bluuebunny/crossref_metadata_2025_split huggingface dataset:
X‐ray spectroscopic investigation of beryllium by polarized K‐emission valence bands
Authors: Dräger G. , Brümmer O. 
Published: July 2007

AbstractA theory of polarized K‐emission bands is given and a special method of measuring the spectra by means of a grating spectrometer is described. In this way polarized components of the radiation emitted by a monocrystalline sample are recorded. The spectra of optically anisotropic crystals measured in such a way possess a higher content of information by additional applicable selection rules for polarized radiation compared with the spectra of polycrystalline samples. In the case of the polarized Be K‐emission valence bands conclusions can be drawn on the existence of bonding charges. A more unambiguous correlation of spectra structures with van Hove singularities of the band structure provides a better experimental check up of several recent band structure calculations for metallic beryllium. This is especially demonstrated for the discontinuities at the points Γ and M.
Mixing of frenkel excitons and ionic excited states of a linear molecular crystal with two molecules in the unit cell. II. Physical consequences
Authors: Petelenz P. 
Published: July 2007

AbstractThe direct generalization of the Merrifield theory for a linear crystal with two molecules in the unit cell, presented in part I of this paper, is applied to a model crystal with nearest‐neighbour interactions. The parameters of resulting Merrifield‐type effective Hamiltonian are calculated for several parameter sets along with the intensities of the transitions induced by the mixing. The electric field dependence of the results is discussed. Differences with the Merrifield theory for a crystal with one molecule in the unit cell are pointed out.
Deformation of the vanadium phonon spectrum due to additions of uranium atoms
Authors: Syrykh G. F. , Zheknov A. P. , Zemlyanov M. G. , Chernoplekov N. A. , Solovev G. I. 
Published: July 2007

AbstractMeasurements are made of inelastic incoherent scattering of neutrons by a V0.97U0.03 alloy using a time‐of‐flight spectrometer with cold neutron source. The presence of quasi‐local vibrations (QLV) in the vibrational spectrum of the V0.97U0.03 alloy is revealed and the variation in the efficient force constants (FC) of the impurity atom‐matrix atom interaction is estimated and compared with the interaction between matrix atoms. It is shown that the revealing of the QLV in the spectra of inelastic incoherent neutron scattering essentially depends on the relation between the total amplitudes of neutron scattering by impurity atoms and matrix atoms.
Submillimeter conductivity of thermal and warm electrons in GaAs and InP
Authors: Philipp A. , Kuchajr F. , Seeger K. 
Published: July 2007

AbstractExperimental and theoretical results of the transmittance of submillimeter radiation (890 GHz) by n‐GaAs and n‐InP show that the relaxation time obtained from the do mobility can be used for describing high‐frequency effects in this range of the spectrum too. The investigation of heating the conduction electrons by the electric field of radiation (“warm electrons”) yields the energy relaxation time τε in GaAs and InP. The experimental values of τε are larger than those calculated by a simple electron temperature model. This discrepancy can be removed by assuming a “two‐electron‐temperature” model.
Wavelength‐modulated reflectivity spectra of ZnSe and ZnS from 2.5 to 8 eV
Authors: Theis D. 
Published: July 2007

AbstractWavelength‐modulated reflectivity spectra of ZnSe and ZnS are presented in the energy range from 2.5 to 8 eV. Around the fundamental transition Eo qualitative evidence for the occurrence of excitonic polariton and exciton‐phonon quasi‐bound complex is given. The structure at higher energies is compared to calculated derivative reflectivity curves of Walter et al. The agreement between theory and experiment is good.
Surface effects in spin‐wave resonance in thin YIG films
Authors: Ramer O. G. , Wilts C. H. 
Published: July 2007

AbstractFerromagnetic resonance data from thin YIG films are compared with calculated data using the surface layer and tensorial models of surface pinning. The consistency with which the surface layer model explains the resonance phenomena is considered strong evidence that the boundary conditions are provided by surface regions with magnetic properties different than the bulk.
A new approach to overlap repulsion
Authors: Sharma M. N. , Kant A. , Misra K. D. 
Published: July 2007

