Anomalous dependence of the critical current of 45° grain boundaries in YBa₂Cu₃O_7−x on an applied magnetic field

Abstract

Grain boundaries grown by the biepitaxial technique do not show Fraunhofer-type critical current I _c vs. magnetic field H _a dependences, which are a hallmark of standard Josephson junctions. To clarify the reason for this unusual behavior, we have fabricated asymmetric 45° grain boundaries using the bicrystal technique and analyzed their I _c (H _a) characteristics. These characteristics crisply show a number of remarkable features, which suggest that due to the particular orientation of these junctions, their I _c (H _a) dependence is intrinsically non-Fraunhofer. Conventional models, relying on standard tunneling and a superconducting order parameter with s-wave symmetry, do not account for the I _c (H _a) characteristics observed.