Anharmonic melting of the 3D charge-density wave in the kagome metal CsV3Sb5

The CDW mechanism and resulting structure of the AV3Sb5 family of Kagome metals has posed a puzzling challenge since their discovery four years ago. Thus far, attempts to theoretically study the CDW have overlooked the importance of anharmonic effects. Here, we employ a non-perturbative treatment of anharmonicity to conduct a comprehensive exploration of the mechanisms governing the formation and melting of the initial charge-density wave (CDW) transition in CsV3Sb5. Our results reveal that the CDW