bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic States of the Dilute Transiation Metal–Simple Metal Alloy According to the Resonant Model Potential Method
Authors: Dagens L.
Published: July 2007

The transition metal–simple metal alloy problem is formulated in the framework of the resonant model potential theory. It is reduced to a formally equivalent problem involving a tight‐bindinglike Hamiltonian, with non‐hermitian and energy dependent matrix elements. The s–d hybridization potential is exactly described by this non‐hermiticity. The Friedel‐Anderson virtual bound state (VBS) is recovered in the dilute limit. The theory is then applied to the 3d‐transition impurities in aluminium and the results are compared with observed values of the VBS widths.
Drift Diffusion in Grain Boundaries under the Influence of Stress Fileds
Authors: Trinkaus H.
Published: July 2007

The diffusive‐elastic coupling between a misfitting inhomogeneity in a grain boundary and the related stress field is analysed within a continuum approach. The scaling behaviour of the basic equations allows some general conclusions on the time dependence of various processes like the spreading of an inhomogeneity, the diffusional creep and the growth of grain‐boundary voids. The coupled Green's functions for the spreading of an inhomogeneity and the related stress field over an infinite plane grain boundary are derived. They show that in the early stage the width of such an inhomogeneity develops in time as t1/3 (grain‐boundary diffusion), in the late stage as t1/2 (volume diffusion). Possible applications to the quantitative treatment of creep and void growth are indicated.
Effects of Surface Spin Waves and Surface Anisotropy in Magnetic Thin Films at Finite Temperatures
Authors: Levy Diep‐The‐Hung, J. C. S. , Nagai O.
Published: July 2007

The validity of the Green function theory developed by Haubenreisser et al. for ferromagnetic thin films (TF) is investigated. Their method, although to rough for the calculation of magnetization spatial distribution, is found useful for the calculation of critical temperature. By using their method, the effect of surface spin wave modes on the critical temperature is analyzed of both ferro‐ and antiferromagnetic TF. The film thickness dependence and the surface anisotropy dependence of the critical temperature are presented. The condition of the appearance of the so‐called “hard surface” and “pinned spin wave” is discussed. The effect of temperature in ferro‐ and antiferromagnetic resonance is also studied.
The Scattering of γ‐Rays in the Regime of Double Resonance
Authors: Bashkirov Sh. Sh. , Belyanin A. L. , Sadykov E. K.
Published: July 2007

The resonance scattering of γ‐quanta is considered theoretically when a radiofrequency field of large intensity with a frequency equal to hyperfine splitting is imposed on the scatterer. The spectra of scattered radiation and the dependence of the line intensities on rf field amplitude are derived. The accordance between experimental results obtained earlier and the obtained results is outlined.
Electro‐Optic Effects Due to the Stark Splitting of Energy Bands
Authors: Enderlein R. , Renner P. , Scheele M.
Published: July 2007

The effect of the electric field induced interband coupling on the electronic structure and optical properties of cubic materials is investigated theoretically. The three independent coefficients of the quadratic electro‐optic effect are expressed by band structure parameters in the case of the E0, E0 + Δ0 transition of diamond type crystals and of the blue and violet exciton series in Cu2O. In the latter case relations between the band parameters of Cu2O are obtained from a comparison with experimental results.
Excitonic Molecule Transitions in ZnO
Authors: Hvam J. M.
Published: July 2007

By means of excitation spectroscopy, the M‐band emission in ZnO is identified to be mainly due to decay of excitonic molecules (EM) into longitudinal B excitons and upper B polaritons. The latter process is easily stimulated and gives rise to a band (M2) of positive optical gain. Formation of EM by two‐photon resonance absorption and by induced absorption is observed in excitation spectra as well as directly in transmission. From the experiments the EM energy is deduced to EM = 6.740 eV.
Measurement of the Cyclotron Masses in n‐Conducting BiSb under High Hydrostatic Pressure
Authors: Krüger H. , Herrmann R. , Fellmuth B. , Kraak W.
Published: July 2007

For a series of n‐type Bi1−xSbx samples (0.08 < x < 0.14), the pressure dependence of the small cyclotron mass mC1c (B | C1) and of the anisotropy m3c/mC1c (B ⊥ C2) is determined up to the gap‐less state (p = pc). At low pressure values the small mass decreases proportionally to the pressure, thereafter reaching a constant value depending on the charge carrier concentration, which leads to a reduction of the anisotropy of the cyclotron masses. The results, when discussed with regard to galvanomagnetic measurements, present problems in their interpretation by means of the known models.
Magnetic Excitations in Amorphous Ferromagnets
Authors: Continentino M. A. , Rivier N.
Published: July 2007

The propagation of magnetic excitations in amorphous ferromagnets is studied from the point of view of the theory of random frequency modulation. It is shown that the spin waves in the hydrodynamic limit are well described by perturbation theory while the roton‐like magnetic excitations with wave vector about the peak in the structure factor are not. A criterion of validity of perturbation theory is found which is identical to a narrowing condition in magnetic resonance.
Magnetoreflectance of exciton‐polaritons in cubic II–VI compounds. I. Comparison of the magnetic field behaviour of excitons and polaritons
Authors: Uihlein Ch. , Feierabend S.
Published: July 2007

