Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>La</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Sr</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>CuO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>

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 ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ 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 ${\ensuremath{\alpha}}_{{T}^{*}}$ systematically decreases from ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}0.6(1.3)$ for $x=0.15$ to ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}4.4(1.1)$ for $x=0.06$, corresponding to increasing ${T}^{*}$ and decreasing superconducting transition temperature ${T}_{c}$. Both ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ exhibit a linear doping dependence with different slopes and critical end points [where ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ fall to zero] at ${x}_{c}(^{16}\mathrm{O})=0.201(4)$ and ${x}_{c}(^{18}\mathrm{O})=0.182(3)$, indicating a large positive OIE of ${x}_{c}$ with an exponent of ${\ensuremath{\alpha}}_{{x}_{c}}=0.84(22)$. 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.

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