Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV3Sb5 (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV3Sb5. Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M-L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV3Sb5 originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.
@article{arxiv.2510.19790,
title = {Soft Mode Origin of Charge Ordering in Superconducting Kagome CsV$_3$Sb$_5$},
author = {Philippa Helen McGuinness and Fabian Henssler and Manex Alkorta and Mark Joachim Graf von Westarp and Artem Korshunov and Alexei Bosak and Daisuke Ishikawa and Alfred Q. R. Baron and Michael Merz and Amir-Abbas Haghighirad and Maia G. Vergniory and Sofia-Michaela Souliou and Rolf Heid and Ion Errea and Matthieu Le Tacon},
journal= {arXiv preprint arXiv:2510.19790},
year = {2025}
}