bluuebunny/crossref_metadata_2025_split huggingface dataset:
Orientation Distribution Function Average of Second‐Rank Material Constant Tensors. Susceptibility, Conductivity, and Extinction
Authors: Ganster J. , Geiss D. 
Published: July 2007

AbstractFor triclinic crystal and sample symmetries material properties of polycrystals, which can be represented by symmetrical second‐rank tensors, are expressed by the generalized Fourier coefficientsC(ι = 0, 2) of the orientation distribution function (ODF) and the corresponding single crystal tensors. This relation is also given in terms ofoccurring if the ODF is expanded in real (tesseral)D‐functions. All higher symmetric cases can be treated in terms of(which belong to symmetrizedD‐functions) using the general formula and leading to drastically simplified expressions. An example is discussed. Some special cases often encountered in polymer science are expressed by orientation factors.
Conduction‐Electron Spin Resonance and Thermoelectric Power of the Mixed‐Valence Tetrathiafulvalenium Chloride
Authors: Graja A. , Sekretarczyk G. , Pukacki W. , Nešpůrrek S. , Ryba O. 
Published: July 2007

AbstractA conduction‐electron spin resonance study together with thermoelectric power measurements for the TTF‐Cl0.71 salt are presented. One‐dimensionality and eclipsed stacking of TTF molecules are confirmed by ESR. It is shown that the Coulomb interaction is comparable with the intermolecular exchange interaction. The anomalies of the ESR linewidth, the spin susceptibility, the electrical conductivity, the thermoelectric power, and the specific heat at 250 K are associated with a commensurate Peierls transition.
Approximations for the Scattering of Excited Electrons in Solids
Authors: Fritzsche V. 
Published: July 2007

AbstractThe errors of several approximations for photoelectron scattering are systematically studied as a function of the electron energy in the range from 10 to 1000 eV. The accuracy of the approximations does not improve with rising energy, as it was commonly supposed. The errors increase considerably with the angular momentum of the incident wave. The large discrepancies of the small‐scattering‐centre approximation (SSCA) are substantially reduced by means of effective scattering amplitudes in the modified small‐scattering‐centre approximation (MSSCA). Above all the reduced angular momentum expansion (RAME) has such a high degree of accuracy that allows reliable calculations of multiple‐scattering contributions for all purposes. Both the MSSCA and the RAME are further simplified in order to facilitate applications.
Diffusion Coefficient of F− Ions in Perfect β‐PbF2 Crystals
Authors: Sekar P. , Haridasan T. M. 
Published: July 2007

AbstractIn the quasi‐harmonic approximation, the mean square velocity of F− ions in β‐PbF2, along different directions is estimated. A critical square of the velocity is fixed by equating the experimental value of the migration energy to the minimum kinetic energy (E) required by the F– ion to make the jump. The ratio of the calculated kinetic energy to E is assumed to govern the probability term appearing in the Arrhenius equation. The diffusion coefficient is estimated for migration along 〈100〉 and 〈111〉 directions. The calculation is repeated in the reaction coordinate approach for migration along 〈111〉 direction. The estimated attempt frequencies of the ions in this approach are compared with that from Debye approximation. The diffusion coefficient thus calculated compares well with that of experiment. The requirement that the amplitude of the mobile ion should fluctuate to values just enough to cross the first ring of obstructing ions is found to be justified for these fluorite type crystals.
The Advantage of Internal Force Calculations in the X‐Ray Crystal Structure Analysis
Authors: Pietsch U. 
Published: July 2007

AbstractAccording to the Hohenberg‐Kohn‐theorem the valence electron density (VED) determines the electronic ground state of crystal. The VED can be constructed measuring the X‐ray scattering amplitudes. Parametric density models are developed in order to complete the experimental data and to enable a generalization of the solved structure parameters. Caused by experimental and methodic errors various density models are able to describe the experimental data with comparable accuracy. However, electrostatic criterions can be applied in order to select a density model, which describes also the dynamical structure–property connections of semiconductor. A quantitative calculation of internal forces is possible by use of the generalized Berlin‐Theorem. The electrostatic model contains the ion–ion‐ and the electron–ion interaction in the crystal. The latter one can be calculated in the Fourier space using experimental or theoretical structure amplitudes. A model is proposed specially designed for diamond and zincblende structure compounds. The elastic properties of covalently bonded materials are described sufficiently precise by a bond charge model, taking into account the variation of amount and extension of the bond charge by deformation of lattice. The so fitted VED response of binary compounds can be transfered to solid solutions in order to interprete quantitatively the measured internal deformations in such materials.
Optical Absorption Coefficient in Cylindrical Quantum Wells
Authors: Pérez‐Alvarez R. , Pajón‐Suárez P. 
Published: July 2007