AbstractIntroducing the concept of interorbital distance (δi), a new form of overlap repulsion (ΨR = Aδ) for ionic crystals is suggested. Alkali halide crystals are considered for the present purpose. The theory is based on the assumption that the ions possess a structure like the Bohr atom and this assumption along with other basic considerations exhibits a gap effect which leads to the formation of the present potential energy function. To test the validity of the new approach the compressibility (β) and the elastic constant (C‖) of alkali halide crystals are evaluated. The closeness of theoretical and experimental values strongly supports the present approach. The ionic radii of alkali and halogen ions are also evaluated and discussed in detail in the light of Pauling's values.
Random collinear structures of a dilute magnetic alloy
Authors: Medvedev M. V. , Zaborov A. V. 
Published: July 2007

AbstractIt is shown that the internal molecular field distribution in a dilute magnetic alloy with infinite‐ranged exchange interactions is a Gaussian one. Magnetic properties of two random collinear states – an uniaxial spin‐glass state and a random ferromagnetic state – are investigated for an uniaxial model of magnetic alloy.
On flux pinning by point pinning centres in Type‐II superconductors
Authors: Schmucker R. , Brandt E. H. 
Published: July 2007

AbstractThe maximal flux line displacement s(O) caused by a point force is calculated from the correct elastic properties of the flux line lattice, which were derived in a provioous paper. For bx2> (1  b)(x Ginsburg‐Landau parameter, b  B/Hc2 reduced induction) non‐local effects become important and lead to an s(O) larger than the result of local theory of elasticity by a factor \documentclass{article}\pagestyle{empty}\begin{document}$ {x \sqrt {b/2(1 - b)} }$\end{document} or even more. As a consequence, the volume pinning force Pv for a depinning process becomes larger by this factor. In particular, near H<c2, Pv ∼ \documentclass{article}\pagestyle{empty}\begin{document}$ {\sqrt {1 - b} }$\end{document} is obtained rather than Pv ∼(1 − b) as obtained by previous authors.
Influence of electron‐ion spin coupling on the electronic properties of a ferromagnetic semiconductor
Authors: Nolting W. 
Published: July 2007

AbstractAn attempt is made to obtain theoretical predictions concerning ferromagnetic semiconductors, especially the Eu‐chalcogenides, by approximate determination of relevant spectral densities. Of primary interest is the influence of the localized Heisenberg spin system (4f‐levels) on the conduction band. The electron‐magnon interaction is regarded in the sense of emission and absorption of magnons by conduction electrons and the resulting distortion of the electronic dispersion is discussed for an f.c.c. lattice. The magnetic red shift of the optical absorption edge, as it is experimentally observed for the ferromagnetic Eu‐chalcogenides, can be reproduced nearly quantitatively by discussing the dependence on the band width. Some well‐known, exactly calculable limiting cases, e.g. the Richmond‐criterion, are shown to be fulfilled by the approximation.
Optical absorption in Hg Te
Authors: Szuszkiewicz W. 
Published: July 2007

AbstractOptical absorption in intrinsic HgTe is measured in the wavelength range 2.0 to 25 μm at tem peratures from 10 to 295 K. The Γ6 Γ8 transitions are observed at low temperatures. The ab‐ sorption coefficient is theoretically calculated using Kane's formulae for inverted band structure. The results of the calculations based both on the complete Kane theory and on the approximation of large spin‐orbit splitting are obtained. It is shown that the latter approximation does not describe well experimental absorption data. The agreement between the experimental results and the theoretical curves which are based on the complete Kane theory with the reasonable values of band structure parameters is much better. However, for energies greater than 0.35 eV small discrepancy is observed. This discrepancy increases with the energy.
Large unit cell calculations of solids in the CNDO approximation
Authors: Evarestov R. A. , Lovchikov V. A. 
Published: July 2007

AbstractEquations are obtained describing in the CNDO approximation the electronic states of solids in the framework of the large unit cell approach. The CNDO approximation is applied to first timefor three‐dimensional crystals (diamond and lithium fluoride). The applicability of the CNDO parametrization suggested for molecules for the calculation of electronic states of solids is discussed. The results obtained are compared with those of band structure calculations using the LCAO approximation.
On the third‐order elastic constants of ionic solids with NaCl structure
Authors: Garg V. K. , Puri D. S. , Verma M. P. 
Published: July 2007

