Correlated insulating state via stacking faults in 1T -TaSe₂

Strong electronic correlations can transform metals into Mott insulators, giving rise to a diverse landscape of emergent quantum phases. While the Hubbard model is better understood in the weak- and strong-coupling limits, its intermediate regime—particularly on triangular lattices where frustration suppresses long-range order—remains largely unexplored. Experimental realizations that enable direct spectroscopic access to this regime are exceedingly scarce. Here we demonstrate that a naturally formed surface stacking motif on the surface of 1T-TaSe₂ suppresses interlayer coupling and stabilizes a correlated insulating state with clear intermediate-coupling fingerprints. By combining scanning tunneling microscopy, nano- and micro-spot angle-resolved photoemission spectroscopy, first-principles calculations, and cluster perturbation theory, we resolve a surface-confined flat band whose spectral function evolves non-trivially with doping and temperature. These results establish 1T-TaSe₂ as a platform for accessing the intermediate-coupling regime on a triangular lattice, opening a potential pathway to explore correlation-driven quantum phenomena such as quantum spin-liquid states.