bluuebunny/crossref_metadata_2025_split huggingface dataset:
Type II/1 and type II/2 Superconductors
Authors: de Cesare L. , Mancini F. 
Published: July 2007

AbstractThe problem of a first‐order transition at the lower critical field in type II superconductors is investigated by means of the boson formulation of superconductivity. The critical value ϰc of the Ginzburg‐Landau parameter which separates type II/1 from type II/2 superconductors is theoretically determined and is in good agreement with the experimental value. The distribution of the magnetic field of an isolated flux line and the interaction potential between two flux lines are also computed in the entire region including the core.
A Model for the Electron–Phonon Interaction in Polyvalent Metals: Al
Authors: Hayman B. , Carbotte J. P. 
Published: July 2007

AbstractBross and Bohn have suggested a form for the electron–phonon matrix element which avoids the serious difficulties that are associated with the one OPW approximation for the case of a Fermi surface that intersects zone boundaries. Using this suggested form, we have calculated the electron–phonon mass enhancement, the transport scattering times, and the electrical resistivity. A discussion of the merits and limitations of the Bross‐Bohn matrix element is included.
Drift of Electron–Hole Drops in Exciton Density Gradients
Authors: Balslev I. , Hvam J. M. 
Published: July 2007

AbstractThe motion of electron–hole drops is analysed when nucleated in a non‐uniform density of free excitons. The exciton condensation on the drop surface gives rise to a driving force towards regions with lower densities. This force and the internal friction of the exciton gas lead to a drift velocity which is proportional to the drop radius. In typical excitation geometries this motion is more important than drop diffusion, and the flight towards low density regions influences strongly the drop growth. It is pointed out that very large drops can be formed in a local minimum of the exciton density.
Spin–Spin and Spin–Lattice Equilibrium in TCNQ NH Salt
Authors: Alizon J. , Berthet G. , Blanc J. P. , Gallice J. , Robert H. 
Published: July 2007

AbstractIn order to detect an eventual coupling between the protons of the NH4 cations and the paramagnetic electrons an investigation is made of the free induction decay and the spin–lattice relaxation time of protons of the TCNQ · NH4 salt and of the partly deuterated TCNQD4 · NH4, TCNQ · ND4 salts as functions of temperature. Experimental results are interpreted in terms of two Zeeman reservoirs and make evident an interaction between the paramagnetic electrons and NH4 protons.
Phonon Dispersion in SiC and ZnS Polytypes
Authors: Pandey B. P. 
Published: July 2007

AbstractPhonon dispersion relations in [111] direction for SiC and ZnS polytypes have been computed on the basis of a two‐neighbour short‐range force constant model. Satisfactory agreement is observed between theory and experiment. This agreement shows the important fact that a short range force system is operative in these polytypes, the forces up to second neighbours are the most dominant.
Experimental Studies of Magnetoresistance Tensor Components in Bismuth at Low Temperatures
Authors: Bogod Yu. A. , Krasovitskii V. B. 
Published: July 2007

AbstractThe monotonic and oscillating parts of the magnetoresistance tensor components are studied on a pure single crystals of bismuth. The nondiagonal‐to‐diagonal components ratio is shown to be sufficiently small at helium temperatures, so that σxx = 1/ϱxx with a high degree of accuracy, and thus the experimental deviations from the law ϱxx = αHn (n = 2 for ωcτ ≫ 1) are independent of the electron–hole compensation disturbance in Bi. Within the framework of the ellipsoidal parabolic model taking account of the relaxation time anisotropy, the mobility tensor components at helium temperatures are found whose values coincide with data obtained by other authors in weak (ωcτ > 1) magnetic fields. It has been revealed that for magnetic field directions H near twinning planes the sign of an even constituent of the nondiagonal magnetoresistance tensor components becomes opposite to that for samples with no twinning interlayer. The twinning plane effect is essential up to 70 °K. It is also found that at helium temperatures the even constituent ϱyx(H) changes its sign in high magnetic fields (10 kOe) near H ‖ C3. It is shown that the transition in the H ‖ C3 direction under H rotation in the bisectrix plane causes the oscillation phase of the even constituent ϱyx to change. It is found that in the preultraquantum region of the magnetic fields the oscillating part of ϱyx turns out to be greater at certain H directions than the monotonic one.
Effect of Impurities on the Phase Transition Temperature of a Two‐Dimensional Ising Lattice with Interaction between Some of the Next‐Nearest Neighbours
Authors: Grozdev K. I. , Kosevich A. M. 
Published: July 2007

