bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Anomalous Magneto‐Resistance of Gallium Arsenide in Intense Magnetic Fields
Authors: Jones J. C. , Wallace P. R.
Published: July 2007

The self consistent T matrix (SCTM) and the average T matrix (ATM) approximations are used to calculate the longitudinal magneto‐resistance of n‐type gallium arsenide in intense magnetic fields. Taking into account the improved screening of the impurity centres at high fields the longitudinal magneto‐resistance is found to decrease anomalously with increasing field. Although there are important differences in the details of the density of states using the two approximations, the numerical results show a relative decrease in the magneto‐resistance in qualitative agreement with experiment.
Spin Polarization of X‐Ray Photoelectrons in Nickel
Authors: Sokolov O. B. , Finashkin V. K. , Turov E. A.
Published: July 2007

It is shown that under certain conditions an X‐ray photoelectron spectrum is represented as a sum of additive contributions from each of the atoms separately. Discussing the problem of measuring the electron spin polarization (ESP), conclusions are made about advantages of the X‐ray photoelectron emission as compared with the optical method. The results of the ESP numerical calculations for nickel based on Hubbard's model with a small concentration of electrons (holes) and in the approximation of a local effective interaction are reported. Drastic correlation effects for ESP are found. The correlation interaction may change the sign of ESP at the Fermi level compared with the one‐electron Hartree‐ Fock calculations, and provides an additional hump of the ESP energy distribution at the bottom of the spectrum.
The Mössbauer Emission Spectra of Impurity 57Cobalt in a Halide Matrix
Authors: Maddock A. G. , Williams A. F. , Siekierska K. E. , Fenger J.
Published: July 2007

The Mössbauer emission spectra of 57Co in low concentrations in KF, NaCl, NaF, LiF, and MgF2, and the effects of doping NaF and LiF with La3+ ions are reported. The monovalent halides all give similar spectra showing a broad single line or a doublet at 2.19mm/s and two overlapping doublets at 0.46 and 0.19 mm/s (w.r.t. metallic iron), although relative intensities vary. Previous Mössbauer and EPR spectroscopic data on these systems are reviewed briefly, and qualitative conclusions drawn on the chemistry of such systems.
The Phonon‐Induced Temperature Dependences of Spin‐Hamiltonian Parameters for S‐State Ions
Authors: Bates C. A. , Szymczak H.
Published: July 2007

The temperature dependence of the spin‐Hamiltonian parameters including the hyperfine coefficient for S‐state ions is examined. The starting point in the theory is the dynamical spin‐Hamiltonian with experimentally determined values for the parameters. Using this Hamiltonian the contributions to the static spin‐Hamiltonian from spin‐phonon interactions are calculated. It is shown that the most important contributions to the crystal‐field parameters arise from lattice anharmonicity. Such mechanism cannot explain the temperature dependence of the hyperfine coupling parameters and it is suggested instead, that two‐phonon processes give the dominant contributions.
Effects of Ultraviolet Irradiation on KBr: Gd
Authors: Kristianpoller N. , Rehavy A. , Israeli M.
Published: July 2007

The results of UV irradiation on Gd3+ doped KBr single crystals were investigated. Thermoluminescence (TL) glow curves, excitation spectra, and optical absorption were measured in comparison with pure and Sr2+ doped KBr crystals. The concentration of anion vacancies was found to be markedly enhanced by the presence of impurity ions. TL excitation spectra of Gd3+ doped samples had maxima at the α band and at the 212 nm band, which is attributed to a localized exciton state perturbed by a Gd3+ ion. Results suggest that defects responsible for the TL are created by an excitonic process.
Magnetostriktion und lineare magnetische Doppelbrechung im Jahn‐Teller‐gekoppelten System TmAsO4, mit effektivem Spin S = 1
Authors: Becker P. J.
Published: July 2007

Es wird das Verhalten von TmAsO4, das einen kollektiven Jahn‐Teller‐Phasenübergang bei TD = 6,1 K aufweist, unter dem Einfluß eines entlang der Verzerrungsrichtung gelegten Magnetfeldes Hx diskutiert. Der elektronische Grundzustand des Tm3+‐Ions kann durch einen effektiven Spin S = 1 beschrieben werden. Durch einfache Molekularfeldrechnungen kann gezeigt werden, daß eine kristallographische Verzerrung bei jeder Temperatur oberhalb TD, erzeugt werden kann. Deutlich unterhalb TD vermindert dasselbe Magnetfeld die spontanc Jahn‐Teller‐Verzerrung. Diese Effekte wurden mit Hilfe der Methode der linearen optischen Doppelbrechung gemessen. Quantitative übereinstimmung wurde für Felder bis zu 20 kOe bei Temperaturen oberhalb 3,5 K erhalten. Messungen bei 1,6 K ergaben die erwartete Abnahme der kristallografischen Verzerrung mit ansteigendem Magnetfeld.
Combined Investigations of the Valence Band Structure of Graphite by K X‐Ray Emission Spectroscopy and X‐Ray Photoemission Spectroscopy
Authors: Berg U. , Dräger G. , Brümmer O.
Published: July 2007

