
On the Relativistic Electronic States of a Monatomic and a Diatomic One‐Dimensional Lattice
Authors: Sen Gupta N. D.
Published: July 2007

A new method has been developed to obtain directly wave functions and relations between the energy and momentum of an electron in a linear array of δ‐potentials obeying the Dirac equation, both for a monatomic and a diatomic one‐dimensional lattice.
Type II/1 and type II/2 Superconductors
Authors: de Cesare L. , Mancini F.
Published: July 2007

The problem of a first‐order transition at the lower critical field in type II superconductors is investigated by means of the boson formulation of superconductivity. The critical value ϰc of the Ginzburg‐Landau parameter which separates type II/1 from type II/2 superconductors is theoretically determined and is in good agreement with the experimental value. The distribution of the magnetic field of an isolated flux line and the interaction potential between two flux lines are also computed in the entire region including the core.
A Model for the Electron–Phonon Interaction in Polyvalent Metals: Al
Authors: Hayman B. , Carbotte J. P.
Published: July 2007

Bross and Bohn have suggested a form for the electron–phonon matrix element which avoids the serious difficulties that are associated with the one OPW approximation for the case of a Fermi surface that intersects zone boundaries. Using this suggested form, we have calculated the electron–phonon mass enhancement, the transport scattering times, and the electrical resistivity. A discussion of the merits and limitations of the Bross‐Bohn matrix element is included.
Drift of Electron–Hole Drops in Exciton Density Gradients
Authors: Balslev I. , Hvam J. M.
Published: July 2007

The motion of electron–hole drops is analysed when nucleated in a non‐uniform density of free excitons. The exciton condensation on the drop surface gives rise to a driving force towards regions with lower densities. This force and the internal friction of the exciton gas lead to a drift velocity which is proportional to the drop radius. In typical excitation geometries this motion is more important than drop diffusion, and the flight towards low density regions influences strongly the drop growth. It is pointed out that very large drops can be formed in a local minimum of the exciton density.
Spin–Spin and Spin–Lattice Equilibrium in TCNQ NH Salt
Authors: Alizon J. , Berthet G. , Blanc J. P. , Gallice J. , Robert H.
Published: July 2007

In order to detect an eventual coupling between the protons of the NH4 cations and the paramagnetic electrons an investigation is made of the free induction decay and the spin–lattice relaxation time of protons of the TCNQ · NH4 salt and of the partly deuterated TCNQD4 · NH4, TCNQ · ND4 salts as functions of temperature. Experimental results are interpreted in terms of two Zeeman reservoirs and make evident an interaction between the paramagnetic electrons and NH4 protons.
Phonon Dispersion in SiC and ZnS Polytypes
Authors: Pandey B. P.
Published: July 2007

Phonon dispersion relations in [111] direction for SiC and ZnS polytypes have been computed on the basis of a two‐neighbour short‐range force constant model. Satisfactory agreement is observed between theory and experiment. This agreement shows the important fact that a short range force system is operative in these polytypes, the forces up to second neighbours are the most dominant.
Experimental Studies of Magnetoresistance Tensor Components in Bismuth at Low Temperatures
Authors: Bogod Yu. A. , Krasovitskii V. B.
Published: July 2007

The monotonic and oscillating parts of the magnetoresistance tensor components are studied on a pure single crystals of bismuth. The nondiagonal‐to‐diagonal components ratio is shown to be sufficiently small at helium temperatures, so that σxx = 1/ϱxx with a high degree of accuracy, and thus the experimental deviations from the law ϱxx = αHn (n = 2 for ωcτ ≫ 1) are independent of the electron–hole compensation disturbance in Bi. Within the framework of the ellipsoidal parabolic model taking account of the relaxation time anisotropy, the mobility tensor components at helium temperatures are found whose values coincide with data obtained by other authors in weak (ωcτ > 1) magnetic fields. It has been revealed that for magnetic field directions H near twinning planes the sign of an even constituent of the nondiagonal magnetoresistance tensor components becomes opposite to that for samples with no twinning interlayer. The twinning plane effect is essential up to 70 °K. It is also found that at helium temperatures the even constituent ϱyx(H) changes its sign in high magnetic fields (10 kOe) near H ‖ C3. It is shown that the transition in the H ‖ C3 direction under H rotation in the bisectrix plane causes the oscillation phase of the even constituent ϱyx to change. It is found that in the preultraquantum region of the magnetic fields the oscillating part of ϱyx turns out to be greater at certain H directions than the monotonic one.
Effect of Impurities on the Phase Transition Temperature of a Two‐Dimensional Ising Lattice with Interaction between Some of the Next‐Nearest Neighbours
Authors: Grozdev K. I. , Kosevich A. M.
Published: July 2007

