Topological spin excitations in a three-dimensional antiferromagnet*

The recent discovery of topological semimetals, which possess distinct electron-band crossing with non-trivial topological characteristics in the bulk, has stimulated intense research interest. By extending the notion of symmetry-protected band crossing into one of the simplest magnetic groups, namely by including the symmetry of time-reversal followed by space-inversion, we predict the existence of topological magnon-band crossing in three-dimensional antiferromagnets [1]. The crossing may take the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a "spin-web" compound, Cu₃TeO₆, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using linear spin-wave approximation [1]. Inelastic neutron scattering experiments have been carried out to detect the bulk magnon-band crossing in a single-crystal sample [2]. The highly interconnected nature of the spin-1/2 lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with the linear spin-wave calculations. The predicted topological magnon Dirac points are confirmed. Further studies will be discussed, including determination of non-collinear spin canting in the ground state with neutron diffraction, and search for topological magnon surface states with inelastic neutron scattering.

[1] K. Li et al., Phys. Rev. Lett. 119, 247202 (2017).
[2] W. Yao et al., Nat. Phys. 14, 1011 (2018).