Evidence for a non-s-wave superconducting order parameter in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ with ${\mathit{T}}_{\mathit{c}}$=60 K

Several recent phase-sensitive-tunneling experiments have indicated that ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ is not an s-wave superconductor. In this work we report an analogous experiment on a related cuprate superconductor, ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$, with ${\mathit{T}}_{\mathit{c}}$=60 K. The critical current vs applied field of a Pb-${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$ single crystal corner Josephson junction is measured and compared with the theory. As a control experiment, measurements are made on a Pb-${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$ edge junction. The results are consistent with a d-wave order parameter for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$. It is thus unlikely that the results of previous similar experiments on almost fully oxygenated ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.6}$ may simply be explained by the presence of the Cu-O chains. © 1996 The American Physical Society.