Publications

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of TI-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the, actiation energy for the flux jumps, which resulted to. be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T-c dc SQUID's.

We report vortex imaging and local magnetometry studies near the edge of a YBCO thin film using scanning Hall probe microscopy (SHPM). Our data show some unusual features, namely local hysteresis inversion and negative remanent fields, which are semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinite-long thin superconducting strip.

Epitaxial Tl2Ba2CaCu2Oy and Tl(Ba,Sr)(2)Ca-2 Cu3Oy thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the TI-2212 films showed well connected and smaller sized grains of 1 mum or less. In the case of TI-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm(2) at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm x 10 mm. The temperature dependence of surface resistance of TI-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the TI-1223 exhibits slightly higher values resulting from an inferior surface morphology.

A data acquisition system is reported with a number of improvements over similar systems currently used in Rayleigh scattering of Mossbauer radiation (RSMR) experiments. It is based on a data acquisition board including a Mossbauer function generator with adjustable frequency reference signal delivery, an internal single channel analyzer, and an internal 32 kB random access memory for data storage. The system performances were tested in RSMR measurements on pyrolytic graphite C (002) single crystal by using an iron enriched Fe-57 ( 30 at.%) in rhodium matrix absorber. We want to demonstrate that the reported data acquisition system is suitable for the experimental separation of the elastic and inelastic components of the scattered radiation. The system can be used also in the ordinary Mossbauer measurements as a compact and rapid data acquisition system. In this case it presents the advantage of including also the function generator. (C) 2003 American Institute of Physics.

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.