ab-plane optical properties of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$

We report on absolute ab-plane reflectivity measurements of the single-layer high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_{6+\mathrm{\delta }}$ in both the normal and superconducting states. The reflectivity has been measured from 40 to 40000 ${\mathrm{cm}}^{\mathrm{-}1}$ and the optical conductivity has been calculated from Kramers-Kronig analysis. The unusual form of the low-frequency reflectivity leads to an intense low-frequency feature in the optical conductivity. In the superconducting state, the spectral weight is strongly suppressed at frequencies below ∼800 ${\mathrm{cm}}^{\mathrm{-}1}$ due to the formation of a superfluid fraction. The value of the London penetration depth has been estimated to be 2400±200 Å, larger than that from muon-spin-resonance (μSR) experiments.