Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd2Re2O7

TL;DR

This study reports the first observation of a Goldstone phonon in a crystalline solid, observed via Raman scattering in Cd2Re2O7, revealing insights into symmetry breaking and collective excitations during a structural phase transition.

Contribution

The paper provides the first experimental detection of a Goldstone phonon in a crystalline material, linked to a continuous symmetry breaking during a structural phase transition.

Findings

Goldstone phonon observed below 200K in Cd2Re2O7

Goldstone phonon appears with a soft mode at the phase transition

Order parameter linked to Eu symmetry vibration of O1 atoms

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 200K, 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.