Topological magnon insulator spin excitations in the two-dimensional ferromagnet CrBr$_3$

TL;DR

This study experimentally observes topological magnon excitations in the 2D ferromagnet CrBr3, revealing a nontrivial gap and topological properties consistent with a magnon insulator, combining neutron scattering and theoretical modeling.

Contribution

It provides the first experimental evidence of topological magnon insulator behavior in a 2D ferromagnet, supported by detailed theoretical calculations.

Findings

Observation of a 3.5 meV magnon gap at the K point

Identification of nonzero Chern numbers for magnon bands

CrBr3 classified as a topological magnon insulator

Abstract

Topological magnons are bosonic analogues of topological fermions in electronic systems. They have been studied extensively by theory but rarely realized by experiment. Here, by performing inelastic neutron scattering measurements on single crystals of a two-dimensional ferromagnet CrBr$_3$, which was classified as Dirac magnon semimetal featured by the linear bands crossing at the Dirac points, we fully map out the magnetic excitation spectra, and reveal that there is an apparent gap of $\sim$3.5~meV between the acoustic and optical branches of the magnons at the K point. By collaborative efforts between experiment and theoretical calculations using a five-orbital Hubbard model obtained from first-principles calculations to derive the exchange parameters, we find that a Hamiltonian with Heisenberg exchange interactions, next-nearest-neighbor Dzyaloshinskii-Moriya (DM) interaction, and single-ion anisotropy is more appropriate to describe the system. Calculations using the model show that the lower and upper magnon bands separated by the gap exhibit Chern numbers of $\pm1$. These results indicate that CrBr$_3$ is a topological magnon insulator, where the nontrivial gap is a result of the DM interaction.