Topological magnons in a collinear altermagnet

Abstract: We propose a model with Weyl magnons and nodal-line magnons ($\mathbb{Z}_2$) in a collinear altermagnet on the honeycomb lattice. Altermagnetic magnon bands can be realized by breaking the symmetry of the second nearest neighbor exchange couplings without the Dzyaloshinskii-Moriya interaction (DMI). In addition to the Chern number and $\mathbb{Z}_2$ invariant, chirality is important to describe the magnon topology. The model shows the nonzero magnon spin Nernst effect when a longitudinal temperature gradient exists. We calculate the differential gyromagnetic ratio induced purely by the topology of altermagnetic magnons, which can be probed through the Einstein-de Haas (EdH) effect.