Up to now the magnetic field behaviour of heavy and light excitons of the II‐VI compounds is discussed in the limiting cases of large and small exchange interaction Δexch with respect to the magnitude of the Zeeman terms HZee. In this paper it is shown that the effective g‐value, which is obtained from the rigid shift of the reflectance minimum, agrees with the excitonic g‐value of the limiting case Δexch ≫ HZee even in the opposite limiting case Δexch = 0. This results from the fact that heavy and light excitons are mixed within the relevant polariton states in the same way as in the limiting case of large exchange interaction. The considerable consequences for the determination of gc, χ, and Δexch are not realised in former publications.
The observation of optical transitions from valence band states in CuInTe2
Authors: Thwaites M. J. , Tomlinson R. D. , Hampshire M. J.
Published: July 2007

Some features of the valence band structure of the chalcopyrite semiconductor CuInTe2, are established using an electroreflectance technique. The results are related to the current theoretical model for the valence band of the I–III–VI2 group of compounds.
Raman and infrared spectra in organic layer crystals of squaric acid
Authors: Nakashima S. , Aziza A. , le Postollec M. , Balkanski M.
Published: July 2007

The Raman spectra of squaric acid and its deuterated derivative are measured for temperatures from 77 to 450 K covering both ordered and disordered phases. Infrared absorption spectra are investigated for thin films of the single crystals and compared with the Raman spectra. It is found that the vibrational Davydov splitting due to the interlayer interaction is large for the external modes, while it is small for the internal modes of the squarie molecule. This behavior is explained by the model that the interlayer interaction acts between the hydrogen bonds of adjacent layers and is localized in the region near the hydrogen bonds. The temperature dependence of the frequencies and the linewidths is discussed in relation to the phase transition mechanism.
Treatment of the electron–hole droplet nucleation in the Fokker‐Planck approximation
Authors: Koch S. W. , Haug H.
Published: July 2007

The master equation which governs the nucleation of electron–hole drops in highly excited semiconductors is approximated by a Fokker‐Planck equation which determines the distribution function of the cluster size. From the Fokker‐Planck equation a closed set of equations for the exciton concentration and for different moments of the droplet distribution function is constructed. These equations are solved numerically for various generation rates. Using an eigenvalue analysis of the Fokker‐Planck equation, the time‐development of the full droplet distribution function is found from the time‐dependent moments.
Chemisorption of ordered atomic layers on a model transition metal
Authors: Masuda Kin‐Ichi
Published: July 2007

The effect of chemisorption of ordered atomic layers with p(1 × 1), p(2 × 1), c(2 × 2), p(2 × 2), p(4 × 1), and c(4 × 2) structures on the (001) surface of a tight‐binding (model transition) metal is investigated within the Newns‐Anderson model using the Hartree‐Fock Green's function formalism and the phase shift technique. The self‐consistent adatom charge q, the heat of adsorption ΔE, and the change in the electronic density of states during chemisorption are calculated for the two binding sites (on‐site and centered fourfold‐site). Particular attention is paid to the H/W(001) system and the results are compared with the available experimental results. It is shown that the long‐range order and adsorption geometry of the overlayers are of great importance for the electronic properties of the chemisorbed systems.
Resonance interaction of electronic two‐level states and short‐wave phonon radiation in ruby
Authors: Pauli G. , Renk K. F. , Klimke G. , Kreuzer H. J.
Published: July 2007

The resonance interaction of 874 GHz phonons with the electronic (Ē, 2Ā)‐two‐level‐states in optically pumped ruby is studied. In a resonance volume containing the two‐level states phonon pulses are generated by optical pumping. The occupation number of these phonons is determined and also their escape time from the resonance volume for a large range of concentrations of two‐level states. A simple diffusion model for the phonon propagation in the resonance volume cannot explain the results. It is suggested that the resonant electron–phonon interaction is dominated by a process of spectral and spatial phonon diffusion.
Very‐low‐frequency inelastic light scattering in non‐crystalline films of alkaline‐earth halides
Authors: Harbach F.
Published: July 2007

The Raman scattering of non‐crystalline films of SrCl2, BaCl2, and BaFCl contains a strong “central peak” with a half‐width of about 40 cm−1 at 100 K. It can be explained as a superposition of a Lorentzian line centered at the laser frequency and a Debye‐like density of states. A structural correlation range 2σ ≈︁ 3 Å is deduced from the deviation of the experimental data points from this theoretical model above 36 cm−1.
Multimode jahn‐teller effect for an E‐Term with strong vibronic coupling II. Band shapes of the infrared and raman spectra
Authors: Polinger V. Z. , Bersuker G. I.
Published: July 2007

Infrared and Raman spectra for a Jahn‐Teller (JT) impurity in the E state strongly coupled with crystalline vibrations, as well as vibronic reduction, are investigated. The spectra are shown to represent a superposition of the delta‐like zero‐phonon lines, corresponding to transitions between the rotational levels of the system, with their multiphonon side bands, the latter manifesting the special JT local and resonant states in the crystal density of states, revealed earlier.
Electronic band structure of pseudodirect chalcopyrite semiconductors I. CdSiP2 and ZnSiP2
Authors: Cordts W. , Monecke J. , Heinrich A.
Published: July 2007

The band structure of the chalcopyrite semiconductor CdSiP2 is calculated using the empirical pseudopotential method applied successfully already in the case of ZnSiP2. The band structure is in good agreement with recent experimental data, especially with the inverse crystal field splitting found by Shileika et al. The influence of the three contributions to the deviation of the chalcopyrite structure from a cubic one (antisymmetric cation potential, tetragonal compression and distortion of the anion sublattice) on the band structure and the optical transition energies is investigated in detail for both ZnSiP2 and CdSiP2.
Identification of the absorption bands in KI:Pb2+ crystals
Authors: Tsuboi T.
Published: July 2007

