bluuebunny/crossref_metadata_2025_split huggingface dataset:
Exciton Molecule Creation by Pure and Phonon‐Assisted Two‐Photon Transitions in Solids
Authors: Hassan A. R. 
Published: July 2007

AbstractDirect creation of bi‐exciton states, by direct and phonon‐assisted two‐photon absorption in direct‐ and indirect‐gap semiconductors, is theoretically investigated. A group‐theoretical analysis of the final direct (indirect) bi‐exciton symmetry is given for CuCl(Ge). Numerical applications to the case of CuCl and Si show that the direct and indirect two‐photon absorption coefficient for bi‐excitonic transitions are several orders of magnitude larger than those for the corresponding processes of interband and excitonic transitions. The essential contribution to this enhancement of the absorption coefficient for the transitions to the direct and indirect excitonic molecule states is shown to be from the resonance effect.
Third‐Order Contribution to the Effective Ion–Ion Interaction and Formation of Angular Forces in Crystalline Solids. I. General Formulation of the Expression for the Total Energy
Authors: Milchev A. , Unger K. 
Published: July 2007

AbstractThe importance of the third‐order correction to the effective interatomic potential is investigated making use of the Dirac density matrix formalism. Starting from the perturbation expansion for the one‐electron density matrix a general expression for the total energy up to third order of the perturbing pseudopotential is obtained. A simplified expression for the third‐order filter function is suggested and used in computing the appropriate corrections to the effective ion–ion potential. The building‐up of tetrahedral noncentral forces in the group IV elements is revealed and the connection with tight‐binding models is established.
Unpaired Elastic Force Model in the Lattice Dynamical Study of Rare Gas Solids
Authors: Kushwaha M. S. , Kushwaha S. S. 
Published: July 2007

AbstractThe lattice dynamics of rare gas solids (RGS) is investigated using an elastic force model which involves the central forces and the unpaired forces of the type of Gazis, Herman, and Wallis. The central forces are assumed to arise from the atomic displacement from their mean positions. The unpaired forces, on the other hand, arise from the deformation in certain angles made by the reference atom and two of its nearest and next nearest neighbours. The present model involves five parameters in total and satisfies the condition of invariance under rigid body rotation. As an application, the phonon dispersion relations of argon (Ar) and krypton (Kr) are calculated along all the three symmetry directions [100], [110], and [111]. The comparison of theoretical results reveals reasonably good agreement with the latest available experimental data.
The Crystal Field for Nd3+ in Garnets
Authors: Nekvasil V. 
Published: July 2007

AbstractA crystal field analysis of the reported optical spectra of Nd3+ in the series of aluminium and gallium garnets LuAG, YAG, GGG, LuGG, and YGG is presented. The reliability of the crystal field parameters determined in this way is confirmed by comparison with ESR g‐values given in the literature. The results obtained represent a basis for a further study of the relation between the crystal field parameters of rare earths in garnets and the structure of the coordination dodecahedron.
Theory of Resonance Scattering for Electron–Phonon Interaction in Doped Semiconductors with Zincblende Structure
Authors: Singh Mahiradhwaj 
Published: July 2007

AbstractExpressions for the resonance scattering of phonons by bound donor electrons are obtained in internally strained n‐type semiconductors with ZB structure for the situation, when, besides the usual spin–orbit splitting of the sixfold degenerate state, derived from the triplet, into a quartet and lower lying doublet, the quarted state further splits into two Kramers doublets owing to the internal and dynamic strains present in the crystal. Similarly expressions for the resonance scattering of phonons by bound donor electrons are obtained in presence of an external magnetic field for the most general case when the degeneracy of each Kramers doublet splits into two singlet states. The expressions obtained for the resonance scattering relaxation rates for internally strained semiconductors are used to explain the thermal conductivity of n‐type GaAs and n‐type GaSb. Excellent agreement between theory and experiment is obtained for both the samples.
On the Second‐ and Third‐Order Elastic Constants of ZnS Structure Solids
Authors: Garg V. K. , Puri D. S. , Verma M. P. 
Published: July 2007

AbstractExpressions for the second‐ and the third‐ order elastic constants of solids crystallizing in the ZnS structure are derived from a general potential. As far as known, this study provides the first analysis of the third‐ order elastic constants of ZnS structure solids. It is observed that the internal strain, arising due to the sublattice displacement, affects only the elastic constants C44, C166, C144, and C456. Applications to a number of solids based on the Lundqvist potential lead to values of the third‐ order elastic constants and pressure derivatives of the second‐ order elastic constants which agree fairly well with the available measured values.
Third‐Order Contribution to the Effective Ion–Ion Interaction and Formation of Angular Forces in Crystalline Solids. II. Application to Tetrahedral Solids
Authors: Milchev A. , Unger K. 
Published: July 2007

