Experimental evidence for singlet s-wave pairing in YBa 2Cu 3O 7

Abstract

We report here the results of an experiment to determine the symmetry of the pair states in superconducting YBa 2Cu 3O 7. Using a fluxgate magnetometer we have monitored a persistent current at 4.2 K in a composite lead-YBa 2Cu 3O 7 ring and shown no measurable decay over a period of 190 h. This sets an upper bound on the resistance of the Pb-YBa 2Cu 3O 7 junction ⪡10 -16 Ω. The experiment shows that phase coherence exists around the ring and that no significant depression of the order parameter or local value of T c occurs in the neighborhood of the junctions. This is strong evidence for singlet s-wave pairing in YBa 2Cu 3O 7.