Anomalous oxygen isotope effect in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_4$

We report measurement of the oxygen isotope effect ${\alpha }_{\mathrm{O}}$ in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ as a function of x. For values of x<0.15, ${\alpha }_{\mathrm{O}}$ in the & is large (0.4–0.6) but decreases by about a factor of 5 for x≥0.15, where we obtain ${\alpha }_{\mathrm{O}\mathrm{\approxeq }0.1}$, in good agreement with earlier results. The large values of ${\alpha }_{\mathrm{O}}$ obtained for small x are reproducible when the ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ ${,}^{16}$O sample pairs are cross exchanged. The maximum measured values of ${\alpha }_{\mathrm{O}}$ fall outside the range of standard Bardeen-Cooper-Schrieffer theory and may imply anharmonic phonon-mediated superconductivity. The variability of ${\alpha }_{\mathrm{O}}$ may be related to a nearby lattice instability, similar to that observed in (La,Ba${)}_2$ ${\mathrm{CuO}}_4$. Isotope frequency shifts of the far-infrared transverse-optic phonons of $A_{2u}$ symmetry at 240 and 500 ${\mathrm{cm}}^{\mathrm{-}1}$ also display an anomaly between x=0.11 and 0.15 and, in addition, demonstrate ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ exchange for both oxygen sites.