Symmetry of the phase of the order parameter in YBa2CU3O7-delta

We first describe the basic idea behind phase-sensitive measurements to determine the symmetry of the order parameter in YBa2Cu3O7-delta (YBCO). We then review recent experimental work. While there are still some apparent disagreements, a majority consensus appears to have emerged that the order parameter in YBCO is consistent with a d(x2-y2) symmetry, perhaps with an added s-wave component. To further illustrate the nature of these experiments, we discuss our investigation of the angular dependence of the phase of the order parameter. Using a 4.2 K scanning SQUID microscope, we examined thin-film YBCO-Ag-PbIn SQUIDs and measured the phase of the order parameter in twinned YBCO for 13 different tunneling angles in the a - b plane. We find that the pairing symmetry 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 which breaks time-reversal symmetry. This rules out states such as d(x2-y2) is Or d(x2-y2) + id(xy).