AbstractUsing the envelope function approximation and a one band model, the absorption coefficient of a quantum wire with arbitrary transverse section is studied. The selection rules for direct allowed transitions are reported. The results are particularised for the cylindrical case with infinite potential barrier, and numerical results for GaAs are given.
Effect of Pressure on Electrical Resistivity and Crystal Structure of Quasi‐Two‐Dimensional Mo4O11
Authors: Koyano M. , Ōhara S. , Negishi H. , Sasaki M. , Inoue M. , Nomura M. , Fujiwara H. 
Published: July 2007

AbstractElectrical resistivities ϱ and lattice parameters of quasi‐two‐dimensional crystals of monoclinic η‐Mo4O11 and orthorhombic γ‐Mo4O11 are measured under hydrostatic pressure up to 1.2 GPa at room temperature. Both crystals have negative pressure coefficients of ϱ, which can be qualitatively explained by a phonon stiffening effect. In particular, the pressure dependence of ϱ for γ‐Mo4O11 shows a drastic drop or jump at Pc = 0.3 GPa, whose magnitude and the pressure coefficients are strongly dependent on the sample resistivity or degree of stoichiometry. It is suggested that the change of a small amount of MoO4 tetrahedra into MoO6 octahedra induced by excess oxygen atoms is responsible for such structure‐sensitive properties.
Green Function Formalism and Superfluid Properties of 4He
Authors: Bahadur R. , Kachhava C. M. 
Published: July 2007

AbstractThe excitation spectrum of superfluid HeII is studied using the Lennard‐Jones form of the potential and the reaction matrix of the Green's function method. The parameters of the theory are determined from the experimental data only. The results on energy values and equation of state amply justify the proposed philosophy.
A Phenomenological Model for the Magnetic Resistivity of NM‐TM Spin Glasses
Authors: Sheikh A. W. , El‐Fazani M. 
Published: July 2007

AbstractA phenomenological expression is developed of magnetic resistivity of NM‐TM spin glasses by incorporating “effectively free” spin populations and “interacting” spin populations into modified Kondo and spin‐diffusion theories, respectively. Analyses of the data on AuMn, AgMn, and CuMn through a successful best‐fitting of the model reveal two new conduction electron scattering mechanisms: static magnetic disorder scatterings and spin‐orbit scatterings in addition to s‐d, spin‐spin and non‐magnetic scatterings. It is found that the magnetic resistivity maximum at temperature Tm originates from the suppression of spin‐diffusivity effect and rise of Kondo effect as the temperature increases. The temperature TSG of a new resistivity maximum in “interacting” spin resistivity, is related to Tm. Unlike previous studies, Tm is found to depend explicitly on impurity spins and the ratio of level width with level splitting for spin‐split impurity virtual bound states.
Photocarrier Drift Mobility in Bulk Vitreous GeSe3 in the Temperature Range 60 to 300 K
Authors: Calas J. , Abdel‐Maksoud S. M. , Averous M. 
Published: July 2007

AbstractFrom the transit time technics with two blocking electrodes, the drift mobility for the two types of carriers is determined for the bulk GeSe3 in the temperature range 50 to 300 K. These values at room temperature are respectively μdn = 10−2 cm2 V−1 s−1 and μdp = 10−1 cm2 V−1 s−1. Initial photocarrier densities n(0) = 5 × 107 cm−3 and p(0) = 3 × 107 cm−3 are also reported. Successively microscopic mobilities are deduced respectively, μon = 6 × 10−2 cm2 V−1 s−1 for electrons and μop = 10−1 cm2 V−1 s−1 for holes. From these values an estimation of the ratio Nc/Ntn ≈︁ 1 and Nv/Ntp ≈︁ 0.5 is lastly provided as well as trap densities Ntn ≈︁ 1020 cm−3 and Ntp ≈︁ 1021 cm−3.
Heavy Fermion Superconductivity via Kondo‐Type Pairing
Authors: Svozil K. 
Published: July 2007

AbstractA consistent quantum field theoretical approach to Kondo‐type pairing in heavy fermion superconducting systems is presented. This pairing mechanism allows both “singlet” and “triplet”, as well as l > 1 pairing, depending on the system parameters. Associated with the superconducting transition is the appearance of hybridization (dielectric gap) between localized f‐states and the conduction band. The parquet approximation is used to calculate the superconducting and excitonic (dielectric) gaps.
Many‐Body Effects in Resonantly Excited Dense Exciton Systems
Authors: Schäfer W. , Schuldt K.‐H. , Treusch J. 
Published: July 2007