AbstractA third‐order correction to the structure dependent part of the total energy derived in Part I is used to demonstrate the building‐up of angular forces in tetrahedral solids and simple interpolation expressions are proposed, which reproduce the numerical data.
On the Screening of the Ions in Solids. Band Structure Calculations of Si with Several Screening Variants
Authors: Pasemann L. 
Published: July 2007

AbstractBy using the approach for the Dirac density matrix of a perturbed Fermi gas given by March et al. the complete screening matrix is derived. Furthermore, band calculations of Si are performed using the non‐diagonally screened pseudopotential in second order in the ionic potential, and the diagonally screened pseudopotential in third order in the ionic potential, respectively.
Calculation of Electrical Resistivity and Its Temperature Coefficient of Amorphous Cu60–Zr40 Alloy Using t‐Matrix
Authors: Waseda Y. , Chen H. S. 
Published: July 2007

AbstractThe electrical resistivity of amorphous Cu60–Zr40 alloy is calculated on the basis of the t‐matrix formulation of the Ziman nearly‐free‐electron theory. Experimental structure factors at different temperature are used for calculating the variation of the electrical resistivity with temperature. The calculated values of the electrical resistivity and its temperature coefficient show reasonable agreement with the available experimental data. The observed high value of the electrical resistivity and its negative temperature coefficient for amorphous Cu60–Zr40 alloy is interpreted as due to the proximity of the Fermi momentum to the position of the first peak of the structure factors.
Spin Waves in a Uniaxial Ferromagnet with Random Exchange Bonds of Different Signs
Authors: Medvedev M. V. 
Published: July 2007

AbstractThe spin wave dispersion law Ek and the spin wave damping Γk in a uniaxial ferromagnet with an easy axis anisotropy and random exchange bonds of different signs between the nearest magnetic neighbours are investigated. It is shown that in the long‐wavelength limit (ak ≪ 1) the spin wave dispersion law Ek has the form Ek = Δ + D̃(ak)2, where Δ is the spin‐wave gap and the damping Γk is proportional to ak. In the limit of the zero anisotropy Ek = ρak and Γk ∼ (ak)4 are obtained.
Magnetic Field Dependence of the Photoconductivity in Amorphous Silicon
Authors: Mell H. , Movaghar B. , Schweitzer L. 
Published: July 2007

AbstractWe report the first observation of magnetic field induced changes in the photoconductivity σp of a‐Si produced by the glow discharge technique. The relative change of σp in fields H0 up to 10 kG is of the order of 1%. The dependence of σp on the magnitude of H0 is similar to that of the dark conductivity in evaporated a‐Si and a‐Ge, suggesting that the same type of mechanism is responsible for both phenomena. We explain the change of σp in terms of a spin dependent pair recombination model in which the recombination rate of triplet pairs is correlated with the spin relaxation rate which depends on the applied magnetic field H0.
Crystal Optics of Surface Polaritons in Anisotropic Crystals
Authors: Puchkovskaya G. A. , Strizhevskii V. L. , Frolkov Yu. A. , Chepilko N. M. , Yashkir Yu. N. 
Published: July 2007

AbstractA general theory of the surface polariton states in anisotropic crystals at arbitrary orientation of crystal axes with respect to the surface is developed. General formulae describing dispersive curves and polarization of surface polaritons (SP) are obtained. It is shown that (contrary to the case of SP in isotropic crystals in which they are H‐waves) the anisotropic SP polarization is in general of mixed (EH‐) type, and the spatial decay of SP is nonexponential. Attenuated total reflection spectra of anisotropic SP's in a crystal α‐LiIO3 are investigated. Experimental and theoretical results are in a good agreement.
Annihilation of Positrons in Electron‐Irradiated Silicon Crystals
Authors: Fuhs W. , Holzhauer U. , Mantl S. , Richter F. W. , Sturm R. 
Published: July 2007

