SURFACE DEPRESSION OF THE ORDER-PARAMETER AND PAIR SYMMETRY IN THE JOSEPHSON CRITICAL-CURRENT OF HIGH-TC SIS' AND SIS JUNCTIONS

The Josephson critical current I-c(T) in various high-T-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 in to account an intrinsic depression of the order parameter Delta at the interface between the superconductor and the insulating layer as the cause of the reduced values of I-C(T)R-N(T) and its behaviour vs. T up to T-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 i s unable to describe experimental results in shape with an univocal ra tio Delta(d)/Delta(s), and, in magnitude, it often yields results two or three times greater than those measured. On the contrary, our gap-d epressed model only fails to describe experimental data when junctions ' quality is rather poor, due to the fabrication process and other rea sons related to the quality of interfaces.