The oxygen isotope (${}^{16}\mathrm{O}$/${}^{18}\mathrm{O}$) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{*}$ is investigated for the cuprate superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ as a function of doping $x$ by means of x-ray absorption near edge structure studies. A strong $x$ dependent and sign reversed OIE on $T^{*}$ is observed. The OIE exponent ${\alpha }_{T^{*}}$ systematically decreases from ${\alpha }_{T^{*}}=-0.6\left(1.3\right)$ for $x=0.15$ to ${\alpha }_{T^{*}}=-4.4\left(1.1\right)$ for $x=0.06$, corresponding to increasing $T^{*}$ and decreasing superconducting transition temperature $T_c$. Both $T^{*}\left({}^{16}\mathrm{O}\right)$ and $T^{*}\left({}^{18}\mathrm{O}\right)$ exhibit a linear doping dependence with different slopes and critical end points [where $T^{*}\left({}^{16}\mathrm{O}\right)$ and $T^{*}\left({}^{18}\mathrm{O}\right)$ fall to zero] at $x_c\left({}^{16}\mathrm{O}\right)=0.201\left(4\right)$ and $x_c\left({}^{18}\mathrm{O}\right)=0.182\left(3\right)$, indicating a large positive OIE of $x_c$ with an exponent of ${\alpha }_{x_c}=0.84\left(22\right)$. The remarkably large and strongly doping dependent OIE on $T^{*}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

• Received 9 May 2016
• Revised 13 December 2016

DOI:https://doi.org/10.1103/PhysRevB.95.014514