AbstractThe effect of interstitial impurities on the thermodynamical properties of a plane Union Jack lattice was studied. The impurities are located on the next‐nearest neighbour bonds only and affect the exchange interaction between the corresponding lattice sites. The phase transition temperature as a function of the impurity concentration is obtained exactly. The case in which the impurity cuts the exchange bond between the next‐nearest neighbours is described in detail.
Effect of Crystalline Anisotropy on the Magnetostatic Spin Wave Modes in Ferromagnetic Plates. II. Results for Finite Plates
Authors: Schneider B. 
Published: July 2007

AbstractThe magnetostatic spin wave mode transmission through anisotropic rectangular slabs of YIG has been investigated experimentally and dispersion curves have been derived. The experimental results are interpreted by an approximate application of the theory developed in a former paper. The agreement between theoretical and experimental results is rather good.
Electron Density Redistribution and Configuration Distortion in an (AB)N Chain
Authors: Lubimov V. S. , Ionov S. P. , Makarov E. F. 
Published: July 2007

AbstractThe electron density redistribution and the configuration instability in (AB)N‐chains are considered. It is found that the symmetrical distorted configuration is more stable for chains with Coulomb interaction between the electrons on centres A and B. This configuration is more stable also in the case of delocalized noninteracting electrons, if an odd electron number is on the unit fragment AB. The antisymmetrical distorted configuration energy is the smallest, if there is an even electron number on the AB fragment. Some examples of (AB)N‐chains are considered.
Band Structure and Reflectivity of GaN
Authors: Bloom S. , Harbeke G. , Meier E. , Ortenburger I. B. 
Published: July 2007

AbstractThe E ⊥ c reflectivity has been measured up to 10 eV for hexagonal, single‐crystal GaN. The band structure and reflectivity have been computed by the empirical pseudopotential method and used to identify the observed spectral peaks.
Electronic Polarons in Non‐Metals with Short‐Range and Local‐Field Effects
Authors: Dagens L. , Perrot F. 
Published: July 2007

AbstractThe electronic polaron approximation (EPA) by Toyozawa does not deal correctly with the short range part of the interaction between a quasi‐particle and the polarization field of a non‐metal, owing to the use of a continuum approximation for the polarization field. A modified EPA is derived, without this continuum approximation, from a weak coupling zero order tight‐binding model. It includes short range and local field effects and leads to an electron self‐energy ∑ without the explicit first Brillouin zone cut‐off found when Toyozawa's EPA is used. A simple practical approximation is derived in order to take into account the non‐dipolar contributions which cannot be neglected when large wave vectors are involved. A physical cut‐off, scaled by the atomic or ionic radius is thus incorporated in the theory. The numerical significance of the continuum approximation for the polarization field is tested by comparison, with the elaborate calculations of Kunz. It is found that the short wave length contributions to ∑ may in no way be neglected if one want to improve significantly over the much simpler classical approximation (due to Mott and Littleton), as used by Fowler.
Triplet State Paramagnetic Resonance and Optical Absorption of Z2 Color Centers
Authors: Adlhart W. , Gehrer G. , Lüscher E. 
Published: July 2007

AbstractA metastable Z2 center triplet state excitable by optical irradiation has been found in KCl:Ca, KCl:Sr, NaCl:Ca, NaCl:Sr. Electron spin resonance measurements yield the following symmetry axes: [100] for centers in KCl:Ca, KCl:Sr, NaCl:Ca; [100], [011], and [011] for NaCl:Sr. Optical measurements of Z2 centers in KCl:Ba show the existence of a [100] symmetry axis. Center models are discussed.
Volume Effect Photoemission: Application to Silver at Low Photon Energies
Authors: Schaich W. L. 
Published: July 2007

AbstractA perturbative evaluation is developed of the volume effect approximation to the general elastic photoemission current. The formulae are applied to the (111) and (001) crystal faces of silver for photon energies less than 8 eV. Treating bulk and surface umklapp scattering on an equal footing, the emission efficiency of primary and secondary cones is calculated. The results are compared with experiment, and the successes and failures of the theory discussed. Several further experimental tests of the model are suggested.
Optical Absorption Spectra of the Co2+ Ion in CdCl2 Crystals
Authors: Nasu T. 
Published: July 2007

