bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical anisotropy of SrLaAIO4 and SrLaAl0.75Ga0.25O4 single crystals
Authors: Hora J. , Navratil K. , Humlicek J. , Berkowski M. 
Published: July 2007

AbstractThe 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

AbstractSingle 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractBy 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

AbstractThe 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

AbstractMagnetic 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

AbstractThe 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

AbstractA 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

AbstractOver 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

AbstractMechanical 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

AbstractWe 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

AbstractThe 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.
Pressure dependence of Grüneisen parameter in solids
Authors: Fang Zheng‐Hua 
Published: July 2007

AbstractMaking 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

AbstractWe 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.
Calculation of electron effective masses in AlAs
Authors: Guo Q. , Ong C. K. , Poon H. C. , Feng Y. P. 
Published: July 2007

AbstractThe ab initio pseudopotential method based on the local density approximation is employed to calculate the band structure and the electron effective masses in AlAs. The expected camel's back structure at the X point of the conduction band is not found in the present calculation, in agreement with previous ab initio local density approximation calculation. The transverse and longitudinal electron effective masses are found to be mt = 0.268m0 and m1 = 1.14m0, respectively. The calculated effective masses are in reasonable agreement with the majority of the previously determined values.
The origin of the “high temperature” oscillations of kinetic coefficients of bismuth in the magnetic field
Authors: Bogod Yu. A. , Libinson A. 
Published: July 2007

AbstractThe electrical conductivity of a Bi monocrystal in high magnetic field (ultraquantum limit) was measured. The special quantum oscillations of the kinetic coefficients in the magnetic field (high temperature oscillations ‐ HTO) discovered in 1973 have been considered. The analysis of the experimental data verified one of two alternative models of HTO. The chosen model relates the HTO with the interband electron‐hole transitions near band limits.
Scintillation in cerium‐activated gadolinium‐based crystals
Authors: Kondratiev D. M. , Korzhik M. V. , Fyodorov A. A. , Pavlenko V. B. 
Published: July 2007

AbstractScintillation in cerium‐doped gadolinium‐based crystals results from the energy transfer of excited host matrix Gd3+ ions (6I7/2,11/2; 6P7/2,5/2) to the Ce3+ ions {5d1(2T2)} which radiatively relax to the ground state.
Correlation between ionicity and nonlinear refractive index in alkali halides
Authors: Górski P. , Kin M. , Kucharczyk W. 
Published: July 2007

AbstractA relationship between the generalized Miller δ1111(ω; ω, ω, ω) coefficient and the Phillips‐van Vechten ionicity in alkali halides is presented. The correlation is employed to estimate the nonlinear refractive index of these crystals.
Semiconductors under Uniaxial Strain
Authors: Cardona M. 
Published: July 2007

AbstractUniaxial strain, having a larger number of irreducible components, yields in principle more information than its hydrostatic counterpart. It has, however, the drawback that its magnitude is limited by sample fracture. We discuss the methods of application of uniaxial stress to solid samples for the purpose of performing spectroscopic measurements. We then present some of the highlights of such measurements, concerning phonons and electronic states in semiconductors.
Electronic Structures of Semiconductors under Pressure
Authors: Christensen N. E. , Wenneker I. , Svane A. , Fanciulli M. 
Published: July 2007

AbstractThe dielectric constants of semiconductors under pressure are derived from ab initio calculations of the complex dielectric functions ε(ω). The pressure coefficient of the refractive index is negative for most tetrahedrally bonded compounds and trends are explained within a simple model. Gap deformation potentials as well as “absolute” band edge deformation potentials are derived, and the pressure dependence of the electronic structure of superlattices is discussed. Examples from the group III‐V, in particular nitrides, and II‐VI (ZnS, ZnS/ZnSe) compounds illustrate these investigations. Also, less conventional semiconductors, β‐FeSi2 and ε‐FeSi, are examined. Strong electron correlation effects limit the validity of LDA theory in these cases. It is suggested that experimental pressure studies can shed new light on the properties of Kondo insulators.
Pressure Dependence of the Electronic Band Gap in 6HSic
Authors: Engelbrecht F. , Zeman J. , Wellenhofer G. , Peppermüller C. , Helbig R. , Martinez G. , Rössler U. 
Published: July 2007

AbstractPhotoluminescence experiments on 6H‐SiC doped with nitrogen have been performed at low temperature (T = (29 ± 2) K) under hydrostatic pressure up to 5 GPa. The pressure coefficients of the S0, R0, P0, S02, and R02 emission lines related to the neutral nitrogen donor bound excitons were determined. The pressure coefficient of the indirect gap of 6H‐Sic deduced from the P0 line turns out to be +2.0 meV/GPa. Nonrelativistic band structure calculations within the density‐functional theory based on the local density approximation are used to calculate the pressure coefficients of the indirect band gaps of the 6H, 4H, and 3C SiC polytypes. The comparison with available experimental data shows good agreement with the theoretical results.
Lattice Dynamics of α‐Boron from ab‐initio Calculation and Raman Scattering under High Pressure
Authors: Vast Nathalie , Baroni S. , Zerah G. , Besson J. M. , Polian A. , Grimsditch M. , Chervin J. C. 
Published: July 2007

