Study of the order parameter symmetry in overdoped evidence for enhancement of the sub dominant order parameter amplitude in high transparency contacts

We have measured the differential conductance of Au/Y_1-x Ca_x Ba₂Cu₃O_7-δ point contacts in the regime where it is dominated by Andreev reflection, which enhances its value at low bias. We find that the characteristics can not be fitted by a pure d-wave Order Parameter (OP). Using the formalism developed by Kashiwaya and Tanaka,¹ the best fits are obtained by adding a sub-dominant imaginary OP, whose amplitude appears to depend on the transparency of the contact. At high transparencies it can reach up to 60% of the total amplitude of the OP, for lower transparencies it is substantially smaller. We attribute this enhancement, at high transparencies to a proximity effect between the Au tip and the superconducting electrode.