bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman Spectra and Superconductivity of Various Phases of a High‐Tc Superconductor: NbN
Authors: Kaiser R. , Spengler W. , Schicktanz S. , Politis C.
Published: July 2007

The first‐order Raman spectra of β‐, γ‐, and ϵ‐NbN and the defect‐induced first‐order Raman scattering of cubic NbNx are determined. In nearly stoichiometric δ‐NbN the centre of the Raman spectrum after division by the Bose factor is at the same frequency of 150 cm−1 as the centre of the phonon density from neutron scattering. With increasing deviation from stoichiometry the lowest Raman peak in δ‐NbNx due to acoustic phonons shifts to higher frequencies and Tc decreases sharply. This behaviour indicates the existence of phonon anomalies.
Peierls Distortion and Antiferromagnetism in the Quasi‐One‐Dimensional Peierls‐Hubbard Model
Authors: Mertsching J.
Published: July 2007

The one‐dimensional Peierls‐Hubbard model with a half‐filled band is studied in the Hartree‐Fock approximation. Peierls distortion and antiferromagnetism cannot coexist in this approximation, except in the case of special coupling parameters, when the pure Peierls state and the pure antiferromagnetic state are degenerate.
The Effective Magnetic Interactions between Impurity and Conduction Electrons for the Wolff Model
Authors: Spałek J. , Oleś A. M. , Chao K. A.
Published: July 2007

A new canonical transformation of a Schrieffer‐Wolff (S‐W) type is applied to the Wolff model at the strong correlation limit to derive an antiferromagnetic (kinetic) exchange interaction between the electrons on the impurity site and the host sites. The problem of the singularity in the original S‐W transformation is removed. The condition for the formation of localized moments is obtained by solving the Green's function with a moment conserving decoupling scheme. If localized moments exist, the high temperature susceptibility, derived in the mean field theory, has a Curie‐Weiss form χ = C/(T + θ) with θ > 0, in agreement with experiments.
A Contribution to the Phenomenological Theory of the Anomalies of the Elastic Constants near a Structural Phase Transition
Authors: Wiszt E.
Published: July 2007

Relations for the temperature and frequency dependence of the linear elastic constants for crystals of arbitrary symmetry near a second‐order structural phase transition are derived on the basis of a thermodynamic potential including time and space (one‐, two‐, or three‐dimensional) fluctuations of the order parameter. The calculation is based on a perturbation expansion with respect to the small deformation tensor components and is given in detail for a two‐component order parameter. The results are used for the explanation of the elastic constant anomalies of GMO and Hg2Cl2. The influence of the higher‐order interaction terms above or below the phase transition is investigated with an application to the case of Hg2Cl2.
On Coherent Neutron Diffraction in Nematic Liquid Crystals
Authors: Cvikl B.
Published: July 2007

The differential cross‐section of the coherent neutron diffraction in the nematic phase is derived. It is shown that spontaneous distortions, which enter the theory through the intermolecular part of the structure factor and may be associated with two different diffusive optic soft modes of this phase, influence the neutron diffraction patterns through the correlations in space of the preferred molecular axes. The structure factor is anisotropic and exhibits for 1 Å−1 < Q < 4 Å−1 two distinct peaks depending on the relative orientation of the neutron scattering vector and the uniaxial nematic direction in quantitative agreement with the observations. For 0.4 Å−1 < Q < 1 Å−1 an intense peak is obtained in the calculations.
Electronic Structure and ESR of Ti2+ in Fluorite Type Crystals
Authors: Yunusov N. B. , Zentsov V. P.
Published: July 2007

Expressions are given for the spin‐Hamiltonian parameters for an arbitrary transition metal ion in a cubic lattice, according to the multi‐electron ligand field theory, taking into account all possible single‐electron contributions. These expressions together with the results of the electronic structure calculation of the [TiF8]6− complex are used for the calculation of the magnetic parameters of the 47Ti2+ ion in fluorite type crystals MF2 (M = Cd, Ca, Sr, Ba). The calculated values are in satisfactory agreement with the experimental data.
Transverse and Longitudinal Nernst‐Ettingshausen Effect in p‐Te
Authors: Gavaleshko N. P. , Gorley P. N. , Kavyuk P. V. , Solonchuk L. S. , Tarasyuk I. I. , Shenderovskii V. A.
Published: July 2007

Using a variational method, general formulae for the kinetic tensor components describing the galvanomagnetic phenomena in anisotropic semiconductors of the p‐Te type are obtained in terms of matrix elements of different scattering mechanisms for the case of arbitrary magnitude and direction of the non‐quantizing magnetic field. Theoretical and experimental results of the temperature and field dependences of the transverse and longitudinal Nernst‐Ettingshausen effect in p‐Te are compared and discussed.
Influence of Finite Carrier Concentrations on the Electronic Spectrum of a Ferromagnetic Semiconductor
Authors: Matlak M. , Nolting W.
Published: July 2007

Starting from the atomic limit‐solution of the s‐f model a diagram technique is used to obtain the one‐particle spectrum of a ferromagnetic semiconductor, valid first of all for low carrier concentration n in the small bandwidth case. It turns out that the conduction band is split, due to the s‐f interaction, into two subbands for every electron spin direction. The lower edge of the lower ↑subband is shifted to higher energies with increasing band occupation explaining herewith recent experiments on EuO. The approximation fulfills the exactly calculable (n = 0, T = 0)‐limiting case.
On the Optical Criterion for the Mott Transition
Authors: Popielawski J.
Published: July 2007