AbstractExpressions for the third order elastic (TOE) constants of ionic solids, crystallizing in the NaCl structure, derived earlier from the Lundqvist potential by Puri and Verma are modified to take account of the second neighbour short range repulsive interactions. These expressions are used to evaluate the TOE constants and the pressure derivatives of the second order elastic (SOE) constants of alkaline earth oxides and those alkali halides which crystallize in the NaCl structure. The calculated values give fairly good agreement with available experimental results and in this respect are better than the results reported by other theoretical workers and those obtained in earlier calculations where short range repulsion was limited to nearest neighbours only.
Extended X‐ray absorption fine structure (EXAFS) studies of the lanthanides
Authors: Vijayavargiya V. P. , Gupta S. N. , Padalia B. D. 
Published: July 2007

AbstractExtended X‐ray absorption fine structures (EXAFS) of the L3‐edges of dysprosium, holmium, thulium, ytterbium, and lutetium metals are investigated using a focussing X‐ray crystal spectrograph of transmission type. The present results are interpreted in the light of the EXAFS theories proposed by Hayasi and by Lytle and the interatomic distances in these metals are determined using Lytle's method. It is found that Moseley's law applies to the white lines in the lanthanides. A comparison of the widths of the white lines with the positron annihilation rates in lanthanides reveals interesting correlations.
Etude de la chaleur spécifique de sels doubles de TCNQ à cations non‐radicalaires
Authors: Delhaes P. , Keryer G. , Flandrois S. , Manceau J. P. 
Published: July 2007

AbstractOn étudie la chaleur spécifique de sels doubles de TCNQ à cation diamagnétique du type ammonium quaternaire. Les résultats des mesures effectuées dans certains cas entre 0,2 et 300 K sont comparés aux variations thermiques de susceptibilité magnétique. Deux types de complexes sont examinés: ceux constitués d'un empilement irrégulier des plans de TCNQ et se comportant comme des semi‐conducteurs et ceux constitués d'un empilement régulier possédant une conductivité électrique métallique mais à température ambiante seulement. Ces derniers présentent un terme linéaire de chaleur spécifique et un paramagnétisme non nul aux trés basses températures. Pour en rendre compte, on montre qu'il faut appliquer un modéle de Hubbard “étendu” unidimensionnel. Une analyse théorique de ce modéle où les interactions électron‐électron sont dominantes permet d'interpréter les résultats obtenus.
Long‐wavelength optical phonons in MnxHg1−xTe mixed crystals
Authors: Gȩbicki W. , Nazarewicz W. 
Published: July 2007

AbstractThe infrared reflection spectra of MnxHg1–xTe with 0 ≦ x ≦ 0.69 are measured in the optical region from 50 to 300 cm+1 at 90 and 295 K. Long‐wavelength optical phonon frequencies are obtained by means of the Kramers‐Kronig relations. The behaviour of the phonon spectra is classified as a “two‐mode type behaviour”. The random‐element isodisplacement (REI) model is applied to calculate numerically the long‐wavelength optical phonon frequencies in the mixed crystals as a function of composition. The theoretical results are in good agreement with those obtained experimentally.
Inhomogeneous strain field scattering deformation potential. I. Theory
Authors: Farvacque J. L. , Lenglart P. 
Published: July 2007

AbstractThe influence of a small inhomogeneous deformation strain field on the expression of the crystal Hamiltonian is presented in terms of perturbation of the undeformed crystal Hamiltonian. In the different perturbation terms, leading to the band‐structure shift in semiconductors and which explains the conductivity change in piezoresistance experiments, one identifies an aperiodical term with the scattering potential needed for free carrier mobility calculations.
Inhomogeneous Strain Field Scattering Deformation Potential. II. Free‐Carrier Mobility Associated with Dislocation Strain Fields in Semiconductors
Authors: Farvacque J. L. , Lenglart P. 
Published: July 2007

AbstractUsing the expression of the scattering deformation potential, established in a preceding paper, the reversal mobility of free carriers associated to edge dislocations is calculated and the results are compared to those obtained by using the classical deformation potential of Bardeen and Shockley. Collation with experimental data obtained on edge‐dislocated tellurium, shows quantitative agreement with the present results.
ESR and Conductivity in Amorphous Germanium and Silicon
Authors: Movaghar B. , Schweitzer L. 
Published: July 2007

AbstractA theory is outlined for the temperature dependence of the ESR linewidth in amorphous germanium and silicon. By connecting the lifetime broadening with the motion of the spins, and assuming a distribution of spin relaxation times, the experimental results may be explained, in particular the observed relationship between temperature dependent linewidth and dc conductivity.
Transition‐Active k = O Representations and the Microscopic Description of Phase Transitions
Authors: Agyei A. K. , Birman J. L. 
Published: July 2007

