J. Phys. I France 6 (1996) 2299-2316
Symmetry of the Phase of the Order Parameter in YBa Cu OY. 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 (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.
© Les Editions de Physique 1996