Anomalous oxygen isotope effect on the in-plane FIR conductivity of detwinned YBa2Cu3${}^{16,18}$O${}_{6.9}$

We observe an anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned YBa2Cu3${}^{16,18}$O${}_{6.9}$. For ${}^{18}$O a pronounced low-energy electronic mode (LEM) appears around 240 cm${}^{-1}$. This a-axis LEM exhibits a clear aging effect, after one year it is shifted to 190 cm${}^{-1}$. For ${}^{16}$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 ${}^{16}$O in the matrix of ${}^{18}$O.