bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetooptical Properties of Surface Plasma Waves on Copper, Silver, Gold, and Aluminum
Authors: Haefner P. , Luck E. , Mohler E. 
Published: July 2007

AbstractSurface plasma waves on Cu, Ag, Au, and Al are excited with HeNe laser light (λ = 632.8 nm) by the prism method in the presence of a magnetic field perpendicular to the plane of incidence. The dip at the surface plasma resonance which is observed in the reflectivity when tuning the angle of incidence shows nonreciprocal behaviour with respect to magnetic field or light propagation reversal. Using an ac magnetic field and lock‐in technique the nonreciprocal changes of the reflectivity can be measured with fields of only 0.2 T. The components of the magnetooptical tensor are determined and discussed in the framework of a Drude model.
Photostimulated Phase Transitions in High‐Tc Superconductors and the Origin of the Superconductivity
Authors: Kityk I. V. , Kolinko M. I. 
Published: July 2007

AbstractThe photostimulated phase transition from the semiconducting to the superconducting phase in YBa2Cu3O7−δ (δ > 0.5) single crystals is revealed. The investigations are performed using the time‐resolved dependences of photoresistivity, photovoltage, and optical second harmonic generation. At the same time the time‐resolved Raman and UV‐visible reflection spectra are monitored. A time delay between the second harmonic generation and photocurrent dependences is found. The intrinsic role of two different mechanisms in the occurrence of the photoinduced phase transition is unambiguously shown. To clarify the main features of this transition it is necessary to account for the specific electron‐phonon interaction together with the existence of the Anderson metal‐insulator transition. The performed band energy calculations show the essential role of photoinduced changes in the Fermi energy level location and the total density of states distribution. The intrinsic role of van Hove singularities in the formation of the superconducting gap is shown.
Pump and Probe Techniques for Spin–Lattice Thermalization Measurements. Thermal Requirements
Authors: Callegaro L. , Puppin E. 
Published: July 2007

AbstractPump and probe techniques with ultrafast laser pulses represent a well established tool for the measurements of electron‐electron and electron‐phonon thermalization processes. It is shown that expanding these techniques to magnetic systems for spin‐lattice relaxation measurements requires a careful selection of the thermal characteristics of the samples. More precisely, a simulation of the thermal behavior of thin metal films heated by short laser pulses shows that film thickness and substrate thermal properties are crucial parameters.
A Lattice Dynamics Approach to the Acoustic Waves Emitted during Dislocation Annihilation
Authors: Trochidis A. , Polyzos B. 
Published: July 2007

AbstractThe emission of acoustic waves by annihilating dislocation kink‐antikink pairs during the Frank‐Read source operation is investigated. The proposed approach is based on a Frenkel‐Kontorova model where the rigidity of the substrate is removed to establish the interaction mechanism between a dislocation and transverse lattice vibrations of the substrate. It is shown that this interaction can be described by a D'Alembert wave equation coupled with a sine‐Gordon equation. Within the framework of this model, the emission of sound by annihilating dislocation kinks during the Frank‐Read source operation is considered in detail. The obtained results could be of use in interpreting acoustic emission experimental results during plastic deformation of crystals.
Combined Measurements of Length and Modulus Change and Calculation of Effective Pair Potentials for the Amorphous Alloy Cu64Ti36
Authors: Porscha B. , Neuhäuser H. 
Published: July 2007

AbstractSimultaneous measurements of the length change and of the change of the eigenfrequency at the same specimen are performed for the metallic glass Cu64Ti36. The length change is preferably connected with a long‐range topological relaxation, while the change of the eigenfrequency preferably gives information on the change of the chemical short‐range order. The correlation between the relaxation effects of the two quantities shows a certain change of slope at a characteristic annealing temperature indicating a transition from preferably chemical to topological rearrangements. Assuming a Lennard‐Jones type potential the powers of the effective pair potential can be calculated from the temperature dependence of the measured quantities. It is also possible to get information on the variation of the powers due to the structural relaxation effect.
Charge‐Density Transfer of H Adsorption on the Si(111) (1 × 1) Surface
Authors: Wu B. R. , Cheng C. 
Published: July 2007

