Nodeless Pairing State in Yba₂Cu₃O₇

Muon spin rotation (mu+SR) results are reported on single-crystal YBa2Cu3O7, having a transition temperature of T-c = 91.3 K with Delta T-c < 0.5 K (in zero applied field). Flux motion and de-pinning can mask intrinsic properties such as the true underlying pairing state, and has led to misinterpretations of data. The present data exhibit a non-monotonic behavior for the second moment of the internal field distribution as a function of field as T -> O, which rules out any single pairing state explanation of the data, without including other extrinsic effects. The data are, however, consistent with s-wave (or extended s-wave) pairing, provided that field-dependent and temperature-activated vortex de-pinning are first taken into account. Applying a self-consistent vortex de-pinning model, the data are found to be best described by an underlying two-fluid model, yielding a London penetration depth value Of lambda(ab)(T=0,11=0)=127.6 nm. Attempts to fit the data using BCS theory with varying coupling strengths and a d-wave model produced much poorer fits. In fact, the probability that the d-wave model gives a better fit than the two-fluid model is less that 4 x 10(-6). This work reveals that the d-wave interpretations are incorrect, confirms earlier work (Refs. 1-3) which first established s-wave pairing in YBa2CU3O7 powders and (heavily twinned) crystals, and re-establishes s-wave pairing for superconductivity in this material.