The Mott criterion for disappearance of bounded electron states (metal–non‐metal transition) is generalized to higher excited states. The results are applied to the interpretation of the optical measurements in solid mercury‐xenon mixtures at very low temperatures.
The Biexciton in Polar Crystals
Authors: Bobrysheva A. I. , Vybornov V. I.
Published: July 2007

The effective Hamiltonian of electrons and holes interacting via the Coulomb law and through exchange by virtual longitudinal optical phonon is derived. On the ground of this Hamiltonian the correction to the biexciton dissociation energy is obtained. In GaAs the interaction of electrons and holes with the lattice affects favourably the biexciton stability.
The Motion of an Electron in One‐Dimensional Periodic Potential under the Influence of Uniform Electric and Magnetic Fields
Authors: Jannussis A. D. , Papatheou V. , Tsilimigras P.
Published: July 2007

The properties are discussed of an electron moving in one‐dimensional periodic potential in the presence of homogeneous mutual perpendicular electric and magnetic fields. The perturbation energy is shown, by two different methods, to be a periodic function of the ky quantum number, as required by Harper's conditions, and also a periodic function of the electric field. Furthermore the partition function is calculated to second order in perturbation. The free energy of the system is evaluated, the magnetic moment as well as the magnetic susceptibility and its dependence on the electric field.
Frequency Dependences of Conductivity and Dielectric Permeability of a One‐Dimensional Disordered Conductor
Authors: Gogolin A. A. , Melnikov V. I.
Published: July 2007

The frequency dependences of conductivity and dielectric permeability of a one‐dimensional disordered conductor were calculated by means of the numerical solution of the Berezinskii equations. The experimental results on the frequency dependences of conductivity and dielectric permeability in TTF‐TCNQ are analyzed and a free path time τi−1 ≈︁ 1000 cm−1 is found. In case of isotropic electron‐phonon scattering the temperature dependences of dc conductivity and dielectric permeability in a one‐dimensional disordered conductor is calculated numerically.
Host Nuclear Magnetic Resonance in Copper‐Based Magnetic Alloys
Authors: Thiemann J.
Published: July 2007

The lineshape of the bulk Cu63 NMR signal of the Kondo systems _Cu—Mn, _Cu—Fe, and _Cu—Co is investigated experimentally as a function of the temperature and of the concentration of the dilute magnetic impurities. The measurements are compared with Monte‐Carlo simulated line‐shapes broadened by charge density oscillations as well as spin density oscillations of the conduction electrons in the surrounding of the impurity atoms. The increasing coupling between the impurity and the conduction electrons with decreasing temperatures (T < TK) leads to a reduction in the amplitude of charge density oscillations in the range of the coherence length. The analysis of the experimental data gives the coupling constant |Jsd| of the different Kondo systems, showing the minimum value of |Jsd| in the middle of the 3d series for _Cu—Mn.
Many‐Electron Effects on the Characteristics of Short‐Range Order in the Pseudopotential Approximation
Authors: Khwaja Farid A. , Katsnelson A. A. , Silonov V. M.
Published: July 2007

The effect of the many‐electrons interaction on the local ordering energy and the short‐range order (SRO) parameter for the solid solutions is critically studied using various prevalent forms of exchange and correlation corrections in an electron gas. An estimate about the effect of these corrections on the ordering energy and the SRO parameter is made. Detailed calculations for Al—Zn and Ni—Pt alloys are carried out to investigate the influence of these corrections on the ordering energy and SRO parameter. It is concluded that the use of the incorrect correction for exchange and correlation may bring a disaggreement between calculated and experimentally measured values of α1 even in their signs. It is observed that the exchange and correlation correction of Vashista and Singwi yields the best agreement of the calculation with the experimental values of the SRO parameter α1 for Al—Zn alloys.
Complex Band Structure and Density of States for a One‐Dimensional Model of Transition Metal Alloys
Authors: Richter R. , Goedsche F.
Published: July 2007

The multiple‐scattering theory is applied to determine the electronic spectrum of a one‐dimensional binary alloy of spatially non‐overlapping randomly distributed scatterers. A general expression is derived for the alloy density of states using the average‐T‐matrix approximation (ATA). Within the framework of the ATA both the complex energy bands and the electronic density of states are calculated numerically for a transition metal like alloy.
Surface States in the (111) and (&1mcar;1&1mcar;1&1mcar;) Faces of Zincblende Compounds
Authors: Flores F. , Tejedor C. , Martín‐Rodero A.
Published: July 2007

The surface states in the (111) faces of zincblende compounds are studied within the pseudopotential approach, by introducing surface self‐consistency. The surface state occupation and its energy levels are determined and compared with available experimental results.
Search for Anisotropy of Mössbauer Absorption in Biotite
Authors: Chandra R. , Tripathi R. P. , Lokanathan S.
Published: July 2007

A search for anisotropy of Mössbauer factor ƒ in biotite single crystal is made by observing the angular variation of Mössbauer γ‐absorption directed at various angles to the normal to the plane of the sheet and also by looking for the Goldanskii‐Karyagin effect in the powdered specimen. It is suggested that the absence of any detectable anisotropy of the ƒ‐values in biotite, and indeed in other Fe containing compounds, in general, is possibly due to the large value of the ƒ‐fraction. This is in contrast to tin‐ and rare‐ earth compounds where such anisotropy is detectable even at room temperatures.
Polariton Dispersion and Crystal Optics in Monoclinic Materials
Authors: Claus R.
Published: July 2007

