Distinguishing and controlling Mottness in 1T−TaS2 by ultrafast light

Distinguishing and controlling the extent of Mottness is important for materials where the energy scales of the on-site Coulomb repulsion $U$ and the bandwidth $W$ are comparable. Here, we report the ultrafast electronic dynamics of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ by ultrafast time- and angle-resolved photoemission spectroscopy. A comparison of the electron dynamics for the intermediate phase (I-phase) as well as the low-temperature commensurate charge density wave (C-CDW) phase shows distinctive dynamics. While the I-phase is characterized by an instantaneous response and nearly time-resolution-limited fast relaxation ($\ensuremath{\sim}200$ fs), the C-CDW phase shows a delayed response and a slower relaxation (a few ps). Such distinctive dynamics reflect the different relaxation mechanisms and provide nonequilibrium signatures to distinguish the Mott insulating I-phase from the C-CDW band insulating phase. Moreover, a light-induced bandwidth reduction is observed in the C-CDW phase, pushing it toward the Mott insulating phase. Our work demonstrates the power of an ultrafast light-matter interaction in both distinguishing and controlling the extent of Mottness on the ultrafast timescale.