c-axis Josephson Tunneling in Twinned YBCO Crystals

TL;DR

This paper proposes a surface effect theory explaining unexpectedly large c-axis Josephson tunneling currents in twinned YBCO crystals, highlighting the role of disorder-induced suppression of d-wave components and s-wave coherence.

Contribution

It introduces a new surface effect model accounting for large tunneling currents, contrasting with previous s-wave component hypotheses, and suggests experimental tests.

Findings

Large tunneling current explained by surface s-wave coherence

Twin size estimates match experimental observations

Proposes a corner junction experiment to validate the theory

Abstract

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.