On the Symmetry of the Superconducting Order Parameter and the Critical Current

We have calculated the critical current of Sharvin contacts between two superconducting electrodes for s - and d -wave-type superconductors. Our results differ substantially from those obtained for Josephson tunnelling barriers. We discuss their relevance for grain boundaries junctions in the high- T c oxides and for contacts between a high- T c oxide and a low- T c superconductor.