Topological magnons in CrI3 monolayers: an itinerant fermion description
Abstract
Magnons dominate the magnetic response of the recently discovered insulating ferromagnetic two dimensional crystals such as CrI3. Because of the arrangement of the Cr spins in a honeycomb lattice, magnons in CrI3 bear a strong resemblance with electronic quasiparticles in graphene. Neutron scattering experiments carried out in bulk CrI3 show the existence of a gap at the Dirac points, that has been conjectured to have a topological nature. Here we propose a theory for magnons in ferromagnetic CrI3 monolayers based on an itinerant fermion picture, with a Hamiltonian derived from first principles. We obtain the magnon dispersion for 2D CrI3 with a gap at the Dirac points with the same Berry curvature in both valleys. For CrI3 ribbons, we find chiral in-gap edge states. Analysis of the magnon wave functions in momentum space further confirms their topological nature. Importantly, our approach does not require to define a spin Hamiltonian, and can be applied to both insulating and conducting 2D materials with any type of magnetic order.