Publications

One step synthesis of the superconducting Bi(Pb)-2223 phase during heat treatment for non-equilibrium decomposition of the freeze dried Bi:Pb:Sr:Ba:Ca:Cu = 1.8.0.4: 1.8:0.2:2.2:3.0 nitrate powder has been studied. The heating rate was similar to 100 degrees C min(-1) and samples were maintained in air at 848 degrees C for 0, 5, 10, 20, 30, 60, 90, 240 and 1080 min. Each batch was investigated by x-ray diffraction analysis and ac magnetic susceptibility chi'(T). Evolution of phases and kinetics of phase formarion-decomposition processes were discussed. similar to 80% of Bi(Pb)-2223 has formed indicating the possibility of lowering the processing time of the freeze dried precursor powders by one order of magnitude comparative to the multistep synthesis route (from hundreds to tens of hours). A 30 min heat treatment step at 840 degrees C performed on the way up to the synthesis temperature is a forthcoming operation in order to block the formation of secondary undesired phases (e.g. (Ca1-xSrx)(14)Cu24O41).

Plasma vertical gradient freezing (PVGF) is a method for growing crystals from the melt in a vertical temperature gradient using a DC glow discharge heater. In particular, undoped crystals can be purified during growth because of the complex shape of the vertical temperature gradient. Standard samples of LiF and CaF2 single crystals grown by the PVGF method from material of 99.9% purity have the following parameters: (a) for lambda is an element of [2.5-5 mu m], average transmission coefficient is 97% for LiF and 92.5% for CaF2; (b) repeated (unlimited) pulse power density approximate to 218 w/mm(2) (1 pulse/min, pulse length = 350 mu s, lambda = 2.94 mu m) for LiF samples. CaF2, and LIF single crystals were grown in a nitrogen atmosphere at a pressure of 10(-1)-10(-2) Torr. They are used in laser optics. Some of these fluorides can be doped and used as active media, too. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Low energy hydrogen ion implantation in ZnO creates an accumulation layer near the surface, giving rise to a quantum well. The corresponding self-consistent Hartree problem is solved by taking into account the donor distribution resulting from the implantation process. The values of the surface potential as a function of repelled away electron concentration are derived. The resulting data are compared with experimental values obtained from space-charge capacitance measurements. A comparative study between the semi-classical and self-consistent quantum treatments is presented.

Metal structures in quartz crystals were obtained by electrodiffusion in air with different metal sources at the anode. The dynamics and the properties of these structures, for different conditions of temperature, applied electric field and anode material were studied. Current-voltage characteristics, Arrhenius plot, optical microscopy, electronic microscopy and thermoluminescence glow curve were employed. (C) 1999 Elsevier Science B.V. All rights reserved.

Cubic boron nitride (c-BN) crystals synthesised at high pressures and temperatures are analysed by optical microscopy, transmission electron microscopy, electron diffraction and electron spin resonance. For various growth conditions, the results of these studies indicate that the c-BN crystals contain defects and impurities. This is the first time that dislocation cores have been revealed in c-BN at the atomic level. Atomic resolution at extended dislocations allows us to determine the stacking-fault energy in c-BN, yielding a mean value of 191+/-15 mJ m(-2). This value, which is reported for the first time for c-BN, is of the same order of magnitude as in diamond. (C) 1999 Elsevier Science S.A. All rights reserved.

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

CxByNz layers have been prepared by laser-assisted chemical vapour deposition (CVD) in a gas atmosphere containing C2H4, B2H6 and NH3 where the starting composition ratio could vary in a large range. The characterization of these C-B-N materials was made by XRD, EPMA, XPS, optical and scanning electron microscopy. The chemical composition of C-B-N layers tended to a composition C1B6N3 on the line BN-'B3C'. The turbostratic structure of C-B-N layers could be influenced and modified as a function of composition. XPS investigations confirmed the single-phase nature and the existence of bonding between all the elements. Some planar structures, containing especially CB2N groups, were suggested for the 'unit cell' of these C-B-N solid solutions, in agreement with EPMA and XPS analysis. (C) 1999 Elsevier Science S.A. All rights reserved.

We calculate energy spectra; of a two-dimensional electron system in a perpendicular magnetic field and periodic potentials of short periods. The Coulomb interaction is included within a screened Hartree-Fock approximation. The electrostatic screening is poor, and the exchange interatcion amplifies the energy dispersion We obtain, by numerical iterations, self-consistent solutions that have a hysteresislike property. With increasing amplitude of the external potential, the energy dispersion and the electron density become periodic, and they remain stable when the external potential is reduced to zero. We explain this property in physical terms, and speculate that a real system could memorize short-range potential fluctuations after the potential has been turned off.

In this paper, we present some transport measurements on artificially layered (BaCuO2)(2)/(CaCuO2)(2) superlattices, engineered by the pulsed laser deposition (PLD) technique, and having critical temperatures (zero resistance) of about 80 K. The experimental current-voltage characteristics are discussed in the frame of current-induced unbinding of thermally created vortex-antivortex pairs (in zero-field) and in the frame of vortex glass (VG) transition for fields up to about 10 kG. The transport critical current density is estimated and compared to values reported from magnetisation measurements. The field dependence of the activation energy of vortices in the vortex liquid (VL) phase shows a strong effective pinning, due to the high disorder intrinsic to these artificially layered superconductors, at low magnetic fields, and a transition to a thermally activated plastic motion of an entangled VL in higher fields. From the temperature and field orientation dependencies of the vortex melting field, an anisotropy factor of about 20 is estimated for these superlattices. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

A low-confinement asymmetric GaAs-AlGaAs double-quantum-well molecular-beam-epitaxy grown laser diode structure with optical trap layer is characterized. The value of the internal absorption coefficient is as low as 1.4 cm(-1), while keeping the series resistance at values comparable with symmetrical quantum-well gradient index structures in the same material system. Uncoated devices show COD values of 35 mW/mu m. If coated, this should scale to about 90 mW/mu m, The threshold current density is about 1000 A/cm(2) for 2-mm-long devices and a considerable part of it is probably due to recombination in the optical trap layer. Fundamental mode operation is limited to 120-180 mW for 6.5-mu m-wide ridge waveguide uncoated devices and to 200-300 mW for 13.5-mu m-wide ones, because of thermal waveguiding effects, These values are measured under pulsed conditions, 10 mu s/l ms.