The oxygen isotope effect on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 has been investigated by means of inelastic neutron scattering. For the 16O compound there is clear evidence for the opening of an electronic gap in the normal state at T(*) approximately 170 K far above T(c) = 79 K. Upon oxygen isotope substitution ( 16O vs 18O) 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)>>tau>10(-13) s.

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The oxygen and copper isotope effects on the relaxation rate of crystal-field excitations in the optimally doped high-temperature superconductor La 1.81 Ho 0.04 Sr 0.15 CuO 4 have been investigated by means of inelastic neutron scattering. For the 16 O compound there is clear evidence for the opening of an electronic pseudogap in the normal state at T*Ϸ60 K, far above T c Ϸ32 K. Upon substituting 16 O by 18 O, T* is shifted to about 70 K. On the other hand, no shift is found for the copper isotope substitution, i.e., T*(63 Cu)ϷT*(65 Cu) Ϸ60 K. These results, together with the large oxygen and copper isotope effects on T* for HoBa 2 Cu 4 O 8 , give evidence that the pseudogap formation in the single-layer LSCO and bilayer YBCO type compounds is governed by an additional local mode in the latter.

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