Using high-energy-resolution inelastic x-ray scattering, we observe anomalous softening and damping of the transverse acoustic phonon in ${\mathrm{UPt}}_2{\mathrm{Si}}_2$ as the system is cooled toward the charge density wave (CDW) transition temperature $T_{\mathrm{CDW}}$. The phonon exhibits a marked Kohn-type anomaly around the CDW wave vector $Q_{\mathrm{CDW}}$ and becomes overdamped within a finite momentum range already well above $T_{\mathrm{CDW}}$. 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 $T_{\mathrm{CDW}}$, consistent with a primarily electronic instability rather than a phonon-driven transition. Our results establish ${\mathrm{UPt}}_2{\mathrm{Si}}_2$ as a prime example of a strongly correlated electron CDW system with exceptionally strong electron-phonon coupling driving phonon softening and collapse.

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• Charge density waves
• Phonons
• Strongly correlated systems
• X-ray scattering