Publications

Amorphous thin films of langasite and YAG have been prepared from crystals targets by pulsed laser deposition, at room temperature, in vacuum, on silicon wafers. Bulges of micrometer size are formed on the film surface. Larger size bulges (10-50 micrometers in diameter) are characteristic to the annealed langasite films. Annealing at high temperatures leads to the crystallization of the films. The annealed langasite films (850 degrees C) are polycrystalline and partially oriented with the plane (001) parallel to the surface of the silicon wafer. The bulges break easily in the heat treated films. Their empty structure was remarked on the electron microscope images. The YAG films annealed at 1100 degrees C shows a crystalline YAG phase depleted in Y and traces of Y2O3. An interesting feature is the presence of silver particles spread along the macrodefects (fracture lines) of the film.

Surfaces of GaN films have been investigated by Atomic Force Microscopy (AFM) with implemented Piezoelectric Force Microscopy (PFM) technique. The GaN layers were grown by molecular beam epitaxy (MBE) on Si(111) substrates with an AlN buffer layer. Simultaneous imaging of surface morphology in the AIM and PFM response was performed. The PFM imaging is sensitively dependent on measurement conditions. By increasing the ac voltage the contrast of the PFM images is increased. Using the Voltage-Modulated Force Microscopy (VMFM) regime, with the frequency close to the first resonant frequency of the piezoelectric signal, PFM images with high contrast and high resolution on the nanometer scale were obtained. Regions of opposite piezoresponse were observed and explained by the presence of inversion domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of thermal neutrons irradiation at different fluencies on the superconductivity properties of Li (2 at. %) doped YBa2Cu3O7-delta system has been investigated by direct microwave absorption method. We have noticed changes of the superconducting properties of the 123 phase that depend on the neutron fluency. By increasing fluency, initially, the phase structure supports a disordering, then a re-ordering accompanied by a significant increase of the average critical temperature T-c to 85.7 K and finally we observed a degradation of the 123 the phase accompanied by a diminishing of T-c. The intragranular dephasing magnetic field B-d, i.e. the direct microwave absorption (DMA) parameter associated with the magnetic field dependence of the critical current density exhibited an opposite tendency, initially decreasing and then increasing at a value close to those of unirradiated samples. We also noticed only on the irradiated samples the presence of a flatness of the DMA curves around 0.5 T, consistent with the expression of a peak effect.

Soft ferrites in the CuZnTi system, having the formula Cu1-xZnxTiyFe2-4O4, with 0.50 <= x <= 0.60 and 0.00 <= y <= 0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with controlled T-C and very high change rate of permeability with temperature around their T-C. The effect of Zn and Ti additions was a drastic change of the Curie temperature with about 10-12 degrees C for each atomic percent of Zn and Ti introduced into the spinel lattice while the cooling speed changed the behaviour of magnetic permeability with temperature around the Curie point. An application using such magnetic temperature sensors for a thermostat is presented. (c) 2004 Elsevier B.V. All rights reserved.

The electronic structure of LaNiSn and NdNiSn compounds and their hydrides has been studied by first principles calculations and variable temperature Sn-119 Mossbauer spectroscopy and the nature of the hydrogen-metal bond is discussed. The analysis of the electronic density of states (DOS) in both compounds before and after hydrogenation indicates an hybridization of the Sn, Ni, and H orbitals. The partial Sn-p DOS of LaNiSnH2 gives evidence for a lower symmetry of electron density around tin atoms compared to LaNiSn, according to the larger quadrupole splitting in the corresponding Mossbauer spectrum. Theoretical and experimental Mossbauer parameters agree very well for all samples.

Antiferromagnetic (AF) FeSn2(001) epitaxial and polycrystalline layers were grown in ultrahigh vacuum under various conditions on clean InSb(001) substrates and covered by polycrystalline ferromagnetic Fe layers, forming a new system with exchange bias. Isotopically enriched Fe-57- and (FeSn2)-Fe-57-tracer layers were placed on either side of the Fe/FeSn2 interface for a microscopic investigation of the spin structure and atomic interdiffusion phenomena in near-interfacial regions by Fe-57 conversion electron Mossbauer spectroscopy (CEMS) at room temperature and T=10 K. Several spectral components, assigned to pure bcc Fe, Sn-containing bcc Fe, AF-ordered FeSn2, and paramagnetic FeSn2 were resolved in the CEM spectra. Evidence is provided for interdiffusion across the interface. The temperature dependence of the exchange-bias field H-E was measured by magnetometry. The CEMS data provide evidence for a correlation between H-E at low T and chemical disorder (defects) in the FeSn2 films via the intensity of the paramagnetic line. These results support the assumption that exchange bias is related to the presence of AF domains formed via magnetic defects in the antiferromagnet. The T dependence of H-E suggests interfacial exchange-coupling energies higher than the AF wall energy. (C) 2005 American Institute of Physics.

Scanning Hall probe microscopy has been used to study flux Structures and dynamics in 5 mu m x 5 mu m YBCO thin film squares, which are mesoscopic with respect to the magnetic penetration depth, lambda(T), at temperatures close to T-C. A number of unusual vortex phenomena are observed in these microstructures which differ qualitatively from the expected behaviour of more macroscopic pieces of film. In field-cooled (FC) experiments to similar to 65 K a full Meissner state is generated for cooling fields less than similar to 6 Oe, reflecting the relatively small demagnetization factors in our samples. Cooling in higher fields, however, results in only a very weak diamagnetic response at low temperatures whose magnitude is almost independent of the cooling field. In contrast we observe considerable trapped flux upon field-removal whose magnitude grows monotonically with cooling field. Remarkably, all FC flux distributions exhibit almost perfect rotational symmetry, and can be nearly completely cancelled in a reversible fashion by tuning the field applied to the initially FC state. Our field-cooled and zero-field-cooled results have been analysed in terms of a Bean-like critical state model containing constant edge and bulk current densities, and most of the observed phenomena can be explained by considering the relative weight of these two components. Not all flux profiles call be described by our simple model, however, and under certain circumstances symmetry-breaking 'dipole'-like flux structures can form in several adjacent YBCO squares. We speculate that these are related to the unidirectional At-ion milling process which was used to pattern the squares and could have broken the expected four-fold symmetry. We note that our results could have important implications for the miniaturization of thin film HTS devices.

While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.

Recently we have found superconductivity in a cubic antiperovskite-like compound Li2BPd3. A new pseudo-binary complete solid solution Li2B(Pd1-xPtx)(3), x = 0-1 with similar structure has been synthesized and observation of superconductivity in the entire x-range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T, is decreasing approximately linearly with amount (x) of Pt from 7.2-8 K for Li2BPd3 to 2.2-2.8 K for Li2BPt3. From isothermal magnetization (M-H) measurements, lower critical fields H-cl (138Oe/(x=0), 38Oe/(x=1)), upper critical fields H-c2(WHH) (3.4 T/(x=0), 1 T/(x=1)), coherence length xi(0) (9.8 nm/(x=0), 17.9 nm/(x=1)) and penetration depth lambda(0) (190nm/(x=0), 364nm(x=1)) were estimated and shown to follow approximately linear dependencies with.x, either. Structure and superconducting similarities with MgCNi3, viewed as a bridge between low and high T-C superconductors are increasing the expectations that Li2B(Pd1-xPtx)(3), x = 0-1 superconductor can be included in the same class of `intermediate' superconductors. For x = 0-1 a weak fishtail effect was observed at low and intermediate fields. Apart from this effect, some samples for x = 1 have shown magnetization jumps at fields close to H-c2.