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

Abstract

In the present letter we discuss the effect of the intrinsic surface depression of the order parameter on the temperature dependence of the Josephson critical current in high-T c SIS and SIS′ junctions. The theoretical results are compared to the Ic(T)RN(T) curves we recently obtained from reproducible data of break junctions made at 4.2 K in single crystals of Bi 2 Sr 2 CaCu 2 O 8+x and Bi 2 Sr 2 CuO 6+x as well as to the experimental data of multilevel edge junctions YBa 2 Cu 3 O 7+x/ SrTiO 3/ YBa 2 Cu 3 O 7+x and of planar tunnel junctions YBa 2 Cu 3 O 7+x/native barrier/Pb. In all the studied junctions, the s-wave depressed-gap model can accurately reproduce the large deviations from the BCS behavior, both in shape and magnitude. The comparison with the results of a model that explains the same deviations in YBa 2 Cu 3 O 7+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)RN(T) values, while the s+id model needs some additional current-lowering mechanism to do the same.