The effect of interstitial impurities on the thermodynamical properties of a plane Union Jack lattice was studied. The impurities are located on the next‐nearest neighbour bonds only and affect the exchange interaction between the corresponding lattice sites. The phase transition temperature as a function of the impurity concentration is obtained exactly. The case in which the impurity cuts the exchange bond between the next‐nearest neighbours is described in detail.
Effect of Crystalline Anisotropy on the Magnetostatic Spin Wave Modes in Ferromagnetic Plates. II. Results for Finite Plates
Authors: Schneider B.
Published: July 2007

The magnetostatic spin wave mode transmission through anisotropic rectangular slabs of YIG has been investigated experimentally and dispersion curves have been derived. The experimental results are interpreted by an approximate application of the theory developed in a former paper. The agreement between theoretical and experimental results is rather good.
Electron Density Redistribution and Configuration Distortion in an (AB)N Chain
Authors: Lubimov V. S. , Ionov S. P. , Makarov E. F.
Published: July 2007

The electron density redistribution and the configuration instability in (AB)N‐chains are considered. It is found that the symmetrical distorted configuration is more stable for chains with Coulomb interaction between the electrons on centres A and B. This configuration is more stable also in the case of delocalized noninteracting electrons, if an odd electron number is on the unit fragment AB. The antisymmetrical distorted configuration energy is the smallest, if there is an even electron number on the AB fragment. Some examples of (AB)N‐chains are considered.
Band Structure and Reflectivity of GaN
Authors: Bloom S. , Harbeke G. , Meier E. , Ortenburger I. B.
Published: July 2007

The E ⊥ c reflectivity has been measured up to 10 eV for hexagonal, single‐crystal GaN. The band structure and reflectivity have been computed by the empirical pseudopotential method and used to identify the observed spectral peaks.
Electronic Polarons in Non‐Metals with Short‐Range and Local‐Field Effects
Authors: Dagens L. , Perrot F.
Published: July 2007

The electronic polaron approximation (EPA) by Toyozawa does not deal correctly with the short range part of the interaction between a quasi‐particle and the polarization field of a non‐metal, owing to the use of a continuum approximation for the polarization field. A modified EPA is derived, without this continuum approximation, from a weak coupling zero order tight‐binding model. It includes short range and local field effects and leads to an electron self‐energy ∑ without the explicit first Brillouin zone cut‐off found when Toyozawa's EPA is used. A simple practical approximation is derived in order to take into account the non‐dipolar contributions which cannot be neglected when large wave vectors are involved. A physical cut‐off, scaled by the atomic or ionic radius is thus incorporated in the theory. The numerical significance of the continuum approximation for the polarization field is tested by comparison, with the elaborate calculations of Kunz. It is found that the short wave length contributions to ∑ may in no way be neglected if one want to improve significantly over the much simpler classical approximation (due to Mott and Littleton), as used by Fowler.
On the Static Approximation for the Hubbard Model. II. Normal Phase at Absolute Zero
Authors: Puff H.
Published: July 2007

In the framework of the single‐site approximation of the statically approximated Hubbard model the normal phase is investigated in the limit T →0. The results completely agree with those of other authors using the functional integral method.
Triplet State Paramagnetic Resonance and Optical Absorption of Z2 Color Centers
Authors: Adlhart W. , Gehrer G. , Lüscher E.
Published: July 2007

A metastable Z2 center triplet state excitable by optical irradiation has been found in KCl:Ca, KCl:Sr, NaCl:Ca, NaCl:Sr. Electron spin resonance measurements yield the following symmetry axes: [100] for centers in KCl:Ca, KCl:Sr, NaCl:Ca; [100], [011], and [011] for NaCl:Sr. Optical measurements of Z2 centers in KCl:Ba show the existence of a [100] symmetry axis. Center models are discussed.
Volume Effect Photoemission: Application to Silver at Low Photon Energies
Authors: Schaich W. L.
Published: July 2007

