van der Waals driven anharmonic melting of the 3D charge density wave in $VSe_{2}$

Nature Communications 12(1), 598 (1-7) (2021). doi:10.1038/s41467-020-20829-2
Understanding of charge-density wave (CDW) phases is a main challenge in condensed matter due to their presence in high-Tc superconductors or transition metal dichalcogenides (TMDs). Among TMDs, the origin of the CDW in VSe$_2$ remains highly debated. Here, by means of inelastic x-ray scattering and first-principles calculations, we show that the CDW transition is driven by the collapse at 110 K of an acoustic mode at q$_{CDW}$ = (2.25 0 0.7) r.l.u. The softening starts below 225 K and expands over a wide region of the Brillouin zone, identifying the electron-phonon interaction as the driving force of the CDW. This is supported by our calculations that determine a large momentum-dependence of the electron-phonon matrix-elements that peak at the CDW wave vector. Our first-principles anharmonic calculations reproduce the temperature dependence of the soft mode and the T$_{CDW}$ onset only when considering the out-of-plane van der Waals interactions, which reveal crucial for the melting of the CDW phase.
Published by Nature Publishing Group UK, [London]