bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic Interactions in Co3O4
Authors: Scheerlinck D. , Hautecler S. 
Published: July 2007

AbstractCo3O4 is a normal spinel with only A‐A interactions which becomes antiferromagnetic at TN ≈︁ 40 K. The exchange integrals between one ion and its first and second neighbours are derived from the second moment of energy distributions of neutrons scattered by a polycrystalline sample in its paramagnetic state (155 K) and from a Green's function analysis of TN. The result, J1/kB = −(7.27 ± 0.25) K and |J2|/kB < 0.3 K, indicates that the interaction with the nearest neighbours is by far dominant.
Nonlinear Electromagnetic Properties of Semiconductors with a Superlattice
Authors: Ignatov A. A. , Romanov Yu. A. 
Published: July 2007

AbstractThe nonlinear high‐frequency conductance of superlattices due to the nonparabolicity of minizones has been investigated. Exact expressions for the current excited with a harmonic electric field are obtained. A selftransparence effect is predicted. It is shown that in the presence of a strong high‐frequency field the increase of the low‐frequency oscillations is possible. The frequency doubling and the high‐frequency signal detection in the presence of a strong constant electric field is also investigated.
Temperature Dependence of Raman Spectra of Cuprous Halides
Authors: Fukumoto T. , Nakashima S. , Tabuchi K. , Mitsuishi A. 
Published: July 2007

AbstractThe temperature dependence of the half linewidth and frequency shift of Raman lines in cuprous halides are investigated in the temperature range from 4.2 to 300 K. As for the LO and TO mode of CuI and CuBr, and the LO mode of CuCI, the experimental linewidth and frequency shift are in good agreement with the theoretical ones which have been calculated in terms of three‐phonon interaction. In order to explain the temperature dependence of a strong combination mode in conjunction with the TO phonons of CuCI, the hybridization theory proposed by Ruvalds and Zawadowski is employed in which hybridization between a two‐phonon and a single phonon state is postulated. A numerical analysis is made assuming that the acoustic combination mode (TA + LA) hybridizes with the TO mode. The anomalous frequency shift and line broadening of the strong combination line are explained by the temperature dependence of the coupling constant of two acoustic phonons (between TA and LA mode). The linewidth and frequency shift of the TO mode can be explained in some degree by this hybridization theory.
Isotropic Compton Profile of LiH in the Tight‐Binding Approximation
Authors: Grosso G. , Parravicini G. Pastori , Resta R. 
Published: July 2007

AbstractThe isotropic Compton profile of LiH is considered in the tight‐binding approximation. Firstly, it is shown how to simplify the inversion of the overlap matrix in LCAO calculations of crystals by fully exploiting the lattice symmetry. Then the wave function for the hydride ion in LiH is calculated variationally by performing restricted Hartree‐Fock calculation for a cluster with 27 centers. The corresponding calculated Compton profile is closer to the measured curves.
Approximate Methods for Taking Absorption into Account in the Dynamical Theory of Electron Diffraction
Authors: Andrew J. W. , Sheinin S. S. 
Published: July 2007

AbstractApproximate methods for taking absorption into account in the dynamical theory of electron diffraction can be obtained from the exact theory by assuming that mixing between certain pairs of Bloch waves is unimportant and can therefore be neglected. The assumptions required to reduce the exact theory to the lower order perturbation theories previously published in the literature are examined. This is of interest since an understanding of these assumptions provides a physical basis for comparing these theories as well as giving the insight required to develop new approximations. The details of a new approximate theory, referred to as the sub‐matrix method, are also given. This approach has a number of advantages over other approximate methods. Firstly, it is applicable under conditions which lead to either non‐degenerate, quasi‐degenerate or degenerate Bloch waves. As well the method can give very close agreement with the exact theory while at the same time providing advantages from a computational point of view.
Renormalized Density of States by Conduction Electron–Localized Spin Scattering
Authors: Ausloos M. 
Published: July 2007

AbstractAn analytic expression for the change in density of electronic states in the presence of conduction electron scattering by localized spins is derived. The expression seems to be general enough to be applicable to any scattering by a localized potential. The change in the density of states at the Fermi level with respect to the free electron density of states is also explicitly given for the low temperature region.
Optical Absorption, Fluorescence, and Electron Spin Resonance of Ir4+ on Octahedral Sites in Y3Ga5O12
Authors: Andlauer B. , Schneider J. , Tolksdorf W. 
Published: July 2007

AbstractIr4+, incorporated on the octahedral a‐sites of Y3Ga5O12:Ir, has been investigated by electron spin resonance at 9 and 35 GHz, and by optical absorption spectroscopy in the range 5000 to 6000 cm−l. For an unequivocal optical identification Zeeman measurements were performed. In the same energy range fluorescence from Ir4+ could be observed. Analysis of the data obtained by electron spin resonance and optical techniques reveals that the properties of the 2T2g ground state of Ir4+ are partly determined by interaction with higher excited states.
On the Anomalous Magneto‐Resistance of Gallium Arsenide in Intense Magnetic Fields
Authors: Jones J. C. , Wallace P. R. 
Published: July 2007

AbstractThe 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

AbstractIt 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractEs 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractA 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.
On the Dynamics of Hydrogen‐Bonded Ferroelectrics
Authors: Pak K. N. 
Published: July 2007

AbstractThe 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

AbstractIt 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

AbstractThe 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

AbstractInelastic 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

AbstractThe 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

AbstractAn 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

AbstractThe 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

AbstractAmorphous 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

AbstractThe 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.