bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the New Unrestricted Hartree‐Fock Vortex Solution of the Hubbard Hamiltonian in Two‐Dimensional Systems A Small‐Cluster Study
Authors: Louis E. , Galán J. , Guinea F. , Vergés J. A. , Ferrer J.
Published: July 2007

The properties of a recently discovered unrestricted Hartree‐Fock solution of the Hubbard Hamiltonian is analysed on the square lattice that shows a really two‐dimensional spin polarization of a vortex‐like character. This and other several mean‐field solutions are compared with the exact ground‐state energy and wave function of two and four electrons on a four‐site system (square). Results for two correlated electrons in large 3 × 3 and 4 × 4 clusters are also given and discussed. The vortex mean‐field solution provides the best solution in almost all systems that deviate from half‐filling.
Theory of superconductivity for quasiparticles with parastatistics
Authors: Safonov V. L.
Published: July 2007

The superconducting state of pairs of spinless quasiparticles with parastatistics is considered on the basis of a model Hamiltonian in mean‐field approximation. It is shown that the energy gap δ0 and critical temperature Tc increase as exp (−1 sol;p2λ) at a definite weak‐coupling parameter λ with increasing rank p of parastatistics. The characteristic ratio 2δ0/Tc and the typical pair size diminish. Thermodynamic properties of the system near the phase transition and parameters of the Ginzburg‐Landau theory are calculated. The obtained results are discussed in connection with a set of experimental data.
Superconducting gap substructure in bisoliton HTSC theory
Authors: Ermakov V. N. , Kruchinin S. P. , Ponezha E. A.
Published: July 2007

The conditions and peculiarities of the realization of the superconducting gap substructure in a bisoliton superconductivity model are considered at temperatures much lower than the critical one in the case when the temperature dependence of the gap can be neglected.
TL spectra from optical fibres at 80 K after ultraviolet illumination
Authors: Khanlary M. R. , Townsend P. D.
Published: July 2007

Emission spectra of thermoluminescence below room temperature are reported from several types of optical fibre after ultraviolet illumination at 80 K. The spectra are modified by previous X‐ray irradiation at room temperature. Since the light energy is insufficient to form new intrinsic defects all the signals are thought to arise from phototransfer from deep stable defect states formed during fibre production. An X‐ray irradiation history at 300 K, before cooling to 70 K, significantly enhances subsequent optically generated glow peaks, in particular that at 120 K which emits at 545 nm. The phototransfer efficiency of the fibres is greater than for “pure” bulk silica. It is therefore proposed that the high phototransfer efficiency found in the optical fibres is related to a combination of the fibre dopants and defect retention during the rapid cooling used in fibre drawing.
Electronic structure and optical properties of MgS
Authors: Stepanyuk V. S. , Grigorenko A. A. , Katsnelson A. A. , Farberovich O. V. , Szász A. , Mikhailin V. V.
Published: July 2007

The electronic structure and optical properties of MgS are calculated by the totally self‐consistent linear augmented plane wave method.
Optical reflectivity of Si above the melting point
Authors: Boneberg J. , Yavas O. , Mierswa B. , Leiderer P.
Published: July 2007

The variation of the optical reflectivity of a Si single crystal during irradiation with two successive Nd:YAG laser pulses is investigated with ns resolution. The first pulse melts the surface and therefore the reflection coefficient increases up to the value of the metallic liquid at the melting temperature (1685 K). Upon further heating the surface with the second, time‐delayed pulse, a decrease of the reflection coefficient is observed, resulting from the temperature‐dependent dielectric function of the molten Si. The largest decrease in the reflectivity that could be reached before damaging the surface amounted to 9% for a probe wavelength of 633 nm. The application of a simple Drude model for the optical constants above the melting point is discussed.
Determination of complexes and diffusion mechanisms in a GaP/InP strained‐layer superlattice
Authors: Wang En Ge
Published: July 2007

A theoretical study of the defect characteristics in a GaP/InP strained‐layer superlattice is presented. Based on the calculated charge states, some closely bound interstitial‐antisite pairs are found to form at a distance of a bond length in a stable GaP/InP (1 × 1) structure. The defect‐related states localized in the energy gap suggest that the formation mechanism of the EL2 complex in the GaAs system is unsuitable for GaP/InP multilayer structures. A slow self‐diffusion procedure is proposed for both group III and V atoms in this superlattice. Besides the investigation of some mobile species in silicon diffusion, the kick‐out reactions are suggested to be favored at higher doping concentration.
Lattice dynamics of the superconductor YBa2Cu3O7
Authors: Hu Bengang , Katiyar Ram S.
Published: July 2007

