Nanoscale determination of the metal-insulator transition in intercalated bulk VSe${}_2$

Abstract

Two-dimensional (2D) materials provide unique opportunities to realize emergent phenomena by reducing dimensionality. Using scanning tunneling microscopy combined with first-principles calculations, we determine an intriguing case of a metal-insulator transition (MIT) in a bulk compound, (TBA)${}_{0.3}$VSe${}_2$. Atomic-scale imaging reveals that the initial $4a_0\times 4a_0$ charge density wave (CDW) order in 1T-VSe${}_2$ transforms to $\sqrt{7}{a}_0\times \sqrt{3}{a}_0$ ordering upon intercalation, which is associated with an insulating gap with a magnitude of up to approximately 115 meV. Our calculations reveal that this energy gap is highly tunable through electron doping introduced by the intercalant. Moreover, the robustness of the $\sqrt{7}{a}_0\times \sqrt{3}{a}_0$ CDW order against the Lifshitz transition points to the key role of electron-phonon interactions in stabilizing the CDW state. Our work clarifies a rare example of a CDW-driven MIT in quasi-2D materials and establishes cation intercalation as an effective pathway for tuning both the dimensionality and the carrier concentration without inducing strain or disorder.

22 pages and 5 figures