Observation of anomalous temperature dependence of the critical current in (formula presented) junctions

We have studied the temperature dependence of the critical current (Formula presented) of bilayer (Formula presented) junctions prepared by using a submicron-diameter filament as a shadow mask. (Formula presented) a layered perovskite isostructural with (Formula presented) has a superconducting transition temperature (Formula presented) lower than that of Pb. Below the (Formula presented) of Pb, the critical current (Formula presented) of the junction was found to increase initially with decreasing temperature. As the temperature was lowered to below the (Formula presented) of (Formula presented) however, a sharp drop in (Formula presented) was observed. This downturn in (Formula presented) suggests that superconductivity in (Formula presented) 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 (Formula presented).