The structure of the valence band of graphite is investigated by combined application of X‐ray photoelectron spectroscopy and a special X‐ray emission spectroscopy which measures polarized radiation. The highly resolved K X‐ray emission spectrum (XS) separated into σ‐ and π‐bands by using their different polarisation and the X‐ray photoemission spectrum (XPS) of this valence band are reported and discussed in connection with the calculated density of states. Width and overlapping range of the subbands are determined experimentally and compared with calculated values. The strong energy dependent differences between the intensities of the spectra and the density of states are interpreted by means of specific selection rules. The interpretations of the XPS are carried out with a cross section ratio σ(2s)/σ(2p) from 15 to 35. It is shown that the cross section of XPS for pσ‐like electrons is higher than that for pπ‐like electrons.
On the Magnetic Ordering in Induced Magnets
Authors: Popielewicz A.
Published: July 2007

The conditions of the occurrence of the ordering in real induced magnets at T = 0 K are discussed in hybridized RPA‐MFA. Special attention is paid to the possibility of surface magnetism and to the role of the soft surface mode in this phenomenon. The influence of the thickness and of the orientation of the crystal surface is considered.
Excitons in Pure and Doped Solid Neon
Authors: Pudewill D. , Himpsel F.‐J. , Saile V. , Schwentner N. , Skibowski M. , Koch E. E.
Published: July 2007

A detailed investigation of the optical excitations of pure solid Ne and of Xe, Kr, and Ar impurities in Ne has been carried out in the vacuum ultraviolet using synchrotron radiation. For pure solid Ne the optical constants ϵ1(ω) and ϵ2(ω) are derived via a Kramers‐Kronig analysis. For the guest atoms the energy levels within the band gap are determined. A new value of 21.69eV is established for the gap energy of pure Ne. The binding energies for the impurity excitations are the same as those for pure Ne for n ≧ 2. The Wannier model provides a satisfactory description. There is no linear dependence of the central cell correction ΔEc on the binding energies B1 as previously claimed. Estimates for the oscillator strength of the excitons in pure Ne (both spin components included) gave f(n = 1) = 0.22, f(n = 2) ≈ 0.042, and f(nn = 3)≈ 0.016 by comparing the experimental results with model calculations.
The Low‐Temperature Expansion of the Polarization of KDP‐Like Crystals
Authors: Kapor D. V.
Published: July 2007

The Hamiltonian of a KDP‐type ferroelectric is formulated in terms of Pauli‐operators. Canonical and unitary transformations are applied which bring the Hamiltonian into a form suitable for the Green's function method analysis. The whole theory is linear in the small parameter η = Ω/I(0). The low‐temperature expansion for the polarization is calculated (using the exact Bose‐representation of Pauli‐operators in the first approximation) exibiting T2 and T3 anharmonic corrections. In the case of high temperatures, the Curie‐Weiss law is reproduced with a transition temperature lower than in the mean‐field approximation.
On the First Excited State of the Biexciton in CuCl
Authors: Ekardt W. , Sheboul M. I.
Published: July 2007

The first excited state of the excitonic molecule in CuCl is considered in the framework of the previously derived Bose formulation for the excitonic molecule. A resolution concerning the discrepancies between existing theoretical and experimental two‐photon absorption results is proposed: the first excited (rotational) state of the molecule consists of four irreducible sets: Γ2 ⊕ Γ3 ⊕ Γ4 ⊕ Γ5, two of which (Γ3 ⊕ Γ5) are strongly two‐photon allowed, one is weakly allowed (Γ4) and one is forbidden (Γ2). The three sets Γ3 ⊕ Γ4 ⊕ Γ5 should then be seen in luminescence from the biexciton states to single‐exciton or exciton‐polariton states. A study of the electron‐hole exchange interaction on these states is also made.
On the Ginzburg Criteria for Static and Dynamic Critical Phenomena and the Small Parameter of Perturbation Theory
Authors: Nattermann T.
Published: July 2007

A clear guide is proposed to the classification of those terms in semimicroscopic and phenomenological theories of phase transitions which have to be treated by perturbation theory. It is based on a rescaling of all variables such that the terms not treated as perturbations become temperature‐independent dimensionless quantities of the order of magnitude of unity. The crucial point of this transformation is, that all lengths and the time are measured in appropriate choosen powers of the correlation length ξ. In terms of rescaled variables the perturbations then increase by approaching the critical point for d ≦ dc and d ≦ dc, d in the case of static and dynamical critical phenomena, respectively. The corresponding Ginzburg critical region is defined as the range, where the perturbations become of order of unity and larger. Several applications and possible extensions are discussed.
Effective ElectronHole Interaction in Molecular Crystals II. Phonon Mechanism
Authors: Petelenz P.
Published: July 2007

A possible mechanism of electronhole interaction via phonons in molecular crystals is considered. A linear approximation in the phonon operators is adopted. Possible conse‐ quences are discussed from the viewpoint of the stabilization energy of an ionized state, the equilibrium charge carrier concentration, and the charge carrier lifetime.
A New Form of Magnetic Bloch Functions and Applications to Magnetic Band Structure Calculations
Authors: Jagannathan R. , Ranganathan N. R.
Published: July 2007

Symmetry‐adapted functions for Bloch electrons in a homogeneous rational magnetic field are presented in a new form and the possibility of their direct utilisation in magnetic energy band structure calculations is indicated.
On the Dynamics of Hydrogen‐Bonded Ferroelectrics
Authors: Pak K. N.
Published: July 2007

The dynamics of hydrogen‐bonded ferroelectrics described by the Ising spin system in a transversal field is investigated within the framework of the continued fraction method for correlation functions by Mori. The complete set of dynamical equations for the longitudinal and transversal relaxation functions derived in this paper is reduced in the high temperature limit to that obtained previously by Moore and Williams making use of the technique of Blume and Hubbard. It is shown that in this limit a short‐range Gaussian memory function related to the second‐order random force in the longitudinal relaxation function is a more appropriate choice than an exponential form which is equivalent to assume a three‐pole structure for this relaxation function. The critical behaviour near the transition point is briefly discussed.
The Use of Modified Relaxation Time in the Transport Theory of Non‐Crystalline Semiconductors
Authors: Krempaský J. , Barančok D.
Published: July 2007

