Publications

A typical facility for fabrication of devices based on high temperature superconducting (HTS) films grown on the less-expensive CeO2-buffered sapphire substrates comprises a transfer chamber, a chamber for CeO2 growth, a chamber for YBa2Cu3Oy (YBCO) growth, and a chamber for Au deposition at room temperature. Here we present the adaptation of our previously-reported method to create artificial pinning centres in HTS films through self-assembled nano-scale islands for this type of facility without making any modifications. We succeeded to grow Au nano-dots from an ultra-thin layer of Au grown at room temperature subsequently annealed in the chamber for YBCO deposition, in the same conditions as for YBCO growth. AFM and RBS were used to investigate the formation of Au nano-dots, while XRD and susceptibility measurements were used to study the properties of nano-structured YBCO films.

Micro and nanorods of potassium acid phthalate (KAP) with diameters ranging from a hundred nanometers to tens of microns were obtained by solution growth in polycarbonate (PC) ion track templates. The ion track templates were produced by swift heavy ion irradiation of PC foils (e.g. U with 11.4 MeV/nucleon specific energy) and subsequently etching. After the growth the host templates were partially dissolved in dichlormethane and imagined by scanning electron microscopy. The template approach allows the fabrication of arrays of uniform nanostructures with a controlled morphology.

A new anisotropic size-broadening model based on a spherical-harmonics representation allowing determination of both volume-and area-averaged apparent crystallites and convenient for implementation into Rietveld programs is described. The model effectiveness is demonstrated on a ZnO powder pattern exhibiting strongly anisotropic size broadening and pronounced super-Lorentzian peak shapes. Moreover, it is shown how the apparent crystallites can be interpreted in terms of physical models by using ellipsoidal and cylindrical crystallites with lognormal size distributions. This interpretation is critically assessed and it is argued that both simplified physical models and a priori complementary information (obtained by transmission electron microscopy, for instance) are often needed to avoid unstable and non-unique solutions.

We investigate the output performance of the 1.06 mu m(4)F(3/2)-->I-4(11/2) transition in Nd:GdVO4 and Nd:YVO4 thin-disk lasers under multi-pass pumping with diode lasers at 0.81 mu m and at 0.88 mu m, which corresponds to direct in-band pumping of the F-4(3/2) emitting level. The use of a pump module with 24 passes through the crystal allowed the realization of an in-band pumped Nd:GdVO4 thin-disk laser with 14.9 W of continuous wave (cw) output power at 1.06 mu m; the overall optical-to-optical efficiency was 0.50 and the slope efficiency with respect to the incident pump power was 0.52. Intracavity frequency-doubling of the Nd:GdVO4 thin-disk laser with a LiB3O5 (LBO) nonlinear crystal yielded 9.1 W of cw output power in the green at 0.53 mu m with an overall optical-to-optical efficiency of 0.31.

Several hole trapped centres, namely Tl2+, self-trapped hole and A-type centres, were observed by ESR in PbBr2:Tl single crystals after X-ray irradiation at 77 K. The corresponding spectra are visible up to 200 K for all three defect centres. The STH centres are observed at higher temperatures than in the undoped PbBr2 single crystals, probably due to the stabilising effect of the Tl+ impurities. The ESR parameters of the Tl2+ centre point to stronger covalency effects in PbBr2 than in the isostructural PbCl2 crystals. All hole trapped centres started decaying around 160 K, probably due to recombination with nonparamagnetic electron trapped centres. Besides a small concentration of self-trapped electron centres, no other electron trapped centres were observed. (c) 2008 Elsevier B.V. All rights reserved.

The hyperfine interactions and dynamics aspects in nanoparticles system of the xSnO(2)-(1-x)alpha-Fe2O3 (x = 0.0-1.0) have been investigated by Sn-119-Mossbauer and XRD techniques. The change revealed by the XRD (XRDSs) and Mossbauer spectra (MSs) vs. x was discussed in terms of the phase and lattice-dynamics changes. The evidences of the triple hyperfine magnetic fields' distributions (MHFDs) and their dependencies x (< 0.175) are discussed taking into account the super-transferred hyperfine interactions (STHI), the local disorder Sn4+/Fe3+ around the tin isotope probe and the reduced dimension effect.

Single crystallites of superhard, semiconducting, n-type, amber colored and p-type, blue colored, Be-doped cubic boron nitride have been irradiated either with UV (350 nm) light or with an intense beam of accelerated (1 MeV) electrons. The examination of the irradiated samples at low temperatures by high frequency W (95 GHz)-band electron spin resonance reveals several new, radiation-induced, isotropic paramagnetic centers. The UV irradiation of both types of crystals yields centers involving very likely protons. In the amber c-BN crystals the irradiation with I MeV electrons results in the formation of vacancy associated paramagnetic defects and quasi-free electrons in colloidal particles. (C) 2008 Elsevier B.V. All rights reserved.

Angle-resolved X-ray Photoelectron Spectroscopy (ARXPS) has been performed on (100) Si surfaces in different conditions as regards the oxides nature - native oxides, thermally formed oxides, and also after a cleaning procedure by Ar+ ion sputtering (e.g. E= 2 keV). The most sensitive angle to surface oxidation states (Si1+, Si2+, Si3+, Si4+) was the take-off-angle (TOA): 25 degrees. Native oxides and thermally oxide phases on Si surfaces consists in a mixture of Si2O, SiO, Si2O3 and SiO2. The XPS measurements have putted into evidence a higher concentration of suboxides in high oxidation state (Si3+) on naturally oxidized Si substrates. The thermally oxidized Silicon surfaces contain suboxides in a low oxidation state (Si1+). The Ar+ ion sputtering removed rapidly the superficial silicon oxides. The analysis and computation of SiO2 film thickness on crystalline Silicon was done in the frame of modified Bethe equation for Electron inelastic mean free path (IMFP) and taking into account the intensity of SiO2 peak and Si peak together with the TOA in the XPS measurement. Native oxide thickness for SiO2 was d(oxy) similar to 24 angstrom and for thermally oxidized sample was d(oxy) similar to 27 angstrom. Due to the slightly difference in the nature oxidation states it is suggested that the silicon dioxide appeared from two different kinetics in the oxidation processes.

In this work we focus on the microstructural and magnetic characterization of CuNb/Nb3Sn wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.