National Institute Of Materials Physics - Romania

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

An attempt has been made to put in evidence the modified V-K centres in low temperature UV irradiated Kl:Cl crystals. The interpretation of the results seems to favour a new type of V-K centre localised near an impurity halogen instead of a heteronuclear V-K centre.

Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration field H* was determined, at different temperatures, from the field dependence of chi '' in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T-c, which are consistent with the usually reported data for this material.

The spectrum and the eigenstates of a finite two-dimensional tight-binding electronic system, with Dirichlet boundary conditions, in a magnetic field and with an external linear potential are studied. The eigenstates show an equipotential character, and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated between the usual integer-quantum-Hall-effect plateaus. This ideals confirmed by a numerical calculation within the four-terminal Landauer-Buttiker approach.

Pd-Tm/gamma-Al2O3 catalysts containing 0.1% Pd-1% Tm, 0.3% Pd-1% Tm and 0.5% Pd-1.% Tm, respectively have been obtained by successive impregnation of gamma-Al2O3, first with palladium and then with thulium. Introduction of Tm leads to an increase in the conversion of vinylbenzenes compared to that with monometallic Pd/gamma l-Al2O3 catalysts. For monocomponent Pd/gamma-Al2O3 catalysts, turnover frequency (TOF) is unchanged. For bicomponent Pd-Tm/gamma-Al2O3 catalysts, if one considers that the reaction occurs only on the Pd particles, then introduction of Tm seems to increase the TOF. However, by taking Tm into consideration when calculating the TOF, then the bicomponent catalysts are seen to behave as the monocomponent catalysts. This behaviour is highlighted also by using the Koros-Nowak criterion. If Tm is not considered to be an active species, vinylbenzene hydrogenation appears to be controlled by mass transfer processes. However, the texture characteristics of the bicomponent catalysts are not sufficiently modified by Tm introduction to support such a hypothesis. On the contrary, by considering the Tm participation in the reaction, the slope of the Koros-Nowak dependence is included in a range typical for kinetically controlled reactions.

Current-voltage characteristics of multifilamentary Bi2Sr2Ca2Cu3O10/Ag tapes (short samples) produced by the 'powder in tube' technique were measured at different temperatures close to the mean-field critical temperature, and in zero applied magnetic field. After performing the required corrections due to the current flowing in the silver matrix, the I-V curves were interpreted in terms of current-induced unbinding of the thermally created vortex-antivortex pairs. Two possible mechanisms for appearance of a finite critical current in zero applied magnetic field are discussed: the Jensen-Minnhagen quasi-two-dimensional (2D) approach, that takes into account the interlayer Josephson coupling, and a model of size limitation of vortex fluctuations. From our analysis, it seems that the latter model is more suitable for this kind of superconducting material, due probably to an accentuated intrinsic anisotropy.

The dependence of the displacement damage induced by pions in silicon has been calculated as a function of their kinetic energy, between 50 and 1000 MeV, using data on pion-silicon interaction. The region of the delta resonance has been carefully considered. because it corresponds to a sharp maximum in the spectra of simulated pion production in the central cavity of LHC. Results are reported on previous calculations and on experimental damage data measured in silicon detectors irradiated with pions.

Four contact bulk samples and p(+)n junction diodes produced from high resistivity n-type silicon wafers of the same ingot have been irradiated with 1 MeV-neutron fluences between 10(12) and 3 x 10(13) cm(-2). The effective impurity concentration N-eff has been calculated from CV measurements in irradiated diodes, and bulk resistivity has been measured with the Van der Pauw method on four contact samples. The experimental resistivity and N-eff dependence on the irradiation fluence, self-annealing time and storage temperature have been modeled considering the exponential shallow donor decay and the creation of acceptor traps in the irradiated silicon. A numerical fit to the experimental data, based on the solution of the neutrality and Poisson equations, has been carried out considering an equivalent deep acceptor defect created during and after irradiation in the silicon lattice. The energy level and the introduction rate of this defect, evaluated as best-fit parameters in the numerical procedure, are E-t = 0.60-0.65 eV above the valence band edge and b similar to 0.06 cm(-1).

The UV/Vis diffuse reflectance (DR) spectra of NiO-Al2O3 systems obtained by mechanical mixing of oxides followed by calcination between 350 and 1400 degrees C are used for a detailed discussion and the reactualisation of an earlier band assignment. From temperature-programmed reduction (TPR) curves, a percent distribution of the total nickel content among various oxidic species, identified by hydrogen consumption peaks, was estimated. An attempt was made to estimate the fraction of octahedral Ni2+ ions in the spinel forms from the same TPR data. The results were correlated with XRD data and magnetisation measurements on the reduced samples.