AbstractOptical absorption spectra of the Co2+ ion in CdCl2 crystals were measured in the energy region from 4000 to 40000 cm−1. The temperature dependence of the spectra was also measured in the temperature range from 80 to 710 K. Observed absorption bands were assigned to forbidden d–d transitions of the Co2+ ion allowed by vibrations and to charge transfer transitions. From the analysis of the structure of the absorption band, the strength of the trigonal crystal field is estimated.
Inelastic Neutron Scattering by Cerium Hydrides. III. Localized Vibrations of Interstitial Hydrogen in the F.C.C. CeH2 Lattice
Authors: Vordekwisch P. , Hautecler S. 
Published: July 2007

AbstractExplicit formulas have been derived for the frequencies of the localized modes of an impurity at the octahedral interstitial sites of a f.c.c. CaF2‐type structure, describing the dynamics of the host lattice by short‐range central forces and assuming that the impurity interacts with the first and second neighbouring atoms of the host lattice. A threefold degeneracy is predicted. A small peak observed at 74 meV in the neutron data on CeH1.98 is attributed to such localized modes, due to an occupation of a few percent of the octahedral sites by hydrogen atoms. A unique set of the parameter values for the coupling of the interstitial hydrogen to the atoms of the host lattice could not be derived from the position of this peak alone; a set is proposed from a comparison with the parameter values for the host lattice which have been previously determined.
Theory of Electron Scattering by Quasi‐Periodic Static Fluctuations in the Presence of Electric and Magnetic Fields
Authors: Hsu W. Y. , Falicov L. M. 
Published: July 2007

AbstractAn extension of the Zener breakdown theory is presented. It corresponds to the case in which the periodic lattice potential is replaced by a quasi‐periodic static fluctuation of mean wave vector g The scattering probability for a wave packet which satisfies the “Bragg condition” to change its wave vector by g is found to be a constant Q0 much smaller than one for fields up to a very high value E0, and a decay as Q0(E0/E) beyond that value. The classical Zener case is obtained as one limit. Extensions to magnetic field induced effects are discussed.
Calculation of the B‐Band Shape in the In+ Center in Alkali Halides
Authors: Matsushima A. , Fukuda A. 
Published: July 2007

AbstractThe spectral band shapes for all of the absorption bands due to the electronic transition a→ have been calculated for the In+ center in alkali halides using the Franck‐Con‐don approximation and the Monte‐Carlo integration method. The calculated result can explain the characteristic features of the observed B‐ as well as A‐ and C‐ band shape, specifically the fact that the B band, unlike the A and C bands, does not show any well‐resolved structure at high temperatures though it has a shoulder on the low energy side at low temperatures.
Calculation of the Electron Mobility Limited by Two‐Phonon Mode Polar Optical Scattering
Authors: Nag B. K. 
Published: July 2007

AbstractAn iteration method is presented for the accurate calculation of the electron mobility limited by two‐phonon polar optical phonon scattering. Test calculations indicate that the results obtained by applying Mathiessen's rule may be in error by as much as 15% in some cases.
Kinetics and Temperature Dependences of Polarized Emission of Anisotropic Tin Centres in Alkali Halides
Authors: Hizhnyakov V. , Zazubovich S. , Soovik T. 
Published: July 2007

AbstractTemperature and time dependences of the polarized A emission of tetragonal Sn2+v centres in alkali halides have been investigated under excitation in the A absorption band. The change of polarization during the relaxation time, low‐temperature depolarization due to the tunnel transitions between the minima of the lowest excited states, evidence of the metastable state in emission processes, and high‐temperature depolarization due to the jumps of cation vacancy (v) around Sn2+, have been observed. A model is proposed for the excited states of Sn2+, in which the electron‐lattice interaction is considered to be the main interaction, while the Spin–orbit interaction is taken into account as an additive correction. The formulae have been obtained for temperature and time dependences of polarization and decay time. Some parameters of Sn2+v centres in KCl:Sn have been found from comparison of the formulae with experimental data.
Polymorphic Phase Transition in a Potassium Chloride Crystal
Authors: Chakrabarti S. K. 
Published: July 2007

