Temperature dependence of the magnetic penetration depth in the high-$T_c$ superconductor ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$: Evidence for conventional $s$-wave pairing

Transverse-field muon-spin-relaxation ($\mu \mathrm{SR}$) measurements have been performed on ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$ ($\delta =2.1\mathrm{}\pm 0.05$) above and below its superconducting transition temperature. The temperature dependence of the magnetic penetration depth, deduced from the $\mu \mathrm{SR}$ data, is that of an ordinary $s$-wave (not $d$-wave with nodes in the gap function) superconductor. The data at 6 K indicate a magnetic penetration depth of $\lambda \approx 1400$ Å which, in the limit of extreme anisotropy, reduces to $\lambda \approx 1065$ Å.