J. Phys. I France
Volume 6, Numéro 12, December 1996
Page(s) 2299 - 2316
DOI: 10.1051/jp1:1996219
J. Phys. I France 6 (1996) 2299-2316

Symmetry of the Phase of the Order Parameter in YBa $\mathsf{_2}$Cu $\mathsf{_3}$O $\mathsf{_{7-\delta}}$

Y. Gim, A. Mathai, R.C. Black, A. Amar and F.C. Wellstood

Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742-4111, U.S.A.

(Received 7 May 1996, revised 12 August 1996, accepted 19 August 1996)

We first describe the basic idea behind phase-sensitive measurements to determine the symmetry of the order parameter in YBa 2Cu 3O $_{7-\delta}$ (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.

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