Experimentally feasible CrBr<sub>3</sub> monolayer: Electronic and magnonic topological states correlated with rotation symmetry

<p>Topological magnets have been the focus of recent interest with both the nontrivial fermions and bosons extensively explored. However, their emergence within one realistic material remains largely unexplored. Here, we demonstrate the emergence of electronic and magnonic topological states, i.e., second-order topological insulator (SOTI) and topological magnon insulator (TMI), in the experimentally feasible 2D CrBr<sub>3</sub> monolayer. For which the electronic SOTI is characterized by the second Stiefel-Whitney number <i>w</i><sub>2</sub>=1 and the emergence of well-localized corner states arising from the rotation symmetry <i>C</i><sub>3</sub>. Moreover, to achieve the TMI phase, we show that <i>C</i><sub>3</sub> serves as a key factor for obtaining the nonzero next-nearest-neighbor Dzyaloshinskii-Moriya interaction that is essential to open the magnon band gap and induce the nontrivial magnon band topology. The calculated Chern number for TMI is <i>C</i>=1, and one chiral magnon edge state is indeed obtained. These explored phenomena and insights not only considerably bridge the topological aspect of fermions and bosons but also enable innovative applications in topotronics devices.</p>