It is shown that the theory of transport characteristics of non‐crystalline semiconductors can be successfully treated within the framework of appropriately modified expressions for relaxation time. The results of this method related to the electrical conductivity, the drift mobility, the thermoelectric effect, the Hall effect, and the magnetoresistance provide the possibility to explain many pecularities of these phenomena from a unified point of view and without any additional assumption. The success of this method lies in the fact that also the electrons in weakly localised states are taken into the calculation.
Analysis of Multiphonon Absorption in Corundum
Authors: Billard D. , Gervais F. , Piriou B.
Published: July 2007

The frequency and temperature dependence of the infrared absorption coefficient of corundum over the range 1400 to 2500 cm−1 is analysed with an anharmonic model based on the concept of phonon self‐energy. A renormalization accounts for important effects of thermal expansion and phonon frequency shifts. First, the analysis of the temperature dependence of the data allows to separate the contributions to the absorption of different w‐phonon summation processes (n = 2 to 5). Results are found to be quite consistent with theoretical predictions. Then each n‐phonon contribution can be investigated in detail, and it is shown that the use of simple forms for average anharmonic potentials is adequate to explain the frequency dependence of the w‐phonon absorptions. The analysis of the results on the basis of a quite different theoretical model is also discussed.
A Study of the 6S5/2‐Term Splitting of an Fe3+ Ion in Zinc Ferrite by Neutron Spectroscopy
Authors: Ligenza S.
Published: July 2007

Inelastic neutron scattering studies are carried out on a zinc ferrite at temperatures between 5 and 293 K. It is found that the 6S5/2‐term of Fe3+ ion at B‐sites is split into six levels. Transitions between these levels are observed both in energy gain and energy loss configurations. The 6S5/2‐term splitting and observed neutron spectra are discussed on the basis of a phenomenological spin‐Hamiltonian assuming combined interaction of a static magnetic field and a static electric field gradient with magnetoelectric moments of 3d5 electron shell, respectively. The quadrupole moment due to deformation process of 3d5 shell is assumed as being caused by trigonal deformation of octahedral B sites and by exchange interaction. It is found that the internal magnetic field acting on Fe3+ (B) ions is about 320 kOe and is directed along one of the equivalent directions [100], [010], or [001]. The parameter of quadrupole interaction is D = −0.667 meV. The shape of 3d5 shell deformation is established as spindle‐like. As a result of mixed magnetoelectric interaction of Fe3+(B) ions, the magnetic moment is lowered to 4.0 BM at 5 K. The obtained data are compared with magnetization against temperature. A test of the proposed magnetization process is carried out by Mössbauer effect measurement in external magnetic fields.
On the Luminescence of KCl:Pb and KBr:Pb Crystals
Authors: Mǎrculescu L. , Ghiţǎ C.
Published: July 2007

The emission spectra of KCl :Pb and KBr:Pb are measured as a function of the impurity concentration in the crystals. The low energy emission depends stronger on the impurity concentration in crystals than the principal emission and is less excited within the A absorption band than within the C band. The low energy emission is well explained by the existence of a resonant transfer between − ν|1P1> + μ|3P1> and μ|1P1> + ν|3P1> electronic states.
An Analysis of the Entropy of Thermally Activated Dislocation Motion Based on the Theory of Thermoelasticity
Authors: Dimelfi R. J. , Nix W. D. , Barnett D. M. , Holbrook J. H. , Pound G. M.
Published: July 2007

An analysis is made of the entropy of thermally activated dislocation motion using the continuum theory of thermoelasticity. A relation for the activation entropy, which is valid for non‐linear, anisotropic elastic solids is derived. The activation entropy is expressed in terms of the general stress state in the solid and is found to be linear in the first temperature derivative of the elastic compliances, a relation often applied to the special case of linear, isotropic elasticity, but not previously derived for the more general case treated here. It is shown that, under the special conditions where pure shear or pure hydrostatic stress states are associated with both the initial and activated states, the present result can be reduced to the one often found in the past for the case of linear isotropic elasticity. However, these special conditions are never satisfied either in the thermally activated motion of dislocations or in diffusion. This suggests that the previous result is not applicable to these situations.
Electron Paramagnetic Resonance of Gd3+ in PrCl3·7H2O
Authors: Misra S. K. , Sharp G. R.
Published: July 2007

The electron paramagnetic resonance spectrum of Gd3+ in PrCl3·7 H2O, with 1/500 dilution of Gd/Pr ions, is studied with an X‐band spectrometer at 295, 79, and 1.60K. The low temperature data are used to determine the signs of parameters. The values of all parameters are determined. Estimates are made of the extent of admixture of the excited 6P7/2 state with the ground state 8S7/2.
An Analytically Tractable Structure Model for Disordered Systems. I. Systems Based on a Primitive Network
Authors: Unger H.‐J.
Published: July 2007

Amorphous structures are described by an ensemble of competing configurations where the probability distribution is constructed by model assumptions. This model leads to a three‐dimensional array of atoms with short‐range order but without long‐range order. The applicability is restricted to finite but sufficiently large clusters which transform at vanishing disorder to a primitive crystalline lattice. An analytical expression is obtained for the radial distribution function depending on one adjustable parameter.
Transient Bleaching of the F‐Band in Alkali Halides
Authors: Dokter G. , Hoffmann H. J.
Published: July 2007

