Optical Study of Localization in the ab-Plane Conductivity of Single Crystals of YBa2Cu306.9s Induced by Ion Damage

We studied the changes in the dc resistivity, Raman scattering, and IR conductivity of single crystals of YBa2Cu306.95 induced by damage from low-energy He + ion bombardment. It appears that T~., transport properties, and optical conductivity are strongly affected by modest irradiation doses whereas the chemical composition of the sample is not modified. Carrier localization is evidenced by the frequency dependence of the optical conductivity in the strongly damaged crystal and is used to explain the suppression of both the superfluid density and Tc upon irradiation. The temperature dependence of the superfluid density is in agreement with the theoretical predictions for a d-wave superconductor. We also show that the intrinsic residual losses in the FIR are dramatically reduced in the disordered crystal