Publications

In this work we investigate the surface morphology and stoichiometry of the ablated targets employed with pulsed laser deposition (PLD) of thin films of the ferromagnetic half-Heusler alloy NiMnSb. The NiMnSb targets were: a pressed and sintered pellet and, respectively, polycrystalline discs grown by RF melting. The films were deposited using a KrF excimer laser (lambda = 248 nm, tau = 30 ns, 600 mJ/pulse maximum) delivering pulses at a repetition rate of 10 Hz. SEM observations show different morphologies of the ablated surfaces in the two cases, which are further correlated with EDX analyses to explore surface segregation phenomena. The corresponding films are also considered. Apparently, the crystalline targets keep better the initial stoichiometry than the pressed ones, which in principle would indicate higher film stoichiometry and hence enhanced quality. (C) 2003 Elsevier Science B.V. All rights reserved.

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra-and inter-grain critical current density J(c) of (Cu,C): 1234 high-T-c superconductors, in the frame of critical state models. The inter-grain J(c) was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J(c) resulted from AC susceptibility measurements and of the impressive increase of intra-grain J(c) due to heavy-ion and neutron irradiation are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

A technique to reduce the size of planar resonators is investigated using a transmission line model and it finite-difference time-domain (FDTD) method. New miniaturized square microstrip resonators are proposed for 900 MHz band. T he resonators offer low impedance sections leading to a better handling of power. The feasibility of compact filters using the proposed resonators is demonstrated. The input/output coupling lines of a two-pole. filter have been positioned in a way that produces a filter response with transmission zeros on the both sides of the pass-band. Sharper skirts and higher out-of-band rejection are obtained for four-pole filters with cross-coupled resonators.

Thin films of infinite-layer compound Sr1-xCaxCuO2-delta have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed (T-c (onset) = 42 and 50 K, T-c(rho=0) = 11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.

In the present paper. the phenomenological model developed by the authors in previous papers has been used to evaluate the degradation induced by hadron irradiation at future accelerator facilities or by cosmic protons in high resistivity silicon detectors. The damage has been analysed at the microscopic (defects production and their evolution toward equilibrium) and at the macroscopic level (changes in the leakage current of the p-n junction). The rates of production of primary defects. its well as their evolution toward equilibrium have been evaluated considering explicitly the type of projectile particle and its energy. Vacancy-interstitial annihilation. interstitial migration to sink. divacancy and complex (VP, VO, V2O, CiOi, CiCs) formation are taken into account for differently doped initial silicon material. The influence of these defects on the leakage current density has been compared with experimental data from the literature, and predictions for the LHC (Large Hadron Collider) radiation fields. as well as for space missions in the near Earth orbits have been done. in the frame of the Schokley-Read-Hall model.

Transmission electron microscopy-selected area electron diffraction studies were performed on as-pyrolyzed sol-gel-derived Pb(Zr1-xTix)O-3 films deposited on Pt electrode layers to elucidate the structural cause behind the large effects of pyrolysis conditions on orientation selection [L. F, G.J. Norga, H.E. Maes, and G. Maes, J. Mater. Res.. 16, 2499 (2001)]. The crystallinity of the intermediate pyrochlore phase, which forms during pyrolysis, was found to strongly depend oil pyrolysis conditions. Specifically, pyrolysis for 10 s at 350 degreesC was seen to result in conversion of the film to a well-crystallized, oxygen-deficient pyrochlore phase with the fluorite crystal structure (disordered pyrochlore). We speculate that formation of the metastable fluorite phase is favored by the reduced oxygen partial pressure, caused by burnoff of residual acetates. Longer pyrolysis times and/or higher pyrolysis temperatures result in a quasi-amorphous intermediate phase. The presence of the well-crystallized fluorite phase in the pyrolyzed film is seen to result in (111)-oriented films after crystallization, while pyrolyzed films consisting of a quasi-amorphous intermediate phase turn out (100) or mixed (100)/(111) oriented after crystallization. An explanation for the observed orientation effects, on the basis of the different surface energetics of fluorite versus perovskite structure oxides, is proposed.

Tl2Ba2CaCu2Oy (Tl-2212) and Tl(Ba,Sr)(2)Ca2Cu3Oy (Tl-1223) superconductor thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr2CaCu2Oy (TlSr-1212) was deposited on top Of CeO2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures (T-c) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density (J(c)) at 77 K measured on 5 x 5 mm. films by inductive technique is as high as 2 MA/cm(2). Surface resistance R-s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films. (C) 2003 Elsevier Science B.V. All rights reserved.

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.

Samples of (Cu0.6C0.4)Ba2Ca3Cu4Oy ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes' the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba2Can-1CunOy series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T-c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J(c) has shown a fish-tail effect. (C) 2003 Elsevier Science B.V. All rights reserved.