Limits to the critical current in Bi2Sr2Ca2Cu3Ox tape conductors: The parallel path model

An extensive overview of a model that describes current flow and dissipation in high-quality Bi2Sr2Ca2Cu3Ox superconducting tapes is provided. The parallel path model is based on a superconducting current running in two distinct parallel paths. One of the current paths is formed by grains that are connected at angles below 4°. Dissipation in this strongly linked backbone occurs within the grains and is well described by classical flux-creep theory. The other current path, the weakly linked network, is formed by superconducting grains that are connected at intermediate angles (4°–8°) where dissipation occurs at the grain boundaries. However, grain boundary dissipation in this weakly linked current path does not occur through Josephson weak links, but just as in the strongly linked backbone, is well described by classical flux creep. The results of several experiments on Bi2Sr2Ca2Cu3Ox tapes and single-grained powders that strongly support the parallel path model are presented. The critical current density of Bi2Sr2Ca2Cu3Ox tapes can be scaled as a function of magnetic field angle over the temperature range from 15 K to 77 K. Expressions based on classical flux creep are introduced to describe the dependence of the critical current density of Bi2Sr2Ca2Cu3Ox tapes on the magnetic field and temperature

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly

We report on three different and complementary measurements, namely magnetisation measurements, positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin.Comment: 10 pages, 4 figures, submitted to PR

Magnetic characterization of sintered MgB2 samples: effect of the substitution or doping with Li, Al and Si

Powdered and sintered MgB2 samples have been characterized through magnetic measurements performed from T = 5 K up to few degrees above the transition temperature of about 39 K. We found that the sintered samples behave as well-connected bodies, showing no trace of granularity. In order to obtain the critical current density value Jc the Critical State Model has been therefore employed in a straightforward way. With the aim either to decrease the electron mean free path or to increase its Jc we have attempted to introduce defects in the MgB2 structure by different procedures: substitution of Lithium on the Magnesium site and doping of the precursor Boron powders with Aluminum and Silicon. The best result in terms of Jc has been achieved by Silicon doping that, moreover, does not significantly affect the transition temperature.Comment: 10 pages, 6 figures, 1 tabl

Growth of c-oriented MgB2 thin films by Pulsed Laser Deposition: structural characterization and electronic anisotropy

MgB2 thin films were deposited using Pulsed Laser Deposition (PLD) and ex-situ annealing in Mg atmosphere. The films presented critical temperatures up to 36K and turned out to be preferentially c-oriented both on Al2O3 (r-cut) and MgO(100) substrates. Synchrotron analyses gave also some indications of in plane texturing. The films exhibit very fine grain size (1200angstromin the basal plane and 100angstrom along c-axis) but the general resistivity behavior and the remarkable extension of the irreversible region confirm that the grains boundaries are not barriers for supercurrents. Upper critical field measurements with the magnetic field perpendicular and parallel with respect to the film surface evidenced a field anisotropy ratio of 1.8. The Hc2 values are considerably higher with respect to the bulk ones, namely when the field lies in the basal plane, and the field-temperature phase diagram for the two magnetic field orientations suggest the possibility of strongly enhancing the pinning region by means of texturing.Comment: 20 pages, 6 figure

Lateral critical current distribution and self-field profile of Bi-2223/Ag conductors: measurements and calculations \ud

The lateral current distribution and the magnetic self-field induced by a transport current were measured independently in three multifilament Bi-2223/Ag tapes with qualitatively different filament layouts. The current and field data from these two experiments, magnetic knife and scanning Hall probe, are compared with each other after straightforward 'forward' or 'inverse' current-to-field calculations. Both datasets show the critical current to be maximal in the central part of the tapes, while the comparison demonstrates that the drop of the critical current distribution at the tape edges is an intrinsic material property and is not caused by the tape's magnetic self-field. Small differences between the datasets reveal the effect of filament bridgin

Properties of doped ex and in situ MgB2 multifilament superconductors

7 pages, 11 figures, 1 table.Four-filament ex and in situ MgB2 wires were prepared with the rectangular wire-in-tube (RWIT) technique. Based on experience with single-core wires, 10 wt% of W was added to the ex situ and 10 wt% of SiC to the in situ powders, which were packed into Fe and Nb/AgMg tubes, respectively, and two-axially rolled into composite conductors. The ex and in situ conductors are compared in terms of field-dependent transport critical current density, effects of filament size reduction and twisting, mechanical behaviour and thermal stability.This work was supported by the Science and Technology Assistance Agency under the contract number APVT-51-029902, and by the EU FP6 contract number NMP3-CT2004-505724. Authors would like to thank T Holúbek for the I–V characteristic measurements.Peer reviewe

Quantum interference and confinement phenomena in mesoscopic superconducting systems

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic Al samples of different topology: lines, open and filld squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j(c), has been achieved. Sharp integer and rational matching peaks in the j(c)(H) curve are observed. The possibility of the ''quantum design'' of the superconducting critical parameters (H-c(T) and j(c)(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated