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 which a conventional superconductor (Pb) is deposited across a single twin boundary of a YBa2Cu3O7-delta (YBCO) crystal. We measure the critical 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 clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed d- and s-wave pairing in YBCO, with a reversal in the sign of the s-wave component across the twin boundary.