We have studied the temperature dependence of the critical current $I_c$ of bilayer ${\mathrm{P}\mathrm{b}/\mathrm{S}\mathrm{r}}_2{\mathrm{RuO}}_4/\mathrm{P}\mathrm{b}$ junctions prepared by using a submicron-diameter filament as a shadow mask. ${\mathrm{Sr}}_2{\mathrm{RuO}}_4,$ a layered perovskite isostructural with ${\mathrm{La}}_2{\mathrm{CuO}}_4,$ 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 ${\mathrm{Sr}}_2{\mathrm{RuO}}_4,$ however, a sharp drop in $I_c$ was observed. This downturn in $I_c$ suggests that superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ 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 ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$

• Received 9 September 1998

DOI:https://doi.org/10.1103/PhysRevB.59.4433