Many-Body Correlation Effects and Excitons in Breathing Kagome Lattice Materials with Flat Bands

Motivated by the recent discovery of flat bands (FBs) in breathing Kagome lattice (BKL) materials, here, we present a detailed first-principles study of the optical response of single-layer (SL) Nb₃Cl₈ using the GW-Bethe-Salpeter equation (GW-BSE) method, incorporating self-energy corrections, band renormalization, and excitonic effects. In contrast to twisted bilayer semiconductors, where FBs are buried within the valence or conduction bands, in SL Nb₃Cl₈ FBs are distinctly visible due to their clear separation from other bands, making them easily accessible for experimental investigation. Our findings unveil a rich spectrum of strongly bound excitons. The key findings are twofold: first, SL Nb₃Cl₈ exhibits a tightly bound dark exciton ground state; second, the first bright exciton peak occurs at 1.13 eV, in excellent agreement with recent ARPES experiments. These findings shed new light on the complex Coulomb interactions between electrons and holes in BKL materials, particularly in the context of FBs.