Absorption and magnetic circular dichroism spectra of KI:Pb2+ single crystals are investigated at temperatures from 80 to 375 K, which are compared with the spectra of KBr:Pb2+. The absorption spectra of KI:Pb2+ at 80 K show five bands centred at 348.0, 290, 273.1, 252.3, and 225 nm, which are assigned to the so‐called A, B, C, D′, and D bands, respectively. The D″ and D bands are attributed to the electron transfer from I− to Pb2+ in a PbI4−6 complex ion.
Symmetry properties of the energy bands of Cd3As2 type crystals
Authors: Dowgiałło‐plenkiewicz B. , Plenkiewicz P.
Published: July 2007

Some symmetry properties of the C space group and the resulting details of energy bands of the Cd3As2 type crystals are reported. The single and spinor representations with the T‐R degeneracies included and the compatibility relations are presented. The spinor SCPW for the A‐axis and two symmetry points Γ and Z are constructed. The selection rules for direct and indirect optical transitions are calculated.
Excitation spectroscopy of the I2 bound exciton and the excitonic molecule in CdS II. Relation between I2‐line and EM‐lines
Authors: Puls J. , Rückmann I. , Voigt J.
Published: July 2007

Using a narrow band dye laser the creation of EM's in CdS via TPA is demonstrated in the transmission spectra. Knowing the excited states of the I2 bound exciton from Part I the connection between TPA and EM emission lines and the relation between the excitation spectra of the I2 bound exciton and the EM are investigated. The creation of a cold EM gas under resonant two‐photon excitation is firstly found in CdS and the intrinsic nature of the M‐lines is shown.
Scattering mechanisms of electrons in ZnSe crystals with high mobility
Authors: Emelyanenko O. V. , Ivanova G. N. , Lagunova T. S. , Nedeoglo D. D. , Shmelev G. M. , Simashkevich A. V.
Published: July 2007

In low‐ohmic n‐type ZnSe crystals with the room temperature specific resistance of (3 to 5) Ω cm and electron concentration from 3 × 1015 to 2 × 1016 cm−3 Hall‐effect, electrical conductivity, and charge carrier mobility is investigated in the temperature interval from 10 to 300 K. The basic parameters of the samples are determined. It is found from the experimental data that the mobility is higher than the theoretical value in the investigated temperature range. It is argued that in the region of ionized impurity scattering this descrepancy between theory and experiment is due to the inadequacy of known scattering based on the Born approximation, as for ZnSe this approximation is valid at T > 140 K, where the phonon scattering is predominant. To square the theory with the experiment in the phonon scattering region a polaron effect is taken into account, which manifests itself in renormalization of the carrier effective mass as well as in alteration of the mobility temperature dependence due to the effect of screening of the interaction with polar vibrations. The theoretical consideration of this question is given which allows to bring the theory and the experiment to a satisfactory correspondence.
Metal—Insulator Transition in Boron Carbide
Authors: Werheit H. , de Groot K.
Published: July 2007

The electrical conductivity and the thermoelectric power of boron carbide are investigated in the temperature range from 80 to 700 K on samples of compositions between B12.94C2.06 and B10.59C4.41. While the extrinsic thermal activation of free carriers dominates the electronic transport at high temperatures, at low temperatures a metal—insulator transition occurs depending on the C content. Lower C contents than B12C3 cause variable‐range hopping (In σ ∼ T−1/4) whereas higher contents lead to impurity‐band conduction. The overlap of two impurity bands (possibly Hubbard bands) attributed to the carbon in boron carbide is assumed.
Luminescence des centres associés Bi3+‐Na+ et Bi3+‐Li+ dans le phosphore CaS
Authors: Asano S. , Yamashita N.
Published: July 2007

Emission and excitation spectra of CaS:Bi3+; Na+ and CaS:Bi3+; Li+ phosphors are measured at 6, 80, and 300 K. Seven excitation bands originating from the associated centers Bi3+‐Na+ and Bi3+‐Li+ are observed in addition to those of a single Bi3+ center. The energy matrices of the associated center is calculated theoretically on the basis of the localized electron model. The energy parameters appearing in the energy matrices are determined so that the positions of observed and calculated excitation bands may coincide with each other.
Raman Scattering of Cadmium Chlorate Single Crystal
Authors: Campelo L. C. , Filho J. Mendes , Mello F. E. A. , Vargas H.
Published: July 2007

A low temperature Raman survey of cadmium chlorate single crystal is presented. An assignment is made of all the optical phonons. The internal and external modes are separated out. The observed chlorate ion internal mode frequencies are seen to agree closely with the values obtained for other metal chlorates.
On the Electronic Structure of Transition Metal Carbonitrides, Carboxides, and Oxinitrides. I. Theoretical Approach
Authors: Weinberger P.
Published: July 2007

In Part I of the paper the theoretical approach as used for a study of the electronic structure of TiCxN1−x, TiCxO1−x, and TiNxO1−x (Part II) and for the calculation of the “4f”‐binding energies in HfCxN1−x (Part III) is summarized. In particular Part I consists of a collection of formulas comprising the main aspects of the relativistic KKR‐coherent potential approximation (KKR‐CPA) especially with respect to its real space single site scattering (averaged t‐matrix, ATA) or multiplescattering (Cluster CPA) approximations as well as with respect to the angular momentum dependent local density of states and intensities for X‐ray emission.
Exciton Ground State in CdMnTe in a Magnetic Field
Authors: Cieplak M. Z.
Published: July 2007