AbstractThe charge‐density transfers from ab initio quantum mechanical calculations for the adsorption of hydrogen atoms on the Si(111) (1 × 1) surface are presented. In the initial stages of the adsorption, the interaction occurs between the hydrogen atom and the surface dangling bond or between the hydrogen atom and the hydrogen overlayer. The interaction has the same characteristics as the initial stages of the hydrogen molecule formation. For the adsorbed systems, the charge‐density transfers exhibit strong ionicity with the adsorbed hydrogen atoms being the more electronegative ones. The hydrogen atoms are negatively charged beneath the Si(111) (1 × 1) surface. This property is retained for the hydrogen atoms in the silicon bulk, no matter whether the system is neutral or charged.
Reference System Approach in the Electron Liquid Theory. IV. Momentum Distribution
Authors: Vavrukh M. , Vavrukh N. 
Published: July 2007

AbstractThe problem of universality and limitation of the local‐field conception in modern theory of many‐particle interacting systems is considered on the example of the calculation of momentum distribution. On the basis of the variational derivative δG(q, v)/δϵk (where G(q, v) is the dynamic local‐field correction function, Ek = h2k2/2m) the momentum distribution in the model of the electron liquid in a wide region of the coupling parameter rs is investigated.
A Unified Total Energy Approach to High‐Temperature Superconductivity and Long‐Range Ordering in Solids
Authors: Mizia J. , Romanowski S. J. 
Published: July 2007

AbstractA novel approach to the long‐range ordering in solids is proposed in which the total energy is minimized with respect to the long‐range ordering parameter. It is shown that the method, on the one hand, reproduces the classic BCS results for superconductivity, whilst also agreeing with the well established results for the itinerant antiferromagnetism. Implications of the method for electronically driven super‐ conductivity are discussed. It is found that the electron transfer around the Fermi energy promotes super‐ conductivity. This transfer is easier in the soft multi‐orbital systems than in the single‐band system.
Deducing Spin‐Hole Interactions from the Three‐Band Anderson Lattice Model for CuO2‐Layered Superconductors
Authors: Kolley E. , Kolley W. , Tietz R. 
Published: July 2007

AbstractAn effective Hamiltonian for localized Cu spins and mobile O holes is deduced from the three‐band Anderson lattice model ascribed to a single CuO2 layer in high‐Tc superconductors. At large but finite Hubbard repulsion strength U on Cu sites the perturbative approach is given completely up to the fourth order in the Cu(d)‐O(p) hybridization amplitude t. Both the Schrieffer‐Wolff transformation and the projection onto the subspace of singly occupied Cu sites are performed by means of a computational algorithm. The resultant interactions in O(t4) include a Kondo‐like spin‐hole exchange in competition to that in O(t2), the spin‐spin superexchange of the Heisenberg‐type, spin‐spin‐hole, hole‐hole, and spin‐hole‐hole correlations. The spin‐hole‐hole coupling, reflecting p‐hole hopping affected by a Kondo exchange, is due to the direct O‐O transfer tp inherent in the periodic Anderson model. It is shown how in the limit tp = 0 the effective Hamiltonian reduces to that originated from the minimal version of the Emery model.
Magnetic Properties of Thin Films Containing Itinerant Electrons Interacting with Localized Spins
Authors: Urbaniak‐Kucharczyk A. 
Published: July 2007

AbstractThe model of ferromagnetism which can be considered as a generalization of the Heisenberg, Stoner, and modified Zener models is applied to the case of thin magnetic films. Calculations are presented within the frame of a Green function formalism of the magnetization distribution across the layer and the thickness dependence of the Curie temperature. The case of h. c. p. thin films with strong surface anisotropy in the high‐temperature region has been discussed in detail.
Scattering of Light on Small Angles in the Conditions of “Devil's Staircase” Existence within the Incommensurate Phase
Authors: Sveleba S. A. , Kapustianik V. B. , Polovinko I. I. , Krochuk A. , Bublyk M. 
Published: July 2007

AbstractInvestigations of small‐angle light scattering in dependence on the temperature change rate, on the distance from the axis of the incident light beam, on the defectivity of a sample, as well as on the polarization of incident light are carried out in [N(CH3)4]2FeCl4 crystals. It is obtained that the small‐angle light scattering in long‐periodic commensurate regions within an incommensurate phase is connected with the scattering on phasons. The appearance of additional scattering of light in the transition regions is caused by the appearance of the perturbation vector qc. Besides, the intensity I(q) in this region depends on the polarization of the incident light testifying the change of the incommensurability wave vector in this region.
Space‐Charge Polarisation and X‐Ray Diffraction Extinction Reduction in Synthetic and Natural Quartz Crystals
Authors: Sebastian M. T. , Klapper H. 
Published: July 2007

