E. Mezzetti

CONTROL OF THE VORTEX MOVEMENT AND ARRANGEMENT BY OUT-OF-PLANE MAGNETIC STRUCTURES IN TWINNED YBa2Cu3O7−x/La0.67Sr0.33MnO3 BILAYER

In this article, we consider the magnetic interaction exerted on vortices in a thin YBa2Cu3O7−x film by a La0.67Sr0.33MnO3 layer. The magnetic coupling of the bilayer system was studied and locally imaged by means of magneto-optics. Twin boundaries in the LaAlO3 substrate cause a clear splitting of the manganite magnetic domains with well-defined in-plane magnetization separated by pinned out-of-plane magnetic structures. The vortices nucleated by the external magnetic field interact with the out-of-plane magnetic moments depending on their local structure and magnetic polarization. Different blocking mechanisms (sink or reservoir) are found for the vortex motion perpendicular to twin boundaries, whereas for vortices moving parallel to the out-of-plane magnetic structures either blocking or channelling effect is observed.

Nanosize texture and transport properties in Y-Ba-Cu-O films

The magnetic-transport properties of thin YBCO films are accounted for by a network of intergranular hidden Josephson junctions. An accurate X-ray analysis reveals the existence of structural patterns with the same distribution function as the one found by fitting the J/sub c/ vs. B curves. The correspondence between the two distributions enables us to establish that the transport mechanism in those films is driven by the nanosize structural pattern behaving as an intergranular hidden array of Josephson junctions. It allows to find out for the first time how the J/sub c/ vs. B curves scale with temperature. The full analysis enables to determine the main parameters to modify in order to exploit the intrinsic properties of this network towards applications.

On the influence of neutron induced point-defects on the transport properties of ceramic YBa2Cu3O7-y

The influence of the neutron induced point defects on the transport properties of ceramic YBa2Cu3O7-δ has been investigated. Pellets of sintered YBa2Cu3O7-δ with three different oxygen contents have been irradiated with fast neutron beams of energies 6.5, 3.3 and 4.4 MeV at fluence of 7.7·104, 1.3·+109 and 1.4·109 n/cm2 respectively. The radiation damage has been investigated by comparing the critical temperature (Tc mid), the zero-resistivity and the onset temperature, before and after neutron irradiation. The critical current as a function of the temperature has been measured for some samples in the same experimental conditions. We argue the existence of two networks of defects, not so tightly connected with intra-grain or inter-grain defects. The first one should be an ordered network of defects and mainly affects the Tc mid temperature; the second one, disordered, affects also the room temperature resistance, the amplitude of the transition and the critical current density

AN IRRADIATION FACILITY FOR IN-VACUUM HEAVY-ION IRRADIATION OF LARGE AREA SUPERCONDUCTORS

An irradiation facility to irradiate long stripe-shaped high-Tc superconductors by means of GeV heavy ions is described. This facility was designed and installed at INFN - Laboratori Nazionali del Sud (LNS), Catania, Italy. It consists into a moving apparatus automatically driven at a given velocity in order to obtain the in-vacuum exposure of contiguous zones of the moving target at a programmed ion fluence. The whole apparatus is automatically controlled by a home-made remote control program. The main parts of the apparatus are the mechanical set-up and the electronic circuit to create feedback signals to control both the beam current and the velocity of the sample holder, the high accuracy system for the measure of the integrated charge and the beam calibration set-up (beam shape and homogeneity). This apparatus is, at our best knowledge, the first one especially designed to create nanostructures across large area superconducting devices by means of the irradiation. Some significant results of 4.2 ...

Quantitative magneto-optical analysis of superconductors

A magneto-optical equipment has been set up and carefully calibrated. In this paper, we shortly review the main characteristic of this equipment, then we focus on the analysis to be performed in order to extract from magneto-optical images the local values of magnetic induction and electrical current density along the superconducting sample. This implies the numerical inversion of the Biot-Savart law. A detailed analysis modeling the interaction of the spontaneous magnetization across the indicator film with the magnetic fields in the specimen region has been performed. In particular, we take into account the generally disregarded interaction of the indicator film with in-plane magnetic fields, i.e. with the component parallel to the indicator surface. A set of experimental data on YBCO films is presents to show the actual impact of the new analysis method, for what concerns both the local values of the magnetic induction and the electrical current density evaluation. A critical comparison of the new obta...