The influence of the magnetic ions on the splitting of the ls exciton state in CdMnTe crystals in the presence of a magnetic field up to 4T is calculated using second order perturbation theory and including electron—hole exchange interaction. The results are compared with experimental data from the literature.
The Core Structure of an Edge Dislocation in NaCl
Authors: Puls M. P. , So C. B.
Published: July 2007

A theoretical study is carried out to determine the core structure of (a/2) < 110 > {110} edge dislocations in NaCl and to investigate whether the dislocations dissociate. The calculations show that they do not. This result is opposite to the predictions of Fontaine and Haasen and the conclusions of Belzner and Granzer. The results are generated with the same point‐ion model (PIM) potential employed by Belzner and Granzer, plus a breathing shell model (BSM) potential due to Corish, Parker, and Jacobs. Core properties such as the stable symmetry type, the strain energy, and the Peierls stress are determined. Both potentials produce an approximately sinusoidally shaped Peierls barrier with a periodicity of one Burgers vector. The PIM and BSM potentials yield Peierls stresses of 80 and 17.3 MPa, respectively. Comparison of these results with low‐temperature flow stress data suggests that the BSM result is consistent with the experimental data whereas the PIM result is not.
Theoretical Investigation of the Current–Voltage Characteristics of Superconducting Niobium–Lead Tunnel Junctions
Authors: Seidel P. , Richter J.
Published: July 2007

A proximity‐effect model for the niobium electrode is presented assuming on the barrier side the existence of a thin surface layer which differs from the other part of the niobium electrode. The resulting dependence of the quasiparticle current–voltage characteristic on temperature and electrode thickness shows good agreement with experimental results.
Analytical Description of the Resonance Dechanneling Effect in the Case of Axial‐To‐Planar Channeling Transition
Authors: Lenkeit K. , Wedell R.
Published: July 2007

The theory of resonance dechanneling under axial‐to‐planar transition conditions is further developed. For describing the structure of the atomic planes an analytical potential expression is used in the Thomas‐Fermi‐Molière approximation on the basis of Lindhards string of strings model. As a result of the investigation of the nonlinear equation of ion motion along the planar channel an expression is derived to calculate the tilt angle αmin required for planar channeling to be effective. This angle increases with increasing transversal coordinate in the planar channel. In this way the angular interval αmin < α < αmax is obtained which separates the axial channeling from the “pure” planar one. In a linear approximation of increasing transversal oscillation amplitude the initial amplitude is calculated, for particles which take part in the resonance of given order. The theoretical results are in sufficient agreement with numerical calculations and experimental values.
The Ground State of the Anderson Model. Variational Approach
Authors: Oleé A. M. , Chao K. A.
Published: July 2007

The ground state of the Anderson model is examined by means of the variational trial function which emphasizes interaction between the impurity electron and the electrons on the Fermi surface. Both, the number of electrons and the phase shift of the conduction electrons at the Fermi level are found to be continuous functions of the model parameters. The results reduce to the exact solutions at both the atomic and the strongly interacting system limits. They are compared with the Anderson solution in the Hartree‐Fock approximation, and the meaning of the local moment in both approaches is discussed. The local moment is shown to cause strong decrease of the impurity resistivity under increasing magnetic field at low temperature.
Renormalization Group and Critical Exponents for Classical Heisenberg Ferromagnet with Dipole–Dipole Interaction
Authors: Fakioǧlu S.
Published: July 2007

The renormalization group technique is applied to the three‐dimensional classical Heisenberg spin system with modified dipole–dipole interaction. For this system, the values and dependence of several critical exponents are investigated. These exponents are: α, the specific heat exponent; β and δ, the magnetization exponents; γ, the susceptibility exponent; η, the correlation function exponent; and v, the correlation length exponent. All the critical exponents except β depend upon the potential range σ. When the potential range σ = 3/2, the modified dipole–dipole results agree with mean‐field values of the critical exponents.
Mechanisms of Current Carrier Scattering in ZnxHg1–xSe
Authors: Gavaleshko N. P. , Gorley P. N. , Khomyak V. V. , Shenderovskii V. A.
Published: July 2007

The molar (vs. x), temperature and concentration dependences of mobility U = σR in ZnxHg1–xSe solid solutions with degenerate carrier concentrations are studied over wide ranges of temperature (77 to 300 K), concentration (1 × 1017 to 6 × 1018cm−3), and composition (0 ≦ x ≦ 0.15). The dominating scattering mechanisms and their peculiarities in the system of solid solutions studied, are established by comparison with U(n, x, T) dependence calculated by taking into account carrier scattering by acoustic and longitudinal optical phonons, impurity ions, and piezoelectric potential. A theoretical approximation is developed in the form of a formula describing the experimental results on band gap variation with molar composition and temperature.
Hyperfine Fields at Nonmagnetic Atoms on Various Sites in Ferromagnetic Heusler Alloys
Authors: Grover A. K. , Pillay R. G. , Nagarajan V. , Tandon P. N.
Published: July 2007