A dispersion relation for long‐wave phonons and phonon‐polaritons in monoclinic polyatomic crystals is derived. It can be brought in the form of a generalized Fresnel equation for the wave normal referring to the principal dielectric axes at a fixed frequency in the visible. Angular dispersion of coupled phonon‐ and polariton‐modes in the crystallographic plane with directional dispersion of the dielectric axes, and the possibility of experiments are discussed.
the Mixing between Frenkel Excitons and Charge Transfer States on Charge Carrier Transfer in Molecular Crystals. II. Physical Consequences
Authors: Petelenz P.
Published: July 2007

The effect of mixing between Frenkel excitons and CT states on the transfer of charge carriers in molecular crystals is considered according to the theory presented in the recent paper. The numerical estimates of the exponential narrowing factors, polaron binding energies, and CT state populations are given for some model cases. Based on these results, the possible methods of experimental verification are suggested.
Higher‐Order Corrections to the Spectrum of the Easy‐Axis Heisenberg Ferromagnet
Authors: Nawrocka W. , Zossel R.
Published: July 2007

The method of thermodynamic two‐time retarded Green functions is applied to calculate the energy spectrum of the easy‐axis Heisenberg ferromagnet. The Dyson‐Maleev transformation is applied what limits the results to the low‐temperature region. The connection of the two‐spin Green function with the scattering matrix is essentially exploited.
A Model Potential and Its Calculations
Authors: Khanna K. N. , Sharma P. K.
Published: July 2007

A one‐parameter model pseudopotential is used to calculate the elastic constants, spin susceptibility, and distortions of the Fermi surface in simple metals. The good agreement with experimental results confirms the general correctness of the model pseudopotential.
Lattice Thermal Conductivity of Solid Neon in the Temperature Range 0.5 to 10 K
Authors: Gupta I. J. , Trikha S. K.
Published: July 2007

The thermal conductivity of neon samples from 0.5 to 10 K is analysed on the basis of Holland's formulation. Fairly good agreement is found between the theoretical and the experimental results of Clemans. It is found for neon samples that longitudinal phonons carry only 15 to 30% of heat, as was shown for argon. The values of the dislocation densities and the average grain sizes of the polycrystalline neon samples are also determined.
Phonon Relaxation in One‐ and Two‐Dimensional Displacive‐Type Ferroelectrics
Authors: Kashcheev V. N.
Published: July 2007

The dampings and frequency shifts (one‐dimensional case) for acoustic phonons as well as for the soft mode due to cubic and quartic anharmonism are derived for the Silverman model of one‐and two‐dimensional displacive‐type ferroelectrics. Contributions from the various phonon–phonon interactions to the phonon dampings are considered and the damping critical exponents are found. In the calculations a self‐consistent formalism in the hydrodynamical limit is used.
Extended‐Hückel‐Theory Calculations for the Positive Divacancy in Silicon
Authors: Ammerlaan C. A. J. , Wolfrat J. C.
Published: July 2007

Calculations are carried out for the positive divacancy in silicon using the extended Hückel theory. These are non‐iterative calculations in the molecular unit cell approach for various sizes of the unit cell and for various points in the Brillouin zone. The effects of two symmetry‐allowed distortion modes of the six nearest‐neighbour atoms of the divacancy are investigated. The parameters for stable Jahn‐Teller distortion are determined by minimizing the extended‐Hückel‐theory energy. A second minimum with slightly higher energy is a saddle point with respect to distortions corresponding to other Jahn‐Teller configurations. The energy barrier for reorientation between these configurations is obtained. From the wavefunction of the unpaired electron the hyperfine interaction with 29Si nuclei in the two nearest‐neighbour shells are calculated. For suitable distortions the results compare favourably with existing experimental data. A large discrepancy is found between the distortion parameters for minimum energy and for optimum match of the hyperfine constants.
Electron Spin Transfer in Alkali Chlorides and Alkali Bromides Doped with Divalent Vanadium
Authors: Vanhaelst M. , Matthys P. , Boesman E.
Published: July 2007

A comparative theoretical analysis of the covalent electron transfer in various alkali chlorides and bromides, doped with V2+, is presented. Using previously studied EPR and optical measurements, it is found that, in spite of the striking difference in spin‐Hamiltonian parameters, the spin density (fractional electron transfer) is of the order of 3% in both the chlorides and bromides which were studied. More systematic differences in the normalized admixture coefficients are observed when, for the same ligand, the interionic distance is increased.
On the Electronic Properties of Disordered Quasi‐One‐Dimensional Systems with Electron Correlation
Authors: Eifrig Th.
Published: July 2007

The one‐dimensional disordered Hubbard model is investigated in alloy analogy and coherent potential approximation. The temperature dependence of the static susceptibility and the dc conductivity is calculated. The numerical investigation shows that especially at low temperatures the results are strongly influenced by disorder. The application of this approximation to the substance NMP‐TCNQ is discussed.
Lifetime of Color Centers in NaBr
Authors: Bosi L. , Longoni A. , Nimis M.
Published: July 2007

New results on the temperature‐dependence of the lifetime of the F‐center in NaBr are obtained by luminescence measurements. The disexcitation energy is evaluated to (0.078 ± 0.006) eV. A fast component ((22.9±1)ns) of the decay spectra is attributed to the M‐center. The data are discussed in respect of previous photoconductivity measurements and of the criterion of occurrence of F‐center luminescence in ionic crystals.
Positronium in Mixed Molecular Crystals
Authors: Goworek T. , Rybka C. , Wawryszczuk J.
Published: July 2007