A perturbative evaluation is developed of the volume effect approximation to the general elastic photoemission current. The formulae are applied to the (111) and (001) crystal faces of silver for photon energies less than 8 eV. Treating bulk and surface umklapp scattering on an equal footing, the emission efficiency of primary and secondary cones is calculated. The results are compared with experiment, and the successes and failures of the theory discussed. Several further experimental tests of the model are suggested.
Optical Absorption Spectra of the Co2+ Ion in CdCl2 Crystals
Authors: Nasu T.
Published: July 2007

Optical absorption spectra of the Co2+ ion in CdCl2 crystals were measured in the energy region from 4000 to 40000 cm−1. The temperature dependence of the spectra was also measured in the temperature range from 80 to 710 K. Observed absorption bands were assigned to forbidden d–d transitions of the Co2+ ion allowed by vibrations and to charge transfer transitions. From the analysis of the structure of the absorption band, the strength of the trigonal crystal field is estimated.
Inelastic Neutron Scattering by Cerium Hydrides. III. Localized Vibrations of Interstitial Hydrogen in the F.C.C. CeH2 Lattice
Authors: Vordekwisch P. , Hautecler S.
Published: July 2007

Explicit formulas have been derived for the frequencies of the localized modes of an impurity at the octahedral interstitial sites of a f.c.c. CaF2‐type structure, describing the dynamics of the host lattice by short‐range central forces and assuming that the impurity interacts with the first and second neighbouring atoms of the host lattice. A threefold degeneracy is predicted. A small peak observed at 74 meV in the neutron data on CeH1.98 is attributed to such localized modes, due to an occupation of a few percent of the octahedral sites by hydrogen atoms. A unique set of the parameter values for the coupling of the interstitial hydrogen to the atoms of the host lattice could not be derived from the position of this peak alone; a set is proposed from a comparison with the parameter values for the host lattice which have been previously determined.
Theory of Electron Scattering by Quasi‐Periodic Static Fluctuations in the Presence of Electric and Magnetic Fields
Authors: Hsu W. Y. , Falicov L. M.
Published: July 2007

An extension of the Zener breakdown theory is presented. It corresponds to the case in which the periodic lattice potential is replaced by a quasi‐periodic static fluctuation of mean wave vector g The scattering probability for a wave packet which satisfies the “Bragg condition” to change its wave vector by g is found to be a constant Q0 much smaller than one for fields up to a very high value E0, and a decay as Q0(E0/E) beyond that value. The classical Zener case is obtained as one limit. Extensions to magnetic field induced effects are discussed.
Calculation of the B‐Band Shape in the In+ Center in Alkali Halides
Authors: Matsushima A. , Fukuda A.
Published: July 2007

The spectral band shapes for all of the absorption bands due to the electronic transition a→ have been calculated for the In+ center in alkali halides using the Franck‐Con‐don approximation and the Monte‐Carlo integration method. The calculated result can explain the characteristic features of the observed B‐ as well as A‐ and C‐ band shape, specifically the fact that the B band, unlike the A and C bands, does not show any well‐resolved structure at high temperatures though it has a shoulder on the low energy side at low temperatures.
Calculation of the Electron Mobility Limited by Two‐Phonon Mode Polar Optical Scattering
Authors: Nag B. K.
Published: July 2007

An iteration method is presented for the accurate calculation of the electron mobility limited by two‐phonon polar optical phonon scattering. Test calculations indicate that the results obtained by applying Mathiessen's rule may be in error by as much as 15% in some cases.
Kinetics and Temperature Dependences of Polarized Emission of Anisotropic Tin Centres in Alkali Halides
Authors: Hizhnyakov V. , Zazubovich S. , Soovik T.
Published: July 2007

Temperature and time dependences of the polarized A emission of tetragonal Sn2+v centres in alkali halides have been investigated under excitation in the A absorption band. The change of polarization during the relaxation time, low‐temperature depolarization due to the tunnel transitions between the minima of the lowest excited states, evidence of the metastable state in emission processes, and high‐temperature depolarization due to the jumps of cation vacancy (v) around Sn2+, have been observed. A model is proposed for the excited states of Sn2+, in which the electron‐lattice interaction is considered to be the main interaction, while the Spin–orbit interaction is taken into account as an additive correction. The formulae have been obtained for temperature and time dependences of polarization and decay time. Some parameters of Sn2+v centres in KCl:Sn have been found from comparison of the formulae with experimental data.
Polymorphic Phase Transition in a Potassium Chloride Crystal
Authors: Chakrabarti S. K.
Published: July 2007