AbstractThe nonlinear optical properties of semiconductors, which are excited by a strong laser pulse at the excitonic resonance are investigated. The concepts to describe the time‐evolution of the initially coherently excited system in the framework of a nonequilibrium Green's function theory are discussed in general. Numerical results are presented for a quasistationary limit and compared with recent experimental data.
Kinetic Theory of Chemically Reacting Systems
Authors: Kremp D. , Schlanges M. , Bornath T. 
Published: July 2007

AbstractA kinetic approach for chemically reacting systems is derived on the basis of a nonequilibrium Green's function method. This approach is used to take into account nonideality effects in the rate equation for the exciton density in a semiconductor. Numerical results, especially for the impact ionization coefficient, are given in the case of germanium.
A Raman Investigation of the Polymorphism of Potassium Nitrite
Authors: Tse W. S. , Fang C. S. , Pai K. F. , Lih W. 
Published: July 2007

AbstractRaman spectra of crystalline KNO2 in the region of lattice and anion internal modes are obtained over the temperature range 80 to 480 K. The temperature dependent results are consistent with the X‐ray and calorimetric work that two transitions occur around 170 and 265 K, respectively.
Sensitized Luminescence of Pure and Doped NaNO2 Single Crystals, III. Excitation Spectroscopy and Time Resolved Trap Fluorescence
Authors: Schmidt Th. , Köhler J. , Kryschi C. , Schmid D. 
Published: July 2007

AbstractEnergy transfer processes in the first excited singlet state of NaNO2 single crystals doped with KNO2 are studied using fluorescence excitation spectroscopy and time resolved spectroscopy of the trap fluorescence. In the latter case either pulse excitation of host excitons or selective resonant pulse excitation of trap states is used. Three pairs of trap states, each of which consists of a shallow trap (trap depth less than 100 cm−1) and a deep trap (trap depth in the order of 1000 cm−1), are identified. At low temperatures energy transfer from the shallow to the deep trap is effective only within the pair. At higher temperatures the shallow traps can also transfer the excitation energy to the host exciton band. The temperature dependence of these processes is discussed on the basis of the generalized master equation.
Curved Wave and Multiple Scattering Corrections in Calculated EXAFS Spectra
Authors: Rennert P. , van Hung Nguyen 
Published: July 2007

AbstractThe curved wave theory RAME (reduced angular momentum expansion) is used to develope an expression for EXAFS. Systematic errors of the standard plane wave evaluation SSCA are found. Partially they can be corrected by a suitable choice of the energy zero E0 in the SSCA expression. But different values E0 have to be used for different neighbour shells at the distance R. An analytical expression E0(R) is given. Furthermore it is shown that there are other important multiple scattering contributions besides those due to the focussing effect for shadowed atoms. The comparision of RAME and the modified small scattering centre approximation MSSCA shows that MSSCA is sufficient accurate for EXAFS. In the XANES region below electron energies of 50 eV RAME is needed. For the first nearest neighbour RAME corrects MSSCA up to 100 eV.
Entropy Optimization in Texture Goniometry. I. Methodology
Authors: Schaeben H. 
Published: July 2007

AbstractPursuing the approach of finite series expansion to the texture problem of reproducing the orientation distribution function (ODF) from its pole distribution functions (PDF) measured in X‐ray or neutron diffraction experiments, the mathematical model of entropy maximization is introduced and elaborated in a discrete setting. The entropy maximizing solution yields the ODF with the smallest information content consistent with the experimental data, thus avoiding artificial textural components for which there is no evidence in the measured PDF's, especially “ghosts” caused by the specific properties of the diffraction experiment. This solution seems favorable particularly when the data are incomplete or afflicted with experimental errors.
Perturbative Approach to Dilute Off‐Diagonal Defects
Authors: Wiecko C. 
Published: July 2007

AbstractA diagrammatic approach is developed to the problem of dilute concentration of off‐diagonal defects for short range interactions. The method is illustrated with the calculation of phonons when impurities that change the central force constant of the harmonic interaction with the nearest neighbors are present in three different structures: on the linear chain, on the square planar lattice, and the simple cubic structure. Systematic rules are obtained for the summation of the series. The procedure can inspire a more general algorithm for matrix inversion based on an eventually summable series.
Relations between Third Order Isothermal and Adiabatic Elastic Constants for an Isotropic Material
Authors: Sachs A. 
Published: July 2007

