  The Mott insulator provides an excellent foundation for exploring a wide
range of strongly correlated physical phenomena, such as high-temperature
superconductivity, quantum spin liquid, and colossal magnetoresistance. A Mott
insulator with the simplest degree of freedom is an ideal and highly desirable
system for studying the fundamental physics of Mottness. In this study, we have
unambiguously identified such an anticipated Mott insulator in a van der Waals
layered compound Nb3Cl8. In the high-temperature phase, where interlayer
coupling is negligible, density functional theory calculations for the
monolayer of Nb3Cl8 suggest a half-filled flat band at the Fermi level, whereas
angle-resolved photoemission spectroscopy experiments observe a large gap. This
observation is perfectly reproduced by dynamical mean-field theory calculations
considering strong electron correlations, indicating a correlation-driven Mott
insulator state. Since this half-filled band derived from a single 2a1 orbital
is isolated from all other bands, the monolayer of Nb3Cl8 is an ideal
realization of the celebrated single-band Hubbard model. Upon decreasing the
temperature, the bulk system undergoes a phase transition, where structural
changes significantly enhance the interlayer coupling. This results in a
bonding-antibonding splitting in the Hubbard bands, while the Mott gap remains
dominant. Our discovery provides a simple and seminal model system for
investigating Mott physics and other emerging correlated states.
