We address the long-standing problem of the ground state of $1T\text{−}{\mathrm{TaS}}_2$ by computing the correlated electronic structure of stacked bilayers using the $GW+\mathrm{EDMFT}$ method. Depending on the surface termination, the semi-infinite uncorrelated system is either band insulating or exhibits a metallic surface state. For realistic values of the on-site and inter-site interactions, a Mott gap opens in the surface state, but it is smaller than the gap originating from the bilayer structure. Our results are consistent with recent scanning tunneling spectroscopy measurements for different terminating layers, and with our own photoemission measurements, which indicate the coexistence of spatial regions with different gaps in the electronic spectrum. By comparison to exact diagonalization data, we clarify the interplay between Mott insulating and band insulating behavior in this archetypal layered system.

• Received 3 February 2022
• Revised 7 April 2022
• Accepted 1 June 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.016402