AbstractRelations between the adiabatic and isothermal third order elastic constants are derived from a general expression for the free energy of an isotropic material. The relations include the linear terms in the temperature variations of the coefficient of thermal expansion and of the second order elastic constants. Numerical calculations are made for aluminium.
Magneto‐Optical Investigations on p‐InSb Bicrystals in the Submillimeter Wavelength Range
Authors: Ludwig F. , Müller H.‐U. , Herrmann R. 
Published: July 2007

AbstractThe magneto‐optical properties of p‐type InSb bicrystals are investigated in the submillimeter wavelength range from λ = 57 to 251 μm at temperatures T ≦ 4.2 K and magnetic fields up to B = 6.5 T. At wavelengths λ = 118.8 and 251 μm the cyclotron resonance of the conduction electrons in the n‐inversion layer adjacent to the grain boundary can be detected. At all other wavelengths strong magnetic field dependent absorption effects in the p‐type bulk mask the relatively small absorption effects in the inversion layer. The acceptor excitation spectrum of the bulk is discussed. From the periodicity of quantum oscillations three occupied electric subbands are identified. The experimental cyclotron resonance spectra are modeled as the superposition of three cyclotron resonance lines belonging to the three occupied electric subbands. Cyclotron masses are obtained with a three‐level model in the triangular‐well approximation and compared to the experimental data.
Size Dependence of Surface Thermodynamic Parameters of Microclusters and the Location of the Gibbs' “Surface of Tension”
Authors: Vogelsberger W. , Fritsche H.‐G. , Müller E. 
Published: July 2007

AbstractThe thermodynamic surface parameters surface stress and surface tension are calculated for icosahedral and cuboctahedral microclusters in dependence on cluster size. Analytical expressions can be obtained. The size dependent behaviour of these expressions is strongly influenced by the position of the surface to which the surface parameters are referred. If the “equimolecular dividing surface” is used the expected decrease of the values of the surface parameters with decreasing cluster size is observed. The location of the Gibbs' “surface of tension” for these clusters is determined. The thermodynamic equilibrium conditions, the equations of Young‐Laplace and of Kelvin, are formulated with emphasize to the importance of the location of the dividing surface.
Effect of Lattice Disorder on the Excitonic States in Superionic Crystals
Authors: Akopyan I. Ch. , Gromov D. N. , Klochikhin A. A. , Novikov B. V. , Ogloblin S. G. 
Published: July 2007

AbstractThe effect of the lattice disorder on excitonic states in superionic crystals is studied experimentally and theoretically. The spectral density of the excitonic states is measured in AgI‐type superionic crystals within a wide temperature region. The theory based on multiple scattering approach is developed. The Lipatov‐Brezin‐Parisi method is applied for the calculation of the averaged scattering matrix in the region of localized excitonic states. The experimental behaviour of the scattering matrix is recovered from the excitonic spectral density after removal of the exciton–phonon interaction effects. A comparison of the experimental and theoretical scattering matrices is made.
The Influence of Compensation on the Hall Effect and Magnetoresistance of n‐Hg0.8Cd0.2Te
Authors: Tsidilkovskii I. M. , Arapov Yu. G. , Zvereva M. L. , Brandt N. B. , Kulbachinskii V. A. , Gorbatyuk I. N. , Elizarov A. I. 
Published: July 2007

AbstractMagnetic field dependences of the transverse (ϱxx) and the Hall (ϱxy) resistances in the temperature range 0.32 ≦ T ≦ 4.2 K on compensated n‐Hg0.8Cd0.2Te samples with 0.4 ≦ K ≦ 0.8 and K ⪆ 0.9 are studied. It is shown that the metal–insulator transition in fields H ⪆ H0xy (H0xy is the field in which ϱxy begins to grow sharply) is a result of the separation of the impurity states from the conduction band and is not connected with the Wigner crystallization. The weakening of ϱxy(H) and ϱxx(H) growth in strong magnetic fields with the increase of K may be satisfactorily explained in the framework of inhomogeneous medium theory.
Electronic Structure of Niobium Nitrocarbides
Authors: Nikiforov I. Ya. , Kolpachev A. B. 
Published: July 2007

