Observation of anomalous temperature dependence of the critical current in Pb/Sr2RuO4/Pb junctions

We have studied the temperature dependence of the critical current I-c of bilayer Pb/Sr2RuO4/Pb junctions prepared by using a submicron-diameter filament as a shadow mask. Sr2RuO4, a layered perovskite isostructural with La2CuO4, has a superconducting transition temperature (T-c) lower than that of Pb. Below the T-c of Pb, the critical current I-c of the junction was found to increase initially with decreasing temperature. As the temperature was lowered to below the T-c of Sr2RuO4, however, a sharp drop in I-c was observed. This downturn in I-c suggests that superconductivity in Sr2RuO4 actually suppresses the Josephson coupling between the two Pb electrodes, which are conventional s-wave superconductors. The implications of this unexpected behavior will be explored, in particularly, in the context of the pairing symmetry in Sr2RuO4.