title: Goldstone-mode phonon dynamics in the pyrochlore Cd2Re2O7.
abstract: We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200 K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.
authors.affiliation: Naval Research Laboratory, Washington, DC 20375-6365, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Kendziora
authors.forename: I A
authors.initials: IA
authors.lastname: Sergienko
authors.forename: R
authors.initials: R
authors.lastname: Jin
authors.forename: J
authors.initials: J
authors.lastname: He
authors.forename: V
authors.initials: V
authors.lastname: Keppens
authors.forename: B C
authors.initials: BC
authors.lastname: Sales
authors.forename: D
authors.initials: D
authors.lastname: Mandrus