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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...

Measurements of the resistance noise in thick films of Y-Ba-Cu-O, in thick films of Bi-Sr-Ca-Cu-O, and in thin films of NbN, at the superconducting transition are reported. The transition to the R =0 state was obtained by either changing the temperature at fixed currents or by lowering the current through the sample at fixed temperatures.In both kinds of experiments, we have observed the divergence of the power spectral density S, /V as the R =0 state is approached. Our results are consistent with a percolative interpretation of the resistive transition of granular superconductors.

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