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

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe thin films synthesized by molecular beam epitaxy on -sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4×√3 periodicity, as opposed to the previously reported hexagonal 4×4 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×√3 CDW periodicity and an energy gap of 2 = (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 4×4 and 4×√3 structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.