Type-II Dirac points and Dirac nodal loops on the magnons of a square-hexagon-octagon lattice

We study topological magnons on an anisotropic square-hexagon-octagon lattice which has been found by a two-dimensional biphenylene network. We propose the concept of type-II Dirac magnonic states where new schemes to achieve topological magnons are unfolded without requiring the Dzyaloshinskii-Moriya interactions (DMIs). In the ferromagnetic states, the topological distinctions at the type-II Dirac points along with one-dimensional closed lines of Dirac magnon nodes are characterized by the Berry phase and the ${\mathbb{Z}}₂$ invariant. We find pair annihilation of the Dirac magnons and use the Wilson loop method to depict the topological protection of the band degeneracy. The Green's function approach is used to calculate chiral edge modes and the magnon density of states. We introduce the DMIs to gap the type-II Dirac magnon points and demonstrate that the Dirac nodal loops are robust against the DMIs within a certain parameter range. The topological phase diagram of magnon bands is given via calculating the Berry curvature and Chern number. We find that the anomalous thermal Hall conductivity gives connection to the magnon edge current. Furthermore, we derive the differential gyromagnetic ratio to exhibit the Einstein--de Haas effect of magnons with topological features.