bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropic Magnetization of Some MnAs1−xSbx Single Crystals
Authors: Kohnke H. J. , Dankelmann V. , Kleeberg Ch. , Schünemann J. W. , Bärner K. , Vetcher A. , Govor G. A.
Published: July 2007

The anisotropy of the magnetization of MnAs1−xSbx single crystals is investigated in the temperature range 4.2 K < T < 600 K, for fields up to 0.6 T and concentrations x = 1, 0.5, and 0.1. The first spin reorientation temperature of MnSb, Ts1 = 270 K, is connected with the influence of the thermal expansion on the local anisotropy constants; the second spin reorientation at Ts2 = 520 K is also due to volume expansion, but after the displacement disorder transition at Td = 458 K has occurred. Anomalies of the angular dependence of the magnetization and nonlinear magnetization versus field curves along the hard axis are related to the occurrence of an easy cone.
To the Problem of the Description of Ordered Structures in Binary Cubic Solid Solutions
Authors: Bugaev V. N. , Chepulskii R. V.
Published: July 2007

Within the framework of the static concentration wave method by means of the geometrically complete procedure a number of new types of ordered substitutional structures with the stoichiometric structural formulae AB, AB3, and A3B5, as well as of corresponding isomorphic interstitial structures, A2X, A4X, and A8X3 (A, B, and X are the types of atoms situated at sites and interstices, respectively) are established that complete the list of all thermodynamically stable with respect to the violations of macroscopic homogeneity (Lifshitz) ordered structures arising from the disordered state of f.c.c. and b.c.c. binary solid solutions.
Determination of Quasi‐Plastic Strains in a Crystalline Plate Based on a Solution of the Inverse Problem of the Theory of Elasticity (One‐Dimensional Case)
Authors: Voloshin A. E. , Smolsky I. L.
Published: July 2007

An approximate solution of the inverse problem of the theory of elasticity is considered in terms of distortions for a crystalline plate with a one‐dimensional inhomogeneous distribution of point defects. It is shown that near the plate surface the distribution function of quasi‐plastic strains can be represented as a linear combination of distortion tensor components, up to second‐order infinitesimals. The errors of determining the distribution function using various approximations to a solution of the corresponding direct boundary‐value problem are evaluated. The stability of the solution is demonstrated by reconstruction of the distribution function using a set of simulated topographic images.
Electromagnetic Forces on Type‐II Superconducting Rotating Cylinders
Authors: Saif A. G. , Refai T. F. , El‐Sabagh M. A.
Published: July 2007

Analytical solutions of the electromagnetic fields are presented for a system composed of an infinitely long superconducting cylinder rotating about its axis and placed parallel to two infinitely long normal conducting wires. Both wires carry the same alternating current. From the obtained electromagnetic fields the electromagnetic power loss on the cylinder surface, electromagnetic forces due to induced currents, electromagnetic torque, and the work opposing the rotation of the cylinder are calculated.
Surface Magnetization of a Semi‐Infinite Spin‐1/2 Ising Model with a Diluted Spin‐1 Surface
Authors: Jaščur M. , Kaneyoshi T.
Published: July 2007

The temperature dependence of magnetization at or near the surface in a semi‐infinite spin‐1/2 Ising ferromagnet with a diluted spin‐1 surface is examined within the framework of an effective‐field theory with correlations. Magnetic behaviors associated with the surface dilution as well as the surface single‐ion anisotropy are discussed. A number of interesting phenomena are obtained, which may be observed experimentally.
A Non‐Dangling Bond Model of Paramagnetic Defects in Ge‐S Glasses
Authors: Černošek Z. , Černošková E. , Frumar M. , Swiatek K.
Published: July 2007

The ESR spectra of the intrinsic defects in GeS glasses are studied at liquid nitrogen temperature in the whole glass‐forming region. The Ge‐related signal is found in glasses of all compositions and this signal is assigned to a two‐atomic defect center (DCI) which is formed by the interaction of broken bonds of Ge with two‐coordinated sulphur atoms forming (Ge4…S3)0 centers, the subscript is for coordination, superscript for the charge, and the dots for a chemical bond of order 1/2. A multi‐line spectrum is found not only in GeS glasses with excess of sulphur but also in glassy sulphur after excitation by Ar+ laser light (454 nm). This signal can be assigned to two defect centers: DCII (S2 … trans‐S3)0 and DCIII (S2 … cis‐S3)0 formed by interaction of a dangling bond of sulphur with a two‐coordinated sulphur atom S in the Sn chains. Sulphur of these chains can be either in trans‐coordination (DCII) or in the cis one (DCIII). The experimentally found compositional dependence of the DC densities is qualitatively explained. The proposed model is the first one which is able to explain the compositional dependence of the intrinsic paramagnetic defects in the whole glass‐forming region. The model can also shed light on the absence of paramagnetic centers in other chalcogenide glasses.
Physical Aspects and Technological Realization of Amorphous Silicon Thin Film Solar Cells
Authors: Wagner H.
Published: July 2007

Thin film solar cells represent the major option for photovoltaic electricity production at economically acceptable costs. Especially cells and modules based on amorphous silicon can be produced on cheap large area substrates and are presently the choice to enter mass production. The understanding of the physical properties of amorphous silicon and amorphous silicon alloys forms the basis for an optimized cell design and the use of appropriate deposition processes. Stacked cell structures and improved interfaces in each single cell minimize the effect of light induced degradation. Multi junction cells of different band gap absorber materials are being developed and are already used which utilize the solar spectrum even better, and lead to higher stabilized efficiencies.
Optical Properties of Silicon Nanostructures
Authors: Stutzmann M.
Published: July 2007

