Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd_2Re_2O_7

摘要

We have measured the polarized Raman scattering spectra of Cd_2Re_2O_7, 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 B_1 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 E_u symmetry) of the O1 atoms which drives the SPT.