Optical and magneto-optical properties of ferromagnetic monolayer CrBr$_3$: A first-principles $GW$ and $GW$ plus Bethe-Salpeter equation study

TL;DR

This study employs first-principles many-body perturbation theory to analyze the optical and magneto-optical properties of monolayer CrBr3, revealing strong excitonic effects, significant quasiparticle band modifications, and potential for optoelectronic applications.

Contribution

It introduces a comprehensive theoretical framework combining $GW$ and BSE methods for 2D ferromagnetic semiconductors, including detailed formalism for magneto-optical effects.

Findings

Large exciton binding energy of 2.3 eV in CrBr3

3.8 eV indirect band gap due to self-energy effects

Magneto-optical signals depend on excitation frequency and substrate refractive index

Abstract

The discovery of atomically thin two-dimensional (2D) magnetic semiconductors has triggered enormous research interest recently. In this work, we use first-principles many-body perturbation theory to study a prototypical 2D ferromagnetic semiconductor, monolayer chromium tribromide (CrBr$_3$). With broken time-reversal symmetry, spin-orbit coupling, and excitonic effects included through the full-spinor $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) methods, we compute the frequency-dependent layer polarizability tensor and dielectric function tensor that govern the optical and magneto-optical properties. In addition, we provide a detailed theoretical formalism for simulating magnetic circular dichroism, magneto-optical Kerr effect, and Faraday effect, demonstrating the approach with monolayer CrBr$_3$. Due to reduced dielectric screening in 2D and the localized nature of the Cr $d$ orbitals, we find strong self-energy effects on the quasiparticle band structure of monolayer CrBr$_3$ that give a 3.8 eV indirect band gap. Also, excitonic effects dominate the low-energy optical and magneto-optical responses in monolayer CrBr$_3$ where a large exciton binding energy of 2.3 eV is found for the lowest bright exciton state with excitation energy at 1.5 eV. We further find that the magneto-optical signals demonstrate strong dependence on the excitation frequency and substrate refractive index. Our theoretical framework for modeling optical and magneto-optical effects could serve as a powerful theoretical tool for future study of optoelectronic and spintronics devices consisting of van der Waals 2D magnets.