Strong visible luminescence of porous silicon has triggered renewed general interest in the optical properties of silicon nanostructures. In addition to porous Si itself, Si‐based molecules, nanometer‐size Si particles, one‐dimensional Si chains, two‐dimensional Si layers, and many Si alloys are known to exhibit optical properties (absorption and luminescence) which strongly deviate from those of bulk crystalline Si. Here, we review and compare the main properties of the above‐mentioned systems and present a brief discussion of the experimental observations in terms of existing theories. Current prospects for Si‐based optoelectronic applications are also discussed.
Two‐Photon Photoemission Spectroscopy of Electronic States at Metal Surfaces
Authors: Steinmann W.
Published: July 2007

In two‐photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon of the same pulse ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the position and the lifetime of both the initial and, in particular, the intermediate state. Most of the work in two‐photon photoemission spectroscopy has been devoted to the investigation of image potential states (image states). They are bound states of electrons in front of metal surfaces in the potential well built by the image potential and the surface. The situation is analogous to the hydrogen atom, with a binding energy of the lowest state of 850 meV in first approximation. Up to four members of the Rydberg series are detected. Experimental results on the binding energy of image states in (111) and (100) surfaces of Ag, Au, Cu, and Pd agree with calculations by a one‐dimensional scattering model. The results for Co, Ni, and Fe are not yet understood. The effective mass is close to unity except for Ag(111). Lifetimes between 5 and 170 fs are measured. They can be explained by decay via electron‐hole pair creation in the bulk. Image states on metal overlayers yield information on film growth and surface morphology. Dielectric overlayers are been investigated. Adsorbate states of atoms and molecules adsorbed on metal substrates can also be studied by two‐photon photoemission spectroscopy. Results for Na, O, CO, and NO on Cu(111) are reported.
Low Energy Electron Microscopy of Nanostructures
Authors: Bauer E.
Published: July 2007

The various imaging modes which are possible in non‐scanning surface microscopy with slow electrons are discussed, in particular the information which can be gained on surface potential, topography, crystal structure, magnetic microstructure, and chemical composition. Applications relevant to semiconductor microelectronics, magnetic storage technology, and heterogeneous catalysis are reported.
Nitrogen Incorporation into Hard Amorphous Carbon Films Obtained by RF Plasma Decomposition of CH4‐N2 Gas Mixtures
Authors: Freire F. L. , Lazaro J. R. , Franceschini D. F. , Achete C. A.
Published: July 2007

Recent results on the incorporation of nitrogen into amorphous hydrogenated carbon films (a‐CNx: H) are reviewed. The films are deposited onto silicon substrates by rf plasma decomposition of methane‐nitrogen mixtures. The samples are characterized by a combination of analytical techniques: nuclear techniques (RBS, NRA, and ERDA), SIMS, infrared absorption, Raman scattering, and positron annihilation spectroscopy (Doppler broadening method). The Vickers hardness and the internal stress of the films are also determined. The results indicate that the incorporation of nitrogen increases the density of voids and induces a progressive graphitization of the films. The incorporation of nitrogen up to 11 at% reduces the internal stress of the films without significant modification of the film hardness. Results on the thermal stability of a‐CNx: H films are also presented: for annealing temperatures higher than 300 °C, hydrogen and nitrogen losses occur as well as the graphitization of the films.
Structure and physical properties of ternary organic conductors of TCNQ, iodine, and double‐charged unsymmetrical cations: PET and PESe
Authors: Abashev G. G. , Russkikh V. S. , Koshta A. , Lapinski A. , Król S. , Pukacki W. , Graja A.
Published: July 2007

Ternary organic salts of TCNQ, iodine, and double‐charged unsymmetrical cations containing sulfur or selenium are investigated. The crystal structure of the salts is determined; their dc electrical conductivity, ESR, and IR spectral properties are studied. The physical properties of the investigated salts are found as typical for Q‐ID organic semiconductors.

Nous avons étudié les sels ternaires du TCNQ, du iode et des cations double chargés, dissymétriques, comprenant le soufre ou le sélénium. La structure crystalline des sels examinés a été déterminée. La conductivité électrique en courant continu, la résonance électronique de spins et les propriétés spectrales en infrarouge ont été étudiés. Nous avons montré que les propriétés physiques des sels examinés sont typiques des semiconducteurs organiques de bas dimensionalité.
Ground‐state properties of quantum dots with a magnetic field
Authors: El‐Said M.
Published: July 2007

The transitions in the spin and orbital angular momentum of the ground state of two interacting electrons, parabolically confined in the quantum dot, in a magnetic field of arbitrary strength are studied.
Temperature dependence of the electrical resistivity of Y3Co11B4 and Gd3Co11B4 compounds
Authors: Kowalczyk A. , Ivanov V.
Published: July 2007

The temperature dependence of the electrical resistivity Q of R3Co11B4 (R = Y and Gd) in the temperature range from 4.2 to 370 K is presented. These compounds crystallize in the Ce3Co11B4‐type structure. The unit cell of the structure contains 18 atoms. The rare earth atoms are distributed in two different crystallographic sites, the Co atoms in three different positions, and boron is located in one type of site. In the low‐temperature region Q is proportional to T2. This quadratic dependence is caused by electron‐spin wave scattering. The experimental results are compared with transport properties of other rare earth‐transition metal compounds.
Photon avalanche in Tm doped CdF2
Authors: Bouffard M. , Jouart J. P. , Mary G.
Published: July 2007

