van der Waals driven anharmonic melting of the 3D charge density wave in VSe2
Josu Diego, Ayman Said, Sanjoy Kr Mahatha, Raffaello Bianco, Lorenzo Monacelli, Matteo Calandra, Francesco Mauri, Kai Roßnagel, Ion Errea, S. Blanco-Canosa
Abstract
Abstract Understanding of charge-density wave (CDW) phases is a main challenge in condensed matter due to their presence in high- T c 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 C D W = (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 C D W onset only when considering the out-of-plane van der Waals interactions, which reveal crucial for the melting of the CDW phase.