Influence of d(x/sup 2/-y/sup 2/) symmetry on device applications of high-T/sub c/ grain boundary junctions

Grain boundary junctions in high-T/sub c/ thin films generally consist of facets with typical dimensions below 100 nm. In combination with a d(x/sup 2/-y/sup 2/) symmetry component of the order parameter, this faceting gives rise to an inhomogeneous critical current density J/sub c/ along the grain boundary. The inhomogeneity is most prominent for asymmetric 45/spl deg/ [001] tilt grain boundaries. For a large fraction of the facets in these boundaries the order parameter orientation gives rise to an additional /spl pi/ phase difference, and therefore to a 'negative' critical current density. This leads to highly anomalous magnetic field dependences of the critical current. Direct imaging with scanning SQUID microscopy provides evidence that magnetic flux is generated spontaneously in these boundaries. These observations have several significant implications, both for understanding the properties of grain boundaries in high-T/sub c/ superconductors and for their applications.