Publications

The spectral characteristics of Er3+ in Sc2O3 transparent ceramics are presented. The Er3+ visible spectra in the ceramics, prepared by solid-state synthesis, show only lines that could be assigned to C-2 sites. The Stark energy levels, cross-sections and emission kinetics of levels involved in the green upconversion emission under infrared excitation in the 800 nm range were determined. The green upconversion emission (H-2(11/2), S-4(3/2)) -> I-4(15/2) characteristics under excitation with a pulsed Ti:sapphire laser in (I-4(15/2)-> I-4(9/2)) absorption were analyzed. The emission kinetics reveal a competition of two-step (I-4(15/2)-> I-4(9/2)) + (I-4(9/2)-> H-2(9/2)) excited state absorption (ESA) and (I-4(11/2), I-4(15/2)) -> (I-4(11/2), F-4(7/2)) energy transfer upconversion (ETU) processes function on the excitation wavelength. (c) 2007 Elsevier B.V. All rights reserved.

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field H-c2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T < 24 K (H <= 5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

The electric, ferroelectric and photoelectric properties of Pb(Zr,Ti)O-3-Nb:SrTiO3 (PZT-STON) junctions were investigated on a broad range of temperatures. It was found that the hysteresis loop is strongly asymmetric, due to the asymmetry in the leakage current. The capacitance-voltage characteristic has a butterfly shape although it is also asymmetric because of the different nucleation and compensation conditions at the two interfaces. The junction shows a strong photovoltaic effect in the 200500 run wavelength range as short-circuit currents Of the order of nA for an illuminated surface of 0.018 mm(2). This makes the PZT-STON junctions attractive for UV optoelectronic applications.

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is "orientation engineering" in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.

We have synthesized new functionalized copolymers of the type divinyl benzene maleic anhydride (DVB-MA) in order to ensure a better compatibility with the penthyl-phenol group of the liquid crystal. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), concentration 5% b.w. (copolymer/LC). We applied cycles of temperature and electric field, corresponding to the TSDC method. We measured the depolarization currents and simultaneously the optical transmission as a function of the electric field and temperature. We obtained information regarding the conduction mechanism, the distribution of the activation energy and the influence of the polarizing voltage oil the optical transmission.

In this paper we summarize the deposition parameters, the crystalline structure and the critical temperature of the in-situ grown YBa2Cu3O7-delta(YBCO) films obtained by pulsed laser deposition (PLD) using our kinetic cool-down regime, described in previous papers. We analyze the resistive transition curves and the corresponding scanning electron microscopy (SEM) pictures of the best in-situ grown YBCO films, in order to better characterize the films and refine the deposition process. SEM pictures reveal the typical surface morphology of both in-situ grown and post-deposition, oxygen atmosphere annealed YBCO films. A significant smoother surface of the in-situ grown films is obtained.

We report the transport and magnetic properties of sintered CrO2-polymer composites. In order to improve the elastic properties of the CrO2-based composites, we investigated two types of styrene-based copolymers: poly(methyl methacrylate)-styrene (9/1 and 5/5) and linear polysiloxane-g-styrene as well as poly(methyl methacrylate)-butadiene 9/1. The electric transport and magnetoresistance are consistent with the spin tunneling model at low temperatures but the contribution of spin independent channels becomes important at high temperature and high fields. All composites display ferromagnetism at room temperature with saturation at low fields and a slightly temperature-dependent coercive field for the samples grafted either with butadiene or with styrene except for CrO2-polysiloxane-g-styrene composite where it exponentially decreases with increasing temperature.

The use of semiconductors as detectors for particle and astroparticle physics imposes the understanding of the processes produced in material, their effects on the electronic properties, in their microscopic structure and at the device level. Calculations of LET and NIEL for different ions including hypothetical exotic particles put into evidence the peculiarities of these interactions.

In this paper a novel configuration of microwave planar filters, with multiple cross-couplings and with a number of poles of attenuation NZ equal to the order N of the filter, is investigated The position of the poles on the frequency axis can be controlled, allowing the design of band-pass filters with improved selectivity with respect to the adjacent channels. The new configurations were designed and verified by em-field simulation. The responses of the designed filters are in good agreement with the specification, confirming the possibilities of designing microwave band-pass filters of a relatively low order with moderate losses and with improved performances.