Dirac Magnon Nodal Loops in Quasi-2D Quantum Magnets

In this report, we propose a new concept of one-dimensional (1D) closed lines of Dirac magnon nodes in two-dimensional (2D) momentum space of quasi-2D quantum magnetic systems. They are termed "2D Dirac magnon nodal-line loops". We utilize the bilayer honeycomb ferromagnets with intralayer coupling $J$ and interlayer coupling $J_L$, which is realizable in the honeycomb chromium compounds CrX$_3$ (X $\equiv$ Br, Cl, and I). However, our results can also exist in other layered quasi-2D quantum magnetic systems. Here, we show that the magnon bands of the bilayer honeycomb ferromagnets overlap for $J_L\neq 0$ and form 1D closed lines of Dirac magnon nodes in 2D momentum space. The 2D Dirac magnon nodal-line loops are topologically protected by inversion and time-reversal symmetry. Furthermore, we show that they are robust against weak Dzyaloshinskii-Moriya interaction $\Delta_{DM}< J_L$ and possess chiral magnon edge modes.}