A lattice dynamical formalism using the rigid ion model is applied to the superconductor YBa2Cu3O7. The model includes the short‐range repulsive interactions of axially symmetric type and long‐range Coulomb interactions among ion pairs. In order to determine the short‐range parameters, the Born‐Mayer approximation and the stability conditions were used. The crystal parameters determined by neutron powder diffration technique and the frequencies from Raman and infrared measurements on single crystal have been utilized to obtain the best fit. The proposed model yields an excellent agreement between the observed and calculated frequencies and gives all phonon modes of YBa2Cu3O7, which can serve as a guide for further experimental and theoretical investigations on YBa2Cu3O7 and other XnBa2Cu3O7 with similar structure. The realistic values of calculated effective charges and bond force constants and the agreement between the observed and calculated frequencies indicate the suitability of the rigid ion model with the proposed approximations to such materials. On the basis of our results, it is found that the atoms Ba and Cu(1) are strongly bonded to out‐of‐plane oxygen atoms. From the eigenvector analysis it appears as if the in‐plane oxygen atoms vibrate alone against the cage lattice with nearly half the frequency for x‐ and y‐directions in comparison to the z‐direction motion. This may have significance in connection with superconducting phenomena below Tc.
Vibrational modes around a moving soliton in trans‐polyacetylene (CH)n
Authors: Ye Zhen
Published: July 2007

The vibrational modes of a soliton in trans‐polyacetylene are studied. Using a quasi‐realistic continuum model which generalizes the Takayama‐Lin‐Liu‐Maki model by including acoustic‐phonon effects, a self‐consistent mean‐field eigenequation for the vibrational modes of the soliton is derived. Then it is shown that these vibrational modes are damped due to the acoustic‐phonon effects, which contributes to the linewidths in the lineshape observation.
Influence of tunneling on trapping kinetics in thin insulating layers
Authors: Schmidt M. , Köster H.
Published: July 2007

The process of trap occupation in Si/SiO2/M structures might be accompanied by a tunneling (detrapping) process near the adjacent contact (x < 5 nm), depending on the distance, the trap energy relative to the Fermi level of the adjacent contact, and the actual trap filling state. The equilibrium between the two opposing processes gives rise to a dynamical trapping behaviour and adjust itself at xg, the boundary position between stationarily occupiable and unoccupiable traps. For a given tunneling rate constant the equilibrium depends on the capture rate constant and therefore on the current density j only. The realistic values of the capture cross‐section σ and of the effective trap concentration Neff can differ by more than one order of magnitude from the value determined by using a simple first‐order kinetic which neglects tunneling phenomena. The developed dynamic trapping model gives a reliable basis for trap analysis in thin oxide layers. For hole traps at E = 6.3 eV we determined σh = 1.5 × 10−14 cm2 and Neff = 1.47 × 1013 cm−2.
The electronic structure and superconductivity of the lanthanum monochalcogenides LaX (X = S, Se, Te)
Authors: Sankaralingam S. , Jaya S. Mathi , Pari G. , Asokamani R.
Published: July 2007

The first band structures and calculated electronic properties of the compounds LaSe and LaTe are reported. The energy band structure of lanthanum monochalcogenides are obtained using the self‐consistent LMTO method within the atomic sphere approximation. The band structure outputs were used to calculate the physical properties such as the electronic specific heat, the Pauli paramagnetic susceptibility, and the superconducting transition temperature. The calculated results are compared with the available experimental values.
The influence of structural defects on the electronic properties of amorphous Ta2O5
Authors: Khanin S. D. , Ivanovskii A. L.
Published: July 2007

The non‐empirical self‐consistent method of discrete variation is employed to calculate the electronic structure of clusters simulating amorphous Ta2O5, which contains anionic vacancies, hydrogen impurities, and substitutional defects (in anionic (N and F) and cationic (Mo and W) sublattices). The influence of these defects on the electronic structure, as well as interatomic bonds, charge distributions, and dielectric properties of amorphous Ta2O5 is discussed.
Bound energy of the Wannier exciton in similar heterogeneous double structures
Authors: Boichuk V. I. , Bilynskii I. V.
Published: July 2007

The problem of the ground state of the Wannier‐Mott exciton in a thin semiconductor film is solved by a variational method. The test wave function has been chosen with two variational parameters. Electron‐hole interaction is considered Coulombic, logarithmic, and general. The binding energy of the exciton in dependence on L−2 is calculated for a WSe2‐type crystal. Qualitative agreement of theory with experiment is obtained in the region of small L.
Interpretation of the semiconducting and metallic phases in the sodium titanium bronze Na1–xTi4O8
Authors: Colaitis D.
Published: July 2007

The electron transfer process is systematically examined in Na1–xTi4O8 as a direct consequence of cation‐anion orbital overlaps. The critical interatomic distance below which the overlap occurs is presumed to be nearly 0.195 nm. On the basis of distortions present in the octahedra of the host lattice, it is shown that molecular‐type orbitals can be imagined in the semiconducting state orthogonally to the infinite chains of octahedra. In the metallic state two infinite ‐TiO‐ chains with transverse correlations are considered as conveying the collective d‐electrons. The occurrence of a Peierls distortion is discussed and excluded. An additional deformation modulation of Ti atoms is proposed, triggered by the fundamental modulation of the interstitial site occupation. The model can describe, in terms of molecular‐type orbitals, the electrical and magnetic behaviour with varying temperature and Na content.
Vapor of pairing particles near the Fermi level of high‐Tc superconductors
Authors: Davydov A. S.
Published: July 2007

The effect of the presence of isolated excited pairs of quasi‐particles at finite temperatures in a cloud over the Fermi level is investigated. It is shown that at inelastic scattering of particles or radiation the decay of pairs from a condensate just as the decay of pairs from a cloud of isolated pairs introduces a contribution to the temperature dependence of the effective gap.
Magnon modes in quasi‐periodic ferromagnets
Authors: Liu Tian‐Shi , Wei Guo‐Zhu
Published: July 2007

