Flat-Bands-Enabled Triplet Excitonic Insulator in a Di-atomic (Yin-Yang) Kagome Lattice*

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure of narrow-gap semiconductors towards exciton formation. Here we show that the flat valence and conduction bands of a Yin-Yang Kagome lattice, as exemplified in a superatomic graphene lattice, conspire to enable an interesting state of triplet EI, based on first-principles calculations combined with many-body GW and Bethe-Salpeter equation. As a natural manifestation of flat bands, highly localized electron and hole wavefunctions significantly reduce the screening and enhance the exchange interaction. This leads to an unusually high triplet exciton binding energy (~1.2 eV) exceeding the GW band gap by ~0.2 eV and a huge singlet-triplet splitting of ~0.4 eV. The singlet exciton binding energy is still very large with a long intrinsic lifetime. The flat-bands-enabled triplet EI state also points to the possibility of complete population inversion between the Yin-Yang topological flat bands for the realization of excited quantum spin Hall effect.