Mott state of flat bands in a 2D metal-organic Kagome framework

Mott states of flat bands (FBs), as a representative strong correlation effect, have recently attracted much attention. However, observation of Mott states has been mostly confined to two-dimensional (2D) inorganic materials. Here, we report the observation of Mott states associated with FBs in a 2D organic quantum material. We have synthesized a large-scale uniform 2D Ag-(BPhen)₃ metal-organic framework (MOF) of a Kagome lattice on the Ag(111) surface. Scanning tunneling microscopy/spectroscopy measurements show high density of states around the charge neutrality point within the MOF, which is consistent with density functional theory calculations that predict an FB bundle (consisting of an FB and flattened Dirac bands) located at the Fermi level (E _F). Interestingly, the FB at the E _F is observed to split into upper/lower Hubbard bands (UHBs/LHBs) with a Mott insulating gap of ∼85-103 meV, as evidenced by the inverted contrast of dI/dV maps between the LHB and UHB. Furthermore, temperature-dependent measurements show a Mott transition temperature of ∼15 K, at which the Mott gap closes. By deposition of K atoms, the gap size is seen to be reduced in the electron-doped framework, which indicates the filling of the correlated Mott states distinguishable from a trivial band insulator. Also, an insulator-to-metal transition was observed approaching a K-doped defect site. Our studies demonstrate a proof of concept for correlated Mott states of topological FBs in artificially synthesized 2D MOFs, opening a new avenue to organic FB superconductivity and exotic quantum many-body phenomena in organic systems.