Goldstone-mode phonon dynamics in the pyrochlore Cd2Re2O7.
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.
Kendziora
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CA
Naval Research Laboratory, Washington, DC 20375-6365, USA.
Sergienko
I A
IA
Jin
R
R
He
J
J
Keppens
V
V
Sales
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BC
Mandrus
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D