OPTICAL STUDY OF LOCALIZATION IN THE AB-PLANE CONDUCTIVITY OF SINGLE-CRYSTALS OF YBA2CU3O6.95 INDUCED BY ION DAMAGE

We studied the changes in the dc resistivity, Raman scattering, and IR conductivity of single crystals of YBa2Cu3O6.95 induced by damage fro m low-energy He+ ion bombardment. It appears that T(c), transport prop erties, and optical conductivity are strongly affected by modest irrad iation doses whereas the chemical composition of the sample is not mod ified. Carrier localization is evidenced by the frequency dependence o f the optical conductivity in the strongly damaged crystal and is used to explain the suppression of both the superfluid density and T(c) up on irradiation. The temperature dependence of the superfluid density i s in agreement with the theoretical predictions for a d-wave supercond uctor. We also show that the intrinsic residual losses in the FIR are dramatically reduced in the disordered crystal.