Abstract
Josephson tunnelling between YBa2Cu3O7-δ and Pb withthe current flowing along the c-axis of theYBa2Cu3O7-δ is presumed to arise from an s-wavecomponent of the superconductivity in YBa2Cu3O7-δ.Experiments on multi-twin samples are not entirely consistent withthis hypothesis. The sign changes of the s-wave order parameteracross the NT twin boundaries should give cancellations,resulting in a small ((NT)1/2) tunnelling current. The actualcurrent is larger than this. We present a theory of thisunexpectedly 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 anon-random tunnelling current. We solve the case of an orderedarray of d + s and d-s twins, and estimate that the twinsize at which s-wave surface coherence occurs is consistent withtypical sizes observed in experiments. In this picture, there is aphase difference of π/2 between different surfaces of thematerial. We propose a corner-junction experiment to test thispicture.
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