The hyperfine fields at Fe and Sn, as determined by Mössbauer effect and NMR, in a new Heusler alloy Ru2FeSn, are reported. By comparison with corresponding data in rhodium based Heusler alloys, it is inferred that a preferential R–Sn (R = Ru or Rh) disorder exists in these alloys. The hyperfine field data on Rh atoms occupying different sites in rhodium Heusler alloys helps to narrow the range of the parameter incorporated to account for the preasymptotic behaviour in the Blandin‐Campbell model of the conduction electron polarization. The calculations indicate for the elements of the s–p series occupying D site in traditional Heusler alloys that, if the change in sign of the hyperfine field occurs at the s2–p2 element (the hyperfine fields at s2–p1 and s2–p3 elements on D site being always observed to be negative and positive, respectively), then for s–p elements on (A, C) site, the change in sign will occur at the s2–p1 element and on B site, it will occur at the s2–p2 element but with the phase reversed i.e., the field at s2–p1 and s2–p3 is positive and negative, respectively. These predictions find strong support from recent experimental data in Heusler alloys.
Lattice Specific Heat of α‐Phase Alloys Based on Copper and Silver
Authors: Tiwari M. D.
Published: July 2007

The low temperature lattice specific heat is studied in detail in the α‐phase f.c.c. solid solutions of the following systems based on Cu or Ag: CuZn, CuGa, CuGe, CuNi, AgGa, AgGe, and AgIn. Except in cases of CuZn and AgIn where a reduction in lattice specific heat is obtained, other systems give an enhancement due to introduction of impurities. A defect model which includes the change in mass at the impurity site and the changes in the impurity–host crystal interactions around the impurity is considered. The effects of lattice expansion or contraction due to the substitution of impurities are also taken into account. For the systems having more than 5 at% of impurities the low‐concentration theory is modified. The calculated results compare well with the experimental data. It is observed in both systems (copper or silver based) that the changed lattice specific heat is quite sensitive to force constant changes. The contribution due to F1u‐symmetry modes dominates over all irreducible representations.
Phase Diagrams of the SiGe System
Authors: Soma T.
Published: July 2007

The phase diagrams of the Si–Ge system are investigated using the previous electronic theory based on pseudopotentials. The obtained curves near the melting point show reasonable agreement with experimental data. The calculated results predict the presence of a mixture of two phases at low temperatures.
Exciton States in 1,2,4,5‐Tetrachlorobenzene Crystals and Dimers
Authors: Davidovich M. A.
Published: July 2007

A calculation of the exciton interactions of the lowest triplet and singlet states in 1,2,4,5‐tetrachlorobenzene crystals and dimers is described. The results, including the effect of vibrations on the electronic interactions, are: (a) The splittings of the triplet and singlet dimer states for the equivalent dimer, located along the shortest crystal axis a, are, respectively, 4 and 16 cm−1; for the inequivalent dimer, located at the inequivalent sites in the unit cell, they are 0.01 and −0.4 cm−1, respectively. (b) The triplet band structure is essentially one‐dimensional. The triplet bandwidth and Davydov splitting are, respectively, 8 and −0.04 cm−1. (c) The crystal singlet Davydov splittings range from 2 cm−1, for k ‖ a−1, to 11 cm–1, for k ‖ c−1. The values obtained for the triplet interactions are one order of magnitude larger than the experimental ones. Possible reasons for this discrepancy are discussed. No experimental values on the singlet interactions are available for comparison.
On Magnon Damping in Antiferromagnets in Zero External Magnetic Fields
Authors: Breus S. A. , Sobolev V. L.
Published: July 2007

It is shown that the dipole–dipole part of the antiferromagnet Hamiltonian makes a small contribution both to the amplitudes of magnon–magnon interactions and to the coefficients of magnon damping. Possible relaxation mechanisms explaining the temperature‐independent part of damping, which is observed in the experiment, are proposed.
Temperature Dependence of the EPR Spectra of Cr3+ and Cr5+ Ions in CaV2O6 and BaV2O6 Crystals
Authors: Grunin V. S. , Patrina I. B. , Zonn Z. N.
Published: July 2007

An investigation is made of EPR spectra of CaV2O6 and BaV2O6 single crystals doped with chromium. It is found that the point symmetry of the paramagnetic Cr3+ and Cr5+ ions depends on temperature. It follows from the symmetry of the Cr5+ spectrum in BaV2O6 and its superhyperfine structure at T = 77 K that the Cr5+ ion is displaced to one of two neighbouring V atoms. The observed change of the impurity ion symmetry cannot be explained both by the off‐centre ion theory and the Jahn‐Teller effect.
Third‐Order Elastic Constants and the Pressure Derivatives of the Second‐Order Elastic Constants of Rare Gas Solids
Authors: Lehri S. , Verma M. P.
Published: July 2007

Expressions für the third‐order elastic constants (TOEC) and the first‐order pressure derivatives of the second‐order elastic constants (SOEC) of rare gas solids are derived from a many‐body potential proposed in an earlier paper. The model parameters are evaluated from the three SOEC, the equilibrium condition, and (1) the experimental values of the pressure derivatives of the bulk modulus (dKT/dP) or (2) by approximating the three‐body potential to an exponential function. The values of the TOEC and the pressure derivatives of the SOEC evaluated by the two methods are tabulated.
On the Electronic g‐Faetor in n‐Type Silicon Inversion Layers
Authors: Englert Th. , von Klitzing K. , Nicholas R. J. , Landwehr G. , Dorda G. , Pepper M.
Published: July 2007

