Anomalous dependence of the critical current of 45 degrees grain boundaries in YBa2Cu3O7-x on an applied magnetic field

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 degrees 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 Ic(H?a) characteristics observed.