Considering ferromagnetic exchange energies J1 and J2 between layers, which are arranged in quasi‐periodic Fibonacci sequence, and the exchange energy J0 in a layer, a rescaling approach to magnons in quasi‐periodic superlattice ferromagnets is presented. An exact decimation transformation is derived for magnon modes of α‐ and β‐layers in ferromagnets. Iteration of the transformation provides numerial results for the local density of states (LDOS) and the magnetization, when J1 + J2 = 2J0 fixed, λ = J2/J1 ( > 1). The band width of LDOS of layer α is 16SJ0(2 + λ)/(1 + λ), but the band width of LDOS of layer β is 24SJ0, the magnetizations of layers α and β decrease as λ is increasing for a given temperature, the magnetization of layer β is always bigger than that of layer α.
Cathodoluminescence of copper‐doped GaAs and its relation to EL2 centres
Authors: Kimpel B. M. , Schulz H.‐J.
Published: July 2007

Several series of GaAs:Cu crystals are studied with respect to cathodoluminescence properties at cryotemperatures, together with undoped or Si‐doped blank specimens for comparison. A one‐electron model is proposed which comprises the various emission processes related to copper centres in this host material. The presence of EL2 defects in the samples is proved and their energy levels are included in the model. The tendency of Cu and EL2 centres to form complexes is considered. Especially the level hitherto named CuB is now related to an EL2 defect plus a neighbouring copper ion. The 1.0 eV emission of GaAs:Cu thus becomes comprehensible as electron capture at an arsenic antisite defect associated with substitutional copper.
The Exact Fokker‐Planck Type Equation for Planar Channeling of Relativistic Positrons
Authors: Doumanis N. A. , Trikalinos Ch. G.
Published: July 2007

The motion of relativistic positrons under the planar channeling regime is investigated. The corresponding Fokker‐Planck type equation is established considering the longitudinal momentum as constant and then taking into account the exact expressions for the transverse and longitudinal energy. Thus we conclude on the exact form of the Fokker‐Planck type equation. An approximate solution is given without taking into account multiple scattering. This solution, although approximate shows that we have the phenomenon of particles that spread out of the channel instead of damping. The exact solution of the equation requires computer calculation. Our result may be used for the explanation of stronger dechanneling of relativistic particles in planar channeling where the intensity of channeling radiation is less than the expected one.
Absorption Spectra and Low‐Temperature Thermal Conductivity of Irradiated MgO Crystals with Impurity OH− Ions
Authors: Abramishvili M. G. , Akhvlediani Z. G. , Kalabegishvili T. L. , Kvachadze V. G.
Published: July 2007

The influence of γ‐irradiation on low‐temperature thermal conductivity χ (T) and the spectra of optical and paramagnetic absorption of MgO crystals with OH− impurity ions is investigated. It is shown that irradiation leads to strong accumulation of hydrogen‐containing centres and ions of Cr2+ and Fe2+ causing significant changes of χ (T) in crystals. The annealing experiments reveal that in the radiative change of χ (T) the contribution of hydrogen‐containing hole centres is significant.
Improved Raman Effect Studies on Boron Carbide (B4.3C)
Authors: Kuhlmann U. , Werheit H.
Published: July 2007

With a FT Raman spectrometer highly resolved and reliable Raman spectra of B4.3C were obtained largely refuting previous results of other authors. The symmetry selection rules of IR and Raman active phonons are proved to hold. The most prominent Raman doublet at 270/320 and that at 869/928 cm−1are due to pentagonal pyramids of the icosahedra symmetrically vibrating relative to the end atoms of the three‐atomic chain. The broad band at 1065 cm−1is attributed to corresponding vibrations of pairs of pentagonal pyramids in adjacent icosahedra. The remaining Raman‐active phonons, mainly attributed to intraicosahedral vibrations, are comparably weak. At higher photon energies the luminescence radiation due to electron transitions is essential.
The Inapplicability of the Effective Mass Equation to Narrow‐Gap Semiconductors
Authors: Vodná L.
Published: July 2007

It is shown that the well known effective mass equation which was successfully applied to many materials may become not sufficient for the heavy hole band in narrow‐gap semiconductors.
Exchange Interaction in the Diluted Semimagnetic Semiconductors Hg1−xMnxSe Under Pressure
Authors: Chudinov S. M. , Yu. Rodichev D. , Mancini G. , Stizza S.
Published: July 2007

Galvanomagnetic properties and Shubnikov‐de Haas oscillations in n‐type Hg1−xMnxSe diluted solutions for Mn concentrations 0.001 ≦ x ≦ 0.23 in magnetic fields up to 7 T at temperatures T = (0.2 to 20) K under hydrostatic pressure up to 1.6 GPa are investigated. New methods based on oscillation measurements are proposed to determine magnetic characteristics (value and sign of exchange interaction energy for magnetic ion clusters and average cluster size) of semimagnetic semiconductor diluted solid solutions. These methods permit to determine the magnetic subsystem characteristics in the range of low temperatures between the spin‐glass transition and the temperature corresponding to the binding energy of magnetic atoms in the cluster. In this temperature range, familiar methods (based on measurements of sample magnetic moments) are not effective. The methods proposed are used to determine the dependence of interaction energy on Mn concentration and pressure. It is found that Mn concentration or pressure increase causes a change in the character of cluster interaction from antiferromagnetic to ferromagnetic. The possible exchange interaction mechanisms and their contributions to the interaction energy are discussed.
Spin Waves in a Semi‐Infinite Field‐Induced Metamagnet with Three‐Ion Anisotropic Interactions II. Application to FeBr2 and FeCl2
Authors: Rudziński W. , Maciejewski W.
Published: July 2007

