Magnetic penetration depth and surface resistance in ultrahigh-purity ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals

We have succeeded in growing very high-purity (99.995%) ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals in ${\mathrm{BaZrO}}_3$ crucibles and have measured $\Delta \lambda \mathrm{}\left(T\right)\mathrm{}$ and $R_s\mathrm{}\left(T\right)\mathrm{}$ at 1 GHz in crystals with various oxygen treatments. For an oxygen vacancy level of $\delta =0.007,$ Δλ and $R_s$ essentially reproduce our previous results and show no sign of the existence of the two order parameter components as recently reported by Srikanth et al. on ${\mathrm{BaZrO}}_3$-grown crystals. For other oxygen concentrations, we have in some cases observed deviations from the linear low T dependence of Δλ, but never any sign of a second transition.