Atomic Touchstone Distinguishing a Mott Insulator from a Band Insulator

In an electronic system with various interactions intertwined, revealing the origin of its many-body ground state is challenging and a direct experimental way to verify the correlated nature of an insulator has been lacking. Here we demonstrate an experimental way to unambiguously distinguish a paradigmatic correlated insulator, a Mott insulator, from a trivial band insulator based on their distinct chemical behavior for a surface adsorbate. Using scanning tunneling microscopy, we observed, in a microscopic field of view, two distinct insulating states with different energy gaps on 1T-TaS₂ surface, depending on the interlayer stacking of the surface layer. We also revealed that these two insulating states have distinguished adsorption behaviors of surface K adatoms, and the adatoms, furthermore, play totally different roles on the electronic states of the surface layer. On one insulating state, an extra electron from each K adatom is highly localized at the adsorption site and kills the empty state, forming a totally different spectral feature. On the other insulating state, electrons from K adatoms are widely spread and induce the global doping, shifting but keeping the overall spectral features. This can be straightforwardly understood from the fundamental distinction between Mott and band insulators: an electron filling, the half- and full-filled electrons, respectively. This work not only resolves a mystery of the insulating ground state in 1T-TaS₂, but also provides a simple and unambiguous experimental way to identify the Mott insulating state.