Effect of stacking order on the electronic state of 1T-TaS2

New theoretical proposals and experimental findings on transition metal dichalcogenide 1T-TaS2 have revived interest in its possible Mott insulating state. We perform a comprehensive scanning tunneling microscopy and spectroscopy experiment on different single-step areas in pristine 1T-TaS2. After accurately determining the relative displacement of the Star of David superlattices in two layers, we find that different stacking orders can correspond to a similar large-gap spectrum on the upper terrace. When the measurement is performed away from the step edge, the large-gap spectrum can always be maintained. The stacking order seems to rarely disturb the large-gap spectrum in the ideal bulk material. We conclude that the large insulating gap is from the single-layer property, which is a correlation-induced Mott gap based on the single-band Hubbard model. Specific stacking orders can perturb the state and induce a small-gap or metallic spectrum for a limited area around the step edge, which we attribute to a surface and edge phenomenon. Our work provides more evidence about the stacking-order effect on the electronic state and deepens our understanding of the Mott insulating state in 1T-TaS2.