  Complex systems are characterized by strong coupling between different
microscopic degrees of freedom. Photoexcitation of such materials can drive
them into new transient and long-lived hidden phases that may not have any
counterparts in equilibrium. By exploiting femtosecond time- and angle-resolved
photoemission spectroscopy, we probe the photoinduced transient phase and the
recovery dynamics of the ground state in a complex material: the charge density
wave (CDW)-Mott insulator 1$T$-TaS$_2$. We reveal striking similarities between
the band structures of the transient phase and the (equilibrium) structurally
undistorted metallic phase, with evidence for the coexistence of the
low-temperature Mott insulating phase and high-temperature metallic phase.
Following the transient phase, we find that the restoration of the Mott and CDW
order begins around the same time. This highlights that the Mott transition is
tied to the CDW structural distortion, although earlier studies have shown that
the collapse of Mott and CDW phases are decoupled from each other.
Interestingly, as the suppressed order starts to recover, a long-lived
metastable phase emerges before the material recovers to the ground state. Our
results demonstrate that it is the CDW lattice order that drives the material
into this metastable phase, which is indeed a commensurate CDW-Mott insulating
phase but with a smaller CDW amplitude. Moreover, we find that the long-lived
state emerges only under strong photoexcitation and has no evidence when the
photoexcitation strength is weak.
