# Magnon-polaron excitations in the noncollinear antiferromagnet Mn$_3$Ge

## TL;DR

This study investigates magnon-polaron excitations in Mn$_3$Ge using inelastic neutron scattering, revealing hybridized spin-phonon modes and magnetoelastic coupling in a noncollinear antiferromagnet.

## Contribution

It provides the first detailed experimental and theoretical analysis of magnon-polaron modes in Mn$_3$Ge, highlighting the hybridization of magnons and phonons in this compound.

## Key findings

- Identification of a 5-meV magnon gap and anisotropic dispersive modes up to 90 meV.
- Observation of zone-center excitations with magnon-phonon hybrid characteristics.
- Confirmation of magnetoelastic coupling consistent with negative thermal expansion.

## Abstract

We present the detailed inelastic neutron scattering measurements of the noncollinear antiferromagnet Mn$_3$Ge. Time-of-flight and triple-axis spectroscopy experiments were conducted at the temperature of 6~K, well below the high magnetic ordering temperature of 370~K. The magnetic excitations have a 5-meV gap and display an anisotropic dispersive mode reaching $\simeq 90$~meV at the boundaries of the magnetic Brillouin zone. The spectrum at the zone center shows two additional excitations that demonstrate characteristics of both magnons and phonons. The \textit{ab initio} lattice-dynamics calculations show that these can be associated with the magnon-polaron modes resulting from the hybridization of the spin fluctuations and the low-energy optical phonons. The observed magnetoelastic coupling agrees with the previously found negative thermal expansion in this compound and resembles the features reported in the spectroscopic studies of other antiferromagnets with the similar noncollinear spin structures.