Flat-Bands-Enabled Triplet Excitonic Insulator in a Di-atomic Kagome Lattice

TL;DR

This paper predicts a triplet excitonic insulator state in a diatomic Kagome lattice with flat bands, showing large exciton binding energies and singlet-triplet splitting, expanding the understanding of excitonic insulators in flat band systems.

Contribution

It demonstrates, through advanced computational methods, that flat bands in a diatomic Kagome lattice can host a triplet excitonic insulator state with significant binding energy and splitting.

Findings

Large triplet exciton binding energy (~1.1 eV) exceeding the GW band gap.

Significant singlet-triplet splitting (~0.4 eV).

Flat bands promote strong electron-hole interactions.

Abstract

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure towards exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can possibly conspire to enable an interesting triplet EI state, based on density functional theory (DFT) calculations combined with many-body GW and Bethe-Salpeter Equation(BSE). Our results indicate that massive carriers in flat bands with highly localized electron and hole wavefunctions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (~1.1 eV) exceeding the GW band gap by ~0.2 eV and a large singlet-triplet splitting of ~0.4 eV. Our findings enrich once again the intriguing physics of flat bands and extend the scope of EI materials.