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We report the magnetic properties of LaFe2As2, La0.8Ba0.2Fe2As2, and La1-xBaxPt0.1Fe1.9As2 (0

Although vortex pinning in laser-ablated YBa2Cu3O7−δ films on (100) SrTiO3 is dominated by threading dislocations (Dam B et al (1999) Nature 399 439), many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, ndisl. While a universal magnetic field B dependence of the transport current density js(B, T) is observed (independently of ndisl, temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to js(B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that js(B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers ndisl, it appears that linear defects stabilize the vortex lattice by means of the vortex–vortex interaction.

Over the past ten years the perception of grain boundaries in YBa2Cu3O7-δ conductors has changed greatly. They are no longer a problem to be eliminated but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high angle boundaries. The time is therefore right for a targetted review of research into low-angle YBa2Cu3O7-δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa2Cu3O7-δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (Rev. Mod. Phys., 74, 485, 2002). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa2Cu3O7-δ coated conductors both individually and as a collective system.

Most measurements of critical current densities in YBa2Cu3O7-delta thin films to date have been performed on films where the c axis is grown normal to the film surface. With such films, the analysis of the dependence of the critical current, j(c), on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of j(c), such as the measurement geometry and disposition of pinning centers, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed flux line lattice structure models on the basis of angular critical current density measurements on c-axis films. Films grown on miscut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of j(c) in YBa2Cu3O7-delta are reviewed. Measurements on YBa2Cu3O7-delta thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the flux line lattice in YBa2Cu3O7-delta. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation, and temperature. As a consequence, a simple scaling law cannot, by itself, describe the observed critical current anisotropy in YBa2Cu3O7-delta. The experimentally obtained j(c)(theta) behavior of YBa2Cu3O7-delta is successfully described in terms of a kinked vortex structure for fields applied near parallel to the a-b planes.

Large increases in the upper critical field B-C2(0) are reported in bulk superconductors that demonstrate another novel property for nanocrystalline materials. Disordered nanocrystalline PbMo6S8 superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional PbMo6S8 has B-C2(0)similar to50 T. The nanocrystalline materials have higher resistivity (rho(N)) and B-C2(0)similar to100 T. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases rho(N) and, hence, B-C2(0) without significantly reducing the electronic density of states or superconducting transition temperature (T-C). Furthermore, the disorder reduces the electron mean-free path to similar to1 nm which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in B-C2(0).

A large increase (similar to 8 X) in critical current density, J(c), was achieved for in situ laser ablated YBa2Cu3O7-delta-Ag films. High-resolution Auger electron spectroscopic investigation indicates that the Ag-doped films are stoichiometric with a relatively low grain-boundary extension (8 nm) in contrast to undoped YBa2Cu3O7-delta films (32 nm). Further analysis suggests that the doped film contains a much lower silver content (

We present measurements of the critical current in an epitaxial Bi2Sr2CaCu2O8 film as a function of the magnetic field, the temperature, and of the angle theta between the field and the a, b planes. The orientational study reveals an increasing anisotropy in J(c) with increasing temperature. The isothermal curves J(c)(H) taken at various angles 0-degrees less-than-or-equal-to theta less-than-or-equal-to 90-degrees are found to collapse to a single curve when the magnetic field is normalized to a scaling function f(theta): J(c)(H,theta)=J(c)[H/f(theta)]. The best fit for f(theta) is obtained using an expression originally derived for H(c2)(T,theta) by Tinkham. The increase in J(c)(0-degrees)/J(c)(90-degrees) with increasing temperature is reflected by the increase in the anisotropy ratio f(0-degrees)/f(90-degrees), consistent with the quasi-two-dimensional Tinkham expression. Consistently, the same model quantitatively describes the increasing anisotropy with increasing temperature.

For the first time, GdBa2Cu3O7−x nanocomposites were prepared by chemical solution deposition following the ex-situ approach. In particular, ~ 220 nm GdBa2Cu3O7−x-HfO2 (GdBCO-HfO2) nanocomposite films were fabricated starting from a colloidal solution of 5 mol% HfO2 nanoparticles. Hereby, one of the main challenges is to avoid the accumulation of the nanoparticles at the substrate interface during the pyrolysis, which would later prevent the epitaxial nucleation of the GdBCO grains. Therefore, the effect of pyrolysis processing parameters such as heating ramp and temperature on the homogeneity of the nanoparticle distribution has been investigated. By increasing the heating ramp to 300 °C/h and decreasing the final temperature to 300 °C, a more homogenous nanoparticle distribution was achieved. This translates into improved superconducting properties of the grown films reaching critical temperatures (Tc) of 94.5 K and self-field critical current densities (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${J}_{\mathrm{c}}^{\mathrm{sf}}$$\end{document}Jcsf) at 77 K of 2.1 MA/cm2 with respect to films pyrolyzed at higher temperatures or lower heating ramps.