Large Kohn Anomaly and Phonon Collapse Induced by Charge Density Wave in UPt_{2}Si_{2}.

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 toward the charge density wave (CDW) transition temperature T_{CDW}. The phonon exhibits a marked Kohn-type anomaly around the CDW wave vector Q_{CDW} and becomes overdamped within a finite momentum range already well above T_{CDW}. The dispersion anomaly is consistent with potential Fermi surface nesting, which together with the extended phonon collapse indicates strong electron-phonon coupling. The transition temperature estimated from the phonon softening is markedly lower than T_{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.