To determine the symmetry of the order parameter in superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO), we use a scanning SQUID microscope at 4.2 K to perform independent experiments on six YBCO-Ag-Pb SQUIDs. We find completely unambiguous evidence for a time-reversal-invariant order parameter with a phase shift of $\pi$ between the $\mathbf{a}$ and $\mathbf{b}$ directions of YBCO. Our results are inconsistent with purely $s$-wave symmetric pairing and are strongly suggestive of $d_{x^2{-y}^2}$ symmetric pairing in YBCO.

• Received 25 July 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.4523