Roberto Gerbaldo

Effective gap at microwave frequencies in MgB2 thin films with strong interband scattering

The microwave properties of polycrystalline MgB2 thin films prepared by the so-called in situ method are investigated. The characterization of the films at microwave frequencies was obtained by a coplanar resonator technique. The analysis of the experimental data results in the determination of penetration depth, surface impedance, and complex conductivity. The aim of this work is to set the experimental results in a consistent framework, involving the two-band model in the presence of impurity scattering. The energy gaps are calculated and the contribution of intra- and interband scattering is considered. From the comparison between the calculated gap values and the experimental data it turns out that the temperature dependence of the penetration depth can be accounted for by an effective mean energy gap, in agreement with the predictions of V. G. Kogan and N. V. Zhelezina Phys. Rev. B 69, 132506 2004. On the other hand, the temperature dependence of the real part of the microwave conductivity and of the surface resistance is accounted for by the single smaller gap, in agreement with the work of B. B. Jin, T. Dahm, A. I. Gubin, E.-M. Choi, H. J. Kim, S.-I. Lee, W. N. Kang, and N. Klein Phys. Rev. Lett. 91, 127006 2003. Since these findings rely on the same calculated gap structure, the required consistency is fulfilled

Effects of surface heavy ion implantation on superconducting melt-textured YBa2Cu3O7−δ

The possibility of predictably enhancing the critical current density of a bulk material with a strong uncorrelated disorder by introducing linearly correlated defects into just the top and bottom surface layers has been investigated. The task was fully demonstrated for melt-textured YBa2Cu3O7−δ having irradiation modified percent thickness ranging from 13% to 5%. The characteristic signatures of vortex localization were observed in the higher range of temperature and magnetic field.

Evidence of rf-driven dendritic vortex avalanches in MgB2 microwave resonators

The influence of dendritic magnetic-flux penetration on the microwave response of superconducting magnesium diboride (MgB2) films is investigated by a coplanar resonator technique. The characteristic feature consists of abrupt jumps in the resonance curves, which are shown to be induced by vortex avalanches freezing magnetic flux inside the resonator. Flux shaking provided by radio frequency currents is the origin of such avalanches, since a mechanism for magnetic instability of the dendrite type is local heating produced from flux motion. Once the external magnetic field has been changed, the interaction between microwave currents and flux lines influences the vortex-relaxation process. As a consequence, the vortex system is maintained close to a nonequilibrium state during the whole frequency sweep (few seconds), resulting in the observation of several events of dendrite nucleation and branching. From the analysis of a robust statistics, it results that avalanche-size distributions before and after flux...

Effects of surface heavy ion implantation on Ag/BSCCO-2223 tapes

Abstract In this paper the effects of surface 0.25 GeV Au ion implantation on the superconductive properties of Ag/BSCCO-2223 tapes are studied. Columnar-defect length was ~5% of the whole sample thickness. Strong localisation phenomena and the occurrence of the localisation characteristic kink in the irreversibility line, IL, are observed at a point phase (Bkink, Tkink). Large shift of this line towards higher fields and temperatures and a very substantial enhancement of the critical current density, Jc, have been found. IL anisotropy and Jc anisotropy of the irradiated samples are switched off up to fields higher than Bkink. The characteristic trends, as well as their dose dependencies, are particular of such pinning strategy. The results give useful hints for further improvement of the Bi-2223 tape technology, concerning the introduction and influence of appropriate surface pinning.

Enhanced critical current density in melt textured YBCO via Au ion surface irradiation

We irradiated both side of bulk melt textured YBCO with 235 MeV Au ions. The thickness of the specimens is significantly larger than the penetration range of the ions so that two surface layers of columnar defects are created. Critical current density enhancements were obtained in temperature and field ranges that are useful for applications. The boundary between reversible and irreversible regime is shifted toward higher temperatures. At higher fields the increase of the relaxation rate with temperature is less steep after irradiation. The general trends are independent of the thickness of the samples, at least in the range from 5% to 13% of volume affected by irradiation.

Critical state analysis in bulk MgB2 by means of a quantitative magneto-optical technique

Abstract This paper is aimed at investigating the superconducting properties of the MgB2 new compound (bulk) with particular emphasis on the correlation and/or competition between microstructural and magnetic granularity. Standard magnetic characterizations are reported in order to provide background information about the material under study at higher magnetic fields. At low fields, two samples of different sizes are analysed with the magneto-optical (MO) technique. The central investigation method is an extended quantitative MO analysis coupled with microstructural analysis. The main results consist in finding out that, however large the sample, and notwithstanding the structural granularity, at low fields the magnetic flux penetrates in an apparent single-crystal-like fashion, if macroscopic cracks are not present. The local current distribution inside the small sample is calculated as a function of field at different temperatures by means of an inverted two-dimensional Biot-Savart model. From the results it emerges that the main open question in MgB2 consists in understanding the nature of the vortices entering the grain boundaries as well as investigating whether these grain boundaries can also act as pinning centres. Some hints on this subject are preliminarily provided by the reported experimental results.

Nanostructuring superconductors by ion beams: A path towards materials engineering

The paper deals with nanostructuring of superconducting materials by means of swift heavy ion beams. The aim is to modify their structural, optical and electromagnetic properties in a controlled way, to provide possibility of making them functional for specific applications. Results are presented concerning flux pinning effects (implantation of columnar defects with nanosize cross section to enhance critical currents and irreversibility fields), confined flux-flow and vortex guidance, design of devices by locally tailoring the superconducting material properties, analysis of disorder-induced effects in multi-band superconductors. These studies were carried out on different kinds of superconducting samples, from single crystals to thin films, from superconducting oxides to magnesium diboride, to recently discovered iron-based superconductors.

MgB2 Thin Films for Radiation Detectors Operating at Microwave Frequencies

The aim of this work is to investigate the potential of superconducting magnesium diboride (MgB2) thin films, prepared by the in‐situ method, as broadband radiation detectors based on the microwave measurement of the complex impedance. The characterization of the films at microwave frequencies has been obtained by a coplanar resonator technique. The data analysis results in the determination of penetration depth, surface impedance and energy gap. In view of their microwave properties, particularly of the gap structure, the possible use of such MgB2 films as photon detectors is discussed in detail.

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.

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 ...