Nanoscale determination of the metal-insulator transition in intercalated bulk VSe$_{2}$

TL;DR

This study uses advanced microscopy and calculations to explore how intercalation induces a metal-insulator transition in bulk VSe2, revealing a tunable energy gap and the role of electron-phonon interactions.

Contribution

It demonstrates that cation intercalation can control the metal-insulator transition and charge density wave order in VSe2 without strain or disorder, highlighting a new tuning method.

Findings

Charge density wave order transforms upon intercalation.

The energy gap is tunable via electron doping.

Electron-phonon interactions stabilize the CDW state.

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.