The spectroscopic and dynamic studies of the Tm3+ doped cadmium fluoride show the existence of a 450 nm emission (1D2 → 3F4 transition), excited around 650 nm (3H4 → 1D2 transition) by a tunable laser. The presence of a power threshold for the density of excitation allows to establish that the upconversion mechanism is the photon‐avalanche process.
X‐ray diffraction study of gallium nitride grown by MOCVD
Authors: Metzger Th. , Angerer H. , Ambacher O. , Stutzmann M. , Born E.
Published: July 2007

An X‐ray diffraction study of gallium nitride grown on the c‐plane (0001) and r‐plane (0112) of sapphire substrates by metal organic chemical vapour deposition using triethylgallium and ammonia was performed. By measuring asymmetric reflections with a ω‐2θ two‐circle diffractometer, lattice constants of α‐GaN could be resolved for both substrate orientations and the epitaxial relationships could be confirmed. Additionally, the existence of β‐GaN on the r‐plane sapphire substrate could be determined. Pole figure measurements show different qualities of in‐plane orientation of the α‐GaN layers for the two substrate orientations.
Bond distribution and structure of amorphous germanium‐nitrogen alloys
Authors: Zanatta A. R. , Chambouleyron I.
Published: July 2007

The Ge 3d core levels of sub‐stoichiometric amorphous germanium‐nitrogen (a‐GeN) alloys are studied by X‐ray photoelectron spectroscopy (XPS). The nitrogen concentration in the samples, as determined from nuclear reaction analysis (NRA), is allowed to vary from zero to ∼ 3 × 1022 atoms cm−3. For the whole series of samples both, the chemical shift and the bonding distribution are studied as a function of the nitrogen concentration in the alloys. The chemical shift per Ge‐N bond is also investigated using different methods to deconvolute core level energy spectra, and found to be approximately (0.5 ± 0.1) eV. Furthermore, the present study shows that the GeN bond distribution, as obtained from the analysis of Ge 3d core level data, is in relatively good agreement with the random bonding model (RBM).
A study of interlayer coupling in Fe/Cu multilayers by spin wave resonance
Authors: Jin Q. Y. , Xu Y. B. , Zhou S. M. , Lu M. , Zhai H. R. , Shen Y. H.
Published: July 2007

Spin wave resonances related to interlayer exchange are observed in the FMR measurements in Fe/Cu multilayers with the Cu spacer layer thickness dCu < 28 Å. The spin wave is demonstrated to propagate through the whole multilayers and the phase change of the spin wave is shared by both Fe and Cu sublayers. The phase change across the Cu atomic layer is found to decrease with the increase of Cu sublayer thickness.
From solar cell research to a photovoltaics industry
Authors: Hill R.
Published: July 2007

Karl Böer was a pioneer in research on thin film solar cells, and a committed builder of bridges between academic research and industrial applications. Amongst his many achievements was the founding of the Institute of Energy Conversion at the University of Delaware and a company SES Inc., to exploit the fruits of the research at the IEC. He also built, in the early 1970's, what was probably the world's first building with an integrated PV roof. The paper outlines the paths by which research on solar cells has led to significant advances in commercial products, and seeks to identify some common themes in this successful transfer of technology from R&D laboratories to industry. This is then extended to consider the routes by which R&D in general may be transferred into industry to create new or improved commercial products.
Recent developments in amorphous silicon‐based solar cells
Authors: Beneking C. , Rech B. , Fölsch J. , Wagner H.
Published: July 2007

Two examples of recent advances in the field of thin‐film, amorphous hydrogenated silicon (a‐Si:H) pin solar cells are described: the improved understanding and control of the p/i interface, and the improvement of wide‐bandgap a‐Si: H material deposited at low substrate temperature as absorber layer for cells with high stabilized open‐circuit voltage. Stacked a‐Si: H/a‐Si: H cells incorporating these concepts exhibit less than 10% (relative) efficiency degradation and show stabilized efficiencies as high as 9 to 10% (modules 8 to 9%). The use of low‐gap a‐Si:H and its alloys like a‐SiGe:H as bottom cell absorber materials in multi‐bandgap stacked cells offers additional possibilities. The combination of a‐Si: H based top cells with thin‐film crystalline silicon‐based bottom cells appears as a promising new trend. It offers the perspective to pass significantly beyond the present landmark of 10% module efficiency reached by the technology utilizing exclusively amorphous siliconbased absorber layers, while keeping its advantages of potentially low‐cost production.
Growth of bulk ZnSe crystals — recent developments
Authors: Siche D. , Hartmann H. , Böttcher K. , Krause E.
Published: July 2007

Starting with a historical review and a general comparison of growth methods, the paper deals with the seeded vapour growth of substrate‐quality ZnSe crystals. Under optimized growth conditions, in contrast to the growth from the melt, in SPVT and SCVT (seeded physical and chemical vapour transport) twin‐free single crystals are prepared. The dislocation density decreases down to ≈104 cm−2 in growth direction away from the seed transition region. SPVT‐grown crystals are the better ones with respect to optical properties showing exciton luminescence and decreasing deep level emission. For the application in homoepitaxy, the SCVT‐grown crystals with lowest defect densities seem to be superior.
Tailoring of Si doping layers in GaAs during molecular beam epitaxy
Authors: Däweritz L. , Kostial H. , Ramsteiner M. , Klann R. , Schützendübe P. , Stahrenberg K. , Behrend J. , Hey R. , Maier M. , Ploog K.
Published: July 2007

