Basic aspects of high-T-c grain boundary devices

Grain boundaries are extensively used as high-quality Josephson junctions in high-T-c superconductors. Their superconducting characteristics can generally be well described by conventional models of strongly coupled Josephson junctions. Here, we report on highly anomalous critical current vs. magnetic field dependencies of grain boundaries in YBa2Cu3O7-x. Direct imaging with scanning SQUID microscopy provides evidence of magnetic flux generated by single grain boundaries. Conventional Josephson junction models cannot explain these effects if a superconducting order parameter with a pure s-wave symmetry is assumed. The results have significant implications for our understanding of the properties of grain boundaries in high-T-c superconductors and for their applications.