AbstractThe Raman spectrum of icosahedral α‐boron presents a very sharp peak at 525cm−1 that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under pressure are compared with ab‐initio lattice dynamics calculations. The very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the 525 cm−1 line which is a highly harmonic librational mode of the icosahedron. This mode is also identified in the Raman spectrum of other icosahedral boron‐rich solids.
Vibrational Properties of InSe under Pressure: Experiment and Theory
Authors: Ulrich C. , Mroginski M. A. , Goñi A. R. , Cantarero A. , Schwarz U. , Muñoz V. , Syassen K. 
Published: July 2007

AbstractThe pressure dependence of the phonon modes in the layered semiconductor γ‐InSe has been investigated experimentally and theoretically for pressures up to 11 GPa. The mode Grüneisen parameters of all Raman‐active zone‐center phonons have been determined by Raman scattering under pressure. In addition, features corresponding to second and third‐order scattering processes are apparent in the Raman spectra under resonance conditions, from which information about zone‐edge modes can be obtained. For the assignment of the observed Raman features to vibrational modes we have calculated the phonon dispersion curves using a rigid‐ion model including couplings to first‐nearest neighbors and long‐range Coulomb interaction. At about 7 GPa the sample turns from transparent to opaque and a new Raman mode appears in the spectra at around 165 cm−1. This is evidence of a pressure‐induced structural instability of γ‐InSe, which is optically detected but not by X‐ray diffraction.
Pressure Dependence of Band Gaps and Deep Centers in Chalcopyrite Semiconductors
Authors: Choi In‐Hwan , Yu P. Y. 
Published: July 2007

AbstractWe have determined the pressure coefficients of fundamental energy gap and deep centers in two chalcopyrite semiconductors AgGaS2and CuGaS2from their absorption and photoluminescence spectra. The pressure coefficient of a deep center in AgGaS2is found to belargerthan that of the band gap. A deep center with similar behavior has been reported already in AgGaSe2. We discuss the effect of p‐d hybridization on these coefficients.
Galvanomagnetic Measurements of the 2D Electron‐Hole Gas in GaSb/InAs/GaSb Quantum Wells under Pressure up to 2.5 GPa
Authors: Dizhur E. M. , Itskevich E. S. , Kashirskaya L. M. , Voronovsky A. N. , Malik T. A. , Stradling R. A. , Yuen W. T. 
Published: July 2007

AbstractGalvanomagnetic measurements at 4.2 K in fields up to 7 T under hydrostatic pressures up to 2.5 GPa are presented for a series of GaSb/InAs/GaSb quantum wells with different cap‐to‐well ratios and interface type. The pressure derivatives of electron and hole concentrations have been obtained from Shubnikov‐de Haas (ShdH) oscillations and from fitting the low field data to the classical two‐carrier model. Above the pressureinduced metal‐insulator transition, a strong negative magnetoresistance occurs which is attributed to localisation associated with the presence of GaSb surface donors.
Spin Splitting in a 2D Hole System under Uniaxial Compression
Authors: Hansen O. P. , Kraak W. , Minina N. , Olsen J. S. , Saffian B. , Savin A. M. 
Published: July 2007

AbstractMagnetotransport experiments under in‐plane [110] uniaxial compression up to 2.4 kbar of a (001) p‐GaAs/Al0.5Ga0.5As single heterostructure have been performed. A pronounced redistribution of carriers in the spin‐split lowest heavy hole subband in favour of the less populated and lighter mass one is established but the total carrier concentration reveals only a 5% decrease. The difference between the effective masses in spin subbands becomes less. The spin subband splitting decreases by a factor of 2.5 under maximum compression.
Hydrostatic Pressure Dependence of the Threshold Current in 1.5 μm Strained Quantum Well Lasers
Authors: Adams A. R. , Silver M. , O'Reilly E. P. , Gonul B. , Phillips A. F. , Sweeney S. J. , Thijs P. J. A. 
Published: July 2007

AbstractResults are presented which show that the radiative current in 1.5 μm quantum well lasers obeys simple theory, but that the total current through the device at room temperature is dominated by Auger recombination. The measured temperature and pressure dependencies of the threshold current show that it is phonon‐assisted Auger recombination that is operative. This is less sensitive than band‐to‐band Auger processes to the exact form of the band structure and explains why inbuilt strain has little effect on the temperature sensitivity of the threshold current and why there is little change in threshold current as one goes from 1.5 to 1.3 μm devices.