The reversible bleaching kinetics of the F‐band is investigated experimentally in several additively coloured alkali halides in that range of temperatures where F'‐centres decay thermally. The results confirm the second‐order recombination model discussed in an earlier publication. From the measurements these thermal ionization energies of the F'‐centres are determined: KCl(0.44 ± 0.02) eV; KBr (0.34 ± 0.02) eV; NaCl(0.94 ± 0.1) eV; RbCl (0.28 ± 0.02) eV; RbBr (0.14 ± 0.02) eV; Rbl (0.24 ± 0.04) eV. Furthermore, the recombination coefficients rα of the halogen‐ion vacancies are estimated from the measurements provided that the recombination directly into the ground state dominates the recombination via the excited state F* with following emission of photons.
Polariton Effects in the Optical Spectra of Excitonic Molecules
Authors: Henneberger F. , Voigt J.
Published: July 2007

Starting from a model Hamiltonian which describes an excitonic molecule‐excitonphoton system at not too high electron‐hole pair densities, an excitonic molecule‐polariton interaction is derived. By means of this interaction the influence of polariton effects on the optical emission, absorption, and scattering processes involving excitonic molecules is studied theoretically. It is shown, that the transition probabilities of these processes are diminished due to polariton effects, especially in substances with small molecule binding energies. Furthermore, some characteristic modifications of the optical spectra (additional lines, changes in the line positions and shapes) are reported.
Electron Paramagnetic Resonance of Two Stoichiometry‐Induced Intrinsic Defect Centres in CuGaS2
Authors: Von Bardeleben H. J. , Goltzené A. , Schwab C.
Published: July 2007

Related to the sulphur annealing conditions, two different EPR signals are observed in CuGaSg2 With liquid sulphur, an isotropic photosensitive line is observed at g = 2.024 ± 0.002. With gaseous sulphur, an anisotropic spectrum, independent of light excitation, is recorded with g∥[001] = 2.114 ± 0.005 and g ⊥ [001] = 2.005 ± 0.005. The suggested models for the intrinsic centres correspond respectively to a hole trapped on a copper vacancy and to a S molecule ion. The formation of both centres, important for the electrical compensation processes in CuGaS, is justified by the molecular composition of sulphur as a function of its physical state.
Calculation of Nonradiative Multiphonon Capture Coefficients and Ionization Rates for Neutral Centres According to the Static Coupling Scheme II. Alternative Trap Models
Authors: Pässler R.
Published: July 2007

On the basis of the static coupling scheme, explicit calculations are performed of nonradiative multiphonon capture coefficients for one‐band‐model traps in a direct semiconductor. The present investigations are more comprehensive than earlier ones in the following respects: 1) the interaction of the trapped carrier with both the longitudinal acoustic and the longitudinal optical crystal modes is taken into consideration, 2) comparable analytical results are derived for the alternative models of a delta‐function potential (isoelectronic traps) and a Coulombic potential (hydrogenic traps), 3) a comparison is made between the one‐ and two‐parameter descriptions of such traps, and 4) explicit results are given both for pseudo‐intraband and ‐interband processes.
Reflectance Measurements on Single Crystals of PbFCl, PbFBr, and PbBr2
Authors: Eijkelenkamp A. J. H. , Vos K.
Published: July 2007

The reflectance spectra of single crystals of PbFCl and PbFBr are measured between 2.8 and 6.5 eV at 84 and 295 K. The absorption spectra are calculated by Kramers‐Kronig transformation. Peaks at 4.55 and 5.13 eV in the absorption spectra of PbFBr and PbFCI, respectively, are attributed to cation exciton transitions. In the reflectance spectra of vacuum‐cleaved single crystals of PbBr2, which are measured at 84 K with different polarization directions, a new fine structure is observed. An interpretation is given based on an ionic model.
Photoconductivity of sublimated rubrene films during oxidation. The magnetic field effect
Authors: Frankevich E. L. , Tribel M. M. , Soroltk I. A.
Published: July 2007

The mechanism of carrier generation in sublimated rubrene films in the course of their oxidation is investigated. The technique of the simultaneous measurement of photoconductivity and fluorescence of the samples and their variation in magnetic fields up to 3 kOe is employed. The excited states whose deactivation leads to both, fluorescence and carrier generation in unoxidized films are found to be rubrene singlet excited molecules 1M1 and in films exposed to air, excited complexes of 1M1 with O2 molecules and with transannular rubrene peroxide molecules MO2. The probability of carrier generation in the interaction of 1N1 with O2 is higher than with MO2. The existence is shown of an efficient channel of generation of triplet rubrene excitons, associated with the deactivation of excited triplet complexes of rubrene with MO2. The influence is investigated of variations in the crystalline structure of films on the behaviour of photoconductivity.
The quadratic response of a Fermi gas: Electronic charge density of valence electrons in zincblende semiconductors
Authors: Pickenhain R. , Milchev A.
Published: July 2007

The electronic charge density of a free electron gas is exactly calculated up to second order in the perturbing potential, using a direct density matrix approach. The effect of the second order correction is analysed for both f.c.c.‐lattice metals and zincblende semiconductors and it is found, that the singularity of the quadratic filter function, introduced in this work, leads to the appearance of a covalent bonding charge in semiconductors. For illustration of the method, the charge distribution of Ge is calculated and compared with results of explicit summation of the wave functions.
Scattering of free carriers due to the strain field of screw dislocations in semiconductors
Authors: Farvacque J. L. , Gerlach E.
Published: July 2007

