Temperature dependence of the critical current in high-Tcsuperconductors

Abstract

We present a study of the temperature dependence of the critical current in bulk samples of the high-T, superconductors Y-Ba-Cu-0 and Dy-Ba-Cu-O. We have found that the critical current varies linearly with (T, -T). This result suggests a percolating network of Josephson-coupled superconducting grains. However, the critical current versus temperature curves are found to have a nonlinear tail close to the critical temperature.We have determined that the temperature at which the I, vs T curves depart from the linear behavior in our samples agrees with what is expected according to theoretical models of granular superconductors.The recent discovery of high-T, superconductors, ' has generated a remarkable amount of activity in the study of the physical properties of these materials. The interest is twofold: From a practical point of view one would like to improve these materials and optimize those properties which are important in applications. To this end, a number of groups have measured the critical current densities that can be achjeved in bulk, thin-film, and singlecrystal samples and how these critical current densities are modified by the presence of a field. From a purely scientific point of view, the challenge is to understand the mechanism which causes superconductivity in these materials, and to determine whether one can use those descriptions that have been so successful in describing low-T, superconductors or if modifications to these approaches will be required in order to properly describe these new materials. Thus, it is of importance to study various properties of these materials and to attempt to understand them in the context of the usual phenomenological theories for superconductors including the necessary modifications to take into account the peculiarities of these materials such as their short coherence length, their anisotropy, as well as their percolative morphology. One of such properties which is well understood in conventional superconductors is the temperature dependence of the critical current density. While brief accounts of this variation are reported in the literature, and specific predictions have been made of its temperature dependence, ' a thorough comparison with experiment is still lacking.In this paper, we present a detailed study of the temperature dependence of the critical current in Y-Ba-Cu-0 and Dy-Ba-Cu-0 compounds. We show that for homogeneous, single-phased samples the linear dependence of the critical current with temperature is consistent with a percolative network of grains coupled to each other through Josephson coupling. In one of our less homogeneous samples the critical current has two clearly different regimes which we interpret as the effect of the existence of two phases with different critical temperatures.In all cases, the Ginzburg-Landau description seems to remain valid in these materials.Our samples were prepared under a variety of conditions. We consider a sample to be of good quality only if it exhibits a resistivity ratio bigger than one and low resisti...

Maxwell quasi-variational inequalities in superconductivity

“…In particular, temperature effects are included due to its central importance in the superconductivity phenomena. As reported in [2], the temperature dependence in the critical current of the Y-Ba-Cu-O bulk superconductor exhibits a continuous and piecewise smooth structure [2, Figure 2]. More precisely, it features a linear behaviour of the type (θ c − θ), if the temperature θ is sufficiently smaller than θ c .…”

“…All assumptions for the data involved in (1) are summarized in Assumptions 2.1 and 4.1. In particular, motivated from the experimental measurements [2,8,9], our analysis relies on local boundedness and local Lipschitz continuity assumptions for the critical current with respect to the temperature. On the other hand, the magnetic field dependency in j c is assumed to be globally bounded and Lipschitz continuous.…”

Grain alignment and transport properties of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8 /grown by laser-heated float zone method

“…5 shows the temperature dependence of J, in zero magnetic field. The critical current obtained in our samples turns out to be much lower than that of single crystal superconductors [7]; its absolute value and temperature dependence are rather similar to that of untextured polycrystalline material [8]. Obviously, J , is still dominated by the weak couplings at the grain boundaries.…”

Thin film Y-Ba-Cu-O/Ag composites for fluxonic devices

“…The critical currents of the films were measured over the reduced temperature range from to The function where is a constant and is the power law dependence of on represents the behavior typically seen in superconducting samples near For a sample with characteristics dominated by superconductor-insulator-superconductor (SIS) coupling between grains, the Ambegaokar and Baratoff theory should be applicable, and should equal one, whereas for SNS or SS'S coupling, [13]- [14]. "Plain" YBCO samples, however, have been observed experimentally to follow the Ginzburg-Landau theory, which gives in the dirty limit [15]- [17].…”