bluuebunny/crossref_metadata_2025_split huggingface dataset:
Domain structure and transverse conductivity of a many‐valley semiconductor in the multivalued sasaki effect
Authors: Gribnikov Z. S. , Mitin V. V. 
Published: July 2007

AbstractThe transverse conductivity (TC) of a semiconductor is studied in the range of the multivalued Sasaki effect in a heating electrical field. The negative TC due to the Erlbach effect does not arise in this field. The TC depends on the surface conditions of the sample and on the domain structure arising in these fields. The case of a small TC caused by a displacement of a domain wall under the influence of a transverse current is possible together with the case of a large positive differential TC. Such conductivity can be both positive and negative depending on the direction of the wall displacement. The case when the driving field and the transverse current are homogeneoils has been studied.
Properties of a 5T2 in iron(II) under the action of arbitrary‐symmetry ligand fields and spin‐orbit coupling: II. On the electric field gradient and susceptibility tensor
Authors: Zimmermann R. 
Published: July 2007

AbstractThe susceptibility tensor χ and the EFG have been treated for a 5T2, ground state of highspin iron(II) under the action of an arbitrary symmetry ligand field and spin‐orbit coupling λL·S. It is shown that the combined evaluation of χ‐tensor and EFG can be split into two nearly independent parts, in one for the determination of the orbital eigenstates from the relative orientation of the χ‐tensor and EFG and in another for the determination of the energy splittings of the orbital states, spin‐orbit coupling constant, and covalency. The determination of the orbital states does not involve the diagonalization of the 15 × 15 matrix of the Hamiltonian of the ligand field and λL·S. The second part is simplified by the fact that all calculations can be done as if the orbital states were those of D2 symmetry, i.e. |yz〉, |xz〉, |xy〉. Furthermore it is discussed that the rhombic symmetries D2 and D′2 and the twofold symmetries C2, and C′2 (e.g. trans and cis configuration) can be differentiated from χ‐tensor and EFG data.
Phonon‐induced hyperfine interactions for 6S‐state ions in tetrahedral symmetry: Mn2+ in ZnS and ZnSe
Authors: Koch B.‐P 
Published: July 2007

AbstractOn the basis of a point‐charge model according to the theory of Šimánek and Orbach equations are derived for the temperature dependence of the hyperfine coupling constant of 6S‐state ions in tetrahedral symmetry without the approximation of small wave vectors for lattice phonons. Acoustical and optical phonons of the entire phonon spectrum are taken into consideration. The temperature functions calculated are compared with the measured values for ZnS:Mn2+ and ZnSe:Mn2+. The curves obtained very well correspond to the temperature dependence measured if always one parameter is taken from the experiment. The calculations, however, supply parameters which deviate by more than one order of magnitude from the experimental ones. The reasons for these deviations are particularly involved in the applied point‐charge model.
Complementary variational principles for lattice thermal conductivity
Authors: Srivastava G. P. 
Published: July 2007

AbstractA method of complementary variational principles for the lattice thermal conductivity is presented. This two sided variational method gives sequences of upper and lower bounds on the exact thermal conductivity coefficient. The sequence of lower bounds can be recognised as the one presented by Benin. The sequence of upper bounds is a subsequence of an infinite sequence, with only odd number of terms present. One finds that the method of complementary variational principles is a very successful method of estimating the exact coefficient: it gives a controlled means of putting the estimated results into a band of desired accuracy around the exact transport coefficient.
Antiferromagnetism in not exactly half‐filled narrow energy bands (hubbard model)
Authors: Nolting W. 
Published: July 2007

AbstractIt is investigated if the ground state of a system of interacting electrons in a narrow energy band can be of antiferromagnetic order within the limits of the Hubbard model. By means of the “method of spectral moments” an attempt is made to find antiferromagnetic solutions for all values of the band occupation, which then are checked for stability toward para‐ and ferromagnetism. The numerical evaluation for the b.c.c. lattice yields that antiferromagnetism is stable toward paramagnetism in a small region around the half‐filled band, but is unstable against ferromagnetism. Special attention is paid to the determination of a spin dependent band shift, which turns out to be a decisive quantity for the possibility of antiferromagnetic ordering.
Many‐phonon vibronic spectra of molecular crystals: I. The role of anharrnonicity of intramolecular vibrations
Authors: Lalov I. J. 
Published: July 2007

