Switching of charge-density wave by carrier tuning in monolayer TiTe2

How charge-density wave (CDW) is influenced by the change in the dimensionality is a fundamental question in condensed-matter physics. Monolayer titanium ditelluride (${\mathrm{TiTe}}_{2}$) undergoes $2\ifmmode\times\else\texttimes\fi{}2$ CDW despite the absence of CDW in the bulk counterpart, whereas the mechanism of CDW is under intensive debate. Here we show that the CDW of monolayer ${\mathrm{TiTe}}_{2}$ can be conveniently switched by carrier tuning via controlling the Te ratio during molecular-beam epitaxy or depositing potassium (K) atoms on the surface. This is demonstrated by monitoring the CDW-induced $2\ifmmode\times\else\texttimes\fi{}2$ band folding using angle-resolved photoemission spectroscopy. Remarkably, the CDW appears only when the inner hole pocket at the $\ensuremath{\Gamma}$ point and the elongated electron pocket at the $M$ point show partial Fermi-surface nesting. The present study suggests that the carrier doping is an effective method to pin down the mechanism of CDW in atomic-layer transition-metal dichalcogenides.