Evidence for a non-s-wave superconducting order parameter in YBa2Cu3O6.6 with T-c=60 K

Several recent phase-sensitive-tunneling experiments have indicated that YBa2Cu3O6.9 is not an s-wave superconductor. In this work we report an analogous experiment on a related cuprate superconductor, YBa2Cu3O6.6, with T-c=60 K. The critical current vs applied field of a Pb-YBa2Cu3O6.6 single crystal corner Josephson junction is measured and compared with the theory. As a control experiment, measurements are made on a Pb-YBa2Cu3O6.6 edge junction. The results are consistent with a d-wave order parameter for YBa2Cu3O6.6. It is thus unlikely that the results of previous similar experiments on almost fully oxygenated YBa2Cu3O6.6 may simply be explained by the presence of the Cu-O chains.