AbstractMany‐phonon vibronic spectra are investigated in molecular crystals in the weak coupling approximation. A theory is developed (using adiabatic approximation) in which the anharmonicity of the intramolecular vibrations is treated as an exciton‐phonon coupling mechanism. This theory is applied to vibronic spectra with slowly migrating phonons of nontotally symmetrical vibrations. Anharmonicity and the change of frequency have approximately the same impact on the intramolecular interaction and on the intensity of vibronic spectra, but due to its smaller constant, the anharmonirity is less essential to the appearance of one‐particle states. Anharmonicity widens the bands of the many particle states by several ten per cent.
Luminescence by exciton‐exciton collision in GaSe
Authors: Catalano I. M. , Cingolani A. , Ferrara M. , Minafra A. 
Published: July 2007

AbstractThe photoluminescence of GaSe at 80 °K at high excitation with one‐ and two‐photon pumping is reported. The investigated energy range includes the emission localized between the indirect and the direct edges. Only the free exciton emission is observed at low pumping intensities. As the pumping increases, an extra band (L band) is observed, 20 meV below the free exciton. This band shows stimulation effects and may be ascribed to an exciton‐exciton collision process. Photocurrent measurements, resonantly excited at the excitonic wavelength, are in agreement with this conclusion.
Dielectric response theory of phonon‐ and plasmon‐assisted cyclotron resonance
Authors: Chiu K. W. , Wallace P. R. 
Published: July 2007

AbstractThe dynamic dielectric response of a polar semiconductor is calculated, taking account of the interaction of the electrons with the longitudinal optical phonons and plasmons, which themselves may in turn be coupled. This permits the calculation of the optical absorption at the “mixed” cyclotron resonances in which the absorption or emission of a boson accompanies a Landau transition. Because of relatively weak coupling, only one‐boson processes need be considered; a canonical transformation eliminating the electron‐boson interaction to first order then gives the required response. The conditions under which two mixed resonances of comparable intensity may be observed are discussed.
Magnetic resonance and related studies in cerium‐tin alloy
Authors: Malik S. K. , Vijayaraghavan R. , Garg S. K. , Ripmeester R. J. 
Published: July 2007

AbstractThe results of nuclear magnetic resonance (NMR) and magnetic susceptibility measurements in CeSn3 are presented together with other available data. The magnetic susceptibility of CeSn3 shows a broad maximum around 135 K and a rapid rise below 30 K. The 119Sn Knight shift in CeSn3 in the temperature region 1.2 t o 300 K is not proportional to the bulk susceptibility. The electrical resistivity of CeSn3 shows a T2 term at low temperatures as expected in localised spin fluctuation systems, and the coefficient of the electronic specific heat is also very large. These results suggest sizeable 4f occupation near the Fermi level in CeSn3.
Tridimensional character of GaSe excitons in the Faraday effect
Authors: Bianchi D. , Emiliani U. , Podini Y. 
Published: July 2007

AbstractFaraday measurements on GaSe were performed :with magnetic field of intermediate intensity (≈︁ 50 kG). Four exciton lines were clearly resolved. A simple analytical model was used to fit the experimental data successfully, obtaining the values of some relevant parameters such as the exciton resonance energies, the damping factor, and the relative oscillator strength. An analysis of the data with the Wheeler and Dimmock model for anisotropic exciton gives α ≈︁ 1, for the anisotropic parameter, and μ‖/μ⊥ = 1.3 for the reduced exciton masses ratio. These data show that in GaSe crystals excitons are isotropic. This agrees with recent experimental results supporting the tridimensional character of the bands responsible for excitons. A new and extremely interesting problem now arises: the different behaviour of GaSe, tridimensional in respect to optical and transport phenomena, bidimensional in respect to mechanical and crystallographic properties.
The free carrier magneto‐microwave effects in p‐type germanium
Authors: Mehra R. M. , Srivastava G. P. 
Published: July 2007

