  Interplay between fermiology and electron correlation is crucial for
realizing exotic quantum phases. Transition-metal dichalcogenide (TMD) 1T-TaS2
has sparked a tremendous attention owing to its unique Mott-insulating phase
coexisting with the charge-density wave (CDW). However, how the fermiology and
electron correlation are associated with such properties has yet to be claried.
Here we demonstrate that monolayer 1T-NbTe2 is a new class of two-dimensional
TMD which has the star-of-David lattice similarly to bulk TaS2 and
isostructural monolayer NbSe2, but exhibits a metallic ground state with an
unusual lattice periodicity root19xroot19 characterized by the sparsely
occupied star-of-David lattice. By using angle-resolved photoemission and
scanning-tunneling spectroscopies in combination with first-principles
band-structure calculations, we found that the hidden Fermi-surface nesting and
associated CDW formation are a primary cause to realize this unique correlated
metallic state with no signature of Mott gap. The present result points to a
vital role of underlying fermiology to characterize the Mott phase of TMDs.
