Renormalization of the Mott gap by lattice entropy: The case of 1T-TaS2

In many transition-metal oxides and dichalcogenides, the electronic and lattice degrees of freedom are strongly coupled, giving rise to remarkable phenomena such as the metal-insulator transition (MIT) and charge-density wave (CDW) order. We study this interplay by tracing the instant electronic structure under ab initio molecular dynamics. Applying this method to a 1T-TaS₂ layer, we show that the CDW-triggered Mott gap undergoes a continuous reduction as the lattice temperature rises, despite a nearly constant CDW amplitude. Before the CDW order undergoes a sharp first-order transition around the room temperature, the dynamical CDW fluctuation shrinks the Mott gap size by half. The gap size reduction is one order of magnitude larger than the lattice temperature variation. Our calculation not only provides an important clue to understanding the thermodynamic behavior in 1T-TaS₂, but also demonstrates a general approach to quantify the lattice entropy effect in the MIT.