The tilted field method is used to measure the coincidence of spin‐split Landau levels of different Landau quantum numbers in (100) n‐type silicon inversion layers. At constant magnetic field perpendicular to the surface the peak values of Shubnikov‐de Haas oscillations are analyzed as a function of the tilt angle ϕ, where ϕ is the angle between the surface normal and the magnetic field. Pronounced maxima of s̀xx for Corbino devices and of s̀xx for Hall devices are found at the coincidence condition. The electronic g‐factor determined from the coincidence is substantially larger than the bulk value contrary to a recent publication, where experimental evidence for g = 2 was claimed. It is also shown that the Hall component does not falsify the results when the angular dependence of the resistivity maxima is investigated at constant total magnetic fields for a fixed Landau quantum number.
Huang Diffuse Scattering from Dislocation Loops
Authors: Larson B. C. , Schmatz W.
Published: July 2007

Numerical diffuse scattering calculations are carried out for interstitial dislocation loops on {111} planes in aluminum using numerical displacement fields calculated for an elastically isotropic medium. Calculations are performed along selected directions near the (400), (200), (220), and (222) reciprocal lattice points. The results are found to be in qualitative agreement with the predictions of analytical diffuse scattering models; however, for quantitative results, detailed numerical calculations are found to be necessary.
Use of the Large Unit Cell Approach for Generating Special Points of the Brillouin Zone
Authors: Evarestov R. A. , Smirnov V. P.
Published: July 2007

A method for generating of special points in the Brillouin zone is suggested. This method is based on the “large unit cell” model and seems to be more general compared with those previously proposed. As an example, some sets of special points generated by this method for cubic crystals of class Oh with face‐centred and body‐centred Bravais lattices are given.
Relativistic Electron‐Phonon Interaction. Niobium and Tantalum
Authors: John W. , Nemoshkalenko V. V. , Antonov V. N. , Antonov Vl. N.
Published: July 2007

The theory of Gaspari and Gyorffy of the electron–phonon interaction is generalized to the relativistic case. Starting from the Dirac equation an expression for the Hopfield parameter η is derived in the rigid muffin‐tin approximation. As an example the Hopfield parameter is calculated for Nb and Ta by the relativistic APW method. The numerical results show that relativistic effects are important in Ta. However the direct influence of spin‐orbit interaction on the electron– phonon interaction is neglegible both, in Nb and Ta.
A Model Study of the Kohn Anomaly in the Optical Phonon Branch of a Binary Compound
Authors: Fong C. Y. , Weber W.
Published: July 2007

Labbé's tight‐binding method is generalized and applied to a linear chain with two atoms per unit cell to study the Kohn anomaly in the optical phonon branch. The effects of various tight‐binding matrix elements on the anomaly are explored. The enhancement of anomaly can be effectively obtained by increasing the electron–phonon interactions.
Role of Three‐Body Interactions in Cohesive and Thermophysical Properties of Alkaline Earth Oxides
Authors: Singh R. K. , Sanyal S. P.
Published: July 2007

The cohesive and thermophysical properties of alkaline earth oxides are thoroughly investigated by extending Hafemeister and Flygare (HP) and Hafemeister and Zarht (HZ) potentials to include the three‐body interaction effects. The extended HZ potential thus obtained is found to yield a better description of the above properties than those revealed by the extended HF potential presented and many other potentials used by earlier workers.
Dynamic Spin Hamiltonian Formalism and Mössbauer Study of Spin—Lattice Relaxation of Paramagnetic Fe3+ (6S) Ions
Authors: Srivastava K. K. P. , Mishra S. N.
Published: July 2007

In a crystal field potential the 6S state of a paramagnetic 57Fe3+ (3d5, 6S) ion splits into three Kramers spin doublets Sz = ± 5/2, ± 3/2, and ± ½ and each doublet produces its own Mössbauer magnetic hyperfine spectrum at low temperatures as a result of the slow spin relaxation rate. These Mössbauer hyperfine spectra are used to study the temperature dependence of Fe3+ spin lattice relaxation rate falling in the region 106 to 109 s−1. The dynamic spin Hamiltonian formalism is utilised to explicitly calculate the spin—lattice relaxation rate of the three Kramers doublets for one‐phonon and two‐phonon processes in different temperature regions. It is further assumed that the various modes of acoustic phonon propagation are direction independent, the phonon spectrum is of Debye type, and the dynamic crystal field parameters are very similar to the static ones. It is found that for Fe3+ ions the one‐phonon relaxation process is most dominant and the theoretical results are in a very good agreement with the experimental observations. These calculations may be equally applicable to other S‐state paramagnetic ions.
Poisson Approximation to a Functional Approach for the Hubbard Model
Authors: Behn U.
Published: July 2007

In a previous paper the Green function for the Hubbard model is represented by a functional average of the Green function for an electron moving in a local time dependent potential Uvi(t) taking only the values 0 and U. Here this functional average is calculated with an ad hoc assumed Poisson distribution governing the jumps of Uvi(t). In a single‐site approximation the Poisson average can be calculated exactly and leads to a self‐consistent ‚dynamic’︁ scheme interpolating between band and atomic limit. For a half‐filled band in the paramagnetic case at zero temperature for strong Coulomb repulsion U ≈ 2 two subbands are obtained separated by a minimum of the density of states.
Raman Spectroscopic Investigation of Ferroelectric Phase Transitions in Rochelle Salt
Authors: Winterfeldt V.
Published: July 2007

