title: Large Isotope Effect on the Pseudogap in the High-Temperature Superconductor HoBa_{2}Cu_{4}O_{8}.
abstract: The oxygen isotope effect on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa_{2}Cu_{4}O_{8} has been investigated by means of inelastic neutron scattering. For the ^{16}O compound there is clear evidence for the opening of an electronic gap in the normal state at T^{*}≈170 K far above T_{c}=79K. Upon oxygen isotope substitution ( ^{16}O vs ^{18}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}≫τ>10^{-13}s.
authors.affiliation: Laboratory for Neutron Scattering, ETH Zürich and PSI Villigen, CH-5232 Villigen PSI, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Rubio Temprano
authors.forename: J
authors.initials: J
authors.lastname: Mesot
authors.forename: S
authors.initials: S
authors.lastname: Janssen
authors.forename: K
authors.initials: K
authors.lastname: Conder
authors.forename: A
authors.initials: A
authors.lastname: Furrer
authors.forename: H
authors.initials: H
authors.lastname: Mutka
authors.forename: K A
authors.initials: KA
authors.lastname: Müller