Recent debate about the nature of the insulating ground state in 1T-TaS₂ has largely hinged around the stacking pattern of its quasi-2D charge ordered layers. The stacking determines whether the 3D unit-cell contains one or two ‘Star-of-David’ (SD) clusters, each of which hosts 13 unpaired Ta orbitals. For simple stacking with one SD per cell, the insulating state must be explained by Mott physics, but for stacking with a dimerized pair of SDs per cell, ab initio calculations predict a band insulator [1,2].
We will describe scanning tunneling microscopy measurements [3] showing two distinct terminations of the 3D charge order - the sign of a stacking pattern with two SDs per cell - and determine the inter-layer stacking vectors. We argue that the persistence of a spectral gap even at the surface which breaks the SD dimers is a manifestation of Mottness. We also identify a stacking vector, extrinsic to the natural pattern, which renders the surface metallic. These results help to understand this material's mysterious insulating state, and its potentially useful metal-insulator transitions.

[1] T. Ritschel, H. Berger and J. Geck, PRB 98, 195134 (2018).
[2] S.-H. Lee, J. S. Goh and D. Cho, PRL 122, 106404 (2019).
[3] C. J. Butler, M. Yoshida, T. Hanaguri and Y. Iwasa, arXiv:1908.08221