EFFECTS OF GRAIN-BOUNDARIES ON THE CRITICAL-CURRENT OF HIGH-T-C SUPERCONDUCTORS

We study the temperature dependence of the Josephson critical current for different symmetries of the superconducting state. We generalize Z aitsev's model for a smooth interface to include diffuse scattering of quasiparticles at the boundary. For d-wave symmetry, we find strong d eviations from the Ambegaokar-Baratoff (AB) formula for SIS junctions. Similar results are obtained for s-wave superconductors provided one assumes large pair-breaking effects at the interface (inelastic scatte ring, magnetic scattering, etc.). Our numerical calculations are able to explain experimental data measured on single grain boundary junctio ns as well as c-axis oriented granular films of high-temperature super conductors (HTSC).