Evidence for very strong electron-phonon coupling in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Y</mml:mi><mml:msub><mml:mi mathvariant="normal">Ba</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

From the observed oxygen-isotope shift of the midinfrared two-magnon absorption peak of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$, we evaluate the oxygen-isotope effect on the in-plane antiferromagnetic exchange energy $J$. The exchange energy $J$ in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$ is found to decrease by about 0.9% upon replacing $^{16}\mathrm{O}$ by $^{18}\mathrm{O}$, which is slightly larger than that (0.6%) in ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$. From the oxygen-isotope effects, we determine the lower limit of the polaron binding energy, which is about $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$ and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$, in quantitative agreement with angle-resolved photoemission data, optical conductivity data, and the parameter-free theoretical estimate. The large polaron binding energies in the insulating parent compounds suggest that electron-phonon coupling should also be strong in doped superconducting cuprates and play an essential role in high-temperature superconductivity.

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