Positronium formation in mixed organic crystals is investigated. The size of the free volume in the neighbourhood of the strange molecule influences the lifetime of the ortho‐positronium and the width of the momentum distribution of the para‐positronium. A systematic change of positronium formation probability with the position of the substituted group in naphtalene derivatives is observed. In pyrene host crystals it is found that after addition of guest molecules a distortion of the whole momentum spectrum appears.
A Study of Excitonic Spectra in CdSxSe1−x Mixed Crystals
Authors: Kozanecki A. , Werner Z. , Rzewuski H. , Loferski J. J.
Published: July 2007

Cathodoluminescence and reflectivity spectra are studied in homogeneous CdSxSe1−x mixed crystals with x = 0.85, 0.64, 0.47, 0.29 over the temperature range 7 to 110 K. It is found that the luminescence at 7 K is dominated by bound exciton emission. The binding energies of exciton complexes are deduced from the spectra and found to decrease linearly with the increase of selenium content in the crystals. It is concluded that excitons in different CdSxSe1−x mixed crystals are bound to the same impurity or defect.
Theoretical Investigations of Haynes' Rule
Authors: Atzmüller H. , Schröder U.
Published: July 2007

In a variety of semiconductors the binding energy EBX of excitons bound to ionized or neutral impurities shows a linear dependence on the ionization energy EIof the corresponding defect. This variation is described in the form of a generalized Haynes' rule,E EBX = A + BAI, and follows from first‐order perturbation theory with a short‐range potential. The constants A and B are calculated for ionized and neutral complexes as a function of σ = m/m. Good agreement of the theoretical results with experimental data is obtained.
Molecular Vibrations in a Polymethine Dye Layer Studied by Inelastic Electron Tunneling Spectroscopy
Authors: Ewert S. , Heiland G. , Lüth H. , Roll U. , Sander W.
Published: July 2007

Inelastic electron tunneling spectroscopy (IETS) at liquid He temperature is used to study the vibrational spectrum of the polymethine dye NH‐rhodanine merocyanine (NH‐RMc). The dye interlayers are prepared by in situ sublimation of the molecules onto the Al2O3layer of (AlAl2O3Pb) tunneling sandwiches. An interpretation of the most prominent vibration bands is given. By comparison with infrared data it is concluded that the dye molecules are arranged in the film with their polymethine chain parallel to the substrate.
Impurity Interaction on Excited Acceptor States in GaP
Authors: Senske W. , Sondergeld M. , Queisser H. J.
Published: July 2007

The lowest excited donor–acceptor pair states in GaP are monitored as a function of the pair separation R by luminescence excitation spectroscopy. They are interpreted in terms of the excited acceptor states 2P3/2, 2P5/2(Γ7) perturbed by the Coulomb field of the donor. The impurity interaction causes an increase ΔE(R) of the corresponding binding energies in the relevant range of R < 30 Å following the empirical law ΔE ∝ exp (−R/R0). The theoretical interpretation of the impurity interaction is based on the Coulomb overlap between the donor ground and acceptor excited states. Within a perturbational scheme hydrogen‐like wave functions are used with modified effective Bohr radii included. The first‐order calculations reflect the main features of the experimental results.
Magnetoresistance and de Haas‐van Alphen Effect in LaAl2
Authors: Reichelt J. , Winzer K.
Published: July 2007

Measurements of the transverse magnetoresistance and the de Haas‐van Alphen effect in LaAl2 are presented for the first time. The results are compared with a theoretical band structure calculation. Several features are in surprisingly good correspondence. Based upon these ones a model of the Fermi surface is proposed. Additionally a few representative cyclotron masses are given.
New Colour Centres in KCl: (Tl+ + Ca2+) and KCl: (Tl+ + Sr2+) Crystals
Authors: Ioan A. , Topa V. , Giurgea M.
Published: July 2007

Electrolytic colouring under unusual conditions (low temperature and high voltage) gives rise to the appearance of three new absorption bands peaking at 3.2, 2.5, and 1.7 eV and at 2.8, 2.1, and 1.5 eV in KCl: (Tl+ + Ca2+) and in KCl: (Tl+ + Sr2+) single crystals, respectively. The modifications of the absorption spectra of the coloured crystals induced by the application of a reversed electric field at the colouring temperature or by heat treatment are investigated. It is likely that the colour centre responsible for the new absorption bands is an aggregate centre which, besides an Tl− complex, contains also at least an Ca(Sr) ion, a trapped electron, and an anionic vacancy.
Dipolar Relaxation Induced by an AC Field in Various Crystallographic Directions of Me2+‐Doped Alkali Halide Crystals. I. Nearest‐Neighbour Approach
Authors: Poźniak J.
Published: July 2007

Theoretical analysis of the relaxation of Me2+–cation vacancy dipoles in NaCl‐type alkali halide crystals in an ac electric field acting along 〈110〉 or 〈111〉 crystallographic direction is carried out. There is also analyzed the possibility of calculating the rotational jump probabilities for lattice cation‐cation vacancy (ω1) and for direct exchange Me2+‐cation vacancy (ω2), on the basis of the experimental data of tan δ = F(ω) measurements.
Temperature and Frequency Dependence of the Dielectric Constant and Proton NMR near the Structural Phase Transition of Squaric Acid
Authors: Maier H.‐D. , Müller D. , Petersson J.
Published: July 2007

