“Chain Scenario” for Josephson Tunneling with π Shift in YBa2Cu3O7

Abstract

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_7$ can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended $s$ symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have a phase shift equal to $\pi$.