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

Song Bao,Xiangang Wan,Jinsheng Wen,Kejing Ran,Shichao Li,D L Abernathy,Di Wang,Zhen Ma,Jian-Xin Li,Wei Wang,Zhengwei Cai,Jinghui Wang,Shun-Li Yu,Zhao-Yang Dong

doi:10.1038/s41467-018-05054-2

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 realisation of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu3TeO6 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 J1. As such, we showcase Cu3TeO6 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.

Highlights

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
Neutron powder diffraction has shown that Cu3TeO6 develops a long-range collinear antiferromagnetic order below the transition temperature TN of 61 K, with spins aligned along the [111] direction[57,58]
inelastic neutron scattering (INS) is a direct approach to visualise magnon bands in the momentum and energy space, which acts as angle-resolvedphotoemission spectroscopy in characterising electronic band structures[5,8,17]

Summary

Introduction

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. We measure the spin excitations in Cu3TeO6 with inelastic neutron scattering (INS), and compare the INS data with the linear-spin-wave calculations performed based on a Hamiltonian dominated by the nearest-neighbour (NN) exchange interaction J1.

Results

Conclusion