SURFACE ORDER-PARAMETER DEPRESSION AND PAIR SYMMETRY IN HIGH-T-C JOSEPHSON-JUNCTIONS

In the present letter we discuss the effect of the intrinsic surface d epression of the order parameter on the temperature dependence of the Josephson critical current in high-T-c SIS and SIS' junctions. The the oretical results are compared to the I-c(T)R(N)(T) curves we recently obtained from reproducible data of break junctions made at 4.2 Ii in s ingle crystals of Bi2Sr2CaCu2O8+x and Bi2Sr2CuO6+x as well as to the e xperimental data of multilevel edge junctions YBa2Cu3O7+x/SrTiO3/YBa2C u3O7+x and of planar tunnel junctions YBa2Cu3O7+x/native barrier/Pb. I n all the studied junctions, the s-wave depressed-gap model can accura tely reproduce the large deviations from the BCS behavior, both in sha pe and magnitude. The comparison with the results of a model that expl ains the same deviations in YBa2Cu3O7+x junctions by assuming a s + id pair symmetry with a dominant d-wave component shows that our sample- independent depressed-gap model alone can fully explain the I-c(T)R(N) (T) values, while the s + id model needs some additional current-lower ing mechanism to do the same.