Unified Treatment of Magnons and Excitons in Monolayer CrI$_3$ from Many-Body Perturbation Theory

TL;DR

This paper uses many-body perturbation theory to unify the description of magnons and excitons in monolayer CrI$_3$, accurately predicting spectra and revealing the nature of magnon spin and excitation amplitudes without relying on specific magnetic interaction models.

Contribution

It provides a model-independent first-principles approach to simultaneously treat magnons and excitons in CrI$_3$, including spin-orbit effects, and offers new insights into magnon properties.

Findings

Optical absorption spectrum matches experimental data

Magnon dispersion agrees with measurements

Estimated magnon gap of 0.3 meV

Abstract

We present first principles calculations of the two-particle excitation spectrum of CrI$_3$ using many-body perturbation theory including spin-orbit coupling. Specifically, we solve the Bethe-Salpeter equation, which is equivalent to summing up all ladder diagrams with static screening and it is shown that excitons as well as magnons can be extracted seamlessly from the calculations. The resulting optical absorption spectrum as well as the magnon dispersion agree very well with recent measurements and we extract the amplitude for optical excitation of magnons resulting from spin-orbit interactions. Importantly, the results do not rely on any assumptions on the microscopic magnetic interactions such as Dzyaloshinskii-Moriya (DM), Kitaev or biquadratic interactions and we obtain a model independent estimate of the gap between acoustic and optical magnons of 0.3 meV. In addition, we resolve the magnon wavefunction in terms of band transitions and show that the magnon carries a spin that is significantly smaller than $\hbar$. This highlights the importance of terms that do not commute with $S^z$ in any Heisenberg model description.