Nodeless Pairing State in Single-Crystal YBa2Cu3O7

Muon spin rotation (mSR) measurements were conducted on a single- crystal of YBa2Cu3O7 with a superconducting transition temperature of Tc ~ 91.3 K and a transition width of D Tc 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 Tesla. 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 dx2-y2 symmetry. This result is consistent with early mSR 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 de-pinning is also accounted for. Our results (i) confirm the s-wave superconductivity character originally observed in 1989, and (ii) show that the features of mSR (and microwave) data claimed by other authors to be evidence for d-wave superconductivity are instead symptomatic of temperature-dependent de-pinning of vortices, which results in long-ranged distortion of the flux lattice. Indeed, the probability that any d-wave model gives a better fit than the two-fluid s-wave model is less than 4´10-6.