Poster: Discovery of a single-band Mott insulator in a van der Waals flat-band compound*

Many-body quantum phenomena can arise when the Fermi level lies within a flat band, because electron-electron interactions greatly exceed the quenched kinetic energy of electrons. For example, Mott insulator-like states and superconductivity at low temperatures were discovered in magic-angle twisted bilayer graphene with flat bands near the Fermi level. However, these low-energy bands are complex and exhibit multi-band features, making experimental verification and theoretical analysis challenging. Here, we report a single-band Mott insulator state in a van der Waals layered compound Nb₃Cl₈. In the high-temperature phase, this Mott insulator state can be ideally described by the typical single-band Hubbard model based on an isolated half filled flat band. The excellent agreement between experiment and theory unambiguously confirms that monolayer Nb₃Cl₈ is a single-band Mott insulator. Further, we revealed that the Hubbard bands form bonding/anti-bonding states in the low-temperature phase, leading to a non-magnetic spin-singlet state as its ground state, in which the Mott-Hubbard physics continues to dominate the insulating electronic state. The Mott insulator state in Nb₃Cl₈, which involves only one single low-energy band, provides an excellent platform for studying the intrinsic physics of the single-band Hubbard model. In the future, additional correlated states and exotic physical phenomena can be identified by tuning the Mott insulator state.