For Si δ‐like doping of GaAs lateral ordering processes and segregation in growth direction have been investigated in real time by monitoring longe‐range and short‐range ordering effects using reflection high‐energy electron diffraction (RHEED) and reflectance anisotropy spectroscopy (RAS). When Si is supplied in pulses to GaAs(001) distinct ordering processes occur which are promoted by misorientation steps on the vicinal surface. The incorporation of Si atoms on Ga sites in a single plane can be completed to a high degree. The depth profile of the electron concentration is closely related to Si segregation during GaAs overgrowth. In the case of a doping layer of extremely high coverage it shows a minimum at the centre of the doping spike. For samples grown on vicinal GaAs(001) surfaces under conditions favourable for wire‐like Si incorporation a considerable enhancement of the exciton photoluminescence (PL) intensity and decay time has been found.
On the temperature dependence of InGaAsP semiconductor lasers
Authors: Haug A.
Published: July 2007

The temperature dependence of the threshold current of InGaAsP lasers can be improved by using quantum well structures and, in particular, strained quantum wells instead of the bulk material.
High‐density effects, stimulated emission, and electrooptical properties of ZnCdSe/ZnSe single quantum wells and laser diodes
Authors: Grempel H. , Diessel A. , Ebeling W. , Gutowski J. , Schüll K. , Jobst B. , Hommel D. , Pereira M. F. , Henneberger K.
Published: July 2007

In II–VI material based laser diode structures, the role of excitonic processes for stimulated emission is still rather controversially discussed. In this paper, the manifestation of biexciton recombination in high‐density luminescence of ZnCdSe/ZnSe quantum wells is investigated by intensity‐ and polarization‐dependent spectroscopy. In spite of their evidence in such spectra, their contribution to stimulated emission in laser diodes with moderately confining wells is not confirmed, instead, indication of strong Coulomb correlation is given when using the new technique of pump‐and‐probe excitation spectroscopy. A strong electrooptic effect yielding saturation is found in laser diodes exactly at their lasing wavelength thus being promising candidates for integrated laser‐modulation devices.
Femtosecond degenerate four‐wave mixing study of exciton coherence in wide‐gap II–VI quantum wells
Authors: Häpl T. , Nickolaus H. , Henneberger F. , Schülzgen A.
Published: July 2007

This paper studies coherent excitons in the presence of alloy disorder by femtosecond degenerate four‐wave mixing on ternary (ZnCd)Se/ZnSe quantum wells. The use of very low excitation densities enables to uncover an impurity‐mediated exciton dephasing time of (16 ± 2) ps and a related homogeneous exciton width of only 80 μeV. At higher densities, the coherence loss speeds up dramatically due to exciton‐exciton interaction. Separating the contributions from inhomogeneous (photon echo) and homogeneous broadening to the spectrally resolved decay transients allows to deduce the energy dependence of the dephasing time across the disorder broadened exciton ensemble. Further, pronounced biexciton resonances are observed in the signal spectrum which exhibit characteristic polarization properties. For cross‐polarized pulses, the biexciton signal shows a striking stimulation when the excitation density is increased.
Photoreflectance studies of (Al, Ga)As/GaAs heterostructures and devices
Authors: Enderlein Rolf
Published: July 2007

Experimental and theoretical PR spectra are reported for (AlGa)As/GaAs single heterostructures, (AlGa)As/GaAs quantum well structures as well as MESFET and HEMT structures. The theoretical spectra are calculated by means of a novel technique which combines the two methods previously used for spatially inhomogeneous systems: the transfer matrix method for light propagation between different layers, and perturbation theory for light propagation within each weakly inhomogeneous layer. The electric field distribution is calculated from an integral equation which holds under quite general conditions including incomplete dopant ionizations and carrier degeneracies. In most of the spectra, pronounced interference effects show up. Due to them, PR spectra from quantum wells are sensitive to the well distance from the surface. Besides interference effects, field inhomogeneities like broadening of Franz‐Keldysh oscillations manifest themselves in the reported spectra. A controversy in the literature concerning PR spectra from (AlGa)As/GaAs single heterostructures is solved by means of the novel calculation technique.
Peculiarities of thermomagnetic effects in gapless semiconductors specified by spatial correlations of impurity charges
Authors: Tsidilkovski I. M.
Published: July 2007

The results of measurements of the thermomagnetic Nernst‐Ettingshausen effect (NEE) in the gapless semiconductor HgSe doped with iron are presented. The iron content in the samples varies from 1 × 1018 to 4 × 1020 cm−3 when the temperature varies from 9 to 100 K. A change of sign of the NEE from negative to positive when the iron content increases is observed at low temperatures. It is shown that the unusual dependence of the NEE on iron content in HgSe: Fe crystals at low temperatures, its sign change, and other peculiarities are caused by scattering of electrons on aggregates of charged donors Fe3+. Such an aggregate constitutes a weakly correlated system in the range of low iron concentrations and a strongly correlated Coulomb liquid in the range of high iron concentrations. A new mechanism of inelastic electron scattering on static defects, bivalent iron ions, is proposed, which allows to explain the anomalous decrease of the Lorenz factor with increasing temperature. All observed peculiarities of the thermomagnetic effects are interpreted quantitatively.
Charge transfer in GaP and InP
Authors: Saravanan R. , Israel S. , Srinivasan N. , Mohanlal S. K.
Published: July 2007

