PI-PHASE-SHIFTS IN BILAYER S-WAVE SUPERCONDUCTORS

We study the order parameter symmetry in bi-layer cuprates such as YBa CuO, where interesting pi phase shifts have been observed in Josephson junctions. pi phase shifts are shown to be a general consequence of r epulsive interactions, independent of whether a magnetic mechanism is operative. While it is known to occur in d-states, this behavior can a lso be associated with (orthorhombic) s-symmetry when the two sub-band gaps have opposite phase.