The phase transition data at 298 °K of potassium chloride crystals are analysed on the basis of two alternative models, (a) the Tosi‐Fumi prescription (TFP) in which overlap parameters are taken to be different in the two phases and (b) the three‐body potential model (TBPM) used by Sarkar and Sengupta [1]. The effect of finite temperature and that of zero point energy is included using the Einstein approximation. It is found that the transition data are well reproduced in both the models with TFP giving slightly better values. The temperature coefficient of transition pressure comes out with the right sign but has too high a magnitude in both the models. In elastic data, however, TFP gives poor agreement while for TBPM the agreement is much better.
Numerical calculations for the effect of a Brownian sublattice on polariton dispersion
Authors: Andrade P. Da R. , Borstel G. , Merten L.
Published: July 2007

Numerical evaluations for the effect of a Brownian sublattice on polariton dispersion were made. Using reasonable physical values for the coupling between long optical phonons and polaritons with a dielectric disorder mechanism a set of theoretical illustrations is calculated for the temperature dependence of the transverse optical mode and the polariton dispersion. The effects appearing give a new insight on phase transitions.
The hall effect and the thermopower of two simple heavily doped semiconductor models
Authors: Fischbeck H. J.
Published: July 2007

The single‐site approximation is used in order to study the effect of randomly distributed point scatterers on the most important transport properties of two simple semiconductor models. The one consists of two parabolic and the other of two simple‐cubic tight‐binding bands. Numerical results are given for energy‐dependent (density of states, conductivity, Hall conductivity) and temperature‐dependent quantities (conductivity, Hall mobility, thermopower).
Doubling of the period of flux quantization in hollow superconducting cylinders due to quantum effects in the normal state
Authors: Bogachek E. N. , Gogadzb G. A. , Kulik I. O.
Published: July 2007

The effect of quantization of normal excitations in thin‐walled hollow superconducting cylinders on the critical temperature versus magnetic flux oscillations is considered. It is shown that the period of oscillations is he/e instead of hc/2e, as in the Parks‐Little effect. The phenomenon is connected with the so‐ called Aharonov‐Bohm effect and disappears if the quantum levels are smeared out, particularly dur to volume or surface scattering or finite temperature.
Polaron electric conductivity tensor in a high laser radiation field
Authors: Balmush N. I. , Klyukanov A. A. , Pokatilov E. P.
Published: July 2007

The method to calculate the electric conductivity tensor for polarons is extended to the case of electrons interacting with a strong electromagnetic field. This field affects the scattering probability of electrons and the polaron mass and leads to a heating of polarons. Calculations are presented of the relaxation time and polaron mass tensors for various electron–phonon interactions. The optical‐polaron effective mass is determined at ω ≈ Ω, ω being the LO‐phonon‐angular frequency and Ω that of the field. The derived equations predict a resonant variation of the polaron mass at ω = Ω caused by phonon emission. The photoconductivity of electrons is considered with account of heating. The theory is consistent with experimental data.
Theory of free‐carrier absorption in polar semiconductors due to electron‐LO‐phonon interaction
Authors: Myciblski J. , Aziza A. , Mycielski A. , Balkanski M.
Published: July 2007

Free‐carrier absorption due to polar interaction of electrons with LO‐phonons is calculated for a multi‐valley semiconductor with spherical and parabolic band minima. The effect of free‐carrier screening on both matrix element and phonon frequency is taken into account for crystals with high static dielectric constant. The theory is compared with recent experimental data on n‐PbSe.
EPR investigations on Y2+ in KZnF3
Authors: Buzabé J. Y. , Leble A. , Fayet J. C.
Published: July 2007

EPR measurements have been made to investigate the covalency for the [VF6]4− complex in KZnF3. A precise determination of the superhyperfine parameters in the spin Hamiltonian was obtained by a computed reconstruction of the experimental spectra. The experimental results have been analysed by calculating the mean value of the superhyperfine Hamiltonian on an LCAO model of the molecular orbitals.
Theory of a possible parametric resonance in hydrogen‐bonded ferroelectrics
Authors: Stamenković S. , Žakiula R. B.
Published: July 2007

