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

Direct-gap materials hold promises for excitonic insulators. 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 a divergence of polarizability when the band gap approaches zero, leading to a 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 TiS3 lend solid support to this principle.