AbstractThe phase transition data at 298 °K of potassium chloride crystals are analysed on the basis of two alternative models, (a) the Tosi‐Fumi prescription (TFP) in which overlap parameters are taken to be different in the two phases and (b) the three‐body potential model (TBPM) used by Sarkar and Sengupta [1]. The effect of finite temperature and that of zero point energy is included using the Einstein approximation. It is found that the transition data are well reproduced in both the models with TFP giving slightly better values. The temperature coefficient of transition pressure comes out with the right sign but has too high a magnitude in both the models. In elastic data, however, TFP gives poor agreement while for TBPM the agreement is much better.
Numerical calculations for the effect of a Brownian sublattice on polariton dispersion
Authors: Andrade P. Da R. , Borstel G. , Merten L. 
Published: July 2007

AbstractNumerical evaluations for the effect of a Brownian sublattice on polariton dispersion were made. Using reasonable physical values for the coupling between long optical phonons and polaritons with a dielectric disorder mechanism a set of theoretical illustrations is calculated for the temperature dependence of the transverse optical mode and the polariton dispersion. The effects appearing give a new insight on phase transitions.
The hall effect and the thermopower of two simple heavily doped semiconductor models
Authors: Fischbeck H. J. 
Published: July 2007

AbstractThe single‐site approximation is used in order to study the effect of randomly distributed point scatterers on the most important transport properties of two simple semiconductor models. The one consists of two parabolic and the other of two simple‐cubic tight‐binding bands. Numerical results are given for energy‐dependent (density of states, conductivity, Hall conductivity) and temperature‐dependent quantities (conductivity, Hall mobility, thermopower).
Doubling of the period of flux quantization in hollow superconducting cylinders due to quantum effects in the normal state
Authors: Bogachek E. N. , Gogadzb G. A. , Kulik I. O. 
Published: July 2007

AbstractThe effect of quantization of normal excitations in thin‐walled hollow superconducting cylinders on the critical temperature versus magnetic flux oscillations is considered. It is shown that the period of oscillations is he/e instead of hc/2e, as in the Parks‐Little effect. The phenomenon is connected with the so‐ called Aharonov‐Bohm effect and disappears if the quantum levels are smeared out, particularly dur to volume or surface scattering or finite temperature.
Polaron electric conductivity tensor in a high laser radiation field
Authors: Balmush N. I. , Klyukanov A. A. , Pokatilov E. P. 
Published: July 2007

AbstractThe method to calculate the electric conductivity tensor for polarons is extended to the case of electrons interacting with a strong electromagnetic field. This field affects the scattering probability of electrons and the polaron mass and leads to a heating of polarons. Calculations are presented of the relaxation time and polaron mass tensors for various electron–phonon interactions. The optical‐polaron effective mass is determined at ω ≈ Ω, ω being the LO‐phonon‐angular frequency and Ω that of the field. The derived equations predict a resonant variation of the polaron mass at ω = Ω caused by phonon emission. The photoconductivity of electrons is considered with account of heating. The theory is consistent with experimental data.
Theory of free‐carrier absorption in polar semiconductors due to electron‐LO‐phonon interaction
Authors: Myciblski J. , Aziza A. , Mycielski A. , Balkanski M. 
Published: July 2007

AbstractFree‐carrier absorption due to polar interaction of electrons with LO‐phonons is calculated for a multi‐valley semiconductor with spherical and parabolic band minima. The effect of free‐carrier screening on both matrix element and phonon frequency is taken into account for crystals with high static dielectric constant. The theory is compared with recent experimental data on n‐PbSe.
EPR investigations on Y2+ in KZnF3
Authors: Buzabé J. Y. , Leble A. , Fayet J. C. 
Published: July 2007

AbstractEPR measurements have been made to investigate the covalency for the [VF6]4− complex in KZnF3. A precise determination of the superhyperfine parameters in the spin Hamiltonian was obtained by a computed reconstruction of the experimental spectra. The experimental results have been analysed by calculating the mean value of the superhyperfine Hamiltonian on an LCAO model of the molecular orbitals.
Theory of a possible parametric resonance in hydrogen‐bonded ferroelectrics
Authors: Stamenković S. , Žakiula R. B. 
Published: July 2007

