  Combination of low-dimensionality and electron correlation is vital for
exotic quantum phenomena such as the Mott-insulating phase and high-temperature
superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked
great interest owing to its unique nonmagnetic Mott-insulator nature coupled
with a charge-density-wave (CDW). To functionalize such a complex phase, it is
essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas
this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a
strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K
in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower
Hubbard band survives under external perturbations such as carrier doping and
photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard
and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice
distortion assisted by strengthened correlations and disappearance of
interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott
insulator phase well above the room temperature. The present result lays the
foundation for realizing monolayer CDW-Mott insulator based devices operating
at room temperature.