Raman scattering spectra of single crystals of Rochelle salt are recorded at 2 K and at several temperatures between 248 and 300 K in the spectral region from 0 to 4000 cm−1. The frequencies of some phonon modes soften or harden in the intermediate ferroelectric phase between the two Curie points which are known to exist in this material. In the paraelectric phases the frequencies remain constant. This behaviour can be explained by applying a simple nonlinear pseudospinphonon coupling theory originally developed for the TGS family and the incommensurate phase transition in thiourea. The anomalous specific heat due to temperature‐dependent optical phonon modes is also discussed.
Creation and Destruction of X Centres in Alkali Halides by X‐Radiation
Authors: Akilbekov A. , Nurakhmetov T. , Elango A.
Published: July 2007

The production (η ) and destruction efficiencies of X−3 centres in NaCl, KCl, KBr, and KI by X‐radiation versus the irradiation temperature (in the range of 80 to 320 K) are investigated by optical absorption and thermoluminescence methods. Both, the increase (80 to 150 K) and decrease (200 to 250 K) of η in KBr and KI at the adsorbed dose of 1019 to 1021 eV cm−3 are shown to be in correlation with the F‐centre creation efficiency (ηF). In chlorides ηF does not decrease with ηX−3 and these crystals are characterized by a high defect creation efficiency at approximately room temperatures. It is supposed that the creation of large halogen aggregates is much more efficient in chlorides than in bromides and iodides.
Four‐Photon Transitions in Semiconductors
Authors: Hassan A. R. , Raouf R.
Published: July 2007

Four‐photon interband electronic transitions are theoretically investigated in semiconductors. Expressions for the absorption of four‐photons are calculated using fourth‐order perturbation theory for four different band models. The results show that the five‐band model gives the dominant contribution to the absorption coefficient. A numerical application for the case of ZnS is in agreement with available experimental results.
The Thermopower and the Thermomagnetic Power in Arsenic—Antimony Alloys at Low Temperature
Authors: Gitsu D. V. , Golban I. M. , Makeichik A. I. , Muntyanu F. M. , Onu M. I.
Published: July 2007

The thermopower and the thermomagnetic power at low temperatures (4.5 to 77 K) is investigated in AsxSb1‐x (x ⩽ 0.3) alloys. Peculiarities of the thermopower are observed, which are associated with the dragging of electrons by phonons. In magnetic fields B > 5 T the Shubnikovde Haas type quantum oscillation of the thermomagnetic power is displaid. It is established, that the AsxSb1‐x (x ⩽ 0.2) alloys are semimetals, and the overlapping of the energy extrema of L electrons and H holes increases, when arsenic is added to alloys.
Influence of Relaxation Processes and Exciton Dispersion on Optical Characteristics of Crystals
Authors: Serikov A. A.
Published: July 2007

The exciton light absorption by a crystal is studied using the microtheory of electromagnetic and exciton field interaction. A comparison of the results obtained with the corresponding conclusions of earlier theoretical papers and experimental data is made.
Coherent Potential Approximation of the Relationship between Short‐Range Order and the Position of the Fermi Level on the State Density Curves
Authors: Batirev I. G. , Katsnelson A. A. , Kertész L. , Szász A.
Published: July 2007

The density of electron states of binary alloys of transitional elements with short‐range order is found using the coherent potential approximation. On the base of these calculated values the contribution of the band structure energy to the configurational part of the total energy is determined and the equilibrium value of the short‐range order parameter on the first coordination sphere is evaluated. The order parameters obtained for most the considered systems agree in sign with the experimental data.
EPR on Pb‐Doped ZnS Single Crystals
Authors: Dziesiaty J. , Backs D.
Published: July 2007

EPR measurements are made on blue luminescent, Pb‐doped ZnS single crystals after excitation with light of 365 nm wavelength. The results are discussed in terms of a model similar to the ZnS A‐centre model assuming a ZnS vacancy associated with a neutral Pb2+ impurity on next cation site.
Electronic Structure of Fe3Si‐Type Alloys II. Correlation between Ordering Type, Band Structure, and Magnetic Moments
Authors: Blau W. , Himsel A. , Kleinstück K.
Published: July 2007

On the basis of the band structure described in Part I an interpolation scheme is discussed which allows to make a transformation of the band structures of the A2‐, B2‐, and DO3‐type lattices into another ones. For disordered alloys the CPA theory in split‐band approximation reveals a d‐band narrowing without changing the shape. For partially ordered alloys a combined model is proposed which gives a dependence of the magnetic moments on the proportion of pair probabilities in first and second neighbourhoods, in contradiction to former assumptions, which presume a dependence on the first sphere alone.
ENDOR Investigation of Atomic Hydrogen at Anion Sites in KCl
Authors: Heder G. , Niklas J. R. , Spaeth J. M.
Published: July 2007

By means of ESR and ENDOR the g‐values, the hyperfine (hf), and superhyperfine (shf) interactions of four shells of neighbours of atomic hydrogen and deuterium at anion sites in KCl are determined. The shf interactions are strongly temperature‐dependent and show large isotope effects. Both observations are due to vibrations of hydrogen. From the temperature dependence of the ENDOR data the frequency of the hydrogen vibrations could be determined.
Quantum Correction to the Classical Plasma Frequency in the Long‐Wavelength Limit
Authors: Pal R. K. , Tripathy D. N. , Mandal S. S.
Published: July 2007

