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

The copper isotope effect ((CU)-C-63 vs. (CU)-C-65) On the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature su perconductor HoBa2Cu4O8 has been investigated by inelastic neutron scatteri ng. For the 63CU compound there is clear evidence for the opening of an ele ctronic gap in the normal state at T* approximate to 160 K far above T-c = 79.0 K. Upon substitution of (CU)-C-63 by (CU)-C-65, T-c decreases marginal ly to 78.6 K, whereas T* is increased to about 185 K. This large copper iso tope shift DeltaT*(Cu) = T*((65) Cu) - T* ((63) CU) 77 approximate to 25 K - together with the corresponding oxygen isotope shift DeltaT*(O) = T*(O-18 ) - T*(O-16) approximate to 50 K found in an earlier investigation - sugges ts that phonons or lattice fluctuations involving both the copper and the o xygen ions are important for the pairing mechanism in high-T-c materials.