Three-dimensional metallic and two-dimensional insulating behavior in octahedral tantalum dichalcogenides

Using density functional theory with added on-site interactions, we study the electronic structure of bulk, monolayer, and bilayer of the layered transition-metal dichalcogenide 1T-TaS₂. We show that a two-dimensional spin-1/2 Mott phase exists for the monolayer in the charge density wave (CDW) state and that such a phase is systematically destroyed by packing of the distorted layers leading to a one-dimensional metal for bulk, CDW-distorted TaS₂. The latter finding is in contrast with previous dynamical mean-field theory predictions---we explain the disagreement by the weak effective interaction felt by the electrons in the CDW state. Experimental observations of insulating behavior may arise from disorder due to stacking faults.