Superconducting order parameter in bilayer cuprates: Occurrence of π phase shifts in corner junctions

We study the order-parameter symmetry in bilayer cuprates such as Y-Ba-Cu-O, where interesting π phase shifts have been observed in Josephson junctions. Taking models that represent the measured spin-fluctuation spectra of this cuprate, as well as more general models of Coulomb-correlation effects, we classify the allowed symmetries and determine their associated physical properties. π phase shifts are shown to be a consequence of repulsive interactions, independent of whether a magnetic mechanism is operative. While it is known to occur in d states, this behavior can also be associated with (orthorhombic) s symmetry when the two subband gaps have opposite phase. Implications for the magnitude of ${\mathit{T}}_{\mathit{c}}$ are discussed.