A many-body characterization of the fundamental gap in monolayer CrI$_3$

TL;DR

This paper uses advanced quantum Monte Carlo methods to accurately predict the fundamental electronic gap of monolayer CrI$_3$, demonstrating the importance of electron correlation effects over spin-orbit interactions in 2D magnetic materials.

Contribution

It applies many-body DMC methods to predict the fundamental gap of monolayer CrI$_3$, providing benchmark results and insights into the role of electron correlation and basis choices.

Findings

Fundamental gap of 2.9(1) eV agrees with experiments and GW calculations.

DMC yields consistent gap values in the thermodynamic limit.

Electron correlation is more critical than spin-orbit effects for the gap.

Abstract

The many-body fixed-node and fixed-phase spin-orbit Diffusion Monte Carlo (DMC) methods are applied to accurately predict the fundamental gap of monolayer CrI$_3$ - the first experimentally-realized 2D material with intrinsic magnetism. The fundamental gap obtained, 2.9(1)~eV, agrees well with the highest peak in optical spectroscopy measurements and a previous $GW$ result. We numerically show that as expected in DMC the same value of the fundamental gap is obtained in the thermodynamic limit using both neutral promotions and the standard quasiparticle definition of the gap based on the ionization potential and electron affinity. Additional analysis of the differences between density matrices formed in different bases using configuration interaction calculations explains why a single-reference trial wave function can produce an accurate excitation. We find that accounting for electron correlation is more crucial than accounting for spin-orbit effects in determining the fundamental gap. These results highlight how DMC can be used to benchmark 2D material physics and emphasize the importance of using beyond-DFT methods for studying 2D materials.