Disorder and superconducting-state conductivity of single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$

The ab-plane optical conductivity of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$ has been studied by infrared reflectance spectroscopy in the presence of disorder introduced by ion irradiation. The frequency dependence of the conductivity in the superconducting state suggests that ion irradiation has caused carrier localization. Localization explains both the suppression of ${\mathit{T}}_{\mathit{c}}$ and the reduction of the superfluid fraction. The temperature dependence of the penetration depth in pure and disordered ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$ supports the hypothesis of non-s-wave superconductivity.