Charge-Density-Wave Transition in bulk and monolayer VSe2 from First Principles*

1T-VSe2 is particularly interesting transition metal dichalcogenide (TMD), as it develops a purely 3-dimensional (3D) charge-density wave (CDW) at T_CDW=110 K. Our fully anharmonic ab initio calculations, together with high resolution inelastic x-ray (IXS) experiments, have revealed that this CDW transition is characterized by the collapse of an acoustic mode at the critical wave vector q_CDW=(2.25 0 0.7) r.l.u. and the critical temperature. The CDW transition is well captured in our calculations  provided that out-of-plane van der Waals interactions are included in the exchange-correlation functional [1]. Besides, the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI). This emphasizes that the CDW's origin is predominantly attributed to EPI with a minor influence from nesting. Finally, anharmonic calculations under pressure reveal that the soft mode remains nearly constant up to 13 GPa at room temperature, only displaying modest softening before transitioning to the high-pressure monoclinic C2/m phase [2].  

On the other side, our non-perturbative anharmonic phonon calculations in monolayer VSe2 show that it develops two independent charge density wave orders associated to √3 × √7 and 4 × 4 modulations that compete as a function of strain. Theory and experiments match showing a clear substrate-dependence on the charge density wave order on the 2D limit of VSe2 [3].