Publications

The EPR spectra of Mn2+ doped K2ZnCl4 show an anomalously large broadening of the Zn site Mn2+ lines between 152.5 K and 145 K, i.e. just above the transition to the low temperature commensurate phase of A1a1 symmetry. The line broadening is related to a distribution of rotations of the ZnCl4 tetrahedra and supports the existence of a second incommensurate phase in this temperature range as reported by Gesi. (C) 1997 Published by Elsevier Science Ltd.

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculated in the four-probe Landauer-Buttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of the density of states at a given magnetic flux are correlated with the behaviour of the mentioned resistances as function of the Fermi energy. The Hall insulator and quantum Hall regimes are evidenced. The magnetic field dependence of the configurational averages of the longitudinal and Hal; resistance is studied in a purely quantum-mechanical approach. Both, negative and positive magnetoresistances are found.

The phase transition from the solid state to the liquid crystal state was investigated for the copper(II) salts of the fatty acids with 10, 12, 14, 16 or 18 carbon atoms in the chain, by measuring the temperature dependence of the conductance and capacitance of a cell containing the material under investigation. The temperatures deduced from these dependences agree reasonably with those found by DSC measurements and with those to be found in the literature. Differences in dielectric behaviour between the samples are qualitatively discussed.

Samples obtained from the hot conditioning and hot performance procedures have been investigated by GEMS. Various types of substrates were considered. The surface phases were assigned to magnetite, alpha-Fe, wustite and to spinel solid solutions. The data have been analyzed taking into account the compound stoichiometry, the number of iron unequivalent positions appeared in each identified phase and the relative ratio of the formed products.

The Optical Charging Spectroscopy (OCS) was applied for the first time to investigate the trapping levels in high resistivity silicon irradiated with neutrons. An analytical formula for the OCS discharging current has been obtained. The theoretical and experimental results proved that the OCS is more sensitive than the usual Thermally Stimulated Currents method.

High power pulsed laser diodes are of interest due to their potential use in medicine and peculiar applications. This paper presents failure analysis for catastrophic damage in GaAs/AlGaAs LOC pulsed laser devices. There is presented failure decrease of optical power as Ig (P/P-0) = f(t) where Po is initial power, P actual power, t-operating time. The damaged devices were investigated by optical and scanning electron microscopy (SEM) and there was performed a electron dispersion spectroscopy (EDS) analysis These experiments advanced the idea of dividing catastrophic degradation in three types, respectively: 1-catastrophic optical mirror damage (COMD), 2- catastrophic damage due to major mechanic defects (CDMMD), 3-catastrophic damage due to metal migration (e.g. Au) (CDMM). There was established an experimental criterion to characterise catastrophic damage.

The solution chemistry of oxalate coprecipitation of Bi, Si, Ca, Cu has frequently! been used in the synthesis of Bi-based ceramic systems, although there are different opinions referring to some major aspects, such as coprecipitation pH and precipitating agent. In the present work we established the precipitation conditions of Bi, Sr, Ca, Cu individual oxalates and of the quaternary! mixture. The initial and thermally treated oxalate powders were characterized bq! XRD, IR, BET methods and TEM observation. The decomposition conditions were established based on a DTA/TG study. The powders resulted from the decomposition of Bi and Cu oxalates contain the corresponding oxides and the Ca and Sr oxalates decomposition lead to the carbonates obtaining. In each case a high reactivity and a very small powder particle size were observed. A very good powder homogeneity has also been noticed in the final mixture of oxalates. Compared to superconducting ceramic materials realized by a classic route, the reactive oxalate powders allow the decrease of both thermal treatment periods and temperature, emphasizing a high microstructure homogeneity of the ceramic body. (C) 1998 Elsevier Science Limited. All rights reserved.

Amorphous multilayers based on Se and CdSe alternated sublayers were succesfully prepared by thermal vacuum evaporation and laser ablation with the periodicity of 22 nm. The multilayer structure is stable up to similar to 70 degrees C. In the samples prepared by laser ablation a large contraction of the multilayer stacking occurs by annealing. Amorphous SiOx/CdSe multilayers with the periodicity of similar to 15 nm were successfully prepared by thermal vacuum evaporation. The structure is stable up to 400 degrees C annealing temperature. Pulse excimer laser irradiation of the SiOx based multilayer produces an expansion of the interlayer distance with similar to 0.33%.