Many-body description of two-dimensional van der Waals ferroelectric $\alpha-$In$_2$Se$_3$

TL;DR

This paper demonstrates that accurately modeling the electronic properties of 2D ferroelectric In$_2$Se$_3$ multilayers requires advanced many-body quasiparticle methods beyond standard density functional theory.

Contribution

The authors extend the Green function capabilities in the Questaal package to perform high-fidelity many-body extit{GW} calculations for 2D ferroelectric materials, revealing limitations of hybrid functionals.

Findings

Standard DFT and hybrid functionals may fail to predict the correct electronic gap.

Many-body extit{GW} calculations show significant deviations from simpler methods.

The electronic structure depends strongly on multilayer polarization configurations.

Abstract

Two-dimensional (2D) van der Waals ferroelectrics are recognized for enabling many applications, from memory and logic to neuromorphic computing, as well as transforming other materials to control electronic phase transitions and topological states. While these materials are typically weakly correlated and expected to have their ground-state properties well described with the commonly used density functional theory, by focusing on bilayers and trilayers of In$_2$Se$_3$ we show that this approach may not be reliable. The underlying electronic structure strongly depends on the polarization structure of the multilayer system and is surprisingly challenging to accurately calculate, requiring a high-fidelity many-body theory of the quasiparticle self-consistent \textit{GW} approximation. We develop this underlying description by extending the capabilities of Green function implementation within the open-source Questaal package. We show that even a sophisticated hybrid functional approach may fail to predict a nonvanishing gap in a bilayer In$_2$Se$_3$ and yields charge density, polarization, and band offsets that strongly deviate from the many-body picture. We discuss the implications of these computational advances for future opportunities in 2D ferroelectrics.