AbstractThe free carrier magneto‐microwave effects, namely, the Kerr effect, the Faraday rotation, and the magneto‐absorption, have been studied in detail in p‐type Ge in Faraday configuration. The experimental analysis applies to guided waves and treats multiple reflections exactly. The theoretical analysis takes account of the complex valence band structure and the contributions from the light and heavy holes, separately. The mixed scattering due to ionized impurities and lattice vibrations has been taken into consideration. The values of the ionized impurity scattering parameter and relaxation time obtained from Kerr effect measurements compare well with the values obtained by Faraday rotation measurements. Magneto‐absorption measurements have been done on the samples used for Faraday rotation measurements and the experimental results are compared with theoretical results using the transport parameters obtained from Faraday rotation measurements. Good agreement has been found between the two. The results of the above three types of measurements indicate the presence of larger impurity scattering and smaller relaxation time for light holes as compared with the heavy holes. The values of the relaxation times which has been estimated at 77 K (by Kerr effect measurements) compare well with the values obtained by earlier workers from cyclotron resonance and magneto‐plasma studies.
Propagation of stimulated polaritons and infrared emission in uniaxial damped crystals − observation in LiNbO3
Authors: Paraire N. , Koster A. , Biraud‐Laval S. , Reinisch R. 
Published: July 2007

AbstractPropagation of stimulated polaritons generated in a damped uniaxial crystal is studied using Maxwell's formalism without any approximation relative to the wave transversality. The polarization of these polaritons is briefly discussed. The transmitted amplitude of the driven beam is calculated and compared to experiments performed in LiNbO2 in the range 10 to 20 cm−1. The observed position of the maximum intensity of the infrared radiation is in good agreement with previous calculations.
The influence of mechanical stress on the structural phase transitions in V3Si and Nb3Sn
Authors: Pietrass B. 
Published: July 2007

AbstractThe influence of external mechanical stress on the lattice transformations in V3Si and Nb3Sn is investigated on the basis of Landau's theory of phase transitions. The calculated phase diagrams for uniaxial [001] and [110] stress are found to have a critical and a tricritical point. Discrepancies are indicated between theoretical results and experimental observations of the effect of an uniaxial [001] pressure in V3Si.
Hopping photoconductivity in amorphous germanium
Authors: Fischer R. , Vornholz D. 
Published: July 2007

AbstractThe photoconductivity of amorphous germanium is measured as a function of temperature, annealing temperature, and intensity of the incident light. The photocurrent depends only weakly on sample temperature and annealing state, in contrast to the behaviour of the dark current. The dependence of the photocurrent on light. intensity is linear to sublinear. The results are explained by hopping photoconduction of excess carriers around the Fermi level. Results of model calculations are in good qualitative agreement with the experiments.
Linear electroabsorption due to substitutional deep impurities in wurtzite‐type crystals: II. Experimental study of Co2+ in CdS and CdSe
Authors: Gardavský J. , Boyn R. 
Published: July 2007

AbstractMeasurements are made of linear electroabsorption spectra in the range of the 4A2 → 4T1(4F) and 4A2 → 4T1(4P) absorption bands of substitutional Co2+ in CdS and CdSe crystals for electric fields parallel and perpendicular to the hexagonal c‐axis, at 77 and 300 K. The observed dependences of these spectra on field and polarization direction can be completely explained on the basis of the theoretical treatment given in the first part of this work. A moment analysis is performed for CdS: Co2+, and this leads to values for the linear Stark splittings and field‐induced changes in transition probability. The axial crystalfield splittings of the T, excited levels of CdS: Co2+ are also determined, and their similarity to the external field splittings for F ‖ c is emphasized.
Functional integral approach for an extended hubbard model. A graph scheme treatment
Authors: Röpke G. , Albani B. , Schiller W. 
Published: July 2007

AbstractFor the extended Hubbard model, generalized susceptibilities are expressed by functional integrals over fictitious stochastic fields. It is shown, that in the static approximation most of these fields can be eliminated. A diagram technique is developed to evaluate the susceptibilities. Corrections to the molecular field approximation are obtained. Phase transitions from the paramagnetic into long‐range‐ordered phases (magnetically or charge‐ordered) are considered for the atomic limit and special parameter values of the Coulomb interaction terms.
EPR investigations on the hole centres in CaWO4 at T = 4.2 K
Authors: Biederbick R. , Born G. , Hofstaetter A. , Scharmann A. 
Published: July 2007