Using a method, based on the equivalence of energy loss and Joule heat the scatterin due to screw dislocations in semiconductors is calculated. In particular the effect of the deformation potential and the piezoelectric effect are considered.
New aspects of the magnetoluminescence of a band to acceptor transition in GaAs
Authors: Rühle W. , Göbel E.
Published: July 2007

The magnetic field dependence of the conduction band to acceptor (e, A°) transition involving the acceptor carbon is analyzed in high‐purity p‐type GaAs at very low and very high excitation intensities. The change in the density of states of the free electrons with magnetic field determines the lineshape and the non‐linear peak shift of these transitions. From the observed splitting into different electron Landau levels an effective mass of m*/m0 = 0.0663 ± 0.006 is determined. The variation of the low‐energy wing of the luminescence line with magnetic field gives an effective mass of m*/m0 = 0.0669 ± 0.0006 if the effect of a linear Zeeman splitting is included.
Systematic investigation of the sign change of the Hall effect in n‐InSb in the quantum limit
Authors: Fantner E. J. , Kuchar F. , Bauer G.
Published: July 2007

At liquid helium temperatures a change of the sign of the Hall effect of n‐InSb is observed in the magnetic freeze‐out regime. The experimental examination of possible explanations and their discussion show that only the contribution of the asymmetric impurity scattering (“skew scattering”) to the Hall effect explains the observed systematic behaviour of the sign change.
Hole mechanisms of H and F centre production in KCl crystals
Authors: Zazubovich S. , Osminin V.
Published: July 2007

The formation of interstitial halogen atoms (H centres) under the photodestruction of hole impurity Tl2+ centres in KCl crystals is observed by the method of thermally stimulated luminescence from 4.2 to 250 K. The existence of two new hole mechanisms of the H centre generation is established. It is shown that the H centres can be produced under recombination of holes (optically released from A2+ centres) with localized electrons (F or A° centres), as well as under a non‐recombination process caused apparently by the decay of holes into α and H centres.
Structural investigation of the laves phase MgZn2 with model potential calculations
Authors: Rennert P. , Radwan A. M.
Published: July 2007

The structure of the intermetallic compound MgZn2 is investigated using Shaw's model potential. The calculations are carried out using different model potential parameters of Zn. Variations of the axial ratio c/a and the parameter x in the Kagomé nets are studied. Charge transfer, which is taken into account in an approximate and simple way, plays an important role in the determination of the stable structure of MgZn2. Staking fault energy is also calculated.
Photoluminescence in amorphous silicon
Authors: Engemann D. , Fischer R.
Published: July 2007

Photoluminescence of amorphous silicon is measured in the range from 77 K to room temperature. The temperature dependence of the three bands, of which the total luminescence spectrum consists, could be determined seperately. It is found that all three of them decrease at high temperature with approximately the same activation energy of 0.13 eV. This decrease sets in at different temperatures for the different bands. The three bands are interpreted as transitions within electron‐hole pairs consisting of more or less deeply trapped carriers. The activated decrease at high temperature is explained by thermally activated dissociation of these pairs. The electron‐hole pair concept also explains why photoconductivity decreases about twice as fast as luminescence when the substrate temperature is lowered.
Cyclotron resonance and quantum oscillations in semimetallic bismuth‐antimony alloys with Eg  0
Authors: Braune W. , Kuka G. , Hess S. , Müller H.‐U. , Jung T.
Published: July 2007

Measurements are made of quantum oscillations of the surface impedance in the MHz range and cyclotron resonances of the electrons in the GHz range in two crystallographic planes in a semi‐metallic bismuth‐antimony alloy. The experimental results arc analyzed on the basis of Abriko‐sov's model of the gap‐less state of the bands at the point L of the Brillouin zone.
The quadratic response of a fermi gas. Building‐up of the covalent bonding in zineblende semiconductors. A nondiagonal density matrix treatment
Authors: Milchev A. , Pickenhain R.
Published: July 2007

The nondiagonal density matrix of a free electron gas, perturbed by a crystalline potential, is considered in the momentum‐space representation. One obtains for the second‐order correction to the unperturbed electron density on the diagonal an exact analytic expression for the quadratic filter function. It is used to evaluate the valence electron densities of Ge, Si, Sn, and GaAs. The influence of the separate pseudopotential formfactors, taken into account, on the bonding charge heaping up is thoroughly investigated.
Bound electron states close to the conduction band in germanium due to 60° dislocations
Authors: Winter S.
Published: July 2007

Electron states with energies close to the conduction band are calculated in germanium with 60° dislocations within the framework of effective mass and deformation potential theories. These states are found to have energies down to 0.10 eV below the conduction band and have wave functions of a considerable spatial extent.
The superposition model in crystal field theory
Authors: Eremin M. V. , Kornienko A. A.
Published: July 2007

A crystal field Hamiltonian is obtained with allowance for metal‐ligand interaction. Apart from the standard terms it contains the operators {U(k′)U(k″)}(p) whose role is discussed. Convenient expressions for the Hamiltonian parameters are obtained which make it possible to take into account the spatial distribution of all cluster electrons. Detailed calculations and comparison with experiments are made on KMgF3,:Mn2+ and KNiF3, crystals.
Factor group analysis of the Raman spectrum of Pb5Ge3O11
Authors: Müller‐Lierheim W. , Suski T. , Otto H. H.
Published: July 2007

