ELECTROMIGRATION STUDY OF OXYGEN DISORDER AND GRAIN-BOUNDARY EFFECTS IN YBA2CU3O7-DELTA THIN-FILMS

We report on the effects of strong electrical currents (greater-than-or-equal-to 1 MA/cm2) applied at approximately 300 K to YBa2Cu3O7-delta, thin-film microbridges that both do, and do not contain high-angle grain boundaries. In the grain-aligned microbridges, below a certain threshold level such currents promote a slow improvement of the normal and superconductive properties of the microbridge. This behavior is attributable to the current-driven increase in basal-plane oxygen order in a manner characteristic of dispersive transport. Above threshold, the effect of the electromigration current is to create rapidly regions of strong oxygen disorder, which increases the microbridge resistance and decreases the transition temperature and critical current. This disorder shows a tendency to segregate under electromigration bias and can be at least partially removed by reversal of the current. The threshold effect permits an estimate of the atomic force required to promote the creation of oxygen disorder in YBa2Cu3O7-delta. The superconducting properties of the disordered regions have the characteristic of a composite superconductor, consisting of a network of superconducting filaments partially shunted by normal conducting paths. Comparison with the behavior of microbridges containing high-angle grain boundaries indicates that such boundaries consist of more or less random connections of these filaments.