Large Kohn anomaly and phonon collapse induced by charge density wave in UPt$_2$Si$_2$

Abstract: Using high-energy-resolution inelastic x-ray scattering, we observe anomalous softening and damping of the transverse acoustic phonon in UPt$2$Si$_2$ as the system is cooled towards the charge density wave (CDW) transition temperature, T${\rm{CDW}}$. The phonon exhibits a marked Kohn-type anomaly around the CDW wave vector, Q${\rm{CDW}}$, and becomes overdamped within a finite momentum range already well above T${\rm{CDW}}$. The dispersion anomaly is consistent with potential Fermi surface nesting, which together with the extended phonon collapse indicate strong electron-phonon coupling. The transition temperature estimated from the phonon softening is markedly lower than T$_{\rm{CDW}}$, consistent with a primarily electronic instability rather than a phonon-driven transition. Our results establish UPt$_2$Si$_2$ as a prime example of a strongly correlated electron CDW system with exceptionally strong electron-phonon coupling driving phonon softening and collapse.