Charge transfer in GaP and InP is analysed using our precise and extensive X‐ray diffraction data. The direction and the amount of charge transfer are inferred by plotting and comparing the structure factors of the components (both observed and calculated). The charge transfer parameters thus obtained are 0.45ē from Ga to P and 0.36ē from In to P.
Investigation of substitutional interactions by means of Zener relaxation
Authors: Blanter M. S. , Kolesnikov E. V.
Published: July 2007

A new method based on Zener relaxation is elaborated for the determination of substitutional interactions in metal solid solutions. The method consists in computer simulation of temperatures of internal friction peaks taking into account the different values of effective interchange energies. Then comparison between the calculated spectra and the experimental ones is performed. It is suggested that substitutional interaction affects the atomic arrangement in creating short‐range order and shifts the energy of the substitutional atom and therefore the activation energy of internal friction. Experimental parameters of Zener relaxation are used to study the interaction energies of Cu‐Zn, AuAg, and CuAl alloys. The energies are close to those determined by diffuse scattering of X‐rays or neutrons.
Equation of state, isobaric specific heat, and thermal expansion of solids with polyatomic basis in the Einstein‐Debye approximation
Authors: Cankurtaran M. , Askerov B. M.
Published: July 2007

A theoretical study of the thermodynamic properties in the Einstein‐Debye approximation is made for solids with polyatomic basis. Analytical expressions are derived for the equation of state, isobaric specific heat (Cp), and linear thermal expansion coefficient (αP). The temperature dependences of Cp and αp are determined by four characteristic parameters of the solid: the Debye temperature θD, the Einstein temperature θE, and the Grüneisen parameters γ and γ associated with the volume dependences of θD and θE, respectively. It is shown that the difference CP−CV between the calculated isobaric and isochoric specific heat capacities is very small at low temperatures (T ≪ θD) and increases markedly at higher temperatures. In the high‐temperature range (T ≫ θE), CP is almost independent of temperature and is determined by a simple combination of the Grüneisen parameters γ and γ. The results obtained for αP, in the low‐ and high‐temperature limits, are in accord with those found for the asymptotic behavior of CV with temperature.
Effective‐mass Hamiltonian for strained superlattices
Authors: Brezini A. , Sebbani M. , Depollier C.
Published: July 2007

For an abrupt heterojunction between two otherwise homogeneous semiconductors in one dimension, the effective‐mass Hamiltonian

is examined with 2α + β = −1, where m(z), a(z), and Ec(z), are the position dependent effective mass, the local lattice parameter, and the local conduction band edge, respectively. The exact Schrödinger equation is compared with that for the effective‐mass equation in the case of an analytically solvable model. In the asymptotic limit of the energy close to the band edge, the conclusions obtained are shown to be dependent on the parity of the bands.
Activation energy of small polarons and conductivity in LiNbO3 and LiTaO3 crystals
Authors: Wang Qi , Leng Shuyan , Yu Yansheng
Published: July 2007

Eliminating pyroelectric and other disturbances, with the aid of the curves of bulk and surface current versus temperature, the conductivity character of LiNbO3(LN) and LiTaO3(LT) is studied at different temperatures. The activation energy is obtained, which is needed for bound small polarons in LN and LT crystals to jump across their potential barrier to give rise to hopping conduction.
Phenomenological theory of the surface nucleation field in anisotropic superconductors
Authors: Klein U.
Published: July 2007

The surface nucleation field for anisotropic (conventional) superconductors is calculated for an arbitrary modification of the superconducting transition temperature near the surface. We use, as a phenomenological description, anisotropic Ginzburg‐Landau theory including appropriate surface terms. A uniaxial situation, as in layered high‐Tc compounds, is considered, with the external magnetic field oriented along one of the main crystallographic axes. The resulting nucleation fields are all determined by a single universal curve (and appropriate direction‐dependent scaling factors). Depending on the surface state, both an enhancement and a suppression of the usual third critical field 1.7Hc2 may occur. The results are relevant for layered compounds, in particular for high‐Tc materials, where the surface conditions depend sensitively on the nature of the first layer on top of the surface.
Energy eigenvalues in square and rectangular quantum wires with finite barrier potential
Authors: Gangopadhyay S. , Nag B. R.
Published: July 2007

Energy eigenvalues have been calculated for square and rectangular quantum wires with finite barrier potential of GaAs/Ga0.63Al0.37As and Ga0.47In0.53As/InP systems, by expressing the wave function in terms of a two‐dimensional Fourier series. Calculated values are compared with those obtained earlier by using the finite element method and by using equivalent circular or elliptic wires. Experimental values of the energy shift of Ga0.47In0.53As/InP wires are also compared with the calculations.
Electrical resistivity and thermoelectric power of (GeTe)1−x(Bi2Te3)x solid solutions (0 ≤x≤ 0.05) in the temperature interval from 80 to 350 K
Authors: Christakudi T. A. , Plachkova S. K. , Christakudis G. Ch.
Published: July 2007

