Strain Engineering a 4 a ×√3 a Charge Density Wave Phase in Transition Metal Dichalcogenide 1T-VSe 2 .

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe2 thin films synthesized by molecular beam epitaxy on c -sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4 a ×√3 a periodicity, as opposed to the previously reported hexagonal 4 a ×4 a structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same 4 a ×√3 a CDW periodicity and an energy gap of 2 Δ CDW = (9.1 ± 0.1) 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 4 a ×4 a and 4 a ×√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.