Critical Josephson Current in a Model Pb/YBa_2Cu_3O_7 Junction

TL;DR

This paper models a Pb/YBa_2Cu_3O_7 Josephson junction, showing that chain-induced symmetry breaking can produce a nonzero current even with d-wave pairing, aligning theory with experimental observations.

Contribution

It demonstrates how chain effects in YBa_2Cu_3O_7 can lead to finite Josephson current despite d-wave pairing, challenging previous interpretations.

Findings

Calculated critical currents match experimental data.

Chain effects can break tetragonal symmetry, enabling current.

D-wave pairing remains consistent with observed Josephson effects.

Abstract

In this article we consider a simple model for a c--axis Pb/YBa_2Cu_3O_{7-\delta} Josephson junction. The observation of a nonzero current in such a junction by Sun et al. [A. G. Sun, D. A. Gajewski, M. B. Maple, R. C. Dynes, Phys. Rev. Lett. 72, 2267 (1994)] has been taken as evidence against d--wave superconductivity in YBa_2Cu_3O_{7-\delta}. We suggest, however, that the pairing interaction in the CuO_2 planes may well be d--wave but that the CuO chains destroy the tetragonal symmetry of the system. We examine two ways in which this happens. In a simple model of an incoherent junction, the chains distort the superconducting condensate away from d_{x^2-y^2} symmetry. In a specular junction the chains destroy the tetragonal symmetry of the tunneling matrix element. In either case, the loss of tetragonal symmetry results in a finite Josephson current. Our calculated values of the critical current for specular junctions are in good agreement with the results of Sun and co-workers.