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

摘要

We report a rectangular charge density wave (CDW) phase in strained1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphiresubstrates. The observed CDW structure exhibits an unconventional rectangular4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reportedhexagonal $4a\times4a$ structure in bulk crystals and exfoliated thin layeredsamples. Tunneling spectroscopy shows a strong modulation of the local densityof 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 attemperatures below 500 mK, indicating a transition to an insulating phase atultra-low temperatures. First-principles calculations confirm the stability ofboth $4a\times4a$ and $4a\times\sqrt{3}a$ structures arising from soft modes inthe phonon dispersion. The unconventional structure becomes preferred in thepresence of strain, in agreement with experimental findings.