Theoretical results for surface and bulk spin‐wave dispersion relations derived in the first paper of this series are applied to field‐induced metamagnets FeBr2 and FeCl2 with a (001) surface. The effect of the three‐ion anisotropy on the surface excitations is examined numerically for both the antiferromagnetic and paramagnetic phases. The obtained results contain modifications of the frequency separation between the surface and bulk modes. Moreover, by allowing the surface three‐ion anisotropy field to differ from the bulk value it is found that in the paramagnetic phase besides optical‐type solutions evaluated in recent analysis of FeBr2 and FeCl2 (001) surface, acoustic‐type surface branches may also occur. A comparison of the results obtained for FeBr2 and FeCl2 confirms the essential influence of the crystal structure on the behaviour of surface spin waves.
On the Interatomic Correlations in Anharmonic Crystals
Authors: Zubov V. I. , Pascual M. F. , Teixeira Rabelo J. N.
Published: July 2007

A correlative method of unsymmetrized self‐consistent field is utilized to study the interatomic correlations in anharmonic crystals. Using the statistical perturbation theory up to second order the expression for binary correlations is derived. Corresponding diagrams are constructed and classified. As a simple application, the correlation moments are calculated for the one‐dimensional model.
Calculation of the Lifetime of Slow Positrons at Metal Surfaces
Authors: Shrivastava S. B. , Gupta V. K.
Published: July 2007

The diffusion equation for positrons at metal surfaces is solved to give the rate of positrons reaching the surface. Rate equations are written to describe the various processes at the surface including the trapping of positrons in near‐surface defects. These are solved to yield the lifetime of positrons at surfaces as a function of temperature and for different incident energies. The calculated results show that the positron lifetime increases with the increase in temperature and decreases with the incident energy. The above results are compared with available experimental values.
Spatial Anisotropy of Electron—Phonon Interaction Hopfield‐McMillan Approach Calculation Method and Results for 5d‐Metals
Authors: Zhalko‐Titarenko A. V. , Evlashina M. L. , Antonov V. N. , Nemoshkalenko V. V.
Published: July 2007

A method for the theoretical investigation of anisotropy of electron—phonon interaction is developed within the Hopfield‐McMillan approximation. Calculations for Ta, W, Ir, and Pt are carried out. The contributions of separate sheets of the Fermi surface to electron—phonon interactions are found, the origin of the anisotropy of both electron—phonon coupling constant and transport electron—phonon coupling constant is established for these metals.
Phonon Dispersion in Multicomponent Superlattices
Authors: Rath S. , Sanyal S. P.
Published: July 2007

The phonon spectra of a three‐component semiconductor superlattice (SSL) with GaAs, GaSb, AlAs as components along the growth direction are studied by using a one‐dimensional transfer matrix method where one of the components, GaSb, is lattice mismatched with the other two, for various thicknesses. The eigen‐displacements are plotted for various frequencies and thicknesses keeping the thickness of the GaSb layer constant. Also, the frequencies of the confined LO modes of AlAs and GaSb are plotted as functions of the effective wave vector for various thicknesses of the constituting components forming the SSL and compared with the bulk dispersion curves. It is observed that the phonon dispersion in a multicomponent SSL is similar to that of a two‐component SSL, but the large magnitude of lattice parameters of GaSb and its small thickness qualitatively change the superlattice dispersion relation, a feature not reported so far.
Plasmons in a Superlattice with a Doped Surface Layer
Authors: Sy H. K. , Chua T. C.
Published: July 2007

Finite semiconductor multilayers consisting of a doped surface layer in addition to a periodic array of underlying two‐dimensional charge layers are studied. The frequency of the plasmons can be obtained in terms of a determinant equation. The possible existence, and the dispersion relation of the surface plasmon modes, are discussed in the semi‐infinite limit. Numerical results are shown for surface densities greater or smaller than the underlying charge density, for both finite and semi‐infinite systems.
Electron Structure and Residual Resistivity of Aluminium Clusters with 3d‐Impurities
Authors: Kurkina L. I. , Gorbunov V. A. , Farberovich O. V.
Published: July 2007

The self‐consistent electron structure and residual resistivity of Al clusters containing substitutional impurity atoms of Fe, Co, and Ni are calculated within the framework of the model “atom embedded in spherical jellium cluster” and local density approximation. The residual resistivity is obtained to oscillate with increasing cluster size.
Resonant Raman Scattering near the Band Gap in 4H‐PbI2, Crystals)
Authors: Baltog I. , Lefrant S. , Mihut L. , Mondescu R.
Published: July 2007

