Large Isotope Effect on the Pseudogap in the High-Temperature Superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>HoBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The oxygen isotope effect on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor ${\mathrm{HoBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ has been investigated by means of inelastic neutron scattering. For the ${}^{16}\mathrm{O}$ compound there is clear evidence for the opening of an electronic gap in the normal state at ${T}^{*}\ensuremath{\approx}170\mathrm{K}$ far above ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}79\phantom{\rule{0ex}{0ex}}\mathrm{K}$. Upon oxygen isotope substitution ( ${}^{16}\mathrm{O}$ vs ${}^{18}\mathrm{O}$) ${T}_{c}$ decreases marginally to 78.5 K, whereas ${T}^{*}$ is shifted to about 220 K. This huge isotope shift observed for ${T}^{*}$ which is absent in NMR and NQR experiments suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range ${10}^{\ensuremath{-}8}\ensuremath{\gg}\ensuremath{\tau}&gt;{10}^{\ensuremath{-}13}\phantom{\rule{0ex}{0ex}}\mathrm{s}$.

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