c-axis Josephson Tunneling in Twinned YBCO Crystals

Details c-axis Josephson Tunneling in Twinned YBCO Crystals c-axis Josephson Tunneling in Twinned YBCO Crystals

1999-07-02

arxiv.org/abs/cond-mat/9907021v2

Physics

Condensed Matter

Superconductivity

5 pages, 4 eps figures

Scientific paper

10.1088/0953-8984/12/37/315

Josephson tunneling between YBCO and Pb with the current flowing along the c-axis of the YBCO is persumed to come from an s-wave component of the superconductivity of the YBCO. Experiments on multi-twin samples are not entirely consistent with this hypothesis. The sign change of the s-wave order parameter across the N_T twin boundaries should give cancelations, resulting in a small $(\sqrt{N})$ tunneling current. The actual current is larger than this. We present a theory of this unexpectedly large current based upon a surface effect: disorder-induced supression of the d-wave component at the (001) surface leads to s-wave coherence across the twin boundaries and a non-random tunneling 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.

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