Anomalous dependence of the critical current of 45° grain boundaries in YBa₂Cu₃O_7-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° 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. © Springer-Verlag 1996.