The complete Raman spectrum of Pb5[GeO4/Ge2O7] is investigated. The results are analyzed by factor group analysis for external and librational modes taking polariton effects into account. The temperature dependence of the intensity of different modes in the depolarized spectra allows to distinguish between modes of pure E symmetry (at the Γ‐point for P3 space group) and those with mixed symmetry. The modes in the frequency region above 200 cm+1 are assigned to internal modes of the GeO4 tetrahedron and the Ge2O7 double tetrahedron. As Pb5Ge3O11 exhibits above its ferroelectric‐paraelectric phase transformation at Tc = 450 K additional anomalies of the birefringence and of the lattice constants the Raman spectrum up to 750 K is investigated.
Triplet exciton diffusion and double photon absorption in tetracene
Authors: Aladekomo J. B. , Arnold S. , Pope M.
Published: July 2007

Delayed fluorescence from tetracene crystal excited at 8600 Å is studied. The luminescence decay is interpreted in terms of a model in which singlets are generated by triplet exciton fusion and by double‐photon absorption. The double‐photon absorption coefficient is estimated to be 5.8 × 10―27 cm s. The triplet exciton diffusion coefficient along the crystal b axis is determined by the spatial intermittency method to be (4 ± 1) × 10―3 cm2/s. The influence of this diffusion coefficient on other processes is discussed.
Ultraviolet Photoelectron Valence Band Studies on Phthalocyanine Compounds
Authors: Battye F. L. , Goldmann A. , Kasper L.
Published: July 2007

Ultraviolet photoelectron spectra excited by HeI and HeII radiation from the valence bands of H2Pc (H2C32H16N8), FePc, CuPc, and PtPc are measured for binding energies down to 18 eV. No satisfying agreement of the photoelectron spectra with available molecular orbital calculations is found. Although weak contributions of the central metal d‐electrons can be identified, the spectra are surprisingly similar for all Pc's and represent essentially the π‐electron ring system. They appear to be a superposition of the spectra of benzene and an inner porphin skeleton. A spectral change in H2Pc around 10 eV as compared to the metal Pc's is interpreted in terms of different σ‐bonds to the central atoms.
Finite Dislocation Elements in Finite Media. I. Basic Theory
Authors: Korner A. , Svoboda P. , Kirchner H. O. K.
Published: July 2007

The interaction of finite co‐planar dislocation pieces in a finite medium is considered with special reference to the case where two partial dislocations split trapezoidally in a thin foil. The calculation is carried out on the force and moment level. The presence of the surface is taken into account by Lothe's theorem. The remaining problem of the interaction of finite pieces in an infinite medium is considered in detail. It is shown that such problems require the execution of integrals involving only the energy E of dislocations (or interaction energy of parallel dislocations) within that plane — no derivatives of E are required. Formulae suitable for numerical evaluation are derived.
A Theory of the Hall Effect in the Hopping Region in Disordered Systems. I. Rate Equation in Presence of Electrical and Magnetic Fields
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

The equation of motion of the density matrix in presence of an electrical and a magnetic field is derived. This equation holds for arbitrary electrical fields. It describes the charge transport in systems with localized charge carriers: in ordered systems in case of small polarons (strong electron–phonon interaction) as well as in disordered systems with an arbitrary strength of electron–phonon interaction. After linearizing this equation with respect to the electrical field it describes in particular the Hall effect in such systems.
Pseudopotential Theory and the Problem of Covalent Bonding. I. General Theory of Covalent Charge
Authors: Krasko G. L. , Makhnovetskii A. B.
Published: July 2007

A theory of covalent bonding is developed, based on the Brillouin‐Wigner perturbation theory. The covalency (multi‐ion interaction) effects are shown to be associated with the existence of a so‐called “covalent charge distribution” ϱcov(g). The latter arises as a result of “coherent” electronic scattering on some Bragg planes. A procedure is suggested of sorting out “the main” terms of the Brillouin‐Wigner series and selfconsistent determination of the covalent charge Fourier‐transforms ϱcov(g), playing the fundamental role in the theory. As an example ϱcov(g) is found for the diamond lattice. Concrete calculations of ϱcov(g) and crystal lattice stability for Si, Ge, and α‐Sn will be presented in part II of the paper.
Two‐Photon Resonance Raman Scattering by Excitons via an Intermediate Excitonic Molecule. I. Bare‐Exciton Approach
Authors: Bechstedt F. , Henneberger F.
Published: July 2007

Two‐photon resonance Raman scattering from excitons via an intermediate excitonic molecule is studied theoretically. The line shape and dispersion of the scattering spectrum is calculated in terms of damping theory for excitation near the resonance energy of the two‐photon absorption for the molecule. By taking into account a finite band width of the exciting laser light the secondary radiation spectrum is decomposed into a Raman‐ and a luminescence‐like part. The relation between these parts is discussed in dependence on the dampings acting on the electronic intermediate and final states of the scattering process as well as the laser band width. Qualitative agreement is found with the experimental results for CuCl and CdS.
Wave‐Vector Dependence of the Electron Energy Losses of Boron Nitride and Graphite
Authors: Büchner U.
Published: July 2007

Wave‐vector (q)‐dependent energy loss functions, Im [−ϵ⟂(ω, q⟂)]−1, and dielectric functions, ϵ2⟂(ω, q⟂), of hexagonal boron nitride and graphite up to q⟂‐values of almost one third of the Brillouin zone and up to energies of 40 eV were derived from electron energy loss measurements. The results are discussed in terms of non‐vertical interband transitions of two‐dimensional band structures.
Deformation Effects on Biexcitons in Wurtzite‐Type Crystals
Authors: Goede O.
Published: July 2007

