Large isotope effect on the pseudogap in the high-temperature superconductor HoBa2Cu4O8

The oxygen isotope effect on the relaxation rate of crystal-field excitatio ns in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 ha s been investigated by means of inelastic neutron scattering. For the O-16 compound there is clear evidence for the opening of an electronic Sap in th e normal state at T* approximate to 170 K far above T-c = 79 K. Upon oxygen isotope substitution (O-16 vs O-18) T-c decreases marginally to 78.5 K, wh ereas 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 lea ding to an isotope effect on the pseudogap has to involve a time scale in t he range 10(-8) much greater than tau > 10(-13) s.