AbstractAn X‐ray diffraction study of α‐quartz crystals with in situ electric field along the different crystallographic axes shows a strong enhancement in the diffracted intensity. The effect is prominent in natural crystals when the electric field is along the c‐axis. This is evidenced by the increase in the intensity of X‐ray rocking curves recorded from several crystals. The X‐ray topographs recorded with in situ electric field show extinction contrast consisting of striations parallel to the c‐axis. This contrast disappears during irradiation in the absence of an electric field. The increase in the diffracted intensity and the contrast on the topographs are explained by a model on negative space‐charge collection.
Point Defect Observations by Molecular Dynamics and Calculated TEM Imaging
Authors: Mattei G. , Mazzone A. M. 
Published: July 2007

AbstractA simulation method which combines molecular dynamics and calculated TEM imaging is presented. The simulation is applied to irradiated crystalline silicon and silver to clarify the minimum amount of disorder which could be experimentally observed. It is found that in Si isolated cascades formed at energies near the displacement threshold are clearly visible, while in Ag observable disordered regions are generated only by the overlapping of many cascades launched with energy well above che energy displacement threshold.
Elastic Constants and Phonon Dispersion of Amorphous Copper with Embedded‐Atom Force
Authors: Xie Qian , Huang Meichun 
Published: July 2007

AbstractElastic constants and phonon dispersion of amorphous copper are studied by assuming that the interatomic potential is transferable from the crystalline state and can be represented by the analytical model of the embedded‐atom method given by Johnson. To test its effectiveness, the force model used to calculate the phonon eigenfrequencies in the copper crystal and the result is in perfect agreement with experimental values measured at 80 K. Based on the embedded‐atom method and in connection with the radial distribution function, the formulae for calculating the elastic constants of the amorphous phase and the modified Takeno‐Goda formula for studying phonon dispersion in amorphous materials are presented. For an easy implementation, the formulae are simplified by using the Bhatia‐Singh approximation, in which only the nearest‐neighbour contribution is considered.
Thermoelectric Power and Thermal Conductivity of Neutron Irradiated Superconductors YBa2Cu4O8
Authors: Pekala M. , Morawski A. , Łada T. , Mąka E. 
Published: July 2007

AbstractThe first thermoelectric investigation of fast neutron irradiated polycrystalline superconductors YBa2Cu4O8 reveals that with a rising fluence the absolute values of thermoelectric power increase gradually, the transition temperature shifts from 77 to 72 K, whereas the thermal conductivity diminishes due to the suppression of electron and phonon mean free paths by the defects created in the CuO2 layers.
35Cl NQR Spin‐Spin Relaxation Mechanism in Mg(D2O)6SnCl6 and Ni(H2O)6SnCl6 Crystals
Authors: Horiuchi K. 
Published: July 2007

AbstractThe temperature dependence of the 35Cl NQR spin‐spin relaxation times T2Q in Mg(D2O)6SnCl6 and Ni(H2O)6SnCl6 is measured. T2Q of the former compound is determined mainly by the magnetic dipolar field at the chlorine nucleus produced by the intramolecular nuclei, the tin nucleus, and five other chlorine nuclei in the [SnCl6]2‐ ion. T2Q of the latter compound is governed mainly by the local magnetic field at the chlorine site produced by the intramolecular nuclei and unpaired electrons in the Ni2+ ions. The correlation time for electron‐spin flips due to the exchange interaction among neighbouring unpaired electrons is evaluated from T2Q. A general behaviour of halogen NQR T2Q in diamagnetic and paramagnetic insulators is summarized.
A Method of Data Processing for X‐Ray L‐Edge Absorption
Authors: Wang Wen‐Cai , Chen Yu , Hu Tian‐Dou 
Published: July 2007

AbstractA method of data processing for the rare earth (RE) L‐edge X‐ray absorption spectrum in materials is presented. Mathematical analyses show that if the L‐edge is decomposed into a Lorentzian line and an arctan curve, their second derivatives are very different and the former represents a symmetric negative peak at the same location as the original Lorentzian peak. The white line (WL) in the RE L‐edge can be separated without decomposition of the edge. This method simplifies the data processing and reduces some artificial factors in data processing. The RE L‐edges in some fluorides and glasses are analyzed by this method and the ordinary method. The changes in the WL intensities at the L‐edges of various materials, which are obtained by two data processing, are consistent to each other.
Structural Properties of II–VI Heterostructures for Blue‐Green Lasers
Authors: Petruzzello J. , Herko S. P. , Gaines J. M. , Marshall T. 
Published: July 2007

