c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions

TL;DR

This paper models c-axis Josephson tunnelling in YBCO-Pb junctions, showing chains dominate the current and that twin orientation affects tunnelling, challenging previous assumptions about d-wave superconductivity.

Contribution

It introduces a microscopic two-band model highlighting the role of chains and twin orientation in Josephson tunnelling in YBCO.

Findings

Chains significantly contribute to tunnelling current.

Twin orientation ratio influences tunnelling measurements.

Evidence suggests twin abundance affects observed Josephson current.

Abstract

Within a microscopic two band model of planes and chains with a pairing potential in the planes and off diagonal pairing between planes and chains we find that the chains make the largest contribution to the Josephson tunnelling current and that through them the d-wave part of the gap contributes to the current. This is contrary to the usual assumption that for a d-wave tetragonal superconductor the c-axis Josephson current for incoherent tunnelling into an s-wave superconductor is zero while that of a d-wave orthorhombic superconductor with a small s-wave component to its gap it is small but non-zero. Nevertheless it has been argued that the effect of twins in YBCO would lead to cancellation between pairs of twins and so the observation of a current in c-axis YBCO-Pb experiments is evidence against a d-wave type order parameter. We argue that both theory and experiment give evidence that the two twin orientations are not necessarily equally abundant and that the ratio of tunnelling currents in twinned and untwinned materials should be related to the relative abundance of the two twin orientations.