Anomalous oxygen-isotope effect on the in-plane far-infrared conductivity of detwinnedYBa2Cu316,18O6.9

We observe a large and anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned YBa 2 Cu 1 6 , 1 8 3O 6 . 9 . For 1 8 O a pronounced low-energy electronic mode (LEM) appears around 240 cm - 1 . This a-axis LEM exhibits a clear aging effect, after 1 y it is shifted to 190 cm - 1 . For 1 6 O we cannot resolve a corresponding a-axis LEM above 120 cm - 1 . We interpret the LEM in terms of a collective electronic mode that is pinned by "isotopic defects," i.e., by the residual 1 6 O in the matrix of 1 8 O.