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 Fraunh ofer-type critical current I-c vs. magnetic field H-a dependences, whi ch are a hallmark of standard Josephson junctions. To clarify the reas on 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 orientat ion 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 f or the Ic(H?a) characteristics observed.