Symmetry of the Phase of the Order Parameter in YBa Cu O

Abstract
We first describe the basic idea behind phase-sensitive measurements to determine the symmetry of the order parameter in YBa ₂Cu ₃O (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.