Disorder and superconducting-state conductivity of single crystals of YBa2Cu3O6.95

The ab-plane optical conductivity of YBa2Cu3O6.95 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 Tc and the reduction of the superfluid fraction. The temperature dependence of the penetration depth in pure and disordered YBa2Cu3O6.95 supports the hypothesis of non-s-wave superconductivity.