New data are given of the resonatn Raman scattering (achieved by incident excitation tuning or by sample temperature variation) fo rthe 4H polytype of pure and doped PbI2 single crystals. A new Raman line, considered by us as a two‐TA‐phonon line, is observed at 30 cm−1. Some new features of the Raman spectra in the region of 75 cm−1 are revealed for pure and doped crystals, when excitation approaches resoncance. As for the two‐LO‐phonon line (224 cm−1) the same enhancement at resonance is found for a new line apperaing at 126 cm−1.
Distributed Relaxation Response for Two Classes of Material Temperature Behavior
Authors: Macdonald J. R.
Published: July 2007

Expressions for the transient and frequency response of materials showing exponential distributions of transition rates are summarized and related to recent work in the field. Responses arising from temperature‐independent distributions (class I) are contrasted to those that depend in a simple way on temperature (class II). The distinction between dielectric and conducting systems is particularly emphasized, and the different temperature dependences of their frequency power‐law exponents for class‐1) situations are discussed in detail. Besides presenting model frequency‐response curves, it is shown that an exponential distribution of activation energies can lead to conductance frequency response very similar to that recently found by Lee et al. for two glasses and several ionically conducting single crystals.
Optical Properties of Multi‐Layer Structures. IV. Non‐Linear Absorption of light in Multi‐Layer Structures
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

The theory of non‐linear response of an electronic subsystem in planar multi‐layer structures (including quantum wells) to a strong electromagnetic field is developed. On the basis of the exact Hamiltonian of the electron‐phonon interaction for such structures a non‐linear dielectric permittivity tensor is obtained by the statistical operator method in the path integration representation. In the three‐layer structures with a thin middle semiconducting layer and moderate values of the dielectric permittivity of the outer dielectric media the contribution to the absorption due to the interaction of electrons with surface phonons is found to prevail over that of bulk phonons and to ensure a substantially more rapid rise in the absorption coefficient with increasing intensity in comparison with the case of a semiconductor crystal.
Phonon Frequencies, Elastic and Grüneisen Constants of Liquid Chromium
Authors: Rao R. V. Gopala , Venkatesh R.
Published: July 2007

The potential function as formulated by Bretonnet and Deroache is used to evaluate the phonon frequencies, elastic and Grüneisen constants. The latter is also evaluated by a different method as well. The elastic constants are also calculated through Schofield's equations. All the results are found to be in agreement with each other. From the parameters used for the computation of the structure factor we obtain S(0), the structure factor in the long‐wave limit and hence the compressibility from which the velocity of the sound is calculated. Assuming Ziman's theory that the structure factor peaks around 2kF, we find that the longitudinal phonon frequency maximum occurs at kF and from this value and sound velocity the maximum of the phonon frequency could be predicted. From the sum rules of frequency dependent structure factor the maximum value of ωL(k) is predicted. The maximum values so obtained by different methods are in satisfactory agreement with each other.
Quantum Mechnical Hartree‐Fock Study of the Elastic Properties of Li2S and Na2S
Authors: Lichanot A. , Aprà E. , Dovesi R.
Published: July 2007

The binding energy and elastic constants of Li2S and Na2S are investigated at an initio all electron Hartree‐Fock level. The comparison between calculated and experimental value leads to satisfactory agreement for Li2S and a substantial difference in the Na2S case. The comparative evolution of the elastic constants in the series of alkaline oxides and sulphides with the size of the cation seems to indicate that the discrepancy is mainly due to an overstimation of the experimental elastic constants of Na2S.
To the Theory of High‐Temperature Superconductivity
Authors: Naimi E. K.
Published: July 2007

A new attitude towards the problem of high‐temperature superconductivity is suggested. The main attention is focused on the principal differences between electrodynamics of high‐temperature and low‐temperature superconducting state. While describing the electromagnetic field in high‐temperature superconductors (HTSC) it is necessary to take into account both conducting currents and displacement currents in general case. It is shown that the transition of HTSC into the superconducting state is accompanied by its spontaneous polarization, so that the external electric field is completely compensated inside the superconductor volume. The Messiner magnetic effect in HTSC is a secondary one. It is found that the excitation spectrum of HTSC has a multi‐gap structure. Estimations of all fundamental parameters of HTSC are given. Comparison of theoretical with experimental results shows good agreement. Some aspects of anisotropy and high‐frequency properties of HTSC are discussed.
The Physical Nature of Changes of Lattice Parameters in Small Particles
Authors: Ya Gamarnik M.
Published: July 2007

The nature of size‐related changes of lattice parameters is determined by the difference in bond energies of surface and inner unit cells of a particle, which (the difference) leads to an excess or a deficit of the intracrystalline pressure in a small particle, stipulating a compession or tension of its lattice. It is shown that the size‐related change of the length of each lattice edge is determined by crystal sizes in perpendicular direction and practically does not depend on the crystal size along this edge. This property, called transversality, is explained by an energetic non‐equivalence of different unit cells, which is realized within cell monolayers or columns perpendicular to the edge. It is shown that the sign of the size effect – decrease or increase of the lattice parameter – is determined by the sign of the interaction energy v of a unit cell with crystal unit cells excepting the nearest ones. To the positive sign of this energy there corresponds compression, to the negative sign tension of a small particle. The structural features are analysed for substances with ionic, covalent, and metallic bond types, determining the sign of v and therefore the sign of the size effect.
Physical Properties of Thorium under Pressure
Authors: Mahalingam A. , Vidya K. , Kalpana G. , Palanivel B. , Rajagopalan M.
Published: July 2007