The effects of uniaxial stress on optical biexciton transitions in wurtzite‐type crystals are discussed in the framework of an effective‐mass approximation using the Bir‐Pikus Hamiltonian and taking into account electron–hole exchange interaction and JJ‐coupling of the two holes. The calculated stress‐induced energy shift, splitting, and polarization of the ground‐state biexciton line for CdS are compared with the available results of piezoemission measurements. Furthermore, the stress dependence of the optical transitions between the AA(Γ1) biexciton ground state and A(Γ6) and B(Γ1, Γ5,) exciton final states are discussed. In CdS the AA(Γ1) → A(Γ6) emission line may hardly be detectable for σ ⟂ c. The AA(Γ1) → B(Γ1, Γ5) lines, however, should be observable for σ ∥ c and also the expected values of the electron‐hole exchange interaction in the exciton states and the JJ‐coupling of the holes in the biexciton states.
Electrical Properties of (DEPE) (TCNQ)4
Authors: Somoano R. B. , Hadek V. , Yen S. P. S. , Rembaum A. , Hsu C. H. , Deck R. J. , Datta T. , Hermann A. M.
Published: July 2007

Conductivity measurements are performed on single crystals of two phases of (1,2‐di(N‐ethyl‐4‐pyridinium)ethylene2+) (7,7,8,8‐tetracyanoquinodimethane), (DEPE) (TCNQ)4. Conductivities are determined by four probe and Montgomery methods, and the highest values measured are those along theb0‐axis for the monoclinic phase (I) crystals, ≈ 50 (Ω cm)−1at room temperature. The resistivities are in general exponentially activated (semiconducting behavior). Thermoelectric power measurements are performed along the b0‐axis of phase I crystals and the thermopower tends to saturate at high temperatures to −60 μV/K, characteristic of a quarter‐filled correlated band. No evidence for the existence of a stable metallic state reported by Ashwell et al. was found. The preliminary crystal structures of both phases are discussed.
Zero‐Field Splitting in Trigonal Symmetry. Cr3+ and Fe3+ in α‐LiIO3
Authors: Karthe W.
Published: July 2007

The constant of zero‐field splitting B: of Cr3+ in α‐LiIO3 is calculated employing the methods of Rimmer and Johnston and Macfarlane. The lattice sums are determined from the structure of the chromium centre and by the use of the quadrupolar constant of lithium in chromium‐doped LiIO3. A good agreement between the theoretically and experimentally determined values of B is achieved. The contributions of most mechanisms to the zero‐field splitting of S‐states investigated so far are calculated for Fe3+ in α‐LiIO3 by using the above lattice sums because the structure of the centre is quite similar to that of the Cr3+ centre. The theoretically determined value proves to be too small by one order of magnitude. The relativistic mechanism in an overlap model is supposed to contribute to a better agreement.
A Theory of the Hall Effect in the Hopping Region in Disordered Systems. III. DC Hall Effect
Authors: Böttger H. , Bryksin V. V.
Published: July 2007

The random resistor network with extrinsic currents, which describes the Hall effect in the hopping region, is averaged by using ideas of percolation theory. Concrete results are obtained for the dependence of the dc Hall mobility on temperature and concentration of sites for weak and strong interaction with phonons. Both the limits of electron and hole conduction are investigated. The sign of the Hall mobility is shown not to change by passing from electrons to holes (p–n anomaly).
The Pair Correlation Function and the Dielectric Response in the Homogeneous Electron Gas
Authors: Brosens F. , Lemmens L. F. , Devreese J. T.
Published: July 2007

Recent developments in the theory of the homogeneous electron gas relate the pair correlation function g(0) at the origin to a) the static limit of the exchange and correlation effects as well as to b) an integral over frequencies of the inverse dielectric function. It is shown that any static local field correction violates at least one of both the conditions, and that from the frequency‐dependent treatment of exchange, only the variational approach satisfies both the requirements.
Calculation of the Energy Band Structures and Carrier Mobilities in γ‐Platinum Phthalocyanine Single Crystals
Authors: Singh Jai
Published: July 2007

Tight‐binding calculations of the energy band structures for excess holes and electrons in γ‐PtPc are presented using single Slater‐type atomic orbitals. Carrier mobilities are also calculated using the constant relaxation time approximation. Hole energy bands containing the contributions from the platinum metal atom are shown to have as large band‐width as 3.6 eV along the crystal axis b. On the basis of the mobility calculations, it is predicted that the holes probably play a dominant role in the carrier transport in γ‐PtPc.
Interaction of Atomic Order with Localized Ferromagnetism
Authors: Billard L. , Chamberod A. , Villemain P.
Published: July 2007

A pair model is used to describe atomic order and an inhomogeneous Ising model to describe magnetic properties of a binary alloy. Both ones are treated simultaneously in the mean field approximation. A comparison is made with experimental results on FeCo and FeNi3 alloys.
Theory of Temperature Dependence of the Magnetization in Rare‐Earth‐Transition‐Meta1 Alloys
Authors: Szpunar B. , Lindgård P‐A.
Published: July 2007

It is shown that the temperature dependence of the magnetic moments and Curie and ferrimagnetic compensation temperatures for Gdl‐xTx (T = Co, Ni, and Fe) and Y1‐xCox can be accounted for by a simple model assuming a RKKY interaction between the rare‐earth moments and the transition‐metal pseudo‐spin. The interaction is mediated by an effective alloy medium calculated using the CPA theory and elliptic densities of states.
Electronic Structure and Superconductivity in Solid Solutions Based on In, Pb, and TI
Authors: Sulkowski C.
Published: July 2007

