Proposed Strongly Correlated Excitonic Insulator in Nitrogen-Boron-Centered Triangulene Honeycomb Lattice

Excitonic insulator (EI) is a long-sought quantum phase with a many-body gap opening due to the Bose-Einstein condensate of electron-hole pairs, providing an intriguing platform for exploring many-body effects. However, due to the large dielectric screening, few materials are reported to be EIs without external fields. Using the Green's function-based many-body perturbation theory calculations and Bardeen-Cooper-Schrieffer (BCS)-like mean field theory, we present a heteroatom-centered triangulene monolayer with a small band gap and bandwidth that exhibits an intrinsic beyond-room-temperature EI ground state. The GW plus Bethe-Salpeter equation calculation results show that the exciton binding energy exceeds the band gap, and the BCS-like model shows an additional excitonic gap existing. We also present a possible fingerprint of this system in the transmittance spectrum for the experimental observation. Our findings show that such precisely synthesized two-dimensional polymers will promise to enhance the exploration of nontrivial excitonic states and many quantum phenomena.