Evidence for magnon-phonon coupling in the topological magnet Cu$_3$TeO$_6$

TL;DR

This study provides evidence of magnon-phonon coupling in Cu$_3$TeO$_6$ through neutron scattering and thermodynamic measurements, revealing a hybrid magnon-polaron mode and its impact on thermal transport.

Contribution

It reports the first observation of a magnon-polaron mode in Cu$_3$TeO$_6$, demonstrating strong magnon-phonon coupling in a topological magnet.

Findings

Detection of a mode at 4.5 K that disappears above Néel temperature

Observation of a hybrid magnon-polaron mode from neutron scattering

Suppression of thermal conductivity indicating magnon-phonon interaction

Abstract

We perform thermodynamic and inelastic neutron scattering (INS) measurements to study the lattice dynamics (phonons) of a cubic collinear antiferromagnet Cu$_3$TeO$_6$ which hosts topological spin excitations (magnons). While the specific heat and thermal conductivity results show that the thermal transport is dominated by phonons, the deviation of the thermal conductivity from a pure phononic model indicates that there is a strong coupling between magnons and phonons. In the INS measurements, we find a mode in the excitation spectra at 4.5 K, which exhibits a slight downward dispersion around the Brillouin zone center. This mode disappears above the N\'{e}el temperature, and thus cannot be a phonon. Furthermore, the dispersion is distinct from that of a magnon. Instead, it can be explained by the magnon-polaron mode, which is new collective excitations resulting from the hybridization between magnons and phonons. We consider the suppression of the thermal conductivity and emergence of the magnon-polaron mode to be evidence for magnon-phonon coupling in Cu$_3$TeO$_6$.