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 $T^{*}\approx 170\mathrm{K}$ far above $T_c=79\mathrm{K}$. 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 ${10}^{-8\mathrm{}}\gg \tau >{10}^{-13\mathrm{}}\mathrm{s}$.

• Received 10 May 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.1990