Anomalous oxygen isotope effect inLa2−xSrxCuO4

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Abstract

We report measurement of the oxygen isotope effect ${\ensuremath{\alpha}}_{\mathrm{O}}$ in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ as a function of x. For values of x<0.15, ${\ensuremath{\alpha}}_{\mathrm{O}}$ in the is large (0.4--0.6) but decreases by about a factor of 5 for x\ensuremath{\ge}0.15, where we obtain ${\ensuremath{\alpha}}_{\mathrm{O}\mathrm{\ensuremath{\approxeq}}0.1}$, in good agreement with earlier results. The large values of ${\ensuremath{\alpha}}_{\mathrm{O}}$ obtained for small x are reproducible when the $^{18}\mathrm{O}$${,}^{16}$O sample pairs are cross exchanged. The maximum measured values of ${\ensuremath{\alpha}}_{\mathrm{O}}$ fall outside the range of standard Bardeen-Cooper-Schrieffer theory and may imply anharmonic phonon-mediated superconductivity. The variability of ${\ensuremath{\alpha}}_{\mathrm{O}}$ may be related to a nearby lattice instability, similar to that observed in (La,Ba${)}_{2}$${\mathrm{CuO}}_{4}$. Isotope frequency shifts of the far-infrared transverse-optic phonons of ${A}_{2u}$ symmetry at 240 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ also display an anomaly between x=0.11 and 0.15 and, in addition, demonstrate $^{18}\mathrm{O}$ exchange for both oxygen sites.

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