Dependence of the infrared properties of single-domain ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ on oxygen content

The first polarized infrared-reflectivity measurements of untwinned crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ as a function of y are reported. We separate the chain and plane contributions to the infrared conductivity, and find that for crystals with ${\mathit{T}}_{\mathit{c}}$≳50 K the prominent normal state midinfrared feature is associated with the chains. A threshold in the plane conductivity at ∼500 ${\mathrm{cm}}^{\mathrm{-}1}$ has a strong temperature dependence just below ${\mathit{T}}_{\mathit{c}}$ suggesting an association with the superconducting gap. Its temperature dependence above and below ${\mathit{T}}_{\mathit{c}}$ is quite similar to that of relevant NMR data, and is suggestive of gap development in the normal state.