A theoretical calculation of the band structure, electrical resistivity, and superconductivity of f.c.c. thorium under pressure is reported. The effect of pressure on the band structure is obtained by means of the linear muffin‐tin orbital (LMTO) method. The superconducting transition temperature (Tc) is calculated using McMillan's as well as Allen‐Dyne's modified formulae. The parameters necessary to calculate Tc are taken from the band structure outputs. The calculated values of Tc initially decrease with the increase in pressure up to 154 × 108 Pa and then begin to increase. The sudden increase in the value of Tc around 181 × 108 Pa is attributed to the change in the Fermi surface topology. The theoretically calculated values of resistivity initially decrease with increase in pressure up to 154 × 108 Pa and then begin to increase in accordance with the experimental results. The change in slope around 181 × 108 Pa may be due to a change in Fermi surface topology. The calculated values of ϱ are in correct trend with the experimental values.
X‐Ray Diffraction and Mössbauer Effect Studies on Smb−x CaxFeO3−y (0 ≦ x ≦ 0.70)
Authors: Li J. , Cai X. , Yang X. , Wang T. M.
Published: July 2007

Samples of Sm1−x CaxFeO3−y are prepared in air for x ranging from 0 to 0.70. X‐ray powder diffraction analysis shows that the samples are the single‐phased perovskite‐type compounds with the space group Di162h‐Pbnm. The accurate lattice constants of Sm1−x CaxFeO3−y are calculated from the observed diffraction peaks by the least‐squares refinement. Sm1−x CaxFeO3−y yields two hyperfine splitting patterns which would be assigned to iron(III) on octahedral sites and iron(III) on tetrahedral ones. The percentage of the iron on the tetrahedral sites increases almost linearly with increasing x, indicating that the oxygen vacancies in the lattice increase with the increasing substitution of Ca for Sm. The compounds prepared contain only trivalent iron, suggesting that they would have the chemical formula Sm1−x CaxFeO3−x/2.
Coherent Relaxation and Acceleration of Atoms in Nonequilibrium Crystals
Authors: Sugakov V. I.
Published: July 2007

It is shown that in a nonequilibrium system of atoms, which are situated along a crystal line, in the presence of two minima of potential energy for every atom and in case of atom localization in the higher‐energy minimum a series of successive collisions resulting in acceleration of one of the atoms may occur. The above‐mentioned systems can appear in the vicinity of the coherent boundary of different phases in crystals or in the case of strong shear deformation. The excited crystal state, in which the successive atom relaxation leads to the energy rise of one of the atoms, is called “acceleron”. The role of various processes, which restrict the acceleron energy, is analyzed. The possibility to manifest the phenomenon is discussed.
Low‐Temperature Mobility of a Two‐Dimensional Hole Gas in GaAs–AlGaAs Heterojunctions
Authors: Datta R. , Chaudhuri S. R. , Kundu S.
Published: July 2007

A theoretical model for the calculation of the mobility of a two‐dimensional hole gas (2DHG) supported by a heterojunction is presented. Different scattering mechanisms that put a limit to the inherent mobility of the 2DHG system, like the background impurity scattering, remote impurity scattering, deformation potential, and piezoelectric acoustic phonon scattering, and polar optical phonon scattering, have been included. Dynamic screening of the hole gas has been derived. Screening plays a very important role in the scattering of the heavier hole gas than for the lighter electrons. A discrepancy between the experimental and the theoretical effective hole masses existing in earlier theoretical models has been overcome in the present communication.
Filling‐Factor‐Dependent Cyclotron Resonance in GaInAs/AlInAs Modulation‐Doped Single Quantum Wells
Authors: Jianc D. S. , Goiran M. , Leotin J. , Askenazy S. , Zhanc Y. H. , Plooc K.
Published: July 2007

The line shape of cyclotron resonance (CR) absorption is investigated for a two‐dimensional electron gas (2DEG) in modulation‐doped GaInAs/AlInAs narrow SQW structures. CR absorption is measured as functions of far infrared (FIR) frequency, temperature, and carrier concentration. At low temperature and short FIR wavelengths, a well‐defined filling‐factor‐dependent quantum oscillation structure is obtained for samples with high 2DEG concentrations, exhibiting clear maxima and minima and a phase change point at the top of the CR peak. The modulated scattering is attributed to short‐range scatterers originating mainly from alloy scattering. A part of the contribution comes from ionized impurity scattering. The analysis of the temperature dependence of the CR line width and Hall mobility gives additional support to this argument. An electron effective mass as high as 0.063mo in In0.53 Ga0.47As is observed due to the non‐parabolicity effect.
Properties of the Ground State in the Hubbard‐Hirsch Model
Authors: Zhong Qiang , Nie Hui‐Quan , Wei Guo‐Zhu
Published: July 2007

Using Bogolyubov's variational principle and the mean‐field theory the properties of the ground state in the Hubbard‐Hirsch model are discussed for weak coupling condition, and the phase diagrams are plotted. It is pointed out that the ground state of the system can be paramagnetic, ferromagnetic, or antiferromagnetic, depending on the intensity of the Coulomb interaction U and the exchange interaction J; the charge‐ordered phase does not exist.
EPR Studies on Dimers in a Diamagnetically Dilute Copper Calcium Acetate Hexahydrate
Authors: Samanta (Sikdar) R. , Pal A. K.
Published: July 2007