The frequency‐and temperature‐dependent dielectric constant is measured in directions parallel to the molecular layers of squarie acid crystals. There is a monodispersive relaxational dispersion step in the low‐frequency regime Which is ascribed to a thermally activated diffusional motion. The dielectric constant, which is the high‐frequency limit of this dispersion step, is influenced by the 100°C structural phase transition in a characteristic manner. In order to account for the observed behaviour different models are discussed. The measured orientational dependence of the proton NMR spectrum is related to the structural data.
On the Electronic Structure of As2O3, Sb2O3, and As2S3
Authors: Wimmer E. , Weinberger P. , Kosakov A. , Leonhardt G.
Published: July 2007

The AsKβ band in As2O3 and the X‐ray photoelectron spectra of crystalline As2O3 and Sb2O3, and amorphous As2S3 are interpreted by means of the SW method in terms of simple solid state clusters using energy dependent transition probabilities. Considering the simplicity of the applied models the theoretical results agree well with experiment.
On the Theory of Resonance Raman Scattering in Disordered Semiconductors
Authors: Esser B.
Published: July 2007

Using the Green's function technique a general expression of the one‐phonon resonance Raman scattering cross‐section is derived. This expression may be used to calculate the influence of a slowly varying random field present in the electronic subsystem on the resonance dependence. The theory is applied to the case of a fluctuating gap and internal random electric field. Local scattering amplitudes are obtained and the configurational average of the product of the scattering amplitudes is investigated. It is shown that this two point average effectively reduces to the averaging of the scattering amplitudes with one point distribution functions.
Exchange Interaction of Manganese 3d5 States with Band Electrons in Cd1−xMnxTe
Authors: Gaj J. A. , Ginter J. , Gałązka R. R.
Published: July 2007

The influence of a system of magnetic‐field‐aligned paramagnetic ions on the conduction and valence band electron states is studied. A model applicable to large‐gap semiconductors is presented, in which the direct influence of a magnetic field on the band electron states is neglected. This assumption allows to calculate the E(k) dependence in the presence of a magnetic field. The calculations are performed in the Kohn‐Luttinger model producing a strongly anisotropic and nonparabolic structure. As an illustration of the obtained results measurements of the exciton reflection structure in a magnetic field are presented on the ternary semiconductor CdMnTe. The ratio of the exchange integrals of conduction band electrons and valence band electrons to manganese ions is estimated to α/β = −0.25.
A quantitative study of excitonic polariton reflectance in CdS
Authors: Broser I. , Rosenzweig M. , Broser R. , Richard M. , Birkicht E.
Published: July 2007

A careful quantitative analysis of the reflectance of CdS crystals in the spectral range of the A‐exciton is presented. Measured spectra for the isotropic and the mixed‐mode orientation of the crystal are fitted by calculations based on the model of excitonic polaritons. Spatial dispersion and Pekar's abc, damping, exciton‐free surface layers, and oblique incidence of light are considered simultaneously. The good agreement between theory and experiment confirms the model, the still existing discrepancies are due to the simplifying assumption of a homogeneous surface layer. The formulae needed are summarized, and the influence of the different polariton parameters on the calculated reflectivity spectra are discussed.
Optical anisotropy of highly oriented pyrolytic graphite. Effect of surface preparation
Authors: Zanini M. , Grubisic D. , Fischer J. E.
Published: July 2007

Polarized reflectivity spectra are obtained at near normal incidence from highly oriented pyrolytic graphite surfaces which contain the c‐axis (i.e. normal to the cleavage surface). Comparison of spectra from polished and sputter‐etched surfaces leads to conclude ϵ∥ ≦ 3.4, consistent with previous determinations by oblique incidence reflectivity and by ellipsometry but substantially lower than recent values based on normal incidence spectra from polished surfaces. The nature of the polish‐induced damage is studied by RHEED, and explains the difference between our data and previous results.
On the quantitative characterization of chemical disorder of In1−xGaxP from wavelength‐modulated No‐phonon absorption
Authors: Strehlow R. , Kühn W. , Friedrich H. , Lange H.
Published: July 2007

Wavelength‐modulated transmission spectra of the no‐phonon structure of In1−xGaxP are quantitatively analysed on the basis of a recently developed theory. From this comparison the Koster‐Slater parameter J characterizing the magnitude of chemical disorder is determined. The theory also well accounts for the experimentally observed lineshape of the no‐phonon structure. From the intensity ratio of the no‐phonon and LA‐phonon‐assisted transitions the quantity J/C is estimated where C is the deformation potential constant for lattice deformation by LA phonons.
Clebsch‐Gordan coefficients of symmetry groups
Authors: van Den Broek P. M. , Cornwell J. F.
Published: July 2007

A method is given for the determination of Clebsch‐Gordan coefficients of finite groups. The Clebsch‐Gordan coefficients may be arranged into vectors which are eigenvectors of certain projection matrices. It is shown how an appropriate set of orthonormal eigenvectors of these matrices may be obtained; this set gives then a unitary matrix of Clebsch‐Gordan coefficients. The symmetry properties of these Clebsch‐Gordan coefficients are studied with special emphasis on the crystallographic space groups.
Electron states below the conduction band in Germanium originating from dissociated 60°‐dislocations
Authors: Winter S.
Published: July 2007

Electron states due to the strain field from dissociated 60°‐dislocations in Germanium are calculated using effective mass methods. The strain field is found to give rise to states with energies down to (Ec − 0.08) eV.
Fading memory as insufficient condition for the exciton diffusion
Authors: Šubrta V. , Čápek V.
Published: July 2007

