C-AXIS JOSEPHSON TUNNELING BETWEEN YBA2CU3O7-DELTA AND PB - DIRECT EVIDENCE FOR MIXED ORDER-PARAMETER SYMMETRY IN A HIGH-T-C SUPERCONDUCTOR

We report a new class of c-axis Josephson tunneling experiments in whi ch a conventional superconductor (Pb) is deposited across a single twi n boundary of a YBa2Cu3O7-delta (YBCO) crystal. We measure the critica l current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a cl ear experimental signature of an order parameter phase shift across th e twin boundary. These results provide strong evidence for mixed d- an d s-wave pairing in YBCO, with a reversal in the sign of the s-wave co mponent across the twin boundary.