X‐band EPR investigations of mixed crystals of the composition Cu1−xCdxCa(Ac)4 · 6 H2O (Ac = ace‐ tate) with x = 0.858, 0.552,0.328, and 0.148 are carried out at room and liquid nitrogen temperatures. Besides monomer spectra, dimer spectra due to transitions within the spin triplet, singlet‐triplet transitions, and half‐field lines (due to ΔM = 2 transitions within the dimer triplet) are observed. Zero‐field splitting is shown to have purely dipolar origin. Singlet‐triplet spectra enable precision determination of the value of the exchange integral J as well as its sign. J has a much greater value than that derived in case of the concentrated copper salt, and unlike the case of the latter it is isotropic, showing thereby that magnetic dilution plays an important role in determining the magnitude of J.
Growth Kinetics of CdS Nanocrystallites in Glass through Quantum‐Size Effects
Authors: Sukumar V. , Doremus R. H.
Published: July 2007

Glasses containing very small particles of CdS are prepared and their properties studied. Changes in the optical absorption spectra of CdS glass result from quantum confinement effects. With very small particle size, large confinement energies of up to 4 eV are obtained, which have not been before. The blue‐shifts in the optical absorption spectra show that the growth rate of the particles at short times are diffusion controlled, and the activation energy for growth is calculated. The band gaps of the CdS nanocrystallites in glass are determined from photoluminescence measurements, which show a high‐energy peak, corresponding to the band gap, that had not been previously observed.
Structural and Electronic Properties of Carbon‐Doped β‐Rhombohedral Boron
Authors: Werheit H. , Kuhlmann U. , Laux M. , Lundström T.
Published: July 2007

β‐rhombohedral boron doped with carbon up to about 1 at% is investigated by X‐ray diffraction, by optical transmission spectroscopy in the spectral range of the absorption edge and its low energy tail, by I R reflection spectroscopy in the lattice vibration range, and by Raman scattering. The carbon atoms substitute for boron sites in the icosahedra shrinking as a consequence of the increased intraicosahedral bonding force. Hence the lattice constants decrease with increasing carbon content. Within the accuracy of measurement the interband transition energies are not influenced by the carbon content, and the p‐type conductivity remains qualitatively unchanged. Additionally to the series of six ionization energies of B12 electron trapping levels, which are equidistantly arranged below the conduction band in pure boron, a second series of B11C trapping levels develops. The traps are esscntially generated by the interaction between one electron each and up to six pairs (probably triples) of defined phonons. These phonons are quenched as a consequence of the occupation of these traps. In correlation with the new optical results ESR investigations of other authors can now be interpreted. Important details of the electronic structure and its modification by carbon doping are determined. The relevance for other boron‐rich solids with icosahedral structure is discussed.
The Bond‐Diluted Spin‐1 Transverse Ising Model with Random Longitudinal Field
Authors: Benyoussef A. , Ez‐zahraouy H.
Published: July 2007

The bond‐dilution effects on the phase diagram of a spin‐1 king ferromagnet in a transverse field with random longitudinal field is investigated within an expansion technique for cluster identities of spin‐ localized spin systems. Longitudinal and transverse magnetizations are studied for several values of thc bond concentration. The critical temperature dependence on the bond concentration, the longitudinal field, and the transverse field are presented. General formulae applicable to structurcs with arbitrary coordination number N are given.
Investigation of the ADC Method for Direct ODF Approximation by Means of Standard Functions
Authors: Raabe D. , Lücke K.
Published: July 2007

Three‐dimensional orientation distributions of grains in polycrystalline aggregates are referred to as crystallographic textures. They can be calculated from centrosymmetric two‐dimensional pole figures by means of series expansion methods or by direct methods. Both techniques lead to inaccuracies, caused by the absence of the odd coefficients and by truncation errors in the first case and by the underdetermination of the set of linear equations combining discrete cells in the pole figures and in the three‐dimensional orientation space in the second case. The direct method of “arbitrary defined cells” (ADC) is investigated by means of the component fit method and by use of standard functions, respectively. The maximum value of the orientation density achieved by the ADC method represents a good approximation. However, weak negative and positive ghost adulterations are identified.
Lattice Dynamics of Cyanogen
Authors: Zeng W. Y. , Anderson A.
Published: July 2007

A simple force constant model is described and applied to the vibrations of crystalline cyanogen, C2N2, at low temperatures. The model uses eleven adjustable parameters (six intermolecular and five intramolecular) to calculate the frequencies of seventeen lattice modes and twenty‐eight components of internal modes, and reasonable agreement with spectroscopic data is obtained. Although cyanogen shows many of the characteristics of a typical molecular crystal, there is some evidence that its large quadrupole moment results in longer‐range forces being more significant than usual, particularly in contributing to the splittings of some internal modes.
About an Increase of Exciton Binding Energy in Layered InSe
Authors: Gnatenko Yu. P. , Zhirko Yu. I.
Published: July 2007