AbstractThe pseudo‐spin formalism is used to describe a possible parametric resonance in hydrogen‐bonded ferroelectrics. In the long‐wavelength approximation, when dipolar interactions play a predominant role, the characteristic strength of a dc electric field and the threshold and limiting frequency of an rf electric field, applied under an arbitrary angle with respect to the polarization axis, are estimated. Within the present theory the mechanism of such a ferroelectric parametric resonance is simply interpreted. The results obtained in terms of pseudo‐spin energy parameters make possible to establish experimentally the favorable physical conditions to utilize the ferroelectries in certain amplifier‐microelectronic circuits.
Polarization properties and dispersion relation for cyclotron waves
Authors: Frandsen J. B. , Hasselberg L.‐E. , Lebech J. , Saermark K. 
Published: July 2007

AbstractOn the basis of (a) a spherical Fermi surface and (b) Halse's Fermi surface of silver, calculations are presented showing the importance of the polarization properties of extraordinary cyclotron waves propagating through metallic samples. In particular, it turns out that the longitudinal component of the electric field of the cyclotron wave in general is non‐negligible. An attempt at a physical description of the large damping observed at the “turning points” of the dispersion curve is given and an example of a comparison between experimental results and a calculated dispersion curve is presented.
The self‐consistent electronic structure of trigonal selenium
Authors: Krusius P. , von Boehm J. , Stubb T. 
Published: July 2007

AbstractThe ab initio self‐consistent energy band structure of trigonal selenium based on the Hartree‐Fock‐Slater formalism has been determined with the self‐consistent symmetrized OPW method (SCSOPW). The influence of lattice constants, exchange potential, and relativistic effects is discussed. The density of states and effective mass components have been determined. Valence band triplet charge densities are presented for discussions of bonding properties. X‐ray form factors have also been evaluated. The determined properties are satisfactory except for the optical gap.
Static approximation for the Hubbard model: Paramagnetic phase at finite temperatures
Authors: Mertsching J. 
Published: July 2007

AbstractThe static approximation for the Hubbard model by Puff and Weller [1] is discussed for finite temperatures. The distribution function of the stochastic site occupation numbers, the single‐particle density of states, and the free energy are calculated for temperatures much smaller than the bandwidth and the correlation energy.
Microscopic theory of lattice dynamics in nonconducting crystals. II. One‐electron approximation
Authors: Mitskevich V. 
Published: July 2007

AbstractIn the one‐electron approximation and taking into account the deformation of the electron clouds of ions (atoms) formulas for the cohesion energy and the dynamical matrix of a crystal which ions have closed shells are derived. Under certain admissions the connection among the different constituents of the dynamical matrix is defined and expressions for the effective ionic charge, the polarizability of ions in a crystal, and the short‐range potential of a lattice are determined. The results obtained are written in the S2‐approximation.
Dielectric properties of monoclinic KD2P04
Authors: Blinc R. , Burger M. , Čižikov S. , Levstik A. , Kadaba P. , Shuvalov L. A. 
Published: July 2007

AbstractHighly deuterated KD2PO4 which is monoclinic at room temperature, undergoes a dielectric phase transition between T = 0 °C (on heating) and Tcll = −25 °C (on cooling). At lower temperatures there is another dielectric anomaly around TcI = −60 °0. The “lower” transition Tci occurs approximately at the same temperature as the tetragonal‐ orthorhombic ferroelectric phase transition, whereas the “higher” transition TCII is similar in character to the tetragonal‐monoclinic “high temperature” transition in “normal” KH2PO4. A tentative model for the phase diagram of KH2PO4 and the tetragonal‐mono‐clinic transition is discussed.
Theory of phonon relaxation due to multilevel impurities
Authors: Joshi Y. P. 
Published: July 2007

AbstractA theory is developed for the calculation of the single‐mode relaxation time of phonons in a crystal containing impurities that have a possibly degenerate spectrum of energy levels. Transitions of the impurities from one level to another under the effect of the lattice waves can result in the scattering of the latter. Following earlier calculations corresponding to the case of nondegenerate two‐level impurities, certain operators are defined to describe these transitions, which allow to describe the phonon‐impurity system with a model Hamiltonian that is suited to the diagrammatic technique of the thermodynamic Green's functions. The relaxation time obtained exhibits resonant behaviour with peaks at frequencies equal to the energy gaps between various pairs of energy levels. The relaxation time is also found to be quadratic in the degeneracy factors of the impurity levels and shows a dependence on temperature through the occupation probability of these levels.