AbstractLifetime and Doppler‐broadening measurements in positron annihilation are used to investigate defects in silicon crystals irradiated with electrons of 1 MeV energy. The lifetime in undoped defect‐free Si is found to be (218 ± 8) ps. Isochronal annealing studies in the temperature range 12 K < Ta < 600 K show that in undoped Si at low temperature positrons annihilate at neutral monovacancies and double vacancies with lifetimes of (266 ± 10) and (318 ± 15) ps, respectively. In boron‐doped crystals before and after irradiation no special defect reveals in the spectra up to 400 K. This result is explained by the different charge of the defect states in the doped Si. The mean lifetime in doped crystals amounts to (233 ± 10) ps.
Solitons in a One‐Dimensional Modified Hubbard Model
Authors: Lindner U. , Fedyanin V. K. 
Published: July 2007

AbstractIt is shown, that in a Hubbard chain modified by coupling to the lattice solitons exist. They are accompanied by solitary lattice distortions. The non‐lincarity is caused by the electron correlation as well as by the electron‐phonon interaction.
Excitation Spectra of Exciton Fission in Organic Crystals
Authors: Albrecht W. G. , Coufal H. , Haberkorn R. , Michel‐Beyerle M. E. 
Published: July 2007

AbstractIn a sensitive experimental arrangement the excitation spectra of singlet exciton fission into a pair of triplet excitons are measured via the magnetic field effect on the prompt fluorescence. For monomeric crystals as anthracene and β‐perylene the excitation spectra show similar structure which in the case of anthracene does not change remarkably upon perdeuteration. Excimer forming crystals as α‐perylene, however, exhibit a blue shift of the fission threshold of ≈ 3500 cm−1. This effect is explained by the competition between fission and excimer formation.
Calculation of Auger Coefficients for III–V Semiconductors with Emphasis on GaSb
Authors: Haug A. , Kerkhoff D. , Lochmann W. 
Published: July 2007

AbstractIn contrast to former approximate treatments an exact calculation of Auger coefficients for III–V compounds is given taking fully into account the restrictions given by the energy and wave vector conservation. In particular the valence band process involving the split‐off band is considered which is essential for semiconductors with nearly equal energy gap and valence band splitting like GaSb. The well‐known conduction band process follows as a special case. The result for GaSb is in reasonable agreement with experiment.
Nouveaux centres paramagnétiques dans les halogénures alcalins mixtes: Ag(Br1) dans KCl:KBr
Authors: Amanis I. K. , Kliava J. G. , Purãns J. J. 
Published: July 2007

AbstractOn a mis en évidence les spectres de RPE des centres paramagnétiques Ag(Br1) dans KCl. Ces centres sont les atomes d'argent neutres situés dans les sites cationiques de réseau où l'un des ions Cl− de la première sphère de coordination est remplacé par Br−. En comparant les spectres expérimentaux à ceux calculés à l'aide d'un ordinateur on a déterminé les valeurs des paramètres du hamiltonien de spin des centres Ag(Br1) pour le cas H ‖ [111]. Les valeus du facteur g, (1,996 ± 0,001), de la constante de structure shf, (—656 ± 10) G pour l'isotope 109Ag à 285 K et de la constante de structure shf résultant du chlore, (12,8 ± 0,1) G à 285 K sont proches des valeurs correspondantes pour les centres Ag dans KCl. La valeur de la constante shf résultant du noyau de brome est considérablement plus grande dans le cas des centres Ag(Br1) ((95,5 ± 0,3) G pour 81Br à 285 K) que dans le cas des centres Ag dans KBr. Ce désaccord démontre que le réseau de KCl est notablement déformé aux environs du centre Ag(Br1).
Frequency Spectra and Thermal Properties of Rare Gas Solids (Argon and Krypton)
Authors: Kushwaha M. S. , Kushwaha S. S. 
Published: July 2007

AbstractThe unpaired elastic force model developed by the present authors is extended for studying the phonon frequencies of solid argon and krypton at all the irreducible points of the first Brillouin zone with finer subdivisions (1000). This is used to calculate the lattice heat capacities and Debye characteristic temperatures at different temperatures in the range 3 to 50 K. The comparison of theoretical values with the recently available experimental data reveals a fairly good agreement.
Luminescence of the Pb2+‐Ion Dimer Center in CaS and CaSe Phosphors
Authors: Asano S. , Yamashita N. , Nakao Y. 
Published: July 2007

AbstractReflection, emission, and excitation spectra of CaS:Pb2+ and CaSe:Pb2+ are measured at 300, 80, and 6K. At higher concentrations of Pb2+ ions, the emission and exeitation bands due to the Pb2+ dimer are observed besides those of the Pb2+ monomer. Theoretical calculation of energy levels of the dimer of Tl+‐type ions is made on the basis of the localized electron model. The values of the spectroscopic parameters appearing in the energy matrices are determined so that the electronic levels obtained experimentally and theoretically may coincide with each other.
The KLL Auger spectra of Na and Mg metal and their plasmon structure
Authors: Steiner P. , Reiter F. J. , Höchst H. , Hüfner S. 
Published: July 2007

