Strong anharmonicity induces quantum melting of charge density wave in 2H-NbSe2 under pressure

The pressure and temperature dependence of the phonon dispersion of 2H-NbSe2 is measured by inelastic x-ray scattering. A strong temperature dependent soft phonon mode, reminiscent of the charge density wave (CDW), is found to persist up to a pressure as high as 16 GPa, far above the critical pressure at which the CDW disappears at 0 K. By using ab initio calculations beyond the harmonic approximation, we obtain an accurate, quantitative description of the (P,T) dependence of the phonon spectrum. Our results show that the rapid destruction of the CDW under pressure is related to the zero mode vibrations - or quantum fluctuations - of the lattice renormalized by the anharmonic part of the lattice potential. The calculations also show that the low-energy longitudinal acoustic mode that drives the CDW transition barely contributes to superconductivity, explaining the insensitivity of the superconducting critical temperature to the CDW transition.