Publications

An analytical approximation was derived for the calculation of the grazing-incidence X-ray reflectivity pattern in a defect-free multilayer. The approximation is valid in the low-reflectivity theta ranges. For k = (4 pi sin theta)/lambda > 0.28 Angstrom(-1) (theta > 2 degrees with Cu K alpha radiation), the formula deviates by at most 1% from the rigorous matricial approach in the case of most metallic multilayers. The main advantage of the approximation is that it allows the identification of features in the reflectance pattern, from which desired structural parameters (average layer thicknesses, external and internal roughness) can be estimated easily. This approach yields average structural parameters, best approximating those of a real multilayer but ignoring possible defects.

The temperature dependence of the electrical resistance of r.f.-sputtered Ni-Si thin films with variable Si content in the range 33-77 at.% (as determined by Rutherford backscattering spectroscopy) was measured between - 190 degrees C and the annealing temperature T-t (T-tmax=300 degrees C). The as-deposited films were investigated by X-ray diffraction and transmission electron microscopy, and were found to exhibit an amorphous structure (different from that of amorphous Si), except those films with a composition favourable for silicide formation. The temperature dependence of the conductivity of these films is interpreted in the framework of the weak localization model and electron-electron interactions. Annealing at 300 degrees C brings about a transformation from an amorphous structure into a microcrystalline structure comprising different silicides, whose electrical resistance displays a classical metallic temperature dependence, except for the sample with x = 77 at.%, which remains amorphous, although its conductivity increases with temperature.

An important goal for the development of SnO2-based gas sensors is finding calibration procedures. In this paper, a long-term calibration procedure is presented, based on the experimental finding that the dependence of the response of SnO2-based gas sensors on ambient atmosphere changes and on aging can be described in a simple way, i.e, by the changes in the exponent of the power law which describes the relationship between the conductance of the sensor and the concentration of a reducing gas. It also was found that the calibration procedure can describe the changes in the response of the sensors to a reducing gas in a changing background of another reducing gas. Using some previous calculations, use of the calibration procedure suggested is proposed.

The thermally activated flux motion in transport-current-carrying epitaxial Bi2Sr2Ca2Cu3O10+ thin films was investigated by conventional resistive measurements, with the magnetic field B applied parallel to the c-axis and ranging between 10 mT and 3 T. It was found that the magnetic field and temperature dependence of the activation energy, as well as the form of the irreversibility line in the thermally assisted flux flow regime, change significantly at a field value B(cr)approximate to 0.1 T. This behavior is similar to that reported by us in the case of Bi2Sr2CaCu2O8+delta thin films, and was interpreted in terms of the occurrence of a dimensional crossover in the vortex system, due to the increase of the tilt modulus at low fields. We determined an anisotropy factor gamma approximate to 140 for our Bi2Sr2Ca2Cu3O10+delta films, which is lower than that observed for the Bi2Sr2CaCu2O8+delta films.

The experimentally determined current-voltage characteristics of high-T-c superconducting ceramics exhibiting the ''peak-effect'' in the magnetic field dependence of the transport critical-current density were described in terms of thermally activated flux creep at grain boundaries; taking into account the collective pinning of intergranular vortices. The peak effect was explained by considering the increase of the intergrain pinning potential at intermediate fields through the interaction of intergranular (Josephson type) vortices and intragranular (Abrikosov) vortices. The magnetic field dependences of the effective pinning potential and of the collective pinning exponent were experimentally determined, and the features of the I-V curves were explained through these dependences.

Within a one-dimensional model of off-centre diffusion it is shown that the diffusion coefficient depends on the number of off-centre sites and on their orientation relative to the host lattice.

Hemoglobin (Hb) interaction with Hg2+ ions was investigated by means of laser photolysis and spectroscopic methods. The binding of the gaseous axial ligands to the 3rd and 4th molecules of a Hb tetramer, 1(3)' and 1(4)', were found to show more relaxed features than in the absence of Hg2+ ions. A heme iron oxidation takes also place. The kinetic parameters of this interaction were compared with those of the systems Hb-Cu2+ and Hb-Sn(IV).

The new Fe(III) complexes with isosceles or equilateral configurations showed atypical magnetic behaviour with a magnetic susceptibility which does not obey the Curie-Weiss law. The experimental data were fitted by using the Heisenberg-Dirac-van Vleck model with a spin Hamiltonian containing different exchange parameters in the case of isosceles configuration. A more elaborate Hamiltonian, taking into account the perturbation arising from a biquadratic exchange interaction was used in the case of equilateral configurations.

The ferric chloride doped polymers change their refractive index under UV irradiation. The Fe:PVA is sensitive to ferric chloride concentration, molecular weight of PVA, and recording beam power. By Mossbauer investigations we tried to correlate the Fe2+ / Fe3+ ratio, as well as the temperature dependence of the energetical parameters with the assumed mechanisms which involve both the valence state of iron and a cross-linking reaction of PVA matrix.

Conductance transients of thick, sintered SnO2 films at step isothermal changes in oxygen partial pressure show exponential decays with time constants which cannot be explained in the frame of the usually accepted model of Schottky barriers at intergranular contacts dominating the conductance behavior. An alternative model, based on a linear relationship between conductance and the surface coverage by chemisorbed oxygen, accounts well for the experimental results. This model proposes the existence of a highly resistive sub-surface layer, isolating the surface from the bulk; it identifies the oxygen chemisorption sites with surface oxygen vacancies. The model is also supported by the temperature-variation of conductance in ''ramp'' experiments with increasing and decreasing temperature, and by the effects of prolonged heat treatments in oxygen upon the conductance and gas sensitivity.