Large Kohn Anomaly and Phonon Collapse Induced by Charge Density Wave in UPt_2⁢Si₂

Using high-energy-resolution inelastic x-ray scattering, we observe anomalous softening and damping of the transverse acoustic phonon in UPt₂⁢Si₂ 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. In conclusion, our results establish UPt_2⁢Si₂ as a prime example of a strongly correlated electron CDW system with exceptionally strong electron-phonon coupling driving phonon softening and collapse.. Authors: Lee, Jooseop [Hongik University, Seoul (Korea, Republic of); Institute for Basic Science, Pohang (Korea, Republic of)] (ORCID:0000000244135412); Chappell, Greta L. [Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab)]; Baumbach, Ryan E. [Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab); Univ. of California, Santa Cruz, CA (United States)] (ORCID:0000000263143629); Said, Ayman H. [Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)]; Zaliznyak, Igor A. [Brookhaven National Laboratory (BNL), Upton, NY (United States)] (ORCID:0000000285487924). DOE Contract: AC02-06CH11357; SC0012704. Subjects: 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Charge density waves; Phonons; Strongly correlated systems; X-ray scattering