Anomalous Dependence of the Critical Current of 45 degree Grain Boundaries in YBa2Cu3O7-x on an Applied Magnetic Field

Grain boundaries grown by the biepitaxial technique do not show Fraunhofer-type critical current Ic vs. magnetic field Ha dependences, which are a hallmark of standard Josephson junctions. To clarify the reason for this unusual behavior, we have fabricated asymmetric 45 degree grain boundaries using the bicrystal technique and analyzed their Ic(Ha) characteristics. These characteristics crisply show a number of remarkable features, which suggest that due to the particular orientation of these junctions, their Ic(Ha) 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(Ha) characteristics observed.