Publications

ac susceptibility measurements as a function of temperature T and the ac magnetic field amplitude, Hac, have been performed on five superconducting Bi1.8Pb0.4Sr2Ca2-xYxCu3Ox polycrystalline samples (x = 0, 0.005, 0.04, 0.15 and 0.36, and with the volume fraction of the Bi2Sr2Ca2Cu3Oy (2223) phase, f approximate to 100%, 100%, 64%, 0% and 0%, respectively. By using the Muller model for granular superconductors, we found that the pinning force density obtained from the imaginary peak of chi ''(T) is strongly dependent on the doping level. To describe this dependence we adopted the multilayer model (a stack of superconducting, S, and normal, N, layers) of a superconductor. In the framework of this model, we demonstrated that (a) the yttrium addition influences the degree of coupling between the adjacent S layers (and thereby, the pinning strength) through the modification of the carrier concentration and of the effective thickness of the N layers, and (b) the intergrowth of Bi2Sr2CaCu2O delta (2212) layers within the 2223 grains has an important influence on the thickness of N. (C) 2011 Elsevier B.V. All rights reserved.

CeO2 films were prepared from solutions of different concentrations (0.05-1.0 M) on textured Ni substrates. Homogeneous nucleation and growth of CeO2 nanocrystals 170 nm (>0.8 M), the transition to classical curvature-driven grain-growth kinetics is evident (at 1000 degrees C, below the literature value of 1100 degrees C). The decrease in the Kohlrausch exponent for these thick films suggests that the grain coarsening through grain boundary migration is responsible for the stretched regime of grain-size relaxation. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Oxyfluoride glass-ceramic in the system SiO2-Al2O3-CaF2-SmF3 containing Sm3+-doped CaF2 nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm3+-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 A degrees C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm2+-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 A degrees C were attributed to the recombination of the electrons released from the Sm2+-traps located mainly at the surface of the CaF2 nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 A degrees C size have been assigned to the recombination of the electrons released from the Sm2+-traps located inside the CaF2 nanocrystals.

In this paper, we report on Raman scattering and Surface Enhanced Raman Scattering (SERS) studies of single walled carbon nanotubes (SWNTs) and carbon nanotube/conjugated polymers composites. We demonstrate that under SERS conditions we induce an abnormal anti-Stokes Raman emission, that can be interpreted as being due to a "single-beam pumped" Coherent Anti-Stokes Raman Scattering (CARS) effect. We also investigate in detail the anti-Stokes/Stokes (aS/S) intensity ratios of the radial breathing modes (RBMs) of SWNTs as a function of several parameters. From calculations, we show that resonance phenomena mostly explain the aS/S intensity ratio anomalies, but only at low frequencies. In addition, we describe results obtained with polymers like poly(bithiophene) (PBTh) polymerized on carbon nanotube thin films which exhibit also an amplification of its high frequency Raman modes in the anti-Stokes branch, generated by the plasmon excitation of metallic tubes. This phenomenon occurs in several other materials such as composites based on SWNTs and conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyparaphenylene-vinylene (PPV) for modes located around 1500 cm(-1). (C) 2011 Elsevier B.V. All rights reserved.

Ferroelectric ceramics Ba0.6Sr0.4TiO3 (BST 40) were prepared, by solid-state reaction in the temperature range 1210-1450 degrees C. Maximum values of the ceramic densities were around 94% of their theoretical value. X-ray diffraction techniques (XRD) and scanning electron spectroscopy (SEM) were used to analyze the structure and the surface morphology of ceramics. Rounded, well defined or abnormal granular growth was observed in the SEM images, vs. sintering conditions and purity of the raw materials. In all samples, BST 40 ceramic is the major phase, but there are also present small amounts of secondary phases, as revealed in XRD diffraction patterns. Permittivity and dielectric loss measurements were performed in the temperature range - 150 to + 150 degrees C, and 150 Hz-5 MHz frequency values. Permittivity values rising from 1200 to 12,500, with increasing sintering temperatures, were recorded. Narrow and well defined transition peaks were noticed at higher sintering temperatures. Curie temperature was around 2 degrees C, for samples with the mentioned composition. Permittivity and losses vs. frequency show different behavior whether BST ceramics are in polar or non-polar state and with the distance toward phase transition. Microwave measurements performed at room temperature have shown lower values of permittivity, compared with similar data at low frequency, and dielectric losses lower than 1% at 0.7 GHz. The sintering conditions (temperatures, sintering time, etc.) and purity of the raw materials lead to important changes of transition temperatures in the polymorphic diagram, which we have built-for the other Ba1 - xSrxTiO(3) compositions (x = 0.25-0.90) sintered at 1260 degrees C for 2 h. (C) 2011 Elsevier B.V. All rights reserved.

We investigate theoretically the transport properties of the side-coupled double quantum dots in connection with the experimental study of Sasaki et al. [Phys. Rev. Lett. 103, 266806 (2009)]. The setup consists of connecting the Kondo dot directly to the leads, while the side dot provides an interference path which affects the Kondo correlations. We analyze the oscillations of the source-drain current due to the periodical Coulomb blockade of the many-level side dot at the variation of the gate potential applied on it. The Fano profile of these oscillations may be controlled by the interdot coupling, level spacing of the side dot, and also by the temperature. The nonequilibrium conductance of the double-dot system exhibits zero-bias anomaly which, besides the usual enhancement, may show also a suppression (a diplike aspect) which occurs around the Fano zero. In the same region, the weak temperature dependence of the conductance indicates the suppression of the Kondo effect. Scaling properties of the nonequilibrium conductance in the Fano-Kondo regime are discussed. Since the Kondo temperature of the single-impurity Anderson model is no longer the proper scaling parameter, we look for an alternative specific to the double dot. The extended Anderson model, Keldysh formalism, and equation-of-motion technique are used.

Nanostructured powders processed by ball milling of a mixture of Fe and Fe3O4 at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe3O4 powder, during milling process, wustite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe3O4 phases until about 450 degrees C. Above this temperature the reduction reaction Fe + Fe3O4 = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe3O4 content from about 550 degrees C and almost complete disappearance of these phases at about 900 degrees C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic-antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder. (C) 2011 Elsevier B.V. All rights reserved.

SrZrO3 powders are obtained by solid state reaction from SrCO3 and ZrO2 precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 degrees C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 mu m in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.

Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn2+ impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed. (C) 2011 Elsevier Inc. All rights reserved.

Hydrogen adsorption in Metal-Organic Frameworks is improved by using platinum and palladium on different carbon support: activated carbon or carbon nanofibres. Although, hydrogen adsorption is directly dependent of the specific surface area of the material, it is not the only factor contributing to the hydrogen adsorption, as presented in this work. Direct synthesis of metal-organic frameworks on supported Pt and Pd catalysts, reduces the surface area significantly, but the presence of the catalysts overcomes this drawback by improving hydrogen adsorption by a factor of 2.2, in case of platinum and by a factor of 1.4 in the case of palladium, compared to the metal-organic framework without catalysts.