# Magnonic Weyl states in Cu2OSeO3

## TL;DR

This paper explores the topological magnon excitations in Cu2OSeO3, revealing Weyl magnon nodes, their splitting due to Dzyaloshinsky-Moriya interactions, and potential measurable thermal Hall effects.

## Contribution

It provides a systematic analysis of magnon Weyl states in Cu2OSeO3, including theoretical predictions and experimental verification of topological properties.

## Key findings

- Weyl magnon nodes with topological charges ±2 identified at high-symmetry points
- Dzyaloshinsky-Moriya interactions cause splitting of Weyl points
- Thermal Hall conductivity linked to Weyl points and tunable by crystal symmetry

## Abstract

The multiferroic ferrimagnet Cu$_2$OSeO$_3$ with a chiral crystal structure attracted a lot of recent attention due to the emergence of magnetic skyrmion order in this material. Here, the topological properties of its magnon excitations are systematically investigated by linear spin-wave theory and inelastic neutron scattering. When considering Heisenberg exchange interactions only, two degenerate Weyl magnon nodes with topological charges $\pm$2 are observed at high-symmetry points. Each Weyl point splits into two as the symmetry of the system is further reduced by including into consideration the nearest-neighbor Dzyaloshinsky-Moriya interaction, crucial for obtaining an accurate fit to the experimental spin-wave spectrum. The predicted topological properties are verified by surface state and Chern number analysis. Additionally, we predict that a measurable thermal Hall conductivity can be associated with the emergence of the Weyl points, the position of which can be tuned by changing the crystal symmetry of the material.