Critical currents and current-voltage characteristics in superconducting ceramic ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Measurements of current-voltage characteristics in ceramic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are reported above liquid-nitrogen temperatures. The measurements were performed in a quasi-dc ‘‘pulse’’ technique on samples with copper or silver evaporated contacts, reducing disturbance by current heating effects. The critical current deduced from these characteristics has a linear dependence on temperature. Two regimes, similar to those observed directly in the resistive transition, have been observed in these characteristics as indicated by the existence or inexistence of a finite supercurrent. The first regime, corresponding to the major drop in resistance, may be interpreted in terms of a network of strongly coupled superconducting islands having a distribution of critical temperatures; due to this distribution this transition is believed to be of a percolative nature that occurs at the island’s percolation threshold. The second ‘‘tail’’ regime, corresponds to a temperature range where the Josephson coupling energy of the grain-boundary weak links is not large enough to overcome thermal energy. When it becomes larger, phase locking and zero resistance are achieved as is usually described by another kind of percolative transition, typical of granular superconductors. At large currents, that reduce the coupling energy, the full transition occurs at concentrations much above the island’s percolation threshold. Thus, it is possible that 2 within the same sample, crossover between two kinds of percolative transitions occurs. An aging that occurred in one of our samples at low temperatures supports this picture. In this aging (presumably due to water incorporation), the sample ceased to be superconducting, though a drop to finite resistance at the previous superconducting transition temperature still occurred. It is likely that grain boundaries ceased to act as weak links due for instance to increase in barrier height and thickness, but that the superconducting islands remained unaffected.