AbstractThe KLL‐Auger electron spectra of clean Na and Mg metal are measured. A quantitative analysis of these spectra is performed taking into account the core hole–conduction electron coupling and the creation of plasmons by electron scattering. The plasmon intensities are found to be equal to those observed in XPS experiments. The relative Auger line intensities differ from those in the vapor phase.
The exciton dispersion in polar semiconductors
Authors: Behnke G. , Büttner H. 
Published: July 2007

AbstractThe influence of optical phonons on the exciton dispersion is studied for various coupling strengths as a function of the exciton radius aex. The variational method yields an effective exciton mass which for not too large radii aex (smaller than the polaron‐radii) is reasonably approximated by the bare mass. Only for very large aex, the exciton mass is that of two independent polarons. Corrections to the quadratic dispersion in higher order of the centre of mass momentum are found to be negligible small. The dispersion curves for the materials TlCl and PbI2 are calculated.
Magneto‐electric splitting of the crystal field Γ2‐level of Cr3+ ions in MgCr2O4 studied by neutron spectroscopy
Authors: Ligenza S. , Ptasiewicz‐Bąk H. 
Published: July 2007

AbstractNeutron spectroscopic studies are made on polycrystalline MgCr2O4 spinel at temperatures between 5 and 293 K. Neutron spectra connected with local spin interaction are observed and interpreted basing on the splitting of Γ2‐level belonging to the 4F3/2‐term of Cr3+ ions situated at octahedral B‐sites. The fourfold Γ2‐level splitting and the neutron excited transitions are analysed in terms of a spin‐Hamiltonian assuming a magneto‐electric interaction of the 3d3 electron shells. In spectroscopic measurements the set of peaks connected with two kinds of B‐sites are observed. According to neutron diffraction measurements the local magnetic‐molecular field at those sites makes the angles θx = 90° (B(x)) and θv = 35° (B(v)) with a trigonal axis of electric field. From spectral analysis the energies of Γ2‐level splitting are determined for Cr3+ ions at B(x) and B(v) sites, and the respective interaction parameters are found. Basing on interaction parameters the magnetic moments of Cr3+ at B(x) and B(v) sites are calculated to be μ(x) = (2.64 ± 0.02) and μ(v) = (1.80 ± 0.02) BM, respectively. From neutron diffraction measurements the magnetic moments at B(x) and B(v) sites are found to be μd(x) = (2.70 ± 0.10) and μd(v) = (1.83 ± 0.10) BM, in good agreement with moments calculated from spectroscopic data.
The influence of an electromagnetic wave on the electronic spectrum of crystals
Authors: Quang Nguyen Vinh 
Published: July 2007

AbstractThe variation of the crystal electronic spectrum under the influence of both monochromatic and nonmonochromatic, both lineraly and circularly polarized electromagnetic wave is studied using a simple method. The difference between the cases of parabolic and nonparabolic, spherical and nonspherical dispersion relations is analysed in detail. The one‐band approximation is justified when the photon energy is much less than the gap energy.
Spectral and time decay analysis of photoluminescence excited in the A band region in KCl:Pb
Authors: Benci S. , Capelletti R. , Fermi F. , Manfredi M. , Damm J. Z. , Mugeński E. 
Published: July 2007

AbstractThe A band spectral region in KCl:Pb is studied from the point of view of the optical absorption spectra, photoluminescence (emission and excitation) spectra, and the luminescence lifetime measurements in the temperature range 15 to 320 K. The sensitive ITC technique is exploited as a parallel tool to monitor and check the solution state of the impurity in order to rule out the additional and unwanted complications arising from the presence of aggregates. As a consequence only very dilute (4 to 6 ppm) and quenched samples are chosen to perform the detailed and systematic analysis of the photoluminescence spectra. The emission and excitation spectra do not depend significantly on the wavelength of the exciting and emitted light respectively, while their shape depends strongly on the temperature. The careful analysis of the emission spectrum, which looks asymmetric and apparently structureless, suggests that two Gaussians are contributing to it (main band and red side band, with associated average phonons of 1.25 × 10−2 and 1.6 × 10−2 eV, respectively) for T < 120 K, while a third weaker component seems to increase on the high energy side (violet side band) for T > 120 K. The luminescence time decay is complex exhibiting three distinct lifetimes: the length of two of them (τ1 = 30 and τ2 = 160 ns) is temperature independent, while the third one τ3 is strongly temperature‐dependent. The comparison between the intensity behaviour versus temperature both, of the time decay components and of the emission spectral components brings to a tentative attribution of τ1 to the main band, τ2 to the violet side band, and τ3 to the red side band. A scheme for the excited state energy levels, from which the emission excited by A light takes place is discussed both, by taking into account the dynamical Jahn‐Teller effect and the role of the neighbouring cation vacancy.
Contribution of the Fe2+ ion to the magnetic anisotropy constant in ferrites
Authors: Watanabe Y. , Urade K. , Saito S. 
Published: July 2007

