Critical Josephson Current in a Model Pb/YBCO Junction

In this article we consider a simple model for a c–axis Pb/YBa2Cu3O7−δ 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 YBa2Cu3O7−δ. We suggest, however, that the pairing interaction in the CuO2 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 dx2−y2 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. 74.50.+r, 74.20.Fg