Anomalous oxygen-isotope effect on the in-plane far-infrared conductivity of detwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3^{16,18}{\mathrm{O}}_{6.9}$

We observe a large and anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3^{16,18}{\mathrm{O}}_{6.9}.$ For ${}^{18}\mathrm{O}$ a pronounced low-energy electronic mode (LEM) appears around $240{\mathrm{cm}}^{-1}.$ This a-axis LEM exhibits a clear aging effect, after 1 y it is shifted to $190{\mathrm{cm}}^{-1}.$ For ${}^{16}\mathrm{O}$ we cannot resolve a corresponding a-axis LEM above $120{\mathrm{cm}}^{-1}.$ We interpret the LEM in terms of a collective electronic mode that is pinned by “isotopic defects,” i.e., by the residual ${}^{16}\mathrm{O}$ in the matrix of ${}^{18}\mathrm{O}.$