The role of interfacial defects in enhancing the critical current density of YBa₂Cu₃O_7-δ coatings

The critical current density (J_c) of very thin YBa ₂Cu₃O_7-δ (YBCO) films can approach 10MAcm^-2 at 77K in self-field, but for such films J_c drops sharply as the film thickness is increased. We have shown previously that this strong thickness dependence results from an enhancement of J_c near the film-substrate interface. In the present paper we investigate interfacial enhancement using laser-deposited YBCO films on NdGaO₃ substrates, and find that we can adjust deposition conditions to switch the enhancement on and off. We find that the 'on' state is accompanied by a dense array of interfacial misfit dislocations, while we do not observe dislocations in films prepared in the 'off' state. This result appears to be but one of many examples in which interfacial properties of electronic film materials are profoundly affected by stress-induced defects at the film-substrate interface; however, to our knowledge the present work is the only case in which electronic properties are shown to be enhanced by such defects.