Surface depression of the order parameter and pair symmetry in the Josephson critical current of high-T/sub c/ SIS' and SIS junctions
The Josephson critical current I/sub c/(T) in various high-T/sub c/ SIS' and SIS junctions of different kind has been analyzed, and its discrepancies with BCS s-wave predictions are consistent with a model which takes into account an intrinsic depression of the order parameter /spl Delta/ at the interface between the superconductor and the insulating layer as the cause of the reduced values of I/sub c/(T)R/sub N/(T) and its behaviour vs. T up to T/sub c/. Comparison with the fits obtained by a model based on s+id pair symmetry with a dominant d-wave component shows that the latter is unable to describe experimental results in shape with an univocal ratio /spl Delta//sub d///spl Delta//sub s/ and, in magnitude, it often yields results two or three times greater than those measured. On the contrary, our gap-depressed model only fails to describe experimental data when junctions' quality is rather poor, due to the fabrication process and other reasons related to the quality of interfaces.