Electrical conductivity σ, Hall coefficient RH, and thermoelectric power (Seebeck coefficient) S are measured for (GeTe)1−x(Bi2Te3)x solid solutions with 0≤ × ≤ 0.05 in the temperature range 80 to 350 K. On the basis of these investigations some information on the carrier scattering mechanisms in the alloys is obtained. The electrical resistivity ϱ = 1/σ is analyzed as function of temperature T and composition x in the above‐mentioned temperature interval and resistivity contributions due to acoustic phonon scattering (ϱac) and to scattering by defects (germanium vacancies, impurity atoms, etc.) and alloy scattering are distinguished. From the results it may be shown that the acoustic phonon resistivity increases nearly linearly with temperature. At low temperatures the resistivity due to defects ϱdef is almost constant and slowly increases at temperatures T > 250 K.
Study of phonon eigenfrequencies in ternary transition metal‐metalloid Fe40Ni40B20 glass
Authors: Gupta A. , Bhandari D. , Jain K. C. , Saxena N. S.
Published: July 2007

The study of the ternary Fe40Ni40B20 metal‐metalloid glass has been made through a partial pair‐potential approach. Six partial pair potentials of this ternary glass have been computed using pseudopotential‐based interatomic forces. In particular, the model employed here for the calculation of the potential is in analogy with the three‐partial‐structure‐factor approach of Sietsma and Thijsse with the assumptions that Fe and Ni atoms have identical distributions with respect to like neighbours and hence the partial pair potentials of Fe‐B and Ni‐B are identical. The calculated effective pair potential using the partial pair potentials is then employed to investigate the longitudinal and transverse phonon eigenfrequencies. Further, from the elastic part of the frequency spectrum, elastic and thermal properties have been computed. The computed results for these properties are found to be in good agreement with the available experimental results.
Presence of soft phonons in a model ferroelectric
Authors: Freidkin E. S. , Horton G. K. , Cowley E. R.
Published: July 2007

We investigate the behaviour of a one‐dimensional ferroelectric crystal. The model includes both short‐range and long‐range interactions. We prove that the system undergoes a phase transition from an ordered to a disordered state and confirm the existence of a soft mode for this particular model.
Quasi‐resonance and confining phenomena in symmetrical rectangular quadruple‐barrier structures
Authors: Sano H. , Yamamoto H. , Taniguchi K.
Published: July 2007

Resonant tunneling is studied theoretically in detail in symmetrical rectangular quadruple‐barrier structures by taking into account the mass difference between the well layer and the barrier layer. The variation of the resonance energy with the outer barrier width is investigated. It is found that the resonance level is rarely affected by a small change of the outer barrier width. Especially, it is derived analytically that the transmission peak is decreased to less than unity when the outer barrier width becomes thinner than a critical value. Furthermore, wave functions of an electron at the resonance energy level are calculated and the confining phenomenon is studied.
An effective potential for high‐temperature superconductivity in cuprates
Authors: Silva P. R.
Published: July 2007

An effective potential is proposed as a means to study the mechanism of pairing of high‐Tc superconductivity in cuprates. The relation between size and energy of bound states evaluated in a first approximation is comparable to recent calculations obtained through quantum electrodynamics in (2 + 1) dimensions. In a second approximation, we consider that pairing occurs at the minimum of this effective potential, the problem being related to the quantum mechanical behavior of a particle described by polar coordinates in a plane. So, according to the present model, the topological constraint determined by the CuO2 planes seems to be most relevant in the understanding of the pairing mechanism.
Optical anisotropy of SrLaAIO4 and SrLaAl0.75Ga0.25O4 single crystals
Authors: Hora J. , Navratil K. , Humlicek J. , Berkowski M.
Published: July 2007

The optical response of uniaxial SrLaAIO4 (SLA) and SrLaAl0.75Ga0.25O4 (SLAG) single crystals is investigated by several techniques ‐ ellipsometry, reflectance, and transmittance. The optical constants in the UV‐VIS region have been obtained from the reflectance and transmittance spectra. A novel method of measurements of the birefringence by spectroscopic ellipsometry is reported. The resulting refractive indices for both polarizations are compared with precise standard measurements performed using a single wavelength null ellipsometer. The reflectivity of SrLaAIO4 crystals is obtained for both polarizations in the infrared region. The fit of the spectra in terms of Lorentzian oscillators provides a quantitative comparison of the phonon mode parameters with that of LaAIO3, and reveals a strong anisotropy of the lattice vibrations polarized along the optical axis and perpendicular to it.
Optical absorption and photoconductivity at the band edge of β‐Ga2−xInxO3
Authors: Vasyltsiv V. I. , Rym Ya. I. , Zakharko Ya. M.
Published: July 2007

Single crystals of β‐Ga2−xInxO3 with x = 0 to 0.2 have been grown by the float zone technique using radiation heating. When x increases from 0 to 0.2, the interionic distance O‐Ga in the gallium octahedron and tetrahedron increase on the average by 1 to 1.5% and, as a consequence, the position of the steep absorption edge of band‐to‐band electron transitions shifts towards smaller energies by 0.3 eV. Simultaneously, a strong and broad absorption band in the energy range 3.5 to 4.5 eV appears, caused by charge transfer electron transitions from 2p oxygen to 5s indium energy levels in octahedral indium clusters.
Phase transition of ferroelastic BiVO4 crystals under stress at various temperatures
Authors: Ran Lim Ae. , Chang Jin‐Hae , Choh Sung Ho
Published: July 2007

