Publications

A sequential algorithm, leading to three-dimensional polytetrahedral dense random packing structures is employed to simulate the atom-by-atom growth of relatively small clusters, under a large variety of physical assumptions. The computer model developed can provide information about the influence of experimental growth conditions on the topological and energetical parameters of the cluster : average coordination number, atomic pair distribution function, the degree of local tetrahedral perfection and specific bonding energy as a function of cluster size. The degree of icosahedral order built-in during the simulated growth process can be estimated from the pair distribution function and is of particular interest, due to its relevance to quasicrystalline thin films investigation.

Films of C-60, at different stages of annealing of T-c = 200 degrees and 300 degrees C have been electrical characterized over the temperature domain from -130 degrees C to T-c. X-ray diffraction revealed a random polycrystalline fee structure with stacking defects of an intrinsic nature, due to deposition conditions. The value of room-temperature conductivity was found to be in the range (6.3-1.0)*10(-10) (Omega cm)(-1). In the stable annealed state the conductivity showed an activated temperature dependence above 423 K and a non-activated dependence below 330-280 K. The activation energies E(2) 0.8 eV (film thickness 0.70 mu m) and E(2) = 1.0 eV (film thickness 2.40 mu m) were in good agreement with the energy gap values (1.63 eV and 2.08 eV) which were deduced from the al;sorption spectral dependence. Annealing decreased the non-activated contribution to conduction, extending the intrinsic conduction temperature range.

We report on the first simultaneaous investigation of the behavior of ligand field and charge transfer bands in doped Bi12GeO20 (BGO) under various illumination and thermal conditions. This was made possible through MCD measurements in both the near IR and the visible spectral regions. The [MnO4](3-) ion is clearly identified in the thermally bleached state. Illumination with blue light leads to the reduction of Mn5+ while the antisite (Bi-Ge(3+) + h) defect is formed. Finally EGO is an interesting host for the popular d(2) ions in a tetrahedral site.

High T-c bulk superconductors Y(Tm)Ba2Cu3O7-x and YSryBa2-yCu3O7-x were pre pared following new synthesis routes by the method of thermal decomposition of polynuclear coordination complexes. The best superconducting samples were obtained using oxalic and malic acids as ligands. The effect of substituting of Pa by Sr nas a decrease of T-c with increasing the Sr:Ba ratio. Electrical resistance, magnetic susceptibility and structural measurements have been performed on the superconducting samples.

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.