c-axis Josephson tunnelling in twinned YBa2Cu3O7-δ (YBCO) crystals

Josephson tunnelling between YBa2Cu3O7-delta and Pb with the current flowing along the c-axis of the YBa2Cu3O7-delta is presumed to arise from an s-wave component of the super conductivity in YBa2Cu3O7-delta. Experiments on multi-twin samples are not entirely consistent with this hypothesis. The sign changes of the s-wave order parameter across the N-T twin boundaries should give cancellations, resulting in a small (root N-T) tunnelling current. The actual current is larger than this. We present a theory of this unexpectedly large current based upon a surface effect: disorder-induced suppression of the d-wave component at the (001) surface leads to s-wave coherence across the twin boundaries and a non-random tunnelling current. We solve the case of an ordered array of d + s and d - s twins, and estimate that the twin size at which s-wave surface coherence occurs is consistent with typical sizes observed in experiments. In this picture, there is a phase difference of pi/2 between different surfaces of the material. We propose a corner-junction experiment to test this picture.