The stress‐strain hysteresis curves of BiVO4 single crystals have been studied by thermal mechanical analysis in the ferroelastic and paraelastic phases. The phase transition temperatures are obtained as afunction of stress at various temperatures. The phase transition temperature is found to decrease as the stress is increased. The experimental resultsindicate that the phase transition temperature of the ferroelastic BiVO4 shifts to the lower temperature side under the influence of externalstress.
Kinetics of segregation and crystallization with stress development and stress relaxation
Authors: Möller J. , Schmelzer J. , Avramov I.
Published: July 2007

The kinetics of cluster growth in segregation and crystallization processes in highly viscous materials taking into account stress development andstress relaxation is investigated theoretically. Different models of stressdevelopment are considered for the case that stresses arise due to the formation and growth of the newly evolving phase. The theory is formulated in terms of differential equations describing the evolution of the cluster size, the change in time of the thermodynamic driving force of cluster growth and other related quantities. Based on the numerical solution of these equations several effects are analyzed which may occur as the result of stress development and stress relaxation.
Beam instability and space‐charge wave amplification caused by the electron injection out of QW into 2deg
Authors: Tralle I. E. , Sizjuk V. A.
Published: July 2007

The amplification of a space‐charge wave in a two‐dimensional electron gas induced by a beam instability is considered. The electronbeam is regarded as being injected out of the quantum well by means of resonant tunneling. The beam‐2DEG interaction is treated in terms of thehydrodynamic equations of a two‐component liquid and the Poisson equation. The numerical solution of the dispersion equation is obtained, the increment of SCW is calculated and the dependence of the increment on the frequency tending to the 2D‐plasmon frequency is considered.
An accurate expression for the generalized fermi‐dirac integral derived from a kane nonparabolic relation and its reversion. An applicationto n‐type degenerate gaas crystals
Authors: Van Cong H. , Khanh B. D.
Published: July 2007

By means of a piecewise cubic Hermitian polynomial approximation and a theorem for the asymptotics used in our previous papers, an accurate expression for the generalized Fermi‐Dirac integral ℱ (z), derived from a Kane nonparabolic dispersion relation, for any values of reduced Fermienergy z and for the dimensionless conduction‐band nonparabolicity parameter β in the interval [0, 0.1493], is investigated witha precision of the order of 2 × 10−7. Then, two short seriesrepresentations for ℱ (z) are also presented and discussed. These accurate values of ℱ (z) (or reduced carrier density u(z) are used to evaluate the error of our two approximate expressions for z(u), appropriate to the lightly degenerate and degenerate cases, z≤4 and z≥4, respectively, and accurate within 0.5%. Finally, taking account of both the electron‐electron interaction effect and the spin‐orbit splitting interaction effect, the reduced Fermi energy z(u) and the electron effective mass in n‐type heavily doped GaAs crystals at 4.2 and 300 K are determined. Their numerical results are also compared with existing experimental results.
Solid state electronic structure of potassium graphite intercalation compounds; the systems KC24 and KC8
Authors: Böhm M. C. , Schulte J. , Schlögl R.
Published: July 2007

The solid state electronic structures of the potassium graphite intercalation compounds (GIC) KC24 and KC8 are investigated by acrystal orbital (CO) approach of the intermediate neglect of differential overlap (INDO) type. KC24 crystallizes in a hexagonal and KC8 in an orthorhombic lattice. The CO results derived for the potassium GIC are compared with CO results of pristine graphite and potassium‐doped C60 fullerides. An incomplete K‐to‐graphite charge transfer (CT) of about 50% is predicted for both GIC studied. This incomplete CT in GIC is correlated with the incomplete CT in potassium‐doped fullerides predicted in a recent study. The present results can be considered as an a posteriori corroboration of our findings in the case of alkali‐doped fullerides. The acceptor capability of graphite is smaller than the one of the C60 soccerball with its pentagon defects. Modifications in the electronic density of states (DOS) distribution in the potassium GIC relative to pristine graphite are analyzed. The influence of theexcess electrons on the graphite electronic structure is described by a parameter which measures the ‘number of covalent bonds’ per CC contact.
Magnetic measurements and the nature of crystal field in Nd2(so4)3 8h2O
Authors: Gupta R. , Ghosh U. S. , Basu C.
Published: July 2007

Magnetic measurements on octahydrated neodymium sulphate have been carried out in the temperature range 80 to 300 K and analysis of the results has been performed using a crystal field of D4th symmetry. The most rigorous approach of direct diagonalization of the Hamiltonian matrix constructed in the complete basis of states belonging to all atomic terms of the Nd3+ ion has been employed to evaluate the set of energy level parameters including crystal field parameters. The Hamiltonian consists of terms representing free ion (Coulomb and spin‐orbit), crystal field, and two‐body configuration interactions. It has also been demonstrated that for a correct evaluation of crystal field parameters it is absolutely necessary to work with a complete basis in the rigorous framework rather than with a partial basis.
Cluster modelling of the oxygen vacancy in the SiO2−GeO2system
Authors: Sulimov V. B. , Sokolov V. O. , Poumellec B.
Published: July 2007

