  We have studied the Josephson coupling between a conventional s-wave
superconductor (In) and Sr2RuO4 and found that the coupling is allowed in the
in-plane direction, but not along the c axis. This selection rule indicates
that the symmetry of superconducting order parameter of Sr2RuO4 is either p- or
alternatively, purely d-wave. If Sr2RuO4 is a p-wave superconductor, as
strongly favored by other experimental evidence, our result suggests that the
pairing state of Sr2RuO4 is Gamma5 with d(k) = z(kx +/- iky), a nodeless state
in which the spins of the superconducting electrons lie in the RuO2 planes.

Superconductivity