Soft Mode Origin of Charge Ordering in Superconducting Kagome CsV$_3$Sb$_5$

TL;DR

This study reveals that the charge-density-wave order in superconducting CsV$_3$Sb$_5$ kagome metal originates from a soft phonon mode at the L point, emphasizing the importance of lattice dynamics in its phase transition.

Contribution

The paper combines experimental and theoretical methods to identify a soft phonon mode as the origin of charge ordering in CsV$_3$Sb$_5$, providing new insight into its microscopic mechanism.

Findings

Identification of a soft phonon mode at the L point

Reproduction of observations through first-principles calculations

Establishment of a soft-mode instability as the CDW driver

Abstract

Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV$_3$Sb$_5$ (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 CsV$_3$Sb$_5$. 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 CsV$_3$Sb$_5$ originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.