Properties of optical features in YBa 2 Cu 3 O 7 − δ

Characteristic optical features in the far-infrared conductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals have been studied as a function of doping. We focus particularly on a reflectivity ``edge'' near 435 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which we find to be both insensitive to carrier concentration (i.e., ${T}_{c}$), and present in the normal state of some samples. These results place tight constraints on possible interpretations of this feature.