The pseudo‐spin formalism is used to describe a possible parametric resonance in hydrogen‐bonded ferroelectrics. In the long‐wavelength approximation, when dipolar interactions play a predominant role, the characteristic strength of a dc electric field and the threshold and limiting frequency of an rf electric field, applied under an arbitrary angle with respect to the polarization axis, are estimated. Within the present theory the mechanism of such a ferroelectric parametric resonance is simply interpreted. The results obtained in terms of pseudo‐spin energy parameters make possible to establish experimentally the favorable physical conditions to utilize the ferroelectries in certain amplifier‐microelectronic circuits.
Polarization properties and dispersion relation for cyclotron waves
Authors: Frandsen J. B. , Hasselberg L.‐E. , Lebech J. , Saermark K.
Published: July 2007

On the basis of (a) a spherical Fermi surface and (b) Halse's Fermi surface of silver, calculations are presented showing the importance of the polarization properties of extraordinary cyclotron waves propagating through metallic samples. In particular, it turns out that the longitudinal component of the electric field of the cyclotron wave in general is non‐negligible. An attempt at a physical description of the large damping observed at the “turning points” of the dispersion curve is given and an example of a comparison between experimental results and a calculated dispersion curve is presented.
The self‐consistent electronic structure of trigonal selenium
Authors: Krusius P. , von Boehm J. , Stubb T.
Published: July 2007

The ab initio self‐consistent energy band structure of trigonal selenium based on the Hartree‐Fock‐Slater formalism has been determined with the self‐consistent symmetrized OPW method (SCSOPW). The influence of lattice constants, exchange potential, and relativistic effects is discussed. The density of states and effective mass components have been determined. Valence band triplet charge densities are presented for discussions of bonding properties. X‐ray form factors have also been evaluated. The determined properties are satisfactory except for the optical gap.
Static approximation for the Hubbard model: Paramagnetic phase at finite temperatures
Authors: Mertsching J.
Published: July 2007

The static approximation for the Hubbard model by Puff and Weller [1] is discussed for finite temperatures. The distribution function of the stochastic site occupation numbers, the single‐particle density of states, and the free energy are calculated for temperatures much smaller than the bandwidth and the correlation energy.
Microscopic theory of lattice dynamics in nonconducting crystals. II. One‐electron approximation
Authors: Mitskevich V.
Published: July 2007

In the one‐electron approximation and taking into account the deformation of the electron clouds of ions (atoms) formulas for the cohesion energy and the dynamical matrix of a crystal which ions have closed shells are derived. Under certain admissions the connection among the different constituents of the dynamical matrix is defined and expressions for the effective ionic charge, the polarizability of ions in a crystal, and the short‐range potential of a lattice are determined. The results obtained are written in the S2‐approximation.
Dielectric properties of monoclinic KD2P04
Authors: Blinc R. , Burger M. , Čižikov S. , Levstik A. , Kadaba P. , Shuvalov L. A.
Published: July 2007

Highly deuterated KD2PO4 which is monoclinic at room temperature, undergoes a dielectric phase transition between T = 0 °C (on heating) and Tcll = −25 °C (on cooling). At lower temperatures there is another dielectric anomaly around TcI = −60 °0. The “lower” transition Tci occurs approximately at the same temperature as the tetragonal‐ orthorhombic ferroelectric phase transition, whereas the “higher” transition TCII is similar in character to the tetragonal‐monoclinic “high temperature” transition in “normal” KH2PO4. A tentative model for the phase diagram of KH2PO4 and the tetragonal‐mono‐clinic transition is discussed.
Investigations of the absorption edge of InP in a strong magnetic field
Authors: Zverev L. P. , Emlin K. V. , Rut O. E.
Published: July 2007

The InP absorption edge spectrum is experimentally investigated in a magnetic field of 270 koe at 77 °K. An absorption peak is observed and assigned to transitions within a donor‐acceptor complex.
Theory of phonon relaxation due to multilevel impurities
Authors: Joshi Y. P.
Published: July 2007

A theory is developed for the calculation of the single‐mode relaxation time of phonons in a crystal containing impurities that have a possibly degenerate spectrum of energy levels. Transitions of the impurities from one level to another under the effect of the lattice waves can result in the scattering of the latter. Following earlier calculations corresponding to the case of nondegenerate two‐level impurities, certain operators are defined to describe these transitions, which allow to describe the phonon‐impurity system with a model Hamiltonian that is suited to the diagrammatic technique of the thermodynamic Green's functions. The relaxation time obtained exhibits resonant behaviour with peaks at frequencies equal to the energy gaps between various pairs of energy levels. The relaxation time is also found to be quadratic in the degeneracy factors of the impurity levels and shows a dependence on temperature through the occupation probability of these levels.