It is argued that in contrast to the usual opinion, the decreasing character of memory functions (with increasing time) in the generalized master equation describing the exciton propagation in molecular solids is not yet a sufficient condition for the exciton propagation to be diffusion‐like.
ESR from boron in silicon at zero and small external stress. II. Linewidth and crystal defects
Authors: Neubrand H.
Published: July 2007

Observations of the ESR lineshape of the shallow acceptor centre B in silicon at zero external stress are reported. The broadening of two ΔM = 1 transitions can be well fitted by a Voigt profile. The Lorentzian part of the fit is shown to behave in full accordance with the theory of strain broadening by point defects. From linewidth measurements for different oxygen and carbon content the elastic strengths of oxygen and carbon in silicon are determined. All obtainable elastic strengths for point defects in silicon are collected. The possibility of determining the total amount of substitutional impurities from the Lorentzian broadening of the ΔM = 1 transitions is discussed.
Solitary bound states of magnons and lattice deformation in ferromagnets with biquadratic exchange
Authors: Pushkarov Kh. I. , Pushkarov D. I. , Vlahov J. P.
Published: July 2007

The existence of solitary bound states of spin deviations and lattice deformation created by them in some ferromagnetic systems with biquadratic exchange is shown.
Cross relaxation studies in the Cr3+‐doped magnetically dilute paramagnetic Al2O3–Fe2O3 system. I. Effect of Cr3+ concentration
Authors: Srivastava J. K. , Dale B. W.
Published: July 2007

Mössbauer measurements are made in undoped and Cr3+‐doped Al2O3–0.04 mol% 57Fe2O3 systems at 4.2 K. The cross relaxation between Fe3+ and Cr3+ ions, dispersed in the diamagnetic Al2O3 lattice, is studied. The spin–spin relaxation time of the Cr3+ ions is reduced with increasing Cr3+ concentration and these faster relaxing Cr3+ spins are found to enhance the Fe3+‐relaxation process, destroying the paramagnetic hyperfine structure observed in the 57Fe Mössbauer spectra of undoped Al2O3–0.04 mol% 57Fe2O3. It is concluded that at higher Cr3+ concentrations, cross relaxation does not take place by the mutual energy exchange between a Cr3+ ion and and Fe3+ ion but a new kind of pair cross‐relaxation process occurs where a Cr3+ ion and an Fe3+ ion first form an exchange coupled pair and then two such pairs cross relax.
Interband optical absorption in semiconductors with one‐dimensional electronic superlattice
Authors: Dubrovskii G. B. , Pavlov S. T. , Vershinin M. S.
Published: July 2007

The shape of the fundamental absorption edge is theoretically analysed for semiconductors with one‐dimensional electronic superlattice. The structure associated with minizone edges is found experimentally in the absorption spectrum of 6H‐SiC crystals. Good agreement with the theoretical predictions is obtained.
Solid silicon selfconsistently with Xα potentials
Authors: Stöhr H. , Bross H.
Published: July 2007

The band structure of solid silicon is selfconsistently determined with the help of local Xα exchange and correlation potentials using the MAPW method. From a detailed comparison with experimental data and regarding the care taken with numerical convergence and approximations one is forced to conclude that the Xα potential at best may only yield a rough understanding of the electronic structure of Si.
Magneto‐Seebeck and Shubnikov‐de Haas effect in n‐type (Bi1−xSbx)2 Te3
Authors: Köhler H. , Ossau W.
Published: July 2007

The magneto‐Seebeck effect in a transverse magnetic field is investigated in n‐type (Bi1−xSbx)2 Te3 mixed crystals (0 ≦ x ≦ 0.5; 20 K ⪅ T ⪅ 130 K) in order to evaluate electronic parameters from the saturation of the thermoelectric power α⟂ (∞) at high magnetic fields. In addition magneto‐quantum phenomena are observed in the Seebeck and Shubnikov‐de Haas effect simultaneously for B ∥ c. The electron concentrations deduced from the saturation value of the Hall constant (−eRH∞)−1 ranged between 1.8 × 1018 and 3.2 × 1019 cm−3. It is shown, that the presence of two types of electrons has to be assumed for the interpretation of the experimental data (α⟂ (∞)), where the energy gap between the two adopted conduction band minima decreases from Bi2Te3 (24.5 meV) towards zero at x ≈ 0.2, re‐opening for x > 0.2 after the different types of conduction band minima have crossed over and therefore the meaning for the lowest and higher conduction bands is substituted compared to x < 0.2.
Electron–phonon coupling constant and critical temperature of some A15 superconducting compounds of Nb and V
Authors: Raychaudhury M. , Chatterjee P. , Chatterjee S.
Published: July 2007

Theoretical calculations are made of the coupling constant, λ, for the A15 compounds Nb3Al, Nb3Sn, V3Si, and V3Ge with the help of the theory of Gaspari and Gyorffy. For this purpose Herman‐Skillman potentials are used for the phase shift calculation, and the density of states and the Fermi energy are taken from the band structure calculations of Mattheiss. Using these values of the coupling constant the Tc's are obtained of these materials. These values of Tc's agree fairly well with the experimental values. Since for the highest Tc material, Nb3Ge, no band structure calculation exists, Tc is plotted as a function of the Fermi energy. From the plot, the experimental Tc of 23.2 K is reached for EF = 1.330 Ryd.
Application of unitary groups for determining tensor components in crystals and selection principles for phase transitions
Authors: Kustov E. F. , Zakharov N. A. , Steczko G.
Published: July 2007

