bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nanocrystalline versus Nanoamorphous Materials
Authors: Gonser U. , Fujita F. E. 
Published: July 2007

AbstractWith the advent of any new material it is highly desirable to determine its structure and, in addition, new definations must be formulated. Regarding nanocrystalline and nanoamorphous materials, the typical structure are in both cases on a nano‐dimensional scale.
How to Distinguish Dislocation Drag Processes from Thermal Activation
Authors: Galligan J. M. 
Published: July 2007

AbstractExperiments are described which demonstrate the influence of viscosity on thermal activation. In addition experiments which delineate the magnetooscillations in the drag coefficient are detailed. The first set of experiments shows that the creep rate, after plastic deformation, exhibits a minimum and a maximum. This is explained by considering the temperature dependence of the electron drag in the superconducting state. The magnetooscillations a allow a distinction to be made between an anharmonic drag mode and a flutter mode; the experiements show that a flutter mode is preferred for temperatures below 5 K.
Driving Force and Mobility for Microstructural Evolutions. The Rate of Grain Rotation across a Grain Boundary
Authors: Martin G. 
Published: July 2007

AbstractThe orientation dependence of the grain‐boundary energy is a driving force for grain rotation across the boundary in a bicrystal. This has been observed experimentally. Since the energy gained during such a rotation must be entirely transformed into heat, and since this process occurs because of the friction of the grain‐boundary dislocations on the lattice, a simple expression of the rate of energy dissipation as a function of the rate of rotation can be established. It is then demonstrated that the rate of rotation of the grains is the product of a “torque” (gain of energy per unit change of misorientation) times a mobility coefficient. The latter is a function of the misorientation angle, a feature which had been overlooked in the classical theory. Close to zero misorientation, where the grain‐boundary energy exhibits a cusp, the mobility decays algebraically with the misorientation angle, while the torque diverges logarithmically only. As a consequence, the rate of rotation decreases with the misorientation, in agreement with experiment. Existing quantitative data are reasonably well fitted by this model.
EBIC Experiments at Dislocations in Germanium
Authors: Hess J. , Schreiber J. , Hildebrandt S. , Labusch R. 
Published: July 2007

AbstractA new kind of EBIC contrast is observed at 60°‐dislocations in thin slabs of germanium. Minority carriers are collected near one surface by the electrostatic potential of the dislocation and are carried along its conducting core to a contact on the opposite side. This experiment nicely demonstrates the existence of the dislocation potential as well as the conductivity of the dislocation core. A theoretical model of the effect which includes also enhanced recombination at the dislocation is developed and evaluated in a first‐order approximation. The results reflect important features of the experimental contrast profiles. Further experimental and theoretical work is expected to provide better insight into the mechanisms of capture and recombination at dislocations.
Solute‐Atom Segregation at Symmetrical Twist Boundaries Studied by Monte Carlo Simulation
Authors: Udler D. , Seidman D. N. 
Published: July 2007

AbstractDetailed Monte Carlo simulations are performed of solute‐atom segregation at (002) twist boundaries in the Au–Pt system at 850 K; the particular single‐phase bicrystal alloys studied are Pt–1 at% Au and Au–1 at% Pt. The emphasis in this paper is on studying the distribution of solute atoms at low‐angle boundaries. For the Pt–1 at% Au alloy the distribution of sites enhanced in the solute species Au is found to form a bipyramid based on the square cells of the orthogonal primary grain boundary screw dislocations. In the case of the Au–1 at% Pt alloy the solute species Pt is found to be depleted and it also forms a similar bipyramidal pattern. The Gibbsian interfacial excesses of Au and Pt are found to be positive and negative, respectively, for the Pt–1 at% Au and Au–1 at% Pt bicrystal alloys. The absolute values of these Gibbsian interfacial excesses both increase with increasing twist angle.
Magnetic Resonance of Conduction Electrons in Dislocated Germanium
Authors: Pilar von Pilchau A. , Alexander H. , Wattenbach M. , Shreter Yu. , Ionov A. 
Published: July 2007