There exist an anomaly in the Tc‐course versus electron concentration Z in indium alloys at the value Z = 2.98 electrons/atom. This anomaly is caused by changed of electronic structure. Results of Tc‐measurements in ternary and quaternary indium solid solutions in the Tc anomaly area are presented. The minimum value of Tc for these alloys occurs at Z = 2.98. The effective Z, denoted Z*, is determined from the alloy composition at the minimum Tc resulting in following numbers: (2.0 ± 0.1) for Mg, (2.6 ± 0.1) for T1, (3.9 ± 0.1) for Pb, (4.0 ± 0.1) for Sn, and (3.25 ± 0.2) electrons/atom for Hg. Unique dependence of Tc on Z* is found for each group of alloys when the effective electron concentrations are calculated from the Z*‐number.
Raman Study of Dynamic Disorder and the Ferroelectric Transition in K4Fe(CN)6.3 H20
Authors: Savatinova I. , Anachkova E.
Published: July 2007

Polarized Raman spectra of ferroelectric K4Fe(CN)6 ˙ 3 H2O are measured at various temperatures from 140 K up to above the phase transition (Tc = 249 K). The transition becomes apparent through strong phonon coupling of a band at 62 cm‐1 with a low‐frequency continuum in the Bg/A″ spectrum. A detailed analysis of the temperature dependence of this phenomenon is made by Fano's expressions. Intensity measurements indicate the dominant contribution of the water disorder mechanism to the continuum. An anomalous broadening of the Bg/A″ bands in the 80 to 180 cm‐1 region is observed. In the vicinity of Tc the C ≡ N stretching vibrations reveal some changes which are assigned to a slight distortion of the [Fe(CN)6]4‐complexes.
Kinematical exciton levels and absorption of light
Authors: Hadžiahmetović D. , Pirić M. , Tošič B. S.
Published: July 2007

It is shown that the kinematical interaction of Frenkel excitons leads to the creation of additional optical excitations. The role of these excitations is considerable in the luminescence processes and in the processes of the absorption of light.
Dielectric response of saturated semiconductors
Authors: dos Reis F. G. , Cerdeira Hilda A. , Luzzi R.
Published: July 2007

The optical properties of a direct‐gap semiconductor are investigated under conditions of a strong optical pumping by an intense laser source of frequency slightly larger than the energy band gap. By taking into account the Coulomb interaction between electrons and holes we found that the energy gap in the quasi‐particle spectrum diminishes. The dielectric constant of the system is calculated in the generalized Hartree‐Fock approximation. The Raman cross‐section is found to vanish for energies below 2Δ, where Δ is the quasi‐particle energy gap.
A simple model for the calculation of heat of formation for high‐concentration substitutional defects in alkali halide crystals
Authors: Paul S. , Sengupta S.
Published: July 2007

An analytical expression for the heat of formation of substitutional defects in high concentration is developed on the basis of a simple model. In a previous paper the authors developed a simple formula for the heat of formation of substitutional defects with low concentration which convenient formula is utilized for calculating the heat of formation when the defect concentration is high. In this model the defect concentration is developed stepwise and in every step the defect crystal is considered as equivalent to a perfect crystal with a modified lattice parameter determined by the Vegard's law. Values for the heat of formation over the entire composition range are calculated and compared to experimental values for eight different solid solutions. For negative ion substitution, the values are in good agreement with the experimental results. The reason for the slightly larger discrepancy for the positive ion substitution may be ascribed to the effect of polarizability of the negative ions.
A non‐diagonally screened model potential for semiconductors applied to the band structure calculation of Si
Authors: Pasemann L.
Published: July 2007

On the basis of a non‐linear approximation of the electronic charge density in the perturbing potential the screening of the ionic model potential by the valence electrons of a semiconductor is described by a screening matrix and the depletion hole charge. Using the bare model potantial of Shaw the constructed screening model is tested by local and non‐local band structure calculations of Si. The band structure calculated with the non‐local model potential is in good agreement with experimentally established data. The local approximation is not appropriate.
Interband electronic Raman scattering by a three‐band semiconductor in the presence of a resonant electromagnetic field
Authors: van Hieu Nguyen , Ai Viet Nguyen
Published: July 2007

The influence is investigated of a resonant electromagnetic field on the interband electronic Raman scattering of a three‐band semiconductor with direct band gaps. Explicit expressions for the cross‐sections are given. The form of the Raman spectra depends on the effective masses of electrons and holes.
A low temperature equation of state for monovalent metals
Authors: Goel S. C. , Midha Y. R. , Nanda V. S.
Published: July 2007

An equation of state for metals based upon a simple model, which ignores the effect of free electrons, was given in an earlier paper. An improved equation of state is obtained for monovalent metals by taking account of the electronic contribution in the free energy expression. This result is used to explain the thermal expansivity of copper in the range between 1 and 12 K. The same set of parameters give theoretical specific heat values which are in fairly good agreement with the reported experimental data in the same range of temperature.
A new approach to lattice properties of alkali halide crystals
Authors: Sharma M. N. , Kant A.
Published: July 2007

A new form of overlap repulsion as a function of interorbital distance as suggested earlier is employed to calculate some more properties of alkali halide crystals, such as infrared absorption frequency (v0), Grüneisen (γ), and Anderson‐Grüneisen (δ) parameter. The results are compared with the observed values and those calculated by earlier workers. In general the results show a marked improvement, establishing the validity of the proposed model.