AbstractIn terms of multiple scattering theory using the crystalline muffin‐tin potential and taking into account the scattering on a finite number of atoms the electronic structure of pseudobinary solid solutions NbCxN1−x is calculated for x = 1.0, 0.75, 0.5, 0.25, 0.12, and 0. A comparison with APW results is made for x = 0 and 1. It is shown, that the local partial densities of electronic states (DOS) of constituents differ from those in stoichiometric substances. The dependence of DOS at the Fermi level on carbon concentration x correlates with the dependence of superconducting transition temperature on x.
Anderson Localization in Ferromagnetic Semiconductors Due to Spin Disorder. II. Wide Conduction Band
Authors: Auslender M. I. , Kogan E. M. , Tretyakov S. V. 
Published: July 2007

AbstractTheoretical investigation are made of the temperature induced metal–insulator transition in doped ferromagnetic semiconductors described by the s–d(f) exchange model. The transition is the result of the mobility edge movement, the disorder being due to interaction of electrons with the magnetic‐ion spin density fluctuations. The electrons are described in effective mass approximation. The mobility edge is obtained from the simple criterion due to Ioffe‐Regel.
The Influence of Disorder on Superconductivity
Authors: Kleinert P. 
Published: July 2007

AbstractUsing many‐particle Green's function techniques the influence of disorder on the transition temperature is investigated theoretically. Depending on the character of the pair‐correlation function of the random potential Tc, can both, decrease and increase with increasing disorder. The theory is applied to a recently proposed BCS mechanism for high‐Tc, superconductivity. For a constant pair‐correlation function an additional increase of Tc up to 3 K is obtained.
Calculations of Magnetotransport Properties of InSb Bicrystals with Several Populated Q2D Subbands
Authors: Nachtwei G. , Schulze D. , Gobsch G. , Paasch G. , Kraak W. , Krüger H. , Herrmann R. 
Published: July 2007

AbstractThe magnetic subband structure of the quasi two‐dimensional electron gas in an inversion layer with several populated electric subbands in InSb bicrystals is calculated using a straightforward and accurate method developed recently. The oscillating part of the magnetoresistance and the magnetic field dependence of the Hall resistance are evaluated by using the Ando model. Two different assumptions concerning the Fermi level position in dependence on the magnetic field are presented and discussed. A good description of the magnetotransport data determined in previous experiments is achieved. It is shown that the simple model of δ‐like Landau energy levels and the resulting discontinous magnetic field dependence of the Fermi energy for a fixed inversion electron density is not able to describe the magnetotransport behaviour determined in the experiments. Estimates of the electron mobilities in the electric subbands from the Landau level broadening lead us to the conclusion that the mobilities obtained from the onset of the Shubnikovde Haas‐oscillations are lower limits.
Surface Phase Diagrams of the Site‐Bond Diluted Surface Ordering Ising Ferromagnet with a Transverse Field
Authors: Song Wei‐Ji , Li Zhen‐Ya 
Published: July 2007

AbstractWithin the framework of an effective‐field theory with correlation phase diagrams for surface ordering in a semi‐infinite Ising ferromagnet with a site‐bond diluted free surface and a transverse field are discussed. A certain characteristic behaviour for the surface ordering is obtained.
Phase Diagram for the Simple Antiferromagnetic Ising Model Using Constant Coupling and Cluster Variation Approximations
Authors: Polat H. , Ekmekçi S. 
Published: July 2007

AbstractAn explicit formula is derived for the critical curve in the H vs. T plane of the two‐dimensional antiferromagnetic Ising square, three‐dimensional antiferromagnetic simple cubic and body‐centered (b.c.c.) lattices and the critical data are evaluated by using constant coupling (CC) and cluster variation (CV) approximations. In the case of H = 0, Onsager's exact solution is obtained for the square lattice. In the case of H ≠ 0, the critical curve dividing antiferromagnetic and paramagnetic regions for each lattice is obtained and compared with other approximations in the literature.
The Optical Properties of AuAl2 and PtAl2
Authors: Chen Liang‐Yao , Lynch D. W. 
Published: July 2007

AbstractThe dielectric functions of AuAl2 and PtAl2 are measured in the 1.5 to 5.5 eV range. The spectra of reflectivity, absorption, energy loss function, and joint density of states are obtained and discussed in detail. The onsets of interband transitions at 2.125 and 2.834 eV for AuAl2 and PtAl2 respectively, are determined by least squares fitting to the absorption edges in the spectra of the joint density of states, which could be attributed to transitions between only the s–p bands for both materials. The remaining stronger absorption in the 2 to 5 eV region can be attributed to transitions between the s–p bands for AuAl2 but between both the s–p bands and the d and s–p bands for PtAl2.