AbstractPakulis and Jeffries first observed Zeeman transitions in germanium crystals which they attributed to the grown‐in dislocations. The authors explained the effect as a change of the electrical conductivity of the crystal due to spin dependent scattering of the carriers. The spin flip was ascribed to dislocation dangling bonds. Using plastically deformed germanium we can show that the signal is due to combined resonance of conduction electrons in one valley of the conduction band the energy of which is lowered by the strain field of dislocations. Additionally two more combined resonance lines are identified which also are assigned to the glide geometry. The delocalised states of electrons causing these lines by spin flip are not yet determined.
Urbach Law and Lifshitz Singularity in Spectra of Localized States of Disordered Systems
Authors: Klochikhin A. A. , Ogloblin S. G. 
Published: July 2007

AbstractThe density of localized states within the energy interval ranging from the mobility threshold up to the Lifshitz border is found in an instantaneous approach with exponetial accuracy for the three‐dimensional two‐component random system described by single‐band Hamiltonian taking into account diagonal disorder. It is shown that just below the mobility threshold an energy region is placed where the density of states dependence is obeying the Urbach law. Exact and approximate expressions for the Urbach parameter are obtained. Its dependence on the model parameters and the estimation of the absolute value show that the experimental data can be accounted for many disordered systems at reasonable parameters of the Hamiltonian. Arguments are presented speaking in behalf of the existence of an Urbach law in one. and two‐dimensional disordered systems.
Isomers and Pairing Mechanism in Alkali Metal‐Doped Fullerides
Authors: Kasperczyk J. , Büttner H. 
Published: July 2007

AbstractA possible pairing mechanism in the alkali metal‐doped fullerides A3C60 is studied. We have calculated the total π‐electron enecrgies of different ions C60n of the Buckminster fullerene and its isomers (with \documentclass{article}\pagestyle{empty}\begin{document}$ n = 0, \mp 1,\, \mp 2, \mp 3, \mp 4, \mp 5, \mp \, {\rm or}\, \mp 6 $\end{document}). It is shown that anions of the isomers are energetically more stable than the ideal Buckminster fullerene. We have calculated the formation energies of electron local pairs on the C60 clusters. For the phascs consisting of a single type isomer such a formation seems not to be possible. For a mixture of two isomers, however, the electron local pair is stable on anions with odd (in average) valencies. This can serve as a purely electronic pairing mechanism for the so‐called local pair model of superconductivity.
Extinction Related Approach to the Neutron Diffraction on Holmium around the 20 K Magnetic Transition
Authors: Pearce A. , Baruchel J. , Kulda J. 
Published: July 2007

AbstractThe 20K phase transition temperature range is investigated in a high quality and purity single crystal of holmium. The sample was characterized through magnetization measurements and investigated by neutron diffraction. The comparison of the neutron diffraction and magnetization results gives clues about the onset of the conical phase, and its relationship to the 1/6 and 2/11 commensurate phases. The severe extinction present in the diffraction results was treated mainly by a primary extinction variant of the RED model. The obtained extinction parameters show, in contrast with results on other magnetic materials, that the perfection of the magnetic lattice can be higher than that of the nuclear lattice. They give, in addition, indications about the way the transition occurs, with no important reduction of the magnetic coherence length.
Upconversion in Er3+‐Doped Fluorite‐Type Crystals Excited at 800 nm
Authors: Jouart J. P. , Oomen E. W. J. 
Published: July 2007

AbstractA study of the green and red emissioins from different Er3+ sites of CdF2, CaF2, SrF2, and BaF2 for excitation around 800 nm by a Ti sapphire laser is presented. For CaF2, the green emission intensity depends on the square of the excitation power, whereas for CdF2 (codoped with Na+), SrF2, and BaF2 a cubic dependence is observed. Excitation mechanisms are proposed and discussed, with, in particular, a three‐step absorption process for CdF2(Na), SrF2, and BaF2.
Internal Friction Studies on Oxygen–Oxygen Interaction in Niobium. II. Application of Long‐Range Interaction Models
Authors: Haneczok G. , Weller M. , Diehl J. 
Published: July 2007

