Publications

Two aspects of the selective epitaxial growth of Si and Si(1-x)Ge(x) will be discussed. First the facet formation as dependent on the oxide wall orientation and the lateral size of oxide openings. Besides the often cited {111}, {113} and {110} planes additional planes were observed, the {119} and the {0 1 12} planes. Second, the reduction of misfit dislocation density by reducing the area of the pads allows strained Si(1-x)Ge(x) layers to grow much thicker than the critical thickness. As an application for the latter the electroluminescence of forward biased PIN diodes with strained Si(0.80)Ge(0.20)/Si(001) will be discussed as being dependent on the thickness of the SiGe layer. It was found that in thicker strained samples the band edge electroluminescence persists up to room temperature. Quantitative modelling of the electroluminescence could explain the temperature and SiGe thickness dependence of the electroluminescence. (C) 1998 Elsevier Science S.A. All rights reserved.

The supercurrent transport properties of epitaxial Bi2Sr2CaCu2O8+delta films in zero applied magnetic field were investigated in a temperature interval of approximate to 20 K below the mean-held critical temperature T-c0. The modification of the shape of the I-V curves observed by varying the temperature was explained in terms of vortex-fluctuation-induced layer decoupling and vortex-antivortex unbinding, revealing a strong probing-length dependence. The change of the effective dimensionality of thermally excited vortices involved in the dissipation process leads to the appearance of a few characteristic regions in the current-temperature diagram. Above a temperature value T*<T-c0, the superconducting layers are decoupled, as predicted by Monte Carlo simulations and renormalization-group analyses. In this region, the resistivity exhibits two-dimensional (2D) behavior corresponding to the superconducting (CuO2)(2) layers (2D-layer behavior. However, the resistive transition seems to be mainly related to the 2D behavior at the film level. In the sensitivity window of our measurements, finite resistance in the limit of small transport currents was detected to occur above a temperature value T-c. <T*, through the dissociation of vortex-string-antivortex-string pairs. By decreasing the temperature and/or by increasing the transport current, the I-V curves in the double logarithmic plot show a clear downward curvature. This can be described in terms of current-induced quasi-2D vortex pair unbinding, with a nonzero critical-current density resulting from the interlayer Josephson coupling. At even lower temperatures and/or higher transport currents, the I-V curves exhibit a crossover from quasi-2D to 2D-layer behavior, due to the decrease of the probing length below the Josephson length, where the interlayer Josephson coupling becomes irrelevant. The temperature dependence of the 2D I-V exponent is in good agreement with recent Langevin simulations of the Coulomb gas model, revealing an anomalous diffusion of vortex fluctuations.

We have investigated the thermally activated behavior of the in-plane electrical resistivity of Bi2Sr2CaCu2O8+delta films for magnetic fields B less than or equal to 10(4) G applied parallel to the c axis. The activation energy in the vortex-liquid state changes suddenly at a crossover field B-cr. The anisotropy reduction generated by oxygen annealing leads to the increase of the crossover field. For BB-cr, U(B,T)similar to(1-T/T-c0)/B-1/2, which corresponds to an entangled vortex fluid. The observation of vortex-liquid entanglement in the presence of relevant quenched disorder is discussed in connection with the relation between the theoretically predicted entanglement length for a clean system and the collective pinning length along the field direction. Our results suggests that, in the case of a pronounced anisotropy and significant collective pinning, the entanglement field B-E=B(cr)approximate to Phi(0)/gamma(2)s(2), where s is the interlayer spacing.

The x-ray-absorption cross section above the L edges in xenon gas has been measured using synchrotron radiation. Multielectron excitation effects are investigated over a few hundred eV region from the Xe L-I, L-II, and L-III edges and the contributions from the effects of shake-up and shake-off to the photoabsorption spectrum are elucidated. Several multielectron excitation features were detected and analyzed by the theoretical energy of the shake-up process. Previous observations of the [2(s,p)5(s,p)] and [2(s,p)4d] transitions have been confirmed. [2(s,p)4p] transitions are clearly identified in the present study. It is very difficult to detect the [2s3s] multielectron transition with the sensitivity of similar to 2.0 x 10(-4) at the L edge jump. The [2s3d] edge was distinguished using the first derivative of the absorption curve. A new transition is found between [2(s,p)4p] and [2(s,p)4d] transitions and identified as a three-electron transition [2(s,p)4d5p]. Moreover, the difference in the behavior of the multielectron excitation as a function of x-ray energy among the L-I and L-II, or L-III edges is confirmed and investigated. [S1050-2947(98)01802-2].

An elastic model for the twisted nematic texture observed in nematic cells containing octyloxybenzoic acid is proposed. According to the model the twisted textures are due to the presence of cybotactic groups which are responsible for the chirality of the liquid crystal. We show that by assuming for the density of cybotactic groups a critical temperature behaviour close to a special temperature, the experimental data can be well interpreted by our mechanical model.

The dependence of the displacement damage induced by pions in GaAs has been evaluated, in the energy range 50 -1000 MeV, using the Lindhard theory and parametrised values of the pion-nucleus interaction. The curves corresponding to the NIEL produced by protons, neutrons, and pions in Si and in GaAs intersect, in the limit of calculation errors, in the region 50 - 80 MeV, at a NIEL of 2 - 5.5 MeV cm(2)/g. The energy dependence of the NIEL of pions in Si and GaAs is discussed in correlation with the simulated pion spectra at new hadron colliders.

We investigate the far-IR absorption of short-period parallel quantum wires in a perpendicular quantizing magnetic field. The external rime-dependent electric field is linearly polarized along the wire modulation. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. We consider the effects of a metal gate grating coupler, with the same or with a different period as the wire modulation, on the absorption. The evolution of the magnetoplasmon in the nonlocal region where it is split into several Bernstein modes is discussed in the transition from narrow to broad wires and isolated to overlapping wires. We show that in the case of narrow and not strongly modulated wires the absorption can be directly correlated with the underlying electronic band structure.

An EPR study of the Tl+-doped PbCl2 crystals x-ray irradiated at low temperature reveals the presence, besides known spectra attributed to hole-trapped Tl2+ centers and Pb-2(3+) self-trapped-electron (STEL) centers, of an additional strongly anisotropic set of lines consisting of an intense doublet and several weaker satellite lines spread over a large magnetic-field range. The quantitative analysis of the EPR spectrum shows the corresponding paramagnetic center to be a bent molecular ion (PbTl)(2+) resulting from electron trapping at a pair of substitutional Pb2+ and Tl+ ions. Its formation suggests the electron trapping at pairs of neighboring cations is a more general characteristic of this material, with possible relevance in photolysis and exciton localization properties.

We compute numerically the time dependent retarded Green function of the polaron within the self-consistent RPA approximation. The results show an approximately Gaussian behaviour at t = 0 changing at later times its concavity to an exponential decay? as it has been predicted in the approximate form of an inverse hyperbolic cosine function. The result contrasts with the non-selfconsistent RPA: where the exponential decay is only a transitory behaviour and the asymptotics is rather oscillatory. Our conclusions are significant in the context of the quantum kinetics with LO-phonons, where the transition from an intitially coherent scattering kinetics to a Markov kinetics with energy conservation is controlled by the time behaviour of the retarded Green function.

A Mossbauer and magnetic study of FeRh has been undertaken in order to understand better the remarkable properties of both melt-spun BCC and ball-milled FCC-phases of the anti-invar FeRh alloy. (C) 1998 Elsevier Science B.V. All rights reserved.