Publications

For power applications of superconducting films. the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centres in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centres was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on another version of this approach proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices). Several materials have been used for the Creation of artificial pinning centres. Here we report on the artificial pinning centres induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Pd and non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films. We observed an important shift of the position of the maximum in the thickness dependence of J(c)(B) towards higher thicknesses compared with pure YBCO films by both approaches. A high J(c)(B) in our quite thick films provides a very high total critical current per cm of the film width. Critical current as high as 800 A/cm width was achieved in a 2.4 mu m thick quasi-superlattice film with non-superconducting YBCO nano-dots. (C) 2009 Elsevier B.V. All rights reserved.

We present the preparation, structural, and magnetic properties of a series of Fe-containing borosilicate glasses as a function of the ratio SiO2/Fe2O3 which is ranging from 1.49 to 2.68. The role of nucleators (Cr2O3 and P2O5) was also investigated. X-Ray diffraction has revealed the formation of magnetite as the major or unique crystalline phase. As SEM micrographs have revealed, the addition of P2O5 give rise to a finer structure as compared with Cr2O3. In addition, the same oxide decreases the temperature of structural transition with almost 100 degrees C. The magnetization data reveal a two step transition at low temperatures: a high temperature transition at T-v = 128 K, which we attribute to the Verwey transition, and a low temperature transition at T-s = 48 K which, most likely, is the result of change in the dynamic of the domain motion.

General aspects of the interactions of ions in silicon are investigated: energy lost in electronic and nuclear processes, as well as their consequences, creation of point and extended defects in the crystal structure. The correlations between irradiation ions, their energy, ionization and nuclear energy loss per atom, radius of interaction and spatial extension of the primary damaged region are also investigated.

Sr2FeTaO6 and Sr2Fe0.7Co0.3TaO6 double perovskites, unsupported and supported on gamma-Al2O3 were prepared, characterized and studied in the catalytic combustion of methane, as a test reaction for volatile organic compounds (VOCs) destruction. These mixed oxides, with double perovskite structure, present good catalytic activities, and after their dispersion - 5 % (wt.) - on gamma-Al2O3 support, the reaction rates increased compared with the unsupported samples.

The phase change materials are the most important materials in the class of chalcogenides (combination of chalcogens (S, Se and Te) with metalloids or metals). The outstanding property of these materials is the switching from a high electrical resistivity state to low electrical resistivity state and back under a moderate voltage. The thin film materials are used in computer memories, CD and DVD devices with performing speed and storage capacity. We have studied several thin solid films made of Ge-Sb-Te in order to assess the switching quality of different compositions. In order to systemize the whole class of chalcogenide phase change materials we have investigated the correlation between different crystallo-chemical parameters and the ionicity of the elements. Binary and ternary compounds are distributed into several distinct groups. The most favorable phase change materials are situated in a specific range of mean ionicity and mean glass formation ability. The results give the possibility to design new compositions with better switching properties.

We report a strong dependence of the angular magneto resistance (AMR) on the current density in Y0.47Pr0.53Ba2Cu3O7-delta single crystal above the critical temperature T-c = 13 K for any applied field up to 14 T. We estimated the current dependence from the angular dependence of the top resistance R-top, as measured on the face where the current is applied, and the bottom resistance R-bot as measured on the opposite face. At any temperature, both below and above T-c, R-top decreases as the field becomes parallel to the current and ab-plane with an angle dependence that suggests an important contribution arising from the vortex flow. R-bot evolves from a monotonic to nonmonotonic angle dependence with three minima and two maxima in the angle range 0 - 180 degrees as the temperature increases. For less Pr-doped samples, Y0.58Pr0.42Ba2Cu3O7-delta (T-c = 39 K) and Y0.68Pr0.32Ba2Cu3O7-delta (T-c = 55 K), where the interplane resistivity is much lower, both R-top and R-bot follow the same monotonic angle dependence in all temperature and field range.

TiO2:Fe thin films prepared by pulsed laser deposition exhibit in some case light dependent saturation magnetization, as determined from Kerr magnetometry measurements performed in dark or by illuminating the sample. This phenomenon is Studied in correlation with local atomic Structure investigated by extended X-ray absorption fine structure, composition and chemical state analyzed by X-ray photoelectron spectroscopy and by Xray absorption near-edge structure. It is found that light-controllable magnetism is a property of a mixture of Fe and oxidized Fe clusters embedded in the anatase TiO2 matrix.

The existence of Strange Quark Matter as a new stable or metastable form of existence of matter has been postulated by various authors quite a few years ago. Because until now a theory to predict the existence and the properties of these strangelets does not exist, it is absolutely necessary to investigate the peculiarities of their interactions with matter versus ordinary nuclear matter interactions. In the present contribution, some peculiarities of the ionization and nuclear energy loss of strangelets with matter are investigated in comparison with ordinary ions, as a useful tool for their search. The rescattering mechanism could produce sub-threshold reactions and thus it is suggested as a possibility to discriminate between ordinary isotopes and very heavy (candidates for) strangelets with the same charge number.

Nanostructured intrinsic and doped titania materials have long been subject to both basic and applied research, in connection with environment and energy related applications. Apart from beneficial effects. such as widening the wavelength range for surface photocatalytic activation, doping TiO2 materials with 3d ferromagnetic cations is a main route to develop ail important class of diluted semiconductors, with potential applications in spintronics. In all the previous investigations the concentration of the ferromagnetic dopant was limited to a few percents. Here, we initiate ail extended investigation of Ti1-xFexO2 within a, wider iron composition range (x = 0 - 0.55) and discuss the magnetic and optical properties of these materials in thin films in relation with their elemental composition, nano-structure and local atomic ordering.

In this paper the problem of designing simple low-cost planar microwave band-pass filters with strongly asymmetric characteristics, filters that can very efficiently attenuate an adjacent channel, is investigated. For a fourth-order filter, the usual quadruplet structure leads to two transmission zeros, located one on each side of the passband For one of the two adjacent channels, a higher attenuation can be obtained by designing filters with two transmission zeros with prescribed positions in the same stopband (upper or lower). The new topology investigated in this paper offers the possibility of designing such quadruplets with two transmission zeroes situated both on the same side of the passband, in prescribed and controlled positions. Filters with this novel structure can provide very high attenuation not only at a single frequency, but in a larger adjacent frequency band.