AbstractThe oxygen Snoek relaxation in niobium is studied under the assumption of a long‐range interaction among interstitial atoms dissolved in b.c.c. metals. Two models describing this interaction in a mean field approximation (MFA), namely (i) the paraelastic and (ii) the random cooperative strain interaction (RCSI) model are discussed in detail. In the RCSI model the MFA treatment is modified by introducing the “Onsager correction”, and a new formula for the internal friction peak of the Snoek effecct is derived. Based on this modified RCSI model, a quantitative analysis of our experimental data from Nb with 0.12 to 0.85 at% O (presented in Part I) is performed by applying nonlinear regression. The model is able to account for all essential experimental results, (i) the shift of the Snoek maximum to higher temperatures with increasing O content, (ii) the broadening of the maximum increasing with O content, and (iii) a constant value of the activation enthalpy (H = 1.15 eV), independent of the oxygen content. The critical temperature characterizing the interaction of oxygen atoms increases from 110 K for 0.12 at% O to 280 K for 0.85 at% O and is, as predicted by the modified RCSI model, proportional to the square root of the oxygen concentration.
Theory of Macroscopic Local Single‐Particle Charge States in Quasi‐Zero‐Dimensional Structures Bulk Local States
Authors: Pokutnyi S. I. , Efremov N. A. 
Published: July 2007

AbstractA theory of macroscopic local one‐particle charge states in quasi‐zero‐dimensional structures is developed. Localization of charge carriers on a small sphere placed into some medium is considered. Within the framework of electrostatics a general expression is obtained for the potential energy of a charge in the field of image forces on a spherical interface of two media with arbitrary values of permittivities. It is shown that under certain conditions local states of various types appear in the field of electrostatic image forces. A new type of local state of charge carriers inside a small particle, namely bulk states, is predicted.
The Equation for an Electron in a Deformed Lattice in Effective Mass Approximation
Authors: Dubrovskii I. M. 
Published: July 2007

AbstractA critical analysis is made of the existing derivations of the effective wave equation for electrons in deformed crystals. A new method is proposed which is applicable also in the case of deformations caused by dislocations. The equation is deduced in the effective mass approximation, i.e., the wavelength of the quasiparticles is supposed to be large compared with the characteristic length of deformation variations. The significance of the region near the dislocation line (dislocation core) is analysed. Effects of terms in the Hamiltonian corresponding to an inhomogeneous effective mass and of the dislocation core on the electron energy spectrum are investigated. The method is applied to a crystal containing a screw dislocation.
Ballistic Transport in Crossed Quantum Wires. An Analytical Approach
Authors: Gaididei Yu. B. , Malysheva L. I. , Onipko A. I. 
Published: July 2007

AbstractAn analytic approach to the ballistic electron transport in crossed 2D wires is presented. It is shown that within and near the first transverse mode the two‐mode approximation gives a correct description of the system. Simple formulas for the bound‐state energies and the single‐mode scattering probabilities are derived.
The Effect of Uniaxial Stress on the Spectrum of a Neutron‐Coloured Sapphire Studied by Spectral Hole Burning
Authors: Malkin E. , Aizengendler M. , Dolindo I. , Sildos I. 
Published: July 2007

AbstractThe influence of uniaxial compressive stress on the 755.6 nm line of colour centres in neutron‐irradiated sapphire at LHeT is studied. To improve the sensitivity spectral hole burning was applied. The shift and the splitting of spectral holes under stress were accounted for by the removal of the orientational degeneracy of colour centres. The corresponding piezospectroscopic constants are found and an assumption about the structure of the centres is made.
Determination of the Quadratic Electrostrictive Coefficients of Some Doped KCl Crystals
Authors: Ramesh N. , Srinivasan K. 
Published: July 2007

AbstractThe quadratic electrostrictive coefficients of KCl containing substitutional impurities (KCl:Br−, KCl0.8Br0.2, KCl0.4Br0.6, KCl:Tl+, KCl:Cd2+, and KCl:Mn2+) are determined from the stress dependence of static dielectric constants. Small changes in the capacitance of the samples under the application of uniaxial stress are measured with the aid of a bridge circuit and a lock‐in amplifier. The values of the components of the electrostriction tensor of the doped systems are found to be enhanced by one to two orders of magnitude compared to those of the pure KCl crystal. The increment of the electrostrictive coefficients of KCl on doping is attributed to (i) the phenomenological description of a point defect as an anisotropic elastic dipole and the orientation of such defects; (ii) an additional deformation of the lattice arising from the reorientation of the I–V dipoles in the case of KCl containing divalent impurities.
Femtosecond Coherent Spectroscopy of GaAs Quantum Wells
Authors: Kim Dai‐Sik , Shah Jagdeep , Schäfer W. , Schmitt‐Rink S. 
Published: July 2007

