Assignment of excitonic insulators in \textit{ab initio} theories: the case of NiBr$_2$

TL;DR

This study uses advanced first-principles methods to analyze NiBr₂'s electronic and optical properties, aiming to determine if it exhibits excitonic insulator behavior by examining excitonic energies.

Contribution

It provides a detailed extit{ab initio} analysis of NiBr₂ to assess its potential as an excitonic insulator, emphasizing the importance of convergence and approximation consistency.

Findings

Highlights the critical role of convergence in GW calculations.

Shows the importance of approximation consistency in identifying excitonic insulators.

Provides insights into the electronic structure of NiBr₂.

Abstract

In this work we perform a detailed first-principles analysis of the electronic and optical properties of NiBr$_2$ within the state-of-the-art $GW$+BSE scheme to determine whether this system displays negative excitonic energies, which would identify it as an (half) excitonic insulator. Particular attention is payed to the convergence of the $GW$ band structure and to the consistency between approximations employed in the ground-state calculations and approximations employed in the linear response calculations. We show that these two issues play a crucial role in identifying the excitonic nature of NiBr$_2$.