Excitons probe intrinsic flat band Mottness in a van der Waals heterostructure

Abstract

Excitons provide a sensitive optical probe of electronic correlations in nearby two-dimensional materials, yet their coupling to intrinsic flat-band Mott systems remains largely unexplored. Here we combine gate-tunable optical spectroscopy with first-principles calculations to study monolayer WSe${}_2$ in direct contact with the van der Waals Mott insulator Nb${}_3$Cl${}_8$. The gate evolution of WSe${}_2$ excitonic resonances reveals signatures of a correlation-reconstructed Mott gap in Nb${}_3$Cl${}_8$ that is absent from the single-particle band picture. In the electron-doped regime, the WSe${}_2$ 2s Rydberg exciton undergoes a multistage evolution and develops into interlayer attractive and repulsive polaron branches, showing that a Rydberg exciton can be dressed by strongly correlated flat-band electrons in an adjacent Mott layer. Under an out-of-plane magnetic field, spin-polarized Nb${}_3$Cl${}_8$ states further induce valley-selective exciton coupling, producing a strongly enhanced circular polarization of the WSe${}_2$ exciton emission. These results extend exciton-based sensing and exciton-polaron physics to intrinsic flat-band Mott materials, providing an optical route to probe and engineer correlation-driven interfacial quasiparticles.

9 pages, 4 figures