Interface resistance and reduction of I cR product in YBa2Cu3O7-Ag-Pb proximity junctions

Studies of the proximity coupling of YBa2Cu3O7 through Ag to a Bardeen–Cooper–Schrieffer (BCS) superconductor have shown that the IcR product is systematically much smaller than the BCS limit of ∼5 mV. These IcR values range from 10 to 100 μV in the YBa2Cu3O7 a direction to apparently zero in the c direction. The reduced IcR in the a direction can be understood as arising from a large boundary resistance, consistent with the mismatch of carrier densities across the YBa2Cu3O7-Ag interface. The same analysis gives an IcR in the c-oriented junctions that is at least three orders of magnitude larger than observed.