Nodeless Ground State Symmetry of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$

Muon spin rotation measurements were conducted on a single-crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (T$_{c}$ = 91.3 K) as a function of temperature and magnetic field [1]. By correctly accounting for the temperature-activated fluxon depinning and disorder known to exist in these and other high-quality YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ crystals grown today, the underlying ground state symmetry was found to be nodeless, consistent with s-wave (or extended s-wave) pairing. This result is in agreement with earlier data acquired on samples with strongly pinned vortices. The best fit was obtained assuming the two-fluid model. Analysis of the data assuming a d-wave gap function proved to be extremely unsatisfactory. In fact, the probability that the d-wave model tested gives a better fit than the two-fluid model is less than 4 $\times $ 10$^{-6}$. By ignoring the effects of fluxon depinning, others have erroneous claimed evidence of d-wave pairing. This result effectively negates that possibility. \begin{enumerate} \item D. R. Harshman et el., Phys. Rev. B \textbf{69}, 174505 (2004). \end{enumerate}