Publications

The temperature T variation of the normalized vortex-creep activation energy U* determined in standard magnetization relaxation experiments for Pb2Sr2Y0.53Ca0.47Cu3O8+delta single crystals with random point disorder exhibits a maximum, which moves to lower-T values by increasing the external magnetic field oriented parallel to the c axis. The nonmonotonic U*(T) dependence is related to the change of the vortex pinning barriers involved in the creep process across the order-disorder transition in the vortex system (accompanied by the occurrence of the second magnetization peak), in a dynamic scenario. The decrease of U* with decreasing T in the low-T region is caused by the shift of the current density J range probed in standard magnetization measurements toward the critical current density, and the significant U*(J) variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior of highly disordered top-seeded melt-grown YBa2Cu3O7-delta crystals at low T, for which no second magnetization peak appears, and U* does not depend on T.

Second harmonic generation from n-GaAs(100) electrodes in sulfuric acid solution has been investigated at an incident wavelength of 1064 nm over a potential range where Faradaic processes are excluded. In the p-in/p-out polarization configuration, the rotational anisotropy in the nonlinear response was found to depend both on the doping level and the applied potential. The effects of the applied potential on the isotropic amplitude of the SHG signal turned out to be closely related to the population of surface states as revealed by impedance spectroscopy, indicating that these states could play a significant role in second harmonic generation. (C) 2004 Elsevier Ltd. All rights reserved.

Pyroelectric current spectroscopy (PCS) was used to investigate the homogeneity of the Nb doped lead zirconate-titanate Pb(Zr-0.92 Ti-0.08)O-3, (PZT) ceramics intended to be used in the infrared (IR) pyroelectric detectors manufacturing. The non-homogeneous composition in the ceramic volume is reflected by the occurrence of several peaks in the PCS spectra, suggesting the presence of PZT phases with different Zr/Ti ratio, thus having different transition temperatures. The non-homogeneity was confirmed by compositional analysis performed using an Energy Dispersive X-ray Spectrometer (EDAX) attached to a Scanning Electron Microscope (SEM). PCS can be a useful tool in optimizing the calcination and sintering parameters (temperature and time) during ceramic preparation. (C) 2004 Elsevier B.V. All rights reserved.

The electronic mechanisms induced by the UV exposure of thin films of polyvinyl alcohol doped with pairs of mixed valence metal ions were studied in relation to their optical behaviour by Mossbauer spectroscopy and optical absorption. The results obtained definitely point to the role of each element from the pair in the electronic mechanism involved, with influence on the optical properties regarding applications in real-time holography and integrated optics.

Superconducting whiskers of Bi-2212 have been grown by an airtight crucible method in elevated magnetic fields, up to 10 T. Growth behavior with and without application of a magnetic field, as well as the SEM detailed study of the growth defects of the whiskers allow to emphasize some aspects of the growth mechanism. The conclusion is that whiskers are growing mainly through continuous crystallization at the base-end and from the so-called 'micro-crucible' formed by the glassy or powdered substrates. The supply of cations by a vapor transfer is very probable, but is taking place to the micro-crucible reaction site rather than to the droplet, as in the classic vapor-liquid-solid (VLS) growth mechanism. Therefore, whiskers grow by a modified VLS-microcrucible mechanism (MVLS-MC). Application of magnetic fields during growth has proven to be a powerful tool to investigate and influence some processes. Observation of stoichiometric Bi(Pb)-2212, CuO and Ca2CuO3 whiskers is also reported. (C) 2004 Elsevier B.V. All rights reserved.

Stable CoFe(2)O(4) nanoparticles have been obtained by co-precipitation using a microwave heating system. Transmission electron microscopy images analysis shows an agglomeration of particles with an average size of about 5 nm, and X-ray diffraction reveals the presence of a pure ferrite nanocrystalline phase. X-ray photoelectron spectroscopy and thermal gravimetric analysis show the presence of organic matter in the range of about 16 wt%. The magnetic response in DC fields is typical for an assembly of single-domain particles. The measured saturation magnetization is slightly larger than the bulk value, probably due to the presence of small amounts of Co and Fe. AC magnetization data indicate the presence of magnetic interactions between the nanoparticles. Crown Copyright (C) 2004 Published by Elsevier Inc. All rights reserved.

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N(T) in irradiated silicon diodes. In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by gamma-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad. (C) 2004 Elsevier B.V. All rights reserved.

High frequency (95 GHz) ESR measurements have been performed down to 5 K, on three cubic boron nitride (c-BN) single crystals doped with beryllium. The measured samples exhibit at low temperature different ESR spectra, which are sensitive to low temperature in situ illumination using a series of Kr+- and Ar+-laser lines. The analysis of the ESR spectra resulted in the identification of several native paramagnetic centers responsible for the observed component spectra lines. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Impedance spectroscopy studies on polyaniline doped by various amounts of HCl are reported. The measurements were performed at room temperature, over a wide range of frequencies [1000 Hz to 45 MHz]. The experimental data shows that the resistance dominates the AC behavior of polyaniline confirming that the charge transport occurs through an one-dimensional hopping process.

Dopant yttrium at 150 p.p.m. level (Y to Al atomic ratio) in ultra -high purity polycrystalline alumina has been found to be, together with the coefficient of porosity, the reason of grain growth : with abnormal grain growth (AGG) or with uniform grains. The powder, green bodies and sintering samples were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and packing models. In this paper we studied by mathematical modeling the correlation between structure of powder and green bodies in correlation with the porosity in sintering compacts. A formula, which describes the packing of the spherical-cylindrical homogeneous particles, has been obtained. The porosity of the low Y doped alpha-Al2O3 was investigated.