Origin of the multiple charge density wave order in 1T-VSe2

Transition-metal dichalcogenide 1T-VSe2 experimentally exhibits multiple charge density wave (CDW) orders, but its origin is still under debate. Using first-principles calculations, we investigate the origin of CDW orders in 1T-VSe2 and clarify the ground state of CDW in the freestanding monolayer. Our results show that both Fermi-surface nesting and electron-phonon coupling account for the 4 x 4 x 3 CDW superstructure in bulk 1T-VSe2, while the momentum-dependent electron-phonon coupling-induced root 7 x root 3 CDW superstructure is most stable in the freestanding monolayer 1T-VSe2. For monolayer 1T-VSe2, the substrate-induced compressive strain can turn the ground state into the 4 x 4 CDW superstructure, while tensile strain preserves the root 7 x root 3 superstructure. Our results demonstrate the origin of the CDW orders in 1T-VSe2 and shed light on the experimental observation of multiple CDW orders in monolayer 1T-VSe2.