Understanding the charge-density-wave in monolayer 1T-NbSe₂ from first principles.

Layered Transition Metal Dichalcogenides (TMDs) host a large variety of charge-density-waves/ periodic lattice distortions that are often associated with superconductivity. One of the most studied material in this family is NbSe₂ in the 2H polytype. Recently, a monolayer of the 1T phase of NbSe₂ has been successfully synthesized for the first time. The observed superlattice structure was suggested to be a commensurate Star-Of-David phase with √13x√13 periodicity and the insulating behavior ascribed to a possible Mott gap, reminiscent of 1T-TaS₂.

Motivated by these findings, we study from first principles the phase diagram of 1T-NbSe₂. Density functional calculations show that the metallic 1T phase is unstable against the formation of an incommensurate charge-density-wave (CDW), suggesting a possible sequence of transitions from normal state to incommensurate to commensurate order. We discuss the mechanism behind this instability and the relevance of the fermiology. We then study the electronic structure of the commensurate structure to resolve the nature of the observed gap.