AbstractEPR investigations at T = 4.2 K show the principal g‐values of the radiation induced hole centres in CaWO4 being different from those obtained at temperatures ≧ 78 K, whereas the principal directions of the g‐tensor remain nearly unchanged. The g‐tensor at T = 4.2 K is quite well explained by a model in which the hole is assumed in an oxygen Pπ‐orbital, including some admixtures from other orbitals of the WO4 tetrahedron. In this model, combined with a weak bonding between two neighboured WO4 tetrahedra, the observed hyperfine interaction of the hole with two inequivalent W183 nuclei can be understood too. The transition to the results at higher temperatures is possible by a faster exchange of the hole between the two WO4 complexes.
Theory of disordered systems. IV. Coulomb interaction and screening
Authors: Fischbeck H. J. 
Published: July 2007

AbstractThe formal theory of interacting disordered systems is developed. Its main characteristic is that an external potential to which the system responds is coupled not only to the electronic but also to the disordered ionic charge density. This allows to describe the screening of individual potentials and potential fluctuations by the response properties of the averaged system. Electron–electron and electron–ion correlations constitute an effective screened interaction function from which the „screened”︁ single‐site approximation is derived.
Valence energy bands of solid hydrogen in H.C.P. structure. Ab initio LCMO calculation
Authors: Iacovo V. Dello , Resca L. , Resta R. 
Published: July 2007

AbstractAn ab initio LCMO calculation of the valence bands of solid ortho‐hydrogen in h.c.p. structure is reported. Firstly the molecular orientations are considered assuming the space group Pca21. Then in a spherical approximation the whole space group P63/mmc of the h.c.p. structure is considered. These two cases are related to each other on the basis of symmetry properties.
S− and Se− centers in the cuprous halides
Authors: Goltzene A. , Meyer B. , Schwab C. , Cho K. 
Published: July 2007

AbstractInfrared absorption and electron spin resonance have been performed on chalcogen‐doped
copper halides. They suggest a substitutional S− or Se− ion at an anionic site of the lattice. A theoretical model, in terms of a dynamic Jahn‐Teller effect on a Γ5 triplet of the (chalco‐gen)‐ ion is given, which is in fair agreement with the experimental results.
Selfconsistent single‐site approximation for structurally disordered solids
Authors: Kramer B. 
Published: July 2007

AbstractThe self‐energy of an electron in a structurally disordered solid is calculated selfconsistently in the single site approximation. Short range order is taken into account by an atom distribution function with sharp peaks. For smooth atom potential form factors a nonlinear system of equations for the self‐energy is derived, which can be solved numerically.
The influence of band degeneracy and hybridization on correlations in narrow bands
Authors: Capellmann H. 
Published: July 2007

AbstractStarting from a Hubbard Hamiltonian for two bands, including exchange and hybridization, a formal theory of correlations is developed. The possibility of different order parameters is pointed out: magnetic, excitonic, and mixed order parameters. A specific model is discussed and the renormalized band structures of three representative states are calculated: The magnetic insulator, the excitonic insulator, and the correlated metal. A comparison of the energies of these states in given.
Surface phonon‐polaritons on general crystal cuts of α‐quartz observed by attenuated total reflection
Authors: Schuller E. , Borstel G. , Falge H. J. 
Published: July 2007

AbstractThe theory of surface modes on general cuts of anisotropic crystals yields excitations of mixed polarization. This fact results in the existence of additional dispersion branches and pseudosurface modes. The dispersion of the pseudosurface phonon‐polaritons may be determined by means of the minima of the calculated reflectivity response. The theoretical results have been examined by attenuated total reflection and show a very good agreement with the experiment.
Optical absorption of defects in V2O5 single crystals: V2O5 doped with Ti, Mo, and Cu
Authors: Clauws P. , Vennik J. 
Published: July 2007

