Large copper isotope effect on the pseudogap in the high-temperature superconductor HoBa

The copper isotope effect (63Cu vs. 65Cu) on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 has been investigated by inelastic neutron scattering. For the 63Cu compound there is clear evidence for the opening of an electronic gap in the normal state at $T^{\ast} \approx$ 160 K far above $T_{\rm c}=79.0$ K. Upon substitution of 63Cu by 65Cu, Tc decreases marginally to 78.6 K, whereas $T^{\ast}$ is increased to about 185 K. This large copper isotope shift $\Delta T^{\ast} {\rm (Cu)}=T^{\ast}(^{65}{\rm Cu})-T^{\ast}(^{63}{\rm Cu}) \approx 25$ K -together with the corresponding oxygen isotope shift $\Delta T^{\ast}{\rm (O)}=T^{\ast}(^{18}{\rm O})-T^{\ast}(^{16}{\rm O}) \approx 50$ K found in an earlier investigation -suggests that phonons or lattice fluctuations involving both the copper and the oxygen ions are important for the pairing mechanism in high-Tc materials