A negative frequency shift is observed to the classical plasma frequency ωpl in metals in the long‐wavelength limit (k = 0) by analysing the zeroes of the dielectric function of a degenerate many‐electron system evaluated beyond the random phase approximation (RPA). This is achieved by accounting for the non‐RPA contributions arising from the virtual plasmon effects of the effective interaction lines associated with the local field correction diagrams. Redefining the critical wave vector kc the experimentally observed frequency shift to ωpl is reproduced at k = 0 in Li, Al, and Na metals. The theory clearly indicates an extension of the plasmon modes to a momentum transfer region beyond kRPAc.
Analytical Theory of Crystal Growth of Two‐Component Systems II. Nonequilibrium Properties
Authors: Pfeiffer H. , Haubenreisser W.
Published: July 2007

The rate equations for two‐component crystal growth developed in part I of this paper are formulated on the basis of nonreactive interfacial elementary rates for attachment, detachment, and diffusional processes of species. For the attachment and detachment processes a symmetrical dynamics of interchange of species is assumed. An evaluation of the rate equations at small deviation from thermodynamic equilibrium is performed in continuous Bragg‐Williams approximation (CBWA) for a steady‐state growth. The in CBWA derived steady‐state nonequilibrium properties (interface profile, concentration profiles of the components, growth rate, kinetic, and distribution coefficient) are described analytically and discussed for ideal binary mixtures with atomically rough interfaces and normal crystal growth mechanism.
Study of the “Blocking Effect” for Fast Electrons in a Silicon Crystal
Authors: Popov D. E. , Plotnikov S. V. , Kaplin V. V. , Vorobiev S. A.
Published: July 2007

The effect of intensity inversion in the patterns of angular distribution of the electrons with energies up to 2.5 MeV scattered along the low‐index directions of a thin silicon crystal is studied. This phenomenon is observed when the angle of beam incidence relative to a low‐index axis of the crystal is large compared with the critical angle of channeling. The discussion is given in the scope of a model based upon bound states of channeled electrons, as a result of excitation of secondary bound states of electrons with atomic rows and planes.
The “Acoustic Local” Mode of H, D, and F in Amorphous Germanium and Silicon
Authors: Shen S. C. , Fang C. J. , Cardona M.
Published: July 2007

The quasi‐local mode of H in a‐Ge and a‐Si associated with the TA‐phonon branch is studied as a function of temperature of measurement and annealing. Isotopic substitution experiments are also performed. The results confirm the mass defect model recently proposed for this local mode. A similar local mode is observed for a‐Si:F, without hydrogen. It is much stronger than the corresponding mode of a‐Si:H for the same dopant concentration. Likewise in a‐Si:F the TO phonon band, as observed in ir absorption, is stronger than in pure and hydrogenated a‐Si and appears shifted to 510 cm−1. This shift, which is not seen in the Raman spectrum, is attributed to the strong polarity of the Si‐F bond.
EPR Spectra of Cr5+ Ions in KCl Crystals
Authors: Alybakov A. A. , Toichiev N.
Published: July 2007

EPR, optical, and IR absorption spectra of X‐irradiated KCl crystals doped with K2CrO4 and K2Cr2O7 are studied. It is ascertained that the impurity enters as chromate in both cases and there are two centre types — isolated CrO ions and CrO ions coupled with divalent cation impurities. The X‐irradiation results in the formation of paramagnetic CrO3−4 centres with monoclinic local symmetry Cs whose excessive negative charge is compensated by an anion vacancy. The EPR spectrum due to these centres is described by a spin‐Hamiltonian for S = 1/2 with gx = 1.948, gy = 1.953, gz = 1.967. The x‐axis of the g‐tensor is oriented along 〈001〉 and two other principal axes along the directions of 〈110〉 type.
Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets III. Non‐Local Landau‐Ginzburg Theory
Authors: Herzer G. , Fähnle M. , Egami T. , Kronmüller H.
Published: July 2007

In amorphous ferromagnets spatially fluctuating exchange interactions give rise to a critical behaviour substantially different from crystalline magnets. Within the framework of a molecular field theoretical approach, a non‐local Landau‐Ginzburg equation is formulated for phase transitions in amorphous ferromagnets taking into account cooperative spin fluctuations. This is done by introducing a correlated molecular field, which can only vary on larger scale than the spin correlation length. Therefore, the manner how the structural disorder affects the critical behaviour changes with temperature. To some extent, the theory interpolates between the so‐called homogeneous and inhomogeneous versions of the molecular field theory so far used in the literature. The theory is capable of reproducing satisfactorily the experimentally observed curvatures of the Arrott plots and the temperature dependence of the paramagnetic susceptibility in disordered ferromagnets.
Perturbation‐Theoretical Model Calculation of the Lattice Mechanics of Ionic Crystals in the Point‐Dipole Approximation
Authors: Basu A. N. , Sengupta S.
Published: July 2007

A perturbation‐theoretic model calculation is developed in the point‐dipole approximation for the study of ionic crystals from a knowledge of the Hartree‐Fock wave functions of the free ions that constitute the solid. The salient features of the present calculation are that it a) provides a method for calculating the different lattice static and dynamic properties of a crystal directly from the free ion wave functions without using any crystal property; b) clearly gives an empirical justification for the major assumption of the phenomenological shell model; c) develops a new method for evaluating the Coulomb overlap interaction; d) suggests a modified short‐range polarisation mechanism; and lastly e) shows how far actually the free ion wave functions are realistic as a starting point. The method is applied to calculate a number of lattice mechanical properties of the two ionic crystals namely, NaCl and KCl. Despite the simplicity of the approach the agreement, with observation for both the crystals is quite impressive. The source of the remaining discrepancy is discussed.