AbstractThe structural characteristics of II–VI separate confinement heterostructure lasers containing Zn1−xMgxSySe1−y quaternary cladding layers, ZnSySe1−y guiding layers, and Zn1–zCdzSe quantum well active layers are investigated with transmission electron microscopy. The dislocation density in these lasers is dramatically reduced because the entire structure is pseudomorphic to the GaAs substrate. The pseudomorphic structures, however, contain stacking faults that cause degradation and shorten the lifetimes of laser devices. The optical degradation of a pseudomorphic laser structure is investigated with photoluminescence imaging and correlated with the propagation of dislocations in the active layer from existing defects as seen by transmission electron microscopy.
ZnSe‐Based MOVPE and MBE Grown LEDs
Authors: Stanzl H. , Reisinger T. , Wolf K. , Kastner M. , Hahn B. , Gebhardt W. 
Published: July 2007

AbstractThe properties of ZnSe‐based heterostructures on (001) GaAs substrates grown by metalorganic vapor phase epitaxy (MOVPE) are compared with those grown by molecular beam epitaxy (MBE). The structural and optical properties are examined with high‐resolution X‐ray diffraction (HRXRD) and photoluminescence (PL). Furthermore the results of C–V measurements of p‐ZnSe layers doped with nitrogen are discussed and I–V characteristics and electroluminescence (EL) spectra of LEDs grown with both techniques are presented. ZnSe/Zn(SSe) MQWs are prepared with MOVPE and (ZnCd)Se/ZnSe MQWs with MBE to improve the optical efficiency of the diodes.
Blue‐Green Ternary and Quaternary Lasers. Technological Problems and Theoretical Investigations
Authors: Jakobs A. , Schüll K. , Behr Th. , Jobst B. , Hommel D. 
Published: July 2007

AbstractTo overcome the constraints imposed by lattice mismatch on the design of ternary laser structures asymmetric and pseudomorphic waveguides are proposed. The latter structures are shown to give high luminescence efficiency indicative for a low defect density and a high differential quantum efficiency. The measured far‐field patterns agree well with theoretical predictions. Several possibilities to reduce carrier leakage out of the active zone are reviewed and investigated by computer simulations. Gain measurements based on the modulation depth of the Fabry‐Perot modes are also presented.
Exciton scattering with bare electrons
Authors: Koch M. , Bastian G. , Hellmann R. , Feldmann J. , Göbel E. O. , Dawson P. 
Published: July 2007

AbstractIt is shown that exciton‐electron scattering is twice as efficient as exciton‐exciton scattering by performing transient four‐wave‐mixing experiments on a mixed type‐I–type‐II GaAs/AlAs double quantum well structure. In addition it is found that the presence of bare electrons in the wide well leads to a quenching of the high‐energetic part of the photoluminescence spectrum. Scattering with electrons releases excitons localized at shallow states and leads to a more complete relaxation of excitons into the energetically lowest quantum well states.
The accurate numerical evaluation of half‐order Fermi‐Dirac Integrals
Authors: Mohankumar N. , Natarajan A. 
Published: July 2007

AbstractAn efficient algorithm for the direct evaluation of Fermi‐Dirac integrals of half orders is presented. The computational scheme consists of a modified trapezoidal rule which exactly corrects for the loss of accuracy caused by the presence of poles of the integrand. The analysis of the scheme leads to useful error bounds which indicate the quadrature scheme to be rapidly convergent. As much as 16 steps of trapezoidal summation along with at most 9 steps of pole correction can provide an accuracy of one part in 1014 for the usual range of values of the argument.
Positron annihilation in narrow‐gap semiconductors
Authors: Bouarissa N. , West R. N. , Aourag H. 
Published: July 2007

AbstractThe valence electron and positron charge densities in InAs and InSb are obtained from wave functions derived in a model pseudopotential band‐structure calculation. It is found that the positron density is maximum in the open interstices and is excluded not only, as usual, from the ion cores but also to a considerable degree from the valence bonds. Electron‐positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in narrow‐gap semiconductors.
On the temperature dependence of the optical absorption edge in nickel oxide
Authors: Pattanayak J. , Becker K. D. 
Published: July 2007

AbstractResults are reported of a high‐temperature investigation (up to 1000 °C) of the optical absorption edge in NiO. An approximately exponential dependence of the absorption coeficient on photon energy is observed. The absorption coeficient is found to be independent of oxygen partial pressure, i.e., of native lattice defects. The temperature dependence of the edge gives evidence of strong exciton‐phonon coupling in NiO.
Model Effective‐Mass Hamiltonians for Abrupt Degraded Heterojunctions
Authors: Brezini A. , Sebbani M. , Marouf S. 
Published: July 2007

AbstractA class of hermitian effective‐mass Hamiltonians is considered whose kinetic energy term is 1/4 (mαpmβpmγ + mγpmβpmα) with α + β + γ = −1. These Hamiltonians are applied to abrupt degraded interfaces between crystals and the matching conditions across the interface are searched for the effective‐mass wave function Ψ and its spatial derivative Ψ′. For α ≠ γ it is found that the ground‐state energy is negative, i.e. without physical bearing for the considered potential. Consequently, the only viable cases are for α = γ. In a model calculation, β ≈ −1 is estimated, to be the more relevant candidate although the case β = 0 cannot be excluded for typical interfaces.
The Preparation of Amorphous SixGeyOz Alloy Films by Sputtering. Preparation Conditions, Chemical Composition, and Vibrational Properties
Authors: Zacharias M. , Stolze F. , Drüsedau T. , Bock W. 
Published: July 2007

AbstractAmorphous SixGeyOz, alloy films with x = 0.17 to 0.60, y = 0.19 to 0.68, and z = 0.11 to 0.42 are deposited using reactive dc magnetron sputtering in an Ar/water atmosphere. The alloy concentrations are measured by secondary neutral mass spectrometry (SNMS). The oxygen content strongly decreases with decreasing silicon concentration. The dependence of the alloy composition on the target coverage with germanium pieces is investigated. IR transmission measurements in the range from 400 to 4000 cm−1 show a predominance of the SiOSi asymmetric stretching mode. The minor absorption band due to GeOGe stretching vibrations totally disappears after annealing the film at 800 °C. An increase of the SiOSi absorption mode is detected.
The Iron States in Cd1−xFexTe in the Fundamental Gap Energy Range
Authors: Hołda A. , Rodzik A. , Mielnikow A. A. , Żukowski P. W. 
Published: July 2007

AbstractThe reflectivity, absorption, photoconductivity, and cathodoluminescence spectra for Cd1−xFexTe (0 ≦ x ≦ 0.05) at 300 and 77 K are investigated. The measurements are carried out in the fundamental gap energy range. From the analysis of the experimental results, the energy positions and properties of Fe states in the CdTe matrix in the fundamental gap region are presented and discussed. It is concluded that the increase of the width of the fundamental gap with the change of temperature from 300 to 77 K leads mainly to the rise of the energy distance between the donor‐like iron state 5E and the bottom of the conduction band, whereas the energy distance between the iron state 5E and the top of the valence band remains unchanged. The increase of the fundamental gap value with the decrease of temperature changes the energy positions of the excitation iron state 3T1 with respect to the bottom of the conduction band. Energy positions of iron states in the fundamental gap energy region as well as the increase of the energy width and density of these states with the rise of the iron concentration in Cd1−xFexTe are reflected in the results obtained with the use of different experimental methods.
Preparation and Characterization of Surface‐Modified Luminescent Porous Silicon
Authors: Dubin V. M. , Vieillard C. , Ozanam F. , Chazalviel J.‐N. 
Published: July 2007

AbstractPorous silicon layers are chemically grafted with trimethylsiloxy groups by exposure to hexamethyldisilazane (HMDS) vapors. After treatment, infrared spectroscopy reveals that part of the initially hydrogenated surface is actually modified. Luminescence of the porous layers appears to be little affected by the treatment. On strongly luminescent samples, the photocarriers exhibit the same infrared absorption as in hydrogenated or oxidized porous silicon, evidencing the similar involvement of localized states in the luminescence process. The treated porous silicon layers also exhibit an increased stability against aging in ambient, as compared to hydrogenated porous silicon. Furthermore, the modification can be removed by simple exposure to HF vapor, thereby restoring the initial hydrogenated surface.