Strain Engineering a $4a\times\sqrt{3}a$ Charge Density Wave Phase in Transition Metal Dichalcogenide 1T-VSe$_2$

TL;DR

This study demonstrates strain-induced formation of an unconventional rectangular charge density wave phase in 1T-VSe2 thin films, revealing a new CDW periodicity and an associated energy gap transition at ultra-low temperatures.

Contribution

It introduces a strain-engineered rectangular CDW phase in 1T-VSe2, supported by experimental observations and first-principles calculations showing stability of this novel structure.

Findings

Observation of a $4a\times\sqrt{3}a$ CDW structure in strained 1T-VSe2

Detection of a $2\Delta_{CDW}=(9.1\pm0.1)$ meV energy gap

Transition to an insulating phase below 500 mK

Abstract

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reported hexagonal $4a\times4a$ structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same $4a\times\sqrt{3}a$ CDW periodicity and an energy gap of $2\Delta_{CDW}=(9.1\pm0.1)$ meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both $4a\times4a$ and $4a\times\sqrt{3}a$ structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.