EXPERIMENTAL INVESTIGATIONS ON THE CRITICAL VORTEX DYNAMICS WITH CONTROLLED DISORDER

The possibility to enhance in a predictable way the superconducting properties behavior of a bulk material by introducing linearly correlated defects within a surface layer has been demonstrated. Ag/BSCCO-2223 tapes were irradiated by means of 0.25 GeV Au ions at different fluences, with a dose equivalent field Bϕ ranging from 1 T to 5 T. The implantation depth was about 15 μm corresponding to about 15% of the total sample thickness. Sample independent enhancements of different a.c and d.c properties show up, due to vortex confinement in the whole sample. The sample independence exhibited by all the experimental findings as well as, on the other side, the dose dependence of the enhancements, allow to design in advance the performances of a monofilamentary tape where defect trenches of nanometric size are scratched.

Long Josephson Junctions Modulated by Defects and Superconducting Microdomains in YBCO Thin Films

The critical current Jc of superconducting films is about two order of magnitude larger than that of single crystals. The dependence of such Jc on field depends on the presence and distribution of defects. We speculate that High Temperature Superconducting (HTS) films can be modeled as an agglomeration of islands coupled via Josephson contacts and then considered as a network of Long Josephson Junctions (LJJ) between superconducting islands. In order to check this hypothesis we further developed a model firstly introduced by Fehrenbacher based on the modulation of critical current densities in LJJ by means of suitable distribution of defects. The defects act by modulating the coupling energy of the junction. Critical current densities are calculated in the case of periodic and random defect distributions.

Pinning and confinement of vortices: Random disorder and correlated disorder

SummaryWe compare temperature and magnetic field dependence of the critical current density of melt textured YBCO, irradiated with monoenergetic 3.5 and 6.5 MeV protons, and with 235 MeV Au ions, respectively. The Au irradiation affected only a surface layer on the top and on the bottom of our bulk samples,i.e. a volume fraction ranging, in different specimens, from 5% to 13% of the total volume. The point-like defects producedJc enhancements decreasing with the temperature up to a (B, T) point where a crossover to damage occurs. On the contrary, surface columnar defects allow to control the vortex dynamics in the full sample, so that the critical current density enhancement increases as the temperature and the magnetic-field increase. While the addition of point-like defects does not significantly modify the irreversibility line (IL), the ILs of the Au irradiated samples present a shift towards higher temperature as well as smooth matching effects near the dose equivalent field. Magnetic relaxation rates andJc measurements give results in accordance with recent theoretical findings.

Defect Network in Superconducting Ceramic Oxides Studied by Neutron and Proton Irradiation

This paper investigates both transport properties and nature of superconductivity breakdown or, conversely, enhancement in oxide ceramics, due to radiation-induced defects. Low-fluence neutrons (≈107 n cm-2 at 3 MeV) can sensitively damage the samples, giving experimental evidence that the breakdown of coherent percolating paths produces decoupled domains. A set of preliminary measurements shows that high-fluence proton implantation can either damage or enhance critical current density in a currently non controllable way. In both cases strongly damaged or enhanced superconducting paths short-circuit the unaffected bulk network

Conductivity Dependence on the Hydrogen Concentration and Thickness of α-Si: H Films

The connection between the evolution of the clustered morphology in disordered semiconductor alloys and the change of their electronic properties is discussed. Starting from a comparison between conductivities as calculated in recent papers we suggest that the transport behaviour of silicon alloys can be related to characterisitc lengths on macroscopic and microscopic scale respectively, and to a suitable morphological factor. The experimental behaviour of conductivity of silicon alloys versus hydrogen or carbon content is explained in terms of the basic arguments of the theory. Preliminary, relative values of the introduced parameters are reported.