Verification of nodeless superconducting pairing in single-crystal YBa ₂Cu₃O₇

The temperature and field dependence of the penetration depth was determined from muon spin rotation (μ⁺SR) measurements on a single crystal of YBa₂Cu₃O₇ having a superconducting transition at T_c ≈ 91.3 K. Data were acquired at applied magnetic fields of 0.05, 1.0, 3.0, and 6.0 Tesla, yielding results inconsistent with any pairing state requiring nodes, including d-wave pairing. These data are, however, completely consistent with s-wave (or extended s-wave) superconductivity, with clear evidence of field-dependent, temperature-activated vortex pinning. Our results confirm the s-wave character originally observed in 1989, and show that the features of μ⁺SR (and microwave) data used by other authors as evidence for d-wave superconductivity are instead due to temperature- and field-dependent vortex pinning/reordering, resulting in significant distortion of the flux lattice.