C-axis Josephson Tunneling Between YBCO and Pb: Direct Evidence for Mixed Order Parameter Symmetry in a High-T_c Superconductor

TL;DR

This study provides direct experimental evidence for mixed d- and s-wave superconducting order parameters in YBCO through c-axis Josephson tunneling across twin boundaries, revealing phase shifts and sign reversals.

Contribution

It introduces a novel c-axis Josephson tunneling method to detect phase shifts, demonstrating mixed order parameter symmetry in high-T_c superconductor YBCO.

Findings

Evidence of phase shift across twin boundary

Reversal of s-wave component sign

Supports mixed d- and s-wave pairing model

Abstract

We report a new class of $c$-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a YBa_2Cu_3O_{7-\delta} crystal. We measure the critical current as a function of magnitude and angle of magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed $d$- and $s$-wave pairing in YBCO, with a reversal in the sign of the $s$-wave component across the twin boundary.