Role of the anisotropy of lead in BSCCO-Pb Josephson tunneling

Recent experimental studies of Josephson tunneling between single crystals of Bi2Sr2CaCu2O8+x (BSCCO) and lead (Pb) films have revealed small, but finite, critical currents along the c axis of BSCCO, despite this being forbidden by symmetry. We show that the known anisotropy of the Pb order parameter would allow quite strong coupling between single crystals of BSCCO and Pb if the tunneling direction were along the BSCCO c axis and the 110] direction of the Pb crystal. This mechanism could account for the experimental results on granular Pb films if there is a few percent preferred orientation in the films. All the current experimental evidence is therefore consistent with BSCCO being a pure d-wave superconductor.