Phonon collapse and van der Waals melting of the 3D charge density wave of VSe2
Abstract
Among transition metal dichalcogenides (TMDs), VSe2 is considered to develop a purely 3-dimensional (3D) charge-density wave (CDW) at TCDW=110 K. Here, by means of high resolution inelastic x-ray scattering (IXS), we show that the CDW transition is driven by the collapse of an acoustic mode at the critical wavevector qCDW= (2.25 0 0.7) r.l.u. and critical temperature TCDW=110 K. The softening of this mode starts to be pronounced for temperatures below 2× TCDW and expands over a rather wide region of the Brillouin zone, suggesting a large contribution of the electron-phonon interaction to the CDW formation. This interpretation is supported by our first principles calculations that determine a large momentum-dependence of the electron-phonon interaction, peaking at the CDW wavevector, in the presence of nesting. Fully anharmonic ab initio calculations confirm the softening of one acoustic branch at qCDW as responsible for the CDW formation and show that van der Waals interactions are crucial to melt the CDW. Our work also highlights the important role of out-of-plane interactions to describe 3D CDWs in TMDs.