Excitonic insulators and superfluidity in 2D bilayers without external fields

TL;DR

This paper investigates intrinsic excitonic insulators in 2D Janus bilayers, revealing their potential for superfluidity and novel excitonic phases through ab initio calculations and mean-field analysis.

Contribution

It introduces a new class of intrinsic excitonic insulators in Janus bilayers, constructed from first-principles data, and identifies candidate materials with superfluid properties.

Findings

Identified 16 vdW Janus bilayers with excitonic insulating ground states.

Demonstrated superfluid properties in these bilayers.

Highlighted the role of spin-orbit coupling and screening in excitonic phases.

Abstract

We explore a new platform for realising excitonic insulators, namely van der Waals (vdW) bilayers comprising 2D Janus materials. In previous studies, Type-II heterobilayers have been brought to the excitonic insulating regime by tuning the band alignment using external gates. In contrast, the Janus bilayers of the current work represent intrinsic excitonic insulators. We first conduct ab initio calculations to obtain the quasiparticle band structures, screened Coulomb interaction, and interlayer exciton binding energies of the bilayers. These ab initio-derived quantities are then used to construct a BCS-like Hamiltonian of the exciton condensate. By solving the mean-field gap equation, we identify 16 vdW Janus bilayers with insulating ground states and superfluid properties. Our calculations expose a new class of advanced materials that are likely to exhibit novel excitonic phases at low temperatures, and highlight the subtle competition between interlayer hybridization, spin-orbit coupling, and dielectric screening that govern their properties.