Evidence for homogeneous superconducting grains in high-Tcoxides

Abstract

We present direct and magnetization critical current data on ceramic $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ which indicate that the "homogeneous superconducting regions" are much larger than the penetration depth and in fact are grain size. Thus, the current-limiting Josephson junctions in the high-$T_c$ oxide ceramics must be intergranular rather than intragranular. In light of these findings, we believe that a high density of twin boundaries probably enhances the intragranular critical current.