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.
Categories: cond-mat.supr-con