The paper is concerned with compound series of unitary groups, of rotation groups and point groups for classification of various tensor components in crystals, which have various components in relation to a rotation group and a permutation group. For this purpose the following series U[γ]γ ⊃ R[γ]γ ⊃ … ⊃ G is being introduced, where γ represents a vector (a pseudovector); then the number of various branches which end with a unit group G determines the number of various tensor components, which have obvious properties for reduction symmetry. When classifying in this way, a new interpretation of selection principles for phase transitions in crystals can be introduced.
Phonon frequencies in off‐symmetry directions of NaCl determined by neutron inelastic scattering
Authors: Nilsson G. G.
Published: July 2007

Phonon frequencies in NaCl at 80 K have been determined using neutron inelastic scattering. 72 off‐symmetry phonons are measured, both in the symmetry plane (110) and in the off‐symmetry plane (301). The experimental frequencies are compared to earlier breathing shell model calculations and found to be in excellent agreement.
An information theoretical method (ITM) applied to the metal–insulator transition of the neutral hubbard model
Authors: Heiner E.
Published: July 2007

By a minimization procedure of the information measure for the single particle spectral weight function (SWF) of fermions a method is found for the approximate calculation of the SWF. Using this method it is possible to overcome the shortcoming of the rigid shape ansatz for the SWF within the moment expansion method. The new method is applied to the neutral Hubbard model and a criterion for the localization of the quasi‐particles is given.
The structure of liquid mercury over a wide range of temperature (−30 to 290 °C)
Authors: Waseda Y. , Miller W. A.
Published: July 2007

X‐ray diffraction patterns are obtained from liquid mercury at −30, 20, 120, 220 and 290 °C. The structure factors obtained are insensitive to temperature and applied to the calculation of the electrical resistivity and the thermoelectric power on the basis of the t‐matrix formulation of the Ziman nearly free electron theory. The calculated values show reasonable agreement with the available experimental data.
Influence of the size effect and surface roughness on phonon transport in bounded systems
Authors: Nonnenmacher Th. F. , Wunderle R.
Published: July 2007

The influence of the size effect and of surface roughness on the thermal conductivity x is investigated. The starting point for treating this problem is the relaxation time approximation of the Boltzmann equation for all bulk scattering mechanisms. The boundary scattering mechanism comes into play via the boundary condition for flux conservation with the result that x can be calculated as a function of the thickness d of the crystal and two roughness parameters L and η characterizing the horizontal and vertical widths of the surface defects, respectively. The resulting formula for x is a composition of a pure bulk term and a term containing a mixture of bulk and surface properties.
Temperature and concentration dependence of the knight shift of 115In in liquid In–Hg alloys
Authors: Ploumbidis D. , Sutter T.
Published: July 2007

Detailed measurements of the temperature and concentration dependence of the Knight shift of 115In in In–Hg system, in a range of 240 to 510 K and 0 to 100 at% Hg, respectively, are carried out. The temperature coefficient of the 115In Knight shift and its concentration dependence at constant volume is determined using only experimental results. A possible interpretation of the concentration dependence of this coefficient is given in the framework of Mott's pseudogap model.
Isotope effects of diatomic TiH and TiD complexes in FeTi crystals
Authors: Kronmüller H. , Vetter K.
Published: July 2007

In ferromagnetic b.c.c. FeTi alloys (2at%, 10at%Ti) the magnetic relaxation spectrum of diatomic TiH and TiD complexes is studied in the temperature range from 10 to 180 K. The relaxation spectra of both the types of complexes are composed of two overlapping relaxation maxima which may be attributed to different relaxation modes of the diatomic complexes. A significant isotope effect can be detected for the activation energies whereas in the case of the pre‐exponential factors no significant isotope effect can be observed.
Distribution of the perturbation potential and the density of states in high‐doped semiconductors
Authors: Breitenstein O. , Unger K.
Published: July 2007

To calculate the potential distribution and density of states for a set of randomly distributed potentials the Laguère integration method is used. The method substitutes the inverse Laplace transformation by a finite sum. Application is given to the broadening of the density of states under the influence of screened Coulomb potentials in high‐doped semiconductors. The screening of the single potentials is taken according to the density of states, resulting from the statistical superposition of these potentials in a self‐consistent way.
Spin reorientation in HoIG investigated by means of faraday effect
Authors: Bałanda M. , Nizioł S.
Published: July 2007

Faraday rotation measurements in pulsed magnetic fields up to 160 kOe and in low dc fields are carried out for Ho3Fe5O12 near the compensation point. Transition to the canted phase is observed and the temperature dependence of the critical field determined. From the holmium sublattice magnetization and Hcr versus. T gradient, values of the two molecular field coefficients are determined.
Temperature shift of the maximum of neutron critical scattering from an iron polycrystal. A test for the dynamie scaling theory
Authors: Ciszewski R. , Milczarek J.
Published: July 2007

It is found, from neutron critical scattering measurements, that the well‐known temperature shift of the maximum of scattering depends not only on the scattering vector, but also on the wavelength of the incoming neutrons. The results obtained are compared with the predictions of the dynamic scaling theory and a new test for the validity of this theory based on the estimation of the critical index v is proposed.
Size effect, valency factor, and point defect interactions in dilute alloys
Authors: Prakash S. , Lucasson P.
Published: July 2007

