Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet

TL;DR

This paper reports the experimental discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet, revealing a new topological state of matter with potential for quantum materials research.

Contribution

It provides the first experimental evidence of 3D topological magnons with coexisting Dirac and triply degenerate nodes in Cu$_{3}$TeO$_{6}$, supported by theoretical modeling.

Findings

Observation of symmetry-protected Dirac and triply degenerate magnon nodes

Spectroscopic evidence from inelastic neutron scattering

Theoretical confirmation via linear-spin-wave theory

Abstract

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realization of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu$_{3}$TeO$_{6}$ with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac-Weyl framework. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linear-spin-wave theory based on a Hamiltonian dominated by the nearest-neighbour exchange interaction $J_1$. As such, we showcase Cu$_{3}$TeO$_{6}$ as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in quantum materials.