Publications

We examined the electrical and optical properties of a new material containing polymer-clay nano-composite dispersed in a nematic liquid. The clay (Cloisite-type) was modified by copolymerization of maleic anhydride and divinyl benzene, using azobisisobutytironitrile as the initiator. The final polymer-clay nano-composite has an intercalated structure according to XRD patterns. We measured the thermally stimulated depolarization currents and determined the activation energies. Simultaneously we measured the optical transmission and we studied the influence of the previously applied polarizing electric fields.

Pt-ZnO-Pb(Zr0.2Ti0.8)O-3-Pt (PZT-ZnO) heterostructures were fabricated by using a sol-gel process. Capacitance-voltage measurements performed on a wide temperature range (20-450 K) have revealed the presence of a hysteresis that undergo a change of direction from clockwise at temperatures below 350 K to counter-clockwise at higher temperatures. In the first case, the hysteresis is produced by charge injection, similar to the case of classical metal-oxide-semiconductor capacitors. In the last case, the hysteresis is the fingerprint of polarization reversal, as reported for metal-ferroelectric-semiconductor (MFS) structures based on n-Si. The memory window at 450 K is about 6 V. This result suggests that PZT-ZnO MFS heterostructures can be used for memory devices working at elevated temperatures, in which the ZnO plays the role of the semiconductor.

We report on the vortex dynamics of a single crystal of the noncentrosymmetric heavy-fermion superconductor CePt3Si. Decays of the remnant magnetization display a logarithmic time dependence with rates that follow the temperature dependence expected from the Kim-Anderson theory. The creep rates are lower than observed in any other centrosymmetric superconductor and are not caused by high critical currents. On the contrary, the critical current in CePt3Si is considerably lower than in other superconductors with strong vortex pinning indicating that an alternative impediment on the flux-line motion might be at work in this superconductor.

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa2Cu3Ox (PrBCO) nano-dots and layers of YBa2Cu3O7-delta (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 rim, and total thickness between 1.13 and 5.31 mu m. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J(c) at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J(c) only in high fields. We have also investigated the frequency dependence of J(c) from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nanodots, both isotropic and c-axis correlated. (C) 2009 Elsevier B.V. All rights reserved.

YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U-o was determined by analyzing the in-plane resistive transition of 200 mu m, wide bridges with the external magnetic field B oriented along the c axis. It was found that U-o is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U-o with the current I in the specimen: U-o exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep - elastic (collective) creep induced by the transport current. (C) 2009 Elsevier B.V. All rights reserved.

Intercalation, i.e. the insertion of guest species in a crystalline layered structure, is an efficient route for generating new materials with novel properties. Thin films and crystalline powder of BiI3 layered semiconductor were intercalated by exposure to ammonia vapors at room temperature. The intercalated compound was studied by thermo-gravimetric analysis, differential scanning calorimetry, X-ray diffraction, UV-vis optical absorption, FTIR spectroscopy and Raman scattering. After exposure of BiI3 to ammonia the formation of a new phase, BiI3(NH3)(3.83), was evidenced by thermal analysis. The intercalation process leads to a blue shift of the BiI3 Optical absorption edge by 0.5 eV. The appearance of new Raman lines at 135 and 353 cm(-1) in the Raman spectrum of intercalated BiI3 is considered as an evidence of the chemical interaction between the ammonia molecules and BiI3 lattice. (C) 2009 Elsevier Ltd. All rights reserved.

The effect of Rashba spin-orbit coupling on the spin interference in a noninteracting one-dimensional ring connected to two leads is studied theoretically within the nonequilibrium Greens' function formalism. We compute the charge and spin currents and analyze their Aharonov-Bohm oscillations. The geometry of the system is conveniently described by the angle delta between the two leads. We show that for delta=180 degrees (i.e., for symmetrically coupled leads), a good filtering of up-or down-spin orientation is obtained around half-integer multiples of Phi/Phi(0). These particular flux values are degeneracy points for clockwise and counterclockwise propagating states, corresponding to the same spin orientation in the local spin frame of the ring. In contrast, for the asymmetric coupling, i.e., delta=135 degrees, the filter efficiency is maximum around integer multiples of Phi/Phi(0). The numerical results suggest that the spin filtering is obtained when the clockwise or counterclockwise states interfere destructively. It turns out that the spin filtering regime is stable against variations in the bias applied on the system. The quasiperiodic oscillations of the charge current, as a function of the Rashba strength, are obtained and discussed.

In-plane gate (IPG) transistors with various channel geometries have been fabricated by focus ion beam implantation technique in positive pattern definition mode on GaAs/Al(x)Ga(1-x)As heterostructures. Both n- and p-type channel transistors were obtained for either n- or p-doped heterostructures, by implantation of the channnel or the gate regions with the complementary dopant type to compensate the initial doping. The current-voltage charactristics showed that the channel can be fully controlled by a gate bias, i.e., it can be completely depleted or enhanced, but two gates are needed to control the channel. Different channel geometries were studied, the most efficient being the "Z-shaped" one. The n-type channel IPG transistors present source-drain currents of few hundreds of microamperes, which is two to three orders of magnitudes higher than that for the p-type channel ones. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Continuous-wave (CW) simultaneous laser emission on the 0.9-mu m F-4(3/2)-> I-4(9/2) transition and the F-4(3/2)-> I-4(11/2) transition at 1.06 mu m is obtained in Nd-based laser crystals of thin-disk geometry and using a multi-pass pumping scheme. A Nd:Y3Al5O12 (Nd:YAG) thin disk emitted simultaneous laser radiation at 946 and 1064 nm with 5.1 W output power, and Nd:YVO4 and Nd:GdVO4 thin-disk lasers with more than 3 W output power at 0.91 and 1.06 mu m were realized. The ratio between the output power at one of the wavelengths and the total output power could be varied by the laser resonator design. An intracavity frequency-doubled Nd:YVO4 thin-disk laser with alternate green at 532 nm and "deep-blue" at 457 nm gener ation of high average output powers is demonstrated.

We studied the synthesis of lanthanum zirconate from lanthanum nitrate and zirconium butoxide in the transition from liquid to amorphous and crystalline phase. Thermal behaviour of lanthanum nitrate dissolved in 2-methoxyethanol is different from that of the salt, which we ascribe to the potential coordination of the solvent to lanthanum atoms. IR analysis shows that lanthanum nitrate remains strongly associated in the solvent. The lanthanum environment in the lanthanum nitrate and lanthanum zirconate sols, determined by EXAFS, is similar to that of the lanthanum nitrate hydrate. The environment is completely changed in the dried lanthanum zirconate precursor powder and powder heated at 500 degrees C due to the decomposition of nitrates. Zirconium species form polynuclear oxo-alkoxide complexes during thermolysis and they are stable from room temperature to 500 degrees C. The powder heated up to 700 degrees C for 1 h is amorphous, while after heating at 800 degrees C for 1h pure pyrochlore phase crystallizes. No link between La and Zr species is established either in the sol or in the dried precursor powder or amorphous powder heated at 500 degrees C. The reaction between the metallic species proceeds as a solid-state reaction. The synthesis route ensures a very good mixing of the species at the nanometre level. (C) 2009 Elsevier Ltd. All rights reserved.