A simple but realistic physical model is suggested to analyse the capture radii and wipe‐out numbers in dilute alloys of copper and aluminium. The valency of impurity is decided according to Friedel criterion and the size effect is included according to Blatt. It is found that the capture radii and wipe‐out numbers are proportional to the magnitude of perturbation caused by both the realistic valence difference and size effect. It is shown that the size effect is dominating in copper alloys while the valence effect is dominating in aluminium alloys. The deduced linearity relations may be quite useful for predicting the capture radii and wipe‐out numbers for new experiments.
Dynamical exchange effects on the plasmon dispersion in aluminum
Authors: Devreese J. T. , Brosens F. , Lemmens L. F.
Published: July 2007

In the dielectric function, obtained from the dynamical exchange decoupling method, exchange effects are described by a frequency‐dependent local field correction G(q, ω). From the behaviour of G(q, ω) in the static limit ω = 0, it follows that the compressibility sum rule is satisfied, and that the pair correlation function g(r) equals 1/2 at the origin. Also, in the static limit, G(q, 0) shows a sharp peak near q = 2kF. The dynamical behaviour of G(q, ω) at high frequencies fulfills the condition imposed by the third frequency moment sum rule, and it also implies that g(0) = 1/2. The same g(0) thus derives from both the static and the dynamical behaviour of G(q, ω), in contrast to earlier theories. The influence of the dynamical exchange effects on the plasmon dispersion is examined. It is found that, compared to RPA, the plasmon is almost unaffected by exchange outside the continuum, but that in the particle‐hole excitation region exchange effects drastically lower the peak position of the structure factor. This tendency has been experimentally observed in Al, where the RPA peak positions at finite wave vector are found at too large frequencies, but where the dynamical exchange decoupling method yields relatively good agreement with the experimental data.
Interband absorption and the Γ‐point band structure parameters in HgSe
Authors: Szuszkiewicz W.
Published: July 2007

Optical absorption in HgSe crystals is measured in a wide energy range (0.04 to 0.9 eV). Transmission experiments are carried out at seven temperatures from 10 to 295 K. Very thin samples with electron concentration ranging from 4 × 1017 to 2 ± 1018 cm−3 at helium temperature are used (the absorption coefficient is measured up to 3 × 104 cm−1). Direct optical transitions Γ → Γ, manifesting themselves as a „step”︁ on the absorption curve, are observed at low temperatures. The relationship between the value of the negative energy gap E0 and the value of the spin–orbit splitting Δ is determined from the energy of the front of the „step”︁ as a function of temperature. The absorption coefficient for HgSe is calculated numerically using Kane's formulae for the inverted band structure. The finite value of the spin–orbit splitting Δ and the parameters describing the interactions with „higher bands”︁ are taken into account. A new set of band structure parameters (E0, dE0/dT, P, Δ, A′, L′, M, L – M – N) is obtained. This set can explain the data measured as well as previous experimental results.
U centres in alkali iodides
Authors: Sebastian R. , Raj G. A. Savari , Haridasan T. M.
Published: July 2007

Exact changes in force constant due to H− substitution (U centres) in alkali iodides are systematically analysed using Green's function technique. NaI:H−, KI:H−, RbI:H−, and CsI:H− systems are considered. There is a weakening of the force constant varying from 50% to 63% depending on the mass of the neighbour in the host lattice.
Effect of ordering on the OH Raman bands of K4Fe(CN)6 · 3 H2O
Authors: Savatinova I. , Anachkova E.
Published: July 2007

A first detailed Raman study is made of the OH stretching modes of KFCT. The analysis of the spectra is based on a comparison with those of the deuterated compound. Polarized Raman spectra of KFCT in all scattering geometries at both sides of Tc are obtained. The measurements do not reveal any gaining of inversion centre in the paraelectric phase of the crystal. All bands observed in the ferroelectric phase continue to be strong above Tc with the only exception of the 3510 cm−1 band. A correlation between the intensity of this band and the long‐range order parameter m is reported. The influence of the phase transition on the frequencies and bandwidths of the OH spectrum is investigated. As the temperature is lowered down to 80 K the bandwidths narrow drastically. It is shown that this effect is associated with the temperature dependent potential barrier restricting the reorientations of the water trimers.
Nuclear‐acoustic‐resonance determination of the core polarization contribution to the knight shift and hyperfine fields in single‐crystal tantalum
Authors: Schanz G. , Müller V. , Unterhorst E. J.
Published: July 2007

By quantitative evaluation of the nuclear acoustic resonance shift, the temperature dependence of the Knight shift in tantalum is determined for the first time. The experiments are performed in the temperature range of 20 to 300 K. The s‐, d‐ and orbital contributions to the Knight shift, the magnetic susceptibility and the hyperfine fields are specified.
Investigations of EPR superhyperfine structure for the cubic Gd3+ center in CdF2
Authors: Gehlhoff W. , Kreissl J.
Published: July 2007

A critical investigation is made of the superhyperfine (SHF) structure in the EPR spectra of the cubic Gd3+ center in CdF2, which arises from the interaction of the Gd3+ ion with its eight nearest‐neighbor 19F nuclear spins. Contrary to previous EPR investigations it is found that the hyperfine splitting produced by the gadolinium isotopes 155Gd and 157Gd cannot be neglected and that the width of the individual SHF line is dependent on the fine‐structure transition. This linewidth behavior is discussed.
Calculation of the broadening of donor–acceptor pair lines due to the potentials of ionized impurities
Authors: Breitenstein O. , Unger K.
Published: July 2007

The broadening of DAP lines under the influence of perturbation potentials from ionized impurities is calculated in the Thomas‐Fermi approximation. The exact treatment is only numerically solvable, but for physically interesting DAP distances less than half the screening length a simple potential well approximation is introduced, leading to analytical results.