AbstractCell preserving transitions that give rise to localized physical quantities are considered from the standpoint of the phase transformations of the individual crystal sublattices. Such transitions are shown to be associated not with irreducible crystal point‐group representations, but with certain generalized transitive imprimitive representations of the transitive permutation groups of the “transforming” sublattices. These representations allow a microscopic treatment of any such transition, and it is shown, in particular, that the thermodynamic order of a crystal transition depends on the microscopic structures of the transforming sublattices. Further, it is shown on the basis of a Landau‐type crystal free‐energy expansion that at least one linear combination of the components of the (generalized) zero‐field inverse susceptibility tensor, χ+1, of the crystal should go to zero at the transition temperature Tc. The case of structural transitions, for which χ+1 is the elasticity tensor, is considered in some detail. It is shown that if a structural transition is brought about by the spontaneous distortion of a sublattice of site point group H ⊆ D4h, then either one of the shear moduli cs = 1/2 (c11 – c12) and c44 will go to zero at Tc.
Channel Model in Isotropic Exchange Theory
Authors: Eremin M. V. , Rakitin Yu. V. 
Published: July 2007

AbstractThe microscopic theory underlying the exchange channel model is described. It is shown that for a large class of isostructural systems the contributions to the exchange coupling parameter J from separate channels (pairs of interacting half‐filled orbitals) depend only slightly on the number of the electrons in the open d‐subshells of the metals, and the value J in the HDVV Hamiltonian is equal to the sum of such contributions divided by the number of interion exchange channels. The applicability of this approach is examplified by the analysis of a number of exchange systems.
Charge Transfer in Aluminium—Magnesium Alloys
Authors: Neddermeyer H. 
Published: July 2007

AbstractFrom Kα1,2 and L2,3 X‐ray emission spectra of Al and Mg in a series of Al—Mg alloys energy shifts of the Al and Mg Is and 2p3/2 core levels are determined. The experimental results indicate a charge transfer from Mg to Al sites, which can consistently be explained by considering the difference in electronegativities of the pure metals.
The Dielectric Tensor Components of Czochralski‐Grown 2:1 Mullite Single Crystals
Authors: Büchner U. , Guse W. 
Published: July 2007

AbstractElectron energy loss measurements up to 40 eV have been performed in order to characterize the dielectric properties of 2:1 mullite single crystals. The dielectric tensor components were determined by application of the Kramers‐Kronig analysis. A band gap of (7.7 ± 0.2) eV was found. The results show a distinct anisotropic electronic structure, which is related to the crystallographic details of the 2:1 mullite.
Influence of the Surface on the Galvanomagnetic Properties of Tellurium
Authors: Englert Th. , von Klitzing K. , Silbermann R. , Landwehr G. 
Published: July 2007

AbstractAt real tellurium surfaces natural accumulation layers exist which can be changed substantially by different etching processes. For pure samples this surface layer may mask the galvanomagnetic properties of the bulk completely. Employing a new etching process the influence of the surface can be reduced drastically, complete elimination is possible using a field effect arrangement. A comparison of the results obtained from samples with different degrees of accumulation at the surface allows a separation of the galvanomagnetic properties of the surface and the bulk.
Infrared Reflectance and Dielectric Properties of Pyrolytic Graphite for E ∥ c Polarization
Authors: Venghaus H. 
Published: July 2007

AbstractNormal incidence reflectance measurements with E ∥ c polarization are performed on polished surfaces of pyrolytic graphite for 15 meV < ħω < 500 meV and the dielectric function ϵ∥(ω) is derived from the data by Kramers‐Kronig analysis. The function ϵ∥(ω) is characterized by a background dielectric constant ϵ0∥ = 5.4 and free carrier contributions below 100 meV. The free carrier contributions can be characterized to a good approximation by Drude‐like behaviour with a plasma energy ħωp∥ (E ∥ c) = (101 ± 5) meV, giving rise to a maximum of the energy loss function Im [−1/ϵ∥(ω)] at (37 ± 2) meV due to higher lying interband transitions. The plasma frequency determined in the present experiments is in agreement with theoretical estimates of the free carrier plasma frequency.
Two‐Photon Absorption by Envelope–Hole Coupled Exciton States in Cubic ZnSe. I. Energy Spectrum of the Hd Term and Valence‐Band Parameters
Authors: Sondergeld M. 
Published: July 2007

