Experimental observation of the selection rule in Josephson coupling between In and Sr ₂ RuO ₄

We have carried out experimental studies on 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 Γ⁻₅, with {d}({k}) = {z}({k}_x±i{k}_y), a nodeless state in which the spins of the superconducting electrons lie in the RuO₂ planes.