Implications of symmetry and faceting for the transport properties of grain boundaries in high superconductors

Abstract

Grain boundaries in high-(Formula presented) superconductors have attracted wide interest for their potential in a variety of applications and in fundamental studies of high-(Formula presented) superconductivity. Two recent experimental results provide a basis for a better understanding of the grain boundary properties, the mechanisms of which, despite their widespread use, are not yet completely understood. First, it is now well established that the order parameter in many high-(Formula presented) cuprates has a predominant (Formula presented) symmetry. Second, microscopy studies have revealed that practical grain boundaries are comprised of facets having various orientations and typical dimensions of the order of 10- 100 nm. We analyze the combined effects of faceting and (Formula presented) symmetry on the transport properties of high-(Formula presented) grain boundaries. It is found that these effects can partially account for the experimentally observed reduction of the critical current density (Formula presented) with increasing grain boundary angle (Formula presented). The angular dependence of (Formula presented) for individual grain boundary facets may deviate considerably from the (Formula presented) dependence observed in standard measurements that employ macroscopic grain boundaries. This also holds for the product of (Formula presented) and the normal state resistivity (Formula presented). The (Formula presented) product measured for standard grain boundary junctions is therefore not a direct measure of the intrinsic barrier properties. The faceting and (Formula presented) symmetry lead to an inhomogeneous current distribution in the grain boundary which is different for the superconducting and the normal states. © 1996 The American Physical Society.