The results of computer modelling of the neutral oxygen vacancy in Ge‐doped silica and Si‐doped germania with the semi‐empirical method MNDO in the molecular cluster approximation are presented. A choice of a model for the oxygen‐deficient center in these glasses is made. The results support the neutral oxygen vacancy as the model for the oxygen‐deficient center.
Mfrg‐dpir study on the arbitrary‐spin ising model with random transverse field
Authors: Song Wei‐Ji
Published: July 2007

A new and effective method combined by the mean‐field renormalization group (MFRG) and the discretized path‐integral representation(DPIR) is used in the study on arbitrary‐spin Ising model in a rando transverse field (RTIM). The phase diagram and other critical properties are investigated.
Magnetic susceptibility of palladium subjected to severe plastic deformation
Authors: Rempel A. A. , Gusev A. I.
Published: July 2007

Over a temperature interval between 300 and 1225 K an annealing experiment is performed aiming to investigate the electronic structure of submicrocrystalline palladium by means of magnetic susceptibility. To produce submicrocrystalline structure Pd specimens severe plastic shear deformation is used. An analysis is made of the essential excess of susceptibility of deformed palladium over that of the initial polycrystalline palladium, an excessthat persists after deformed palladium had been annealed at T≤ 825 K. The excess of susceptibility is explained by the change in the density of states on the Fermi level due to atomic defects introduced by severe plastic deformation. Results of microstructure and microhardness investigations are also presented.
Depinning lines of Fe‐ and Pb‐Doped High‐Tc superconductors HgBa2Ca2Cu3O8+y
Authors: Tian W. , Shao H. M. , Zhu J. S. , Wang Y. N.
Published: July 2007

Mechanical dissipation of Fe‐ and Pb‐doped superconducting Hg‐based cuprates HgBa2Ca2Cu3O8+y as a function of temperature was measured with the vibrating reed technique. The depinning lines (DL) determined from the dissipation peak temperature of doped materials were found to be higher than that of undoped HgBa2Ca2Cu3O8+y samples. The possible reason was discussed in detail.
Electronic transmission in a double‐chain system with random interchain couplings
Authors: Chen Yan , Xiong Shi‐Jie
Published: July 2007

We study, via an exact self‐consistent recursion method, the effect of random interchain hoppings on the electronic transmission through two chains with common collecting electrodes at their ends. Each of the chains may be pure or doped with random impurity dimers. The interchain couplings are random in both strength and position. The results show that although the increase of the strength of the random interchain couplings causes the decrease of the transmission, the increase of the coupling density of the system may first decrease and then increase the transmission. A new type of coherent disorder in interchain couplings is proposed which can enhance thetransmission coefficient remarkably. Our results may shed light on the understanding of the transport properties in the fibril‐form polymers.
Propagation of femtosecond pulses in thin ZnSe layers
Authors: Neukirch U. , Wundke K. , Gutowski J. , Hommel D.
Published: July 2007

The transmission of optical femtosecond pulses through ZnSe/ZnSxSe1−x heterostructures is investigated in both the spectral and the temporal domain. Quantization of the polariton wave vector withinthe thin ZnSe layers (23 to 100 nm) leads to a variety of allowed propagating modes which are clearly resolved in transmission spectra. Interference of all contributing modes gives rise to a very pronounced and complex temporal beating behavior of the transmitted pulses measured with femtosecond resolution. The transmission spectra as well as the transients taken from different samples are consistently described by a model based on three‐oscillator polariton theory using Pekar's additional boundary conditions.
On a 1/n law in texture related single orientation analysis
Authors: Matthies S. , Wagner F.
Published: July 2007

Two problems of practical importance, connected with single orientations have remained still open or have been resolved only in an unsatisfying manner in texture analysis up to now.
Pressure dependence of Grüneisen parameter in solids
Authors: Fang Zheng‐Hua
Published: July 2007

Making the assumption that the value of the second Grüneisen parameter q is constant, we present a relation for the pressure dependence of the Grüneisen parameter y. The tests on four solids (NaCl, Li, Na, and K) show that the agreement between the calculated results given by the relation and the experimental data on the pressure dependence of y is excellent. A comparison between the linear relation suggested by Kumari and Dass and the relation presented in this paper demonstrates that the latter is better.
The effect of electron‐electron interaction on the electric field dependent variable range hopping conduction in quasi‐low‐dimensional systems
Authors: Singh M. , Thompson R. B.
Published: July 2007

We have calculated the variable range hopping conductivity of quasi‐one‐, quasi‐two‐, and anisotropic quasi‐two‐dimensional systems in the presence of an electric field. We considered the effect of electron‐electron interaction through the density of states in the variable range hopping mechanism. In the presence of the electron‐electron interaction, the density of states is proportional to the energy of the localized electrons. Using this density of states, the expressions of the conductivity are calculated by using the method developed by the authors. At zero electric field, our results of the temperature dependence of conductivity reduce to that given in the literature. We found that for β » 1, the logarithim of conductivity is proportional to β−1/2 for quasi‐one and quasi‐two‐dimensional systems and to β−2/3 for anosotropic quasi‐two‐dimensional systems. Here β is proportional to the electric field and inversely proportional to the temperature. Recent electric field dependent conductivity experiments on low‐dimensional B14T11S10 compounds predict a β−2/3 power law at high electric fields. Hence our theory of anisotropic quasi‐two‐dimensional systems is consistent with these experiments. We have also compared our theory with an entire range of electric field dependent conductivity data of Bi14Te11S10 compounds. A good agreement between theory and experiment has also been found on a qualitative basis.