Nodeless pairing state in single-crystal YBa₂Cu ₃O₇

Muon spin rotation (μ⁺SR) measurements were conducted on a single crystal of YBa₂Cu₃O₇ with a superconducting transition temperature of T_c≈91.3 K and a transition width of ΔT_c<0.5 K in zero applied field. Data were taken at applied magnetic fields along the c axis of 0.05, 1.0, 3.0, and 6.0 T. We found, by taking into account the expected field-dependent and temperature-activated flux-line disorder, that our results were in fact consistent with a nodeless (s-wave) superconducting order parameter and that they appeared to be inconsistent with order parameters possessing nodes, such as those having d_x2-y2 symmetry. This result is consistent with early μSR measurements on sintered samples in which (we believe) strong pinning eliminated the temperature and field dependence of the vortex lattice disorder. These data (including their observed dependences on magnetic field) are, however, completely consistent with s-wave (or extended s-wave) pairing, provided that field-dependent and temperature-activated vortex depinning is also accounted for. Our results (i) confirm the s-wave superconductivity character originally observed in 1989, and (ii) show that the features of μSR (and microwave) data claimed by other authors to be evidence for d-wave superconductivity are instead symptomatic of temperature-dependent depinning of vortices, which results in long-ranged distortion of the flux lattice. Indeed, the probability that any published d-wave model gives a better fit than the two-fluid model is less than 4 × 10^-6.