Theoretical study of pseudo two-step metal–insulator transition in monolayer 1T-VSe2

High-temperature metal phase of monolayer 1T-VSe2 may transform into a charge density wave (CDW) phase with a certain pseudo-gap at around 350 K upon cooling and then into an insulating phase with the entire bandgap at around 140 K upon further decreasing temperature. Is such a seeming quantum transformation with two pseudo-gaps opening one by one a real traditional two-stage phase transition? In this study, our theoretical calculations with density functional theory revealed that monolayer 1T-VSe2 actually undergoes a one-step with pre-transformation or pseudo-two-step phase transition over a wide temperature range upon cooling. Its low temperature phase has a 4 × 4 CDW configuration with an insulating state. There exist two types of V–V atomic bonds in the CDW phase. The opening of a pseudo-gap at around 350 K upon cooling is mainly attributed to the forming of shorter V–V bonds, while another is mainly attributed to the forming of longer V–V bonds. In addition, there is a pseudo-Fermi arc around the K point in our theoretical ARPES calculations. Our theoretical calculations predicted that it is universally present for such a pseudo-two-step phase transition in the monolayer 4 × 4 CDW of transition metal dichalcogenides.