title: Anomalous magnetic excitations of cooperative tetrahedral spin clusters.
abstract: An inelastic neutron scattering study of Cu2Te2O5X2 (X=Cl, Br) shows strong dispersive modes with large energy gaps persisting far above TN, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cl2 and the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.
authors.affiliation: Laboratory for Neutron Scattering, ETH Zürich and Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland. krunoslav.prsa@psi.ch
authors.forename: K
authors.initials: K
authors.lastname: Prsa
authors.forename: H M
authors.initials: HM
authors.lastname: Rønnow
authors.forename: O
authors.initials: O
authors.lastname: Zaharko
authors.forename: N B
authors.initials: NB
authors.lastname: Christensen
authors.forename: J
authors.initials: J
authors.lastname: Jensen
authors.forename: J
authors.initials: J
authors.lastname: Chang
authors.forename: S
authors.initials: S
authors.lastname: Streule
authors.forename: M
authors.initials: M
authors.lastname: Jiménez-Ruiz
authors.forename: H
authors.initials: H
authors.lastname: Berger
authors.forename: M
authors.initials: M
authors.lastname: Prester
authors.forename: J
authors.initials: J
authors.lastname: Mesot