title: Anomalous proton dynamics in ice at low temperatures.
abstract: We present incoherent quasielastic neutron scattering measurements on ice Ih (ordinary ice) and Ic (cubic ice) which show the existence of nonharmonic motion of hydrogen at low temperatures, down to 5 K. We show that this dynamics is localized, nonvibrational, and related to the hydrogen disorder since it is absent in ordered ice VIII. A main jump distance of 0.75 A is identified, hence close to the distance between the two possible proton sites along the oxygen-oxygen bond. The dynamics is non-Arrhenius, has a large time rate of 2.7x10(11) s-1, and affects only a few percent of the total number of hydrogen atoms in the crystal. These results give evidence for the existence of concerted proton tunneling in these ice phases.
authors.affiliation: IMPMC, CNRS-UMR 7590, Université P&M Curie, F-75252 Paris, France.
authors.forename: L E
authors.initials: LE
authors.lastname: Bove
authors.forename: S
authors.initials: S
authors.lastname: Klotz
authors.forename: A
authors.initials: A
authors.lastname: Paciaroni
authors.forename: F
authors.initials: F
authors.lastname: Sacchetti