AbstractThe magnetic anisotropy constant K1 originating from the presence of Fe2+ ions in ferrites is calculated, using the theory of Wolf, for a case that the electron hopping between Fe2+ and Fe3+ ions is allowed within a cluster of iron ions around some lattice defects. The calculated value of K1 exhibits a characteristic temperature dependence, i.e. the change of the sign from positive at lower temperatures to negative at higher temperatures. It is shown that the result well agrees with the Fe2+ contribution to K1 estimated from the available data of the anisotropy constant for the Ni1−xFe2+xO4 system.
Characteristic features of the electronic spectrum and the stability of ternary diamond‐like semiconductors
Authors: Altshuler A. M. , Vekilov Yu. Kh. , Umarov G. R. 
Published: July 2007

AbstractThe electronic spectrum and the assessment of parameters and trends of the 31‐st ternary diamond‐like compound band structure are presented and conditions for possible structural change are defined on their basis. Areas of stability of narrow band semiconductors and semimetals are established. The characteristic features of the electronic compound spectrum in various fields of application are considered. Composition and structure of some compounds having anomalously high melting temperatures are predicted.
Non‐isodisplacement of P atoms in long‐wavelength optical phonons in In1–xGaxP
Authors: Jahne E. , Pilz W. , Giehler M. , Hildisch L. 
Published: July 2007

AbstractA study on long‐wavelength optical phonon modes in In1–xGaxP is carried out by infrared reflection and Raman scattering. The oscillator strengths of the single reststrahlen band (one‐mode behaviour) and of the additional band between the TO and LO mode are determined by classical dispersion analysis. The origin of the additional structure is explained by non‐isodisplacement motions of the P atoms in the long‐wavelength phonon modes arising from changes in the nearest‐neighbour force constants with the kind of surrounding atoms. Compositional dependences of the mode frequencies and oscillator strengths are calculated using a virtual crystal model with five types of basic units.
On the electronic structure of hexagonal boron nitride
Authors: Tegeler E. , Kosuch N. , Wiech G. , Faessler A. 
Published: July 2007

AbstractThe anisotropic emission of the K‐X‐radiation of boron and nitrogen in hexagonal boron nitride BN permits the determination of the π‐ and s̀‐subbands of the two X‐ray spectra. Together with the X‐ray photoelectron spectrum a consistent picture of the valence band of BN is obtained. Position and width of the s‐, s̀‐, and π‐bands are determined. None of the band structure and density‐of‐states calculations known so far is in satisfactory agreement with the experimental observations.
Temperature dependence of the electrical resistivity of disordered metallic alloys
Authors: Christoph V. 
Published: July 2007

AbstractThe electrical resistivity of disordered substitutional alloys is calculated starting from a resistivity formula containing force‐correlation functions. Potential and electron–phonon scattering are taken into account. The correlation functions are calculated by solving approximatively the equations of motion of corresponding Green functions. For a simple model the concentration and temperature dependences of the resistivity are discussed.
Band structure and cluster calculations of FeAl systems
Authors: Müller Ch. , Seifert G. , Lautenschläger G. , Wonn H. , Ziesche P. , Mrosan E. 
Published: July 2007

AbstractThe electronic band structure and the density of states of the ordered B2‐type intermetallic compound FeAl are compared with the density of states profiles obtained from scattered wave‐Xα‐ calculations for four different clusters of the system FeAl and with results obtained for localized substitutional Fe impurities in an Al host. Comparing band structure calculations and cluster calculations the influence of long range order and short range order on the electronic structure is discussed. Band structure calculation, cluster calculations, and also the calculation of the additional screening charge around localized substitutional Fe impurities in Al give a charge transfer from the Al atoms to the Fe atoms in the system FeAl.