Charge order with unusual star-of-David lattice in monolayer NbTe2

The authors fabricated a new two-dimensional material, monolayer NbTe${}_{2}$, and investigated its electronic states. They discovered a charge-density wave with a \ensuremath{\surd}19\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}19 periodicity, accompanied by partially occupied metallic star-of-David clusters of Nb atoms, which is in sharp contrast to the fully occupied Mott-insulating monolayer NbSe${}_{2}$. Such an intriguing difference is found to originate from the underlying fermiology and hidden-Fermi-surface nesting. The present result lays the foundation for manipulating the two-dimensional Mott-insulator phase.