  Mott state in 1T-TaS2 is predicted to host quantum spin liquids (QSL).
However, its insulating mechanism is controversial due to complications from
interlayer coupling. Here, we study the Mott state in monolayer 1T-NbSe2, an
electronic analogy to TaS2 exempt from interlayer coupling, using spectroscopic
imaging scanning tunneling microscopy and first principles calculations.
Monolayer NbSe2 surprisingly displays two types of Star-of-David (SD) motifs
with different Mott gap sizes, that are interconvertible via temperature
variation. And, bilayer 1T-NbSe2 shows Mott collapse by interlayer coupling.
Our calculation unveils the two types of SDs possess distinct structural
distortions, altering the effective Coulomb energies of the central Nb orbital.
Our calculation suggests the Mott gap, the same parameter for determining the
QSL regime, is tunable with strain. This finding offers a general strategy for
manipulating the Mott state in 1T-NbSe2 and related systems via structural
distortions, which may be tuned into the potential QSL regime.