AbstractRecent results on femtosecond time‐integrated and time‐resolved four‐wave‐mixing experiments in intrinsic and modulation‐doped GaAs quantum wells are reported. A variety of new phenomena ranging from giant excitonic resonance for intrinsic samples, Fermi liquid behavior in modulation‐doped samples, and interesting temporal evolutions provide new insights into the physical process responsible for the coherent signals.
Coherent Pulse Breakup in Femtosecond Pulse Propagation in Semiconductors
Authors: Peyghambarian N. , Harten P. A. , Knorr A. , Lee S. G. , Jin R. , Brown Colstoun F. De , Wright E. M. , Khitrova G. , Gibbs H. M. , Koch S. W. 
Published: July 2007

AbstractCoherent pulse breakup is observed for below‐exciton resonance femtosecond excitation in a room‐temperature GaAs/AlGaAs multiple quantum well waveguide using the cross‐correlation technique. This pulse breakup is present only for large area pulses with durations less than the dephasing time of the closest exciton. For pulses longer than the exciton dephasing times, but with the same and even larger areas, the pulse breakup is not observed. The breakup is due to neither self‐induced transparency nor temperoral soliton. Instead, calculations based on the coupled semiconductor Maxwell‐Bloch equations show that coherent self‐phase modulation during propragation drives the system out of the initial adiabatic following regime into oscillatory excitation density, and eventually, pulse shape modulations.
Excitonic Molecules in Nonlinear Optical Response
Authors: Keldysh L. V. 
Published: July 2007

AbstractA model which allows to treat self‐consistently many‐body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by an intense electromagnetic field. In the presence of a pump field the normal spectrum of electromagnetic waves in the vicinity of the nonlinear biexciton resonance is shown to be composed of four polariton branches, where each is the superposition of two photons and two excitons. In the self‐consistent field approximation, the structure of the interconnected, strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at sufficiently strong pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron–hole “liquid”. Appearance and temporary evolution of an exciton–biexciton system after switching on a pump is also considered.
Model Calculation for the Transient Luminescence Spectra in Optical Excited GaAs Quantum Wells
Authors: Thoai D. B. Tran , Haug H. 
Published: July 2007

AbstractThe kinetics of the carrier distribution functions in an optically excited GaAs quantum well is determined by numerical integration of the coupled Boltzmann equations for electrons and holes. Calculated transient luminescence spectra are compared with corresponding time‐resolved picosecond measurements by Daiminger et al.
Transient Strongly Anisotropic Luminescence from an Optically Excited Semiconductor
Authors: Ivanov A. L. , Haug H. 
Published: July 2007

AbstractA strongly anisotropic transient luminescence of a semiconductor which has been excited by an intensive short electromagnetic pulse is predicted and described. This phenomenon occurs on a femtosecond time scale and results directly from the unrelaxed total momentum transferred to the dense electron–hole plasma by the initial optical excitation pulse.
Luminescence Measurements of Two‐Dimensional Electrons in the Regime of the Integer and Fractional Quantum Hall Effects
Authors: Pulsford N. J. , Kukushkin I. V. , Hawrylak P. , Ploog K. , Haug R. J. , von Klitzing K. , Timofeev V. B. 
Published: July 2007

AbstractThe radiative recombination of two‐dimensional electrons is studied in GaAs–GaAlAs heterojunctions with holes localised on acceptors in strong magnetic fields. The optical measurements are emphasized as a probe of the properties of the two‐dimensional electron layer. In the case of two subbands occupied, spatial changes in the self‐consistent potential due to a population in the second subband are demonstrated. In the regime of the fractional quantum Hall effect, downward cusps occur in the mean energy of the luminescence at fractional filling factors and values for the quasiparticle energy gaps are derived.
Optical Properties of Barrier‐Modulated InGaAs/GaAs Quantum Wires
Authors: Gré;us Ch. , Orth A. , Daiminger F. , Butov L. V. , Reinecke T. L. , Forchel A. 
Published: July 2007

AbstractThe optical properties of barrier‐modulated InGaAs/GaAs quantum wires with dimensions down to 25 nm are investigated. The wires show a high quantum efficiency that is mainly due to the processing which is free of defects and also due to efficient carrier capture effects. For wire widths below about 50 nm a blue shift of the emission due to lateral quantization, is observed. In high excitation experiments a second subband is observed for narrow wire widths. The experimentally observed wire width dependence of the band edge and of the subband splitting are in agreement with model calculations based on the geometry of our structures.
Excitonic Effects in Neutral Electron–Hole Magnetoplasma in Quantum Wells at Low Temperature
Authors: Kulakovskii V. D. , Butov L. V. 
Published: July 2007

