# Ballistic magnon heat conduction and possible Poiseuille flow in the helimagnetic insulator Cu$_2$OSeO$_3$

## TL;DR

This study reports exceptionally high magnon thermal conductivity in Cu$_2$OSeO$_3$, revealing ballistic magnon and phonon transport and suggesting Poiseuille flow, making it a key system for magnon dynamics research.

## Contribution

It demonstrates ballistic magnon and phonon transport in Cu$_2$OSeO$_3$ and proposes Poiseuille flow as an explanation for enhanced magnon mean-free paths.

## Key findings

- Magnon thermal conductivity reaches ~70 W/mK near 5 K.
- Ballistic transport observed below 1 K for magnons and phonons.
- Evidence of Poiseuille flow indicating superfluid-like magnon behavior.

## Abstract

We report on the observation of magnon thermal conductivity $\kappa_m\sim$ 70 W/mK near 5 K in the helimagnetic insulator Cu$_2$OSeO$_3$, exceeding that measured in any other ferromagnet by almost two orders of magnitude. Ballistic, boundary-limited transport for both magnons and phonons is established below 1 K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free path substantially exceeding the specimen width for the least defective specimens in the range 2 K $<T<$ 10 K. These observations establish Cu$_2$OSeO$_3$ as a model system for studying long-wavelength magnon dynamics.