At T > 25 K an increase of the direct free exciton binding energy ΔE in the exciton absorption spectra of InSe is found. The calculations show that this phenomena may be attributed to an increase of the exciton reduced mass μ due to exciton‐interlayer optical phonon interaction. The dynamical processes of exciton‐phonon intcraction with temperature in laycred InSc are discussed.
Response Approach to Current and Noise Power Spectrum of Cooper Pair Tunneling in Ultrasmall Junctions
Authors: Krech W. , Hädicke A.
Published: July 2007

Using the response theory approach, the mean current due to tunneling of Cooper pairs in microwave driven superconducting junctions coupled to an electromagnetic environment is derived. The noise power spectrum is calculated showing shot noise behaviour due to the stochastic character of Cooper pair tunneling if the Josephson coupling energy is much smaller than the charging energy.
Spectroscopic Studies on the Spin Structures in the Magnetic Phases of Erbium Oxisulphate
Authors: Oreans P.
Published: July 2007

Er2O2SO4, orders below 1.5 K as an Ising‐like antiferromagnet with its moments parallel to the crystallographic c‐axis. In external fields along c, three ferrimagnetic phases with different magnetizations are crossed before the saturated paramagnetic phase is entered. The lines of the optical absorption spectrum are split owing to different moment configurations in the surroundings of the light‐absorbing Er3+ ion. These line splittings and their Zeeman effect in an external field are studied in order to uncover the spin structures in the different magnetic phases. The analysis is substantiated by calculations in mean‐field theory.
On the Influence of Elastic Strains on Crystallization Processes in Solids. I. Spherical Clusters in Solids of Finite Size
Authors: Möller J. , Schmelzer J. , Gutzow I. , Pascova R.
Published: July 2007

The influence of the finite size of a solid sample on evolving elastic fields and, in particular, on the energy of elastic deformations due to the formation of a new phase inside a solid is analyzed. It is shown that, depending on the ratio between the size of the matrix RM and that of the cluster RC, elastic strains may either stimulate or inhibit the growth of the newly evolving phase. Elastic strains may result in a stable two‐phase equilibrium state. The discussed specific finite‐size effects become important for clusters in samples which are not larger than about 5RC. The outlined model is the basis for the discussion of energies of the elastic strain field caused by the process of formation of a cluster of a new phase near or at the surface of a solid, the detailed analysis of which will be presented in Part II.
On the Influence of Elastic Strains on Crystallization Processes in Solids. II. Formation of Clusters near and at Interfaces
Authors: Möller J. , Schmelzer J.
Published: July 2007

Deformations due to the formation of a spherically shaped cluster near a planar free surface are analysed. The dependence of the mechanical energy contained in these deformations on the cluster distance from the surface is computed analytically. The energy is found to be lower for clusters near the surface. Results are obtained for the energy of clusters formed immediately at the surface. It turns out that the lowest value of the elastic energy for clusters of given volume is found for hemispherical clusters. As intermediate steps in the derivation, the independence of the elastic energy of the cluster shape for clusters in an infinite matrix and the general absence of elastic interactions are reconfirmed, provided matrix and cluster are characterized by the same elastic constants. These results explain in this way a number of observations, for example, surface nucleation and surface crystallization.
Phonon Focusing in LiNbO3 Crystals in an Electric Field
Authors: Zubritskii V. V.
Published: July 2007

For the first time in the 0 to 107 V/cm range of electric field (E), taking into account the nonlinear piezoeffect in trigonal lithium niobate, the directions and coefficients of concentration of longitudinal, fast and slow transverse acoustic phonons are calculated and compared with the orientation of the incomplete electric breakdown channels in accordance with the model of incomplete breakdown of crystals along the phonon fluxes. The possibility of controlling focused phonon fluxes by a strong electric field is predicted. The maximum deviations of individual focusing directions of slow transverse phonons in the field E = 107 V/cm can reach 41°. The effects of suppression of some phonon focusing paths and of generation of new ones by the field are revealed. It is shown that from the 24 directions of concentration corresponding to the infinite phonon fluxes, it is possible to find those coinciding with two of the three breakdown channels at E = 0 to 106 and 107 V/cm, respectively. The account of 15 additional local directions of “weak” phonon focusing paths, also sensitive to crystal data, not required by the breakdown model, provides coincidence of all the three discharge channels with individual directions of concentration in the range E = 0 to 106 V/cm.
The Conductivity of a δ‐Doped Parabolic Quantum Well in a Cross Electric Field
Authors: Safronov E. Yu. , Sinyavskii E. P.
Published: July 2007

The conductivity of a δ‐doped parabolic quantum well in cross electric field at low temperatures is studied. An alteration of the cross electric field causes an alteration of the magnitude of the electron–impurity interaction. It allows to alter the mobility of such a system by a cross electric field, which may be used for the creation of new electronic devices. The possibility of the appearance of a direct current in an alternating electric field forming a small angle with the direction of quantization is discussed.
On Antiferromagnetism, Charge Density Wave, and Superconductivity in the Extended Hubbard Model Studied by Means of the Slave‐Boson Method
Authors: Bulka B. R.
Published: July 2007

The ground state of the extended Hubbard model is studied. The stability conditions for the antiferromagnetic, the charge density wave, and the superconducting state are analyzed in the same slave‐boson representation, which is spin‐ and charge‐rotationally invariant. It is found that at the concentration of electrons no = 1 the stable solutions are similar in form.