Realizing intrinsic excitonic insulator by decoupling exciton binding energy from the minimum band gap

TL;DR

This paper proposes a new principle to realize intrinsic excitonic insulators by decoupling exciton binding energy from the band gap in direct-gap materials, supported by first-principles calculations on 2D GaAs and TiS3.

Contribution

It introduces a novel approach to achieve excitonic insulators by exploiting parity of band-edge states, overcoming previous limitations.

Findings

Decoupling of exciton binding energy from band gap demonstrated in 2D materials.

First-principles calculations support the feasibility of the new principle.

Potential pathway for realizing intrinsic excitonic insulators in direct-gap materials.

Abstract

Direct-gap materials hold promises for excitonic insulator. In contrast to indirect-gap materials, here the difficulty to distinguish from a Peierls charge density wave is circumvented. However, direct-gap materials still suffer from the divergence of polarizability when the band gap approaches zero, leading to diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. First-principles calculations of two-dimensional GaAs and experimentally mechanically exfoliated single-layer TiS 3 lend solid supports to the new principle.