Publications

The thermally activated dissipation in highly anisotropic Bi2Sr2CaCu2O8+delta superconducting thin films was investigated by conventional resistive measurements, with the magnetic field B=mu0H applied perpendicular to the (a, b) plane and ranging between 5 mT and 3 T. We observed a significant change in the form of the ''irreversibility line'' (determined with a constant-resistivity criterion) and of the magnetic-field dependence of the activation energy at an applied field value B(cr) almost-equal-to 30 mT. This change is discussed in connection to the predicted ''dimensional crossover'' in the vortex system, in the case of highly anisotropic superconductors. For the investigated specimen, we found that the model of thermally activated plastic flux motion with double vortex kink formation describes the irreversibility line at intermediate field values. This indicates that the ''pancake'' vortices in the (CuO2)2 blocks are not completely decoupled, at least up to 3 T. In the low field region, due to the predicted increase of the tilt modulus in the vortex system, the above model becomes inappropriate, and the form of the irreversibility line can be explained in terms of thermally assisted flux flow in a three-dimensional vortex ensemble, with a coherence length along the flux line of the order of 50 nm.

Due to fast relaxation of magnetization and high critical current densities it is difficult in HTSC to reach the critical state experimentally. In order to obtain complete information about it from subcritical state measurements an extrapolation scheme based on the sweep rate dependence of the hysteresis widths has been developed.

CO conversion to hydrocarbons at various reaction temperatures and atmospheric pressure has been investigated on alumina-supported Fe, Ni and Fe-Ni catalysts. Maxima of activity and selectivity were obtained in hydrocarbon formation as a function of Fe content in the samples, which corresponds to a certain stoichiometry of metallic components.

The surface of alumina supported catalysts containing 10 wt.% metal (Fe and/or Ni) has been spectroscopically investigated by CO adsorption. The stretching vibration range characteristic for CO bonded to metallic species shows very large absorptions, indicating a high surface heterogeneity. A certain Fe:Ni stoichiometry seems to impose the reduction extent and the catalytic properties.

The icosahedral metastable Al-Mn phase was obtained in thin films prepared by the high temperature sequential vapor deposition method. Analysis of X-ray diffraction line profiles allows an estimate of the random phason strain, revealing the presence of a highly disordered icosahedral fraction. Its concentration is correlated with the degree of crystallographic perfection of the Al substrate. Thereby, the crucial role of Al diffusion in the formation of the icosahedral phase is evidenced.

A design of a high power laser structure is presented which is based on an increase of the cavity length as well as a maximization of the stripe width. This requires a low value for the modal attenuation coefficient and a low optical confinement factor. A model is presented from which the modal gain, the confinement factor, the active region thickness, the stripe width, the length and the reflection coefficients can be calculated. A variant for all design parameters needed to reach 1 W emission in the fundamental lateral mode is given. These values are used to design the epitaxial structure.

The temperature-induced surface alignment transitions in nematic liquid crystals are analyzed. It is shown that it is possible to interpret the experimental data by means of an elastic model in which the effective splay-bend surfacelike elastic constant is taken into account. This effective elastic constant is defined as the sum of the usual splay-bend and spontaneous splay elastic constant. In this framework the experimental data seem to indicate that the easy axis characterizing the nematic-substrate interaction is independent of the liquid crystal. It depends only on the evaporation geometry (we use flat glass plates vacuum coated by SiO) and on the surfactant dissolved in the liquid crystal. The value of the effective splay-bend elastic constant is obtained for three different liquid crystals in the nematic phase.

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.