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

Josephson tunnelling between YBa2Cu3O7-delta and Pb with the current flowin g along the c-axis of the YBa2Cu3O7-delta is presumed to arise from an s-wa ve component of the super conductivity in YBa2Cu3O7-delta. Experiments on m ulti-twin samples are not entirely consistent with this hypothesis. The sig n changes of the s-wave order parameter across the N-T twin boundaries shou ld give cancellations, resulting in a small (root N-T) tunnelling current. The actual current is larger than this. We present a theory of this unexpec tedly large current based upon a surface effect: disorder-induced suppressi on of the d-wave component at the (001) surface leads to s-wave coherence a cross 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 t win size at which s-wave surface coherence occurs is consistent with typica l sizes observed in experiments. In this picture, there is a phase differen ce of pi/2 between different surfaces of the material. We propose a corner- junction experiment to test this picture.