Topological magnons in CrI$_3$ monolayers: an itinerant fermion description

TL;DR

This paper introduces a novel itinerant fermion model to describe topological magnons in monolayer CrI$_3$, revealing chiral edge states and topological properties without relying on a traditional spin Hamiltonian.

Contribution

It presents a first-principles derived fermion-based theory for magnons in 2D CrI$_3$, capturing topological features and edge states in a unified framework.

Findings

Magnon dispersion shows a gap at Dirac points with consistent Berry curvature.

Chiral in-gap edge states are observed in CrI$_3$ ribbons.

Wave function analysis confirms the topological nature of magnons.

Abstract

Magnons dominate the magnetic response of the recently discovered insulating ferromagnetic two dimensional crystals such as CrI$_3$. Because of the arrangement of the Cr spins in a honeycomb lattice, magnons in CrI$_3$ bear a strong resemblance with electronic quasiparticles in graphene. Neutron scattering experiments carried out in bulk CrI$_3$ 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 CrI$_3$ monolayers based on an itinerant fermion picture, with a Hamiltonian derived from first principles. We obtain the magnon dispersion for 2D CrI$_3$ with a gap at the Dirac points with the same Berry curvature in both valleys. For CrI$_3$ 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.