SYMMETRY OF THE PHASE OF THE ORDER-PARAMETER IN YBA2CU3O7-DELTA

We first describe the basic idea behind phase-sensitive measurements t o determine the symmetry of the order parameter in YBa2Cu3O7-delta (YB CO). We then review recent experimental work. While there are still so me apparent disagreements, a majority consensus appears to have emerge d that the order parameter in YBCO is consistent with a d(x2-y2) symme try, perhaps with an added s-wave component. To further illustrate the nature of these experiments, we discuss our investigation of the angu lar dependence of the phase of the order parameter. Using a 4.2 K scan ning SQUID microscope, we examined thin-film YBCO-Ag-PbIn SQUIDs and m easured the phase of the order parameter in twinned YBCO for 13 differ ent tunneling angles in the a - b plane. We find that the pairing symm etry of the order parameter is time-reversal invariant and consistent with a d(x2-y2) symmetry, provided the junction surface is not damaged and the tunneling angle is well-controlled. From a detailed analysis of the data, we conclude that there is less than 2% of any component w hich breaks time-reversal symmetry. This rules out states such as d(x2 -y2) is Or d(x2-y2) + id(xy).