AbstractIn the exciton spectrum of cubic ZnSe a fourfold energy splitting of the 2P exciton is observed by two‐photon absorption. The distinct fine structure is the first experimental evidence for the coupling between the exciton envelope and the total hole spin. The energy spectrum of the spin Hamiltonian for Wannier excitons is calculated by use of a variational technique. Hydrogenic p‐ and f‐like trial functions are introduced as solutions, and the two equivalent Bohr radii are treated as independent variational parameters. First experimental data of the hole effective‐mass parameters for the Γ8 valence band are deduced from the observed energy splitting.
Optical Spectra, EPR, and Spin–Lattice Relaxation of Yb3+ Ions in Crystals Having Perovskite‐Type Structure
Authors: Antipin A. A. , Vinokurov A. V. , Davydova M. P. , Korableva S. L. , Stolov A. L. , Fedii A. A. 
Published: July 2007

AbstractThe optical and EPR spectra of centres formed by Yb3+ ions substituted for monovalent and divalent cations are observed on KMgF3 and KZnF3 crystals. From the Stark structure and from the principal values of the g‐tensor the crystal field parameters of cubic, trigonal, and tetragonal centres are found. It is shown that as a result of substitution Yb3+ into the Me2+ site (cubic centres) the radius of the nearest coordination sphere increases by 20%. The spin‐lattice relaxation time for Yb3+ cubic centres has an unusual temperature dependence of the form T = 82.3T + 1.73 × 10−5T11.
Peculiarities of the Electron–Phonon Interaction in the Spectra of the Molecular Ions O, S, and NO in Alkali Halide Crystals
Authors: Freiberg A. , Rebane L. A. 
Published: July 2007

AbstractA systematic low‐temperature study is made of deformations of no‐phonon lineshapes in the luminescence spectra of small molecular ions in alkali halide crystals. It is shown that the main peculiarities of the temperature broadening and shift of lines arc connected with the selective manner of coupling of the electron transition in the centre with low‐frequency pseudolocal vibrations. This conclusion is confirmed also by the study of the change of the total Stokes' losses on crystalline vibrations with temperature. It is found that the homogeneous no‐phonon lineshapes are Lorentzians with a slight low‐frequency asymmetry.
Band Formation in Xenon–Argon Alloys Studied by Photoelectron Spectroscopy
Authors: Nürnberger R. , Himpsel F.‐J. , Koch E. E. , Schwentner N. 
Published: July 2007

AbstractPhotoelectron energy distribution curves for xenon‐argon alloys for concentrations ranging from 0 to 100% are measured by excitation with synchrotron radiation at hv = 13.8, 16.5, and 18.0 eV. With increasing Xe concentration the gradual formation of Xe valence bands starting from the atomic Xe 5p(1/2) and Xe 5p(3/2) states is observed. Similarly with Ar the 3p states are broadened with increasing Ar concentration. Rather high concentrations of Xe or Ar are necessary in order to reach the fully developed Xe or Ar bands, respectively. The results are discussed in terms of a concentration dependent tight binding band structure.
Resonance Scattering of Phonons at Trigonal Jahn‐Teller Centres
Authors: Rueff M. , Sigmund E. , Wagner M. 
Published: July 2007

AbstractThe scattering of degenerate lattice waves by localized degenerate electronic states is considered. The inverse lifetime of phonons of ΓE‐type is calculated by the method of double time thermo‐dynamical Green's functions, the T‐Matrix formalism, and an application of a nonlinear canonical transformation. The behaviour of the lifetime in both the extremal coupling cases is given. The temperature dependence of the lifetime broadening is discussed. The resonance frequency is studied for small and strong interactions.
Photoionization of Triplet Excitons in Tetracene Crystals
Authors: Schlotter P. , Kalinowski J. , Bässler H. 
Published: July 2007

AbstractTransient photocurrents in crystalline tetracene excited by intense ruby laser pulses are investigated. A cubic relationship between charge carrier density and light intensity is observed. The dominant carrier generation mechanism is photoionization of triplet excitons produced by fission of two‐photon‐excited singlet states. The ionization cross‐section is found to be (5 ± 2) × 10−22 cm−2 and the calculated autoionization efficiency of the excited triplet state is about 10−3. From the time response of the photocurrent pulses a rate constant γeh = (1.0 ± 0.5) × 10−6 cm3 s−1 for electron‐hole recombination follows.