AbstractHigh excitation luminescence spectra from a homogeneous electron–hole system are investigated in undoped unstrained InxGa1−xAs/InP quantum wells in magnetic fields up to 12 T. Clear excitonic effects are observed in the optical spectra of neutral, dense magnetoplasmas at low temperatures. These appear as the suppression of the density dependence of transition energies between the uppermost occupied (Je = jh = N) Landau levels in the range of filling factors N < v/2 < N + 1. The effect disappears with increasing temperature. The experimental results are in agreement with theoretical calculations including both the inter‐Landau‐level coupling and screening effects. In a magnetoplasma with an integer filling factor, Landau‐level transitions near the Fermi level are well described in terms of excitons and deexcitons.
Influence of Doping Concentration on Intersubband Relaxation in Modulation‐Doped Quantum Well Structures
Authors: Plödereder U. , Dahinten T. , Seilmeier A. , Weimann G. 
Published: July 2007

AbstractIntersubband scattering in multiple n‐type modulation‐doped quantum well structures is studied by an infrared bleaching technique at room temperature. The repopulation of the lowest subband of the well is directly observed. In GaAs/Al0.35Ga0.65As quantum wells (thickness 5.9 nm) an increase of the relaxation time from 4 to 7.5 ps is found for increasing doping concentrations from 3.2 × 1011 to 1.4 × 1012 cm−2. Carrier transfer between the wells and barrier states is discussed as a process which determines the relaxation time.
Time‐Resolved Investigation of Recombination and Carrier Capture in Spatially Homogeneous 2D Electron–Hole Plasmas
Authors: Daiminger F. , Neppert B. , Forchel A. , Straka J. 
Published: July 2007

AbstractThe time‐resolved photoluminescence of a homogeneous electron‐hole plasma (EHP) is investigated in a 5 nm strained InGaAs/GaAs quantum well. Good spatial homogeneity is achieved by uniformly exciting mesa structures of about 150 μm width with ps laser pulses. Direct comparison of measurements on mesa and planar structures shows that spatial inhomogeneities in the plasma may strongly influence the quantitative evaluation of the time‐resolved data. The transient spectra of mesa structures clearly show band‐filling effects which last for more than 1ns after the excitation. Nonresonant excitation leads to effective trapping of carriers from the barrier layers.
Temperature Dependence of the Lattice Dynamics of the Layered Semiconductor In2Se3
Authors: Julien C. , Balkanski M. 
Published: July 2007

AbstractThe temperature dependence of the infrared reflectivity of the layerd semiconductor In2Se3 is studied in the range from 10 to 500 cm−1. The behaviour of normal modes is reported from 5 K up to room temperature. Reflectivity spectra are fitted with a four‐parameter dispersion model based on the factorized form of the dielectric function. The temperature dependence of the phonon frequencies and damping is reported. An anomaly in the LO frequency which is observed at 160 K, is related with the α′‐phase transition reported by electron microscopy measurements.
ESR Investigation of Phase Transitions in Some Elpasolite‐Type Fluorides
Authors: Dance J. M. , Grannec J. , Tressaud A. , Moreno M. 
Published: July 2007

AbstractStructural phase transition temperatures are determined for some elpasolite‐type fluorides Rb2KMF6 (M = Ga, Cr, Fe) through electron spin resonance. The results are compared with low temperature X‐ray diffraction and calorimetric results. For the new elpasolite Rb2KGaF6 a transition temperature of T ≈ 120 K is detected using Cr3+ and Fe3+ as paramagnetic probes. In the case of Rb2KGaF6:Fe3+ the isotropic superhyperfine constant As is well resolved in the powder spectrum. An analysis of experimental As values found for Fe3+ in different fluoride lattices is reported. In the case of paramagnetic elpasolites, the variation of the linewidth of the ESR spectra of Rb2KCrF6 and Rb2KFeF6 with temperature confirms the occurrence of a phase transition at T(Cr) ≈ 150 K and T(Fe) ≈ 170 K. The Cr3+ or Fe3+ doped K2NaGaF6 elpasolite which remains cubic down to very low temperatures is also checked. As expected no change in the ESR spectra occurs in this case.