AbstractThe absorption spectra in the range from 0.3 to 2.1 eV of V2O5 single crystals doped with Ti, Mo, and Cu are presented. Besides of bands due to impurities, an enhanced absorption from oxygen vacancies is observed in Ti‐doped samples. Models of the various defect centres involved are discussed and it is shown that the absorption bands can be attributed to centres involving V4+ and an impurity. The model of Ballhausen and Gray for vanadyl complexes is used as a working model for the defects. Two weak bands are assigned to ligand field transitions of V4+. The more intense bands are assigned to charge transfer transitions into the orbitals of the neighbouring vanadium ions or the impurities.
Calculation of the electrical conductivity of disordered binary alloys with off‐diagonal randomness
Authors: Eifrig Th. , Kolley E. , Kolley W. 
Published: July 2007

AbstractThe dc conductivity of a disordered binary alloy with off‐diagonal disorder is calculated. On condition that the transfer integrals depend additively on the configuration the application of a modified CPA is possible. In this approximation the vertex corrections vanish. The numerical calculation shows a strong dependence of the conductivity on the occupied fraction of the band.
The influence of polariton motion on the polariton Raman scattering spectra
Authors: Strizhevskii V. L. , Yashkir Yu. N. , Ponath H.‐E. 
Published: July 2007

AbstractThe influence of the relative group velocity finiteness of the scattering polariton on the polariton Raman scattering (RS) intensities is discussed. It is proved that this effect is not very essential only in the region of polariton frequencies ωp near the frequency ωf of the corresponding polar mechanical phonon because here the relative group velocity ξp of the polariton is small. But the influence of the group velocity ξp finiteness appears to be essential in the case of sufficiently distant frequencies ωp from ωf. This influence results not only in essential quantitative changes but also in qualitative changes, in particular, of the angular dependence of the polariton RS intensity. It is manifested particularily in the vicinity of the „critical”︁ scattering angles where the neglection of this effect distorts absolutely the character of the above‐mentioned dependence. The ξp‐finiteness influence is illustrated by numerical calculations for a number of real crystals (ZnO, ZnSe, GaP, CdS, LiIO3). Experimental investigations are made of the RS‐intensity angular dependence in the region of the upper polariton branch in LiIO3 crystals in the vicinity of the critical scattering angle. The results confirm the conclusion of the theory about the essential influence of the effect mentioned above on the polariton RS intensity.
Spin–lattice relaxation of P donors in Si for different concentrations of majority and compensating impurities
Authors: Gershenzon E. M. , Semenov I. T. , Fogelson M. S. , Chambootkin D. P. 
Published: July 2007

AbstractThe influence of compensating impurities on the spin‐lattice relaxation of P donors in Si was studied for the concentration range ND = 8.5 × 1016 to 1.3 × 1018 cm−3 at temperatures 1.7 to 15 °K in the X‐band. For the concentrations ND = 8.5 × 1016 to 1 × 1017 cm−3 the compensation accelerates the relaxation rate Ts−1. Experimental results are in good agreement with Sugihara's theory. For ND ≧ 3 × 1017 cm−3 the increase of the compensation ratio causes a decrease of Ts−1 and increases its dependence on temperature. The latter is explained by weakening of the exchange interaction and retardation of the relaxation by exchange system.
The electron–electron interaction in gapless semiconductors
Authors: Gelmont B. L. , Ivanov‐Omskii V. I. , Ogorodnikov V. K. 
Published: July 2007

AbstractThe effect of the Coulomb interaction on the dielectric function, the mobility, and the energy spectrum of gapless semiconductors is analyzed. It is shown that the hole energy is strongly renormalized due to the nonlocality of the potential. The resulting shape of the valence band and its dependence on the electron concentration is considered. The nonlocality of the lattice potential diminishes the ground state energy of the acceptor.
The Magnetic Properties and the Metal‐Insulator Transitions in V2O3
Authors: Capellmann H. 
Published: July 2007

AbstractBased on a formal theory of correlations in degenerate bands a qualitative description of the physical nature of the transitions in V2O3 is given. It is argued that the low temperature insulating phase is a magnetic insulator, the intermediate metallic phase is described by the correlated metal, and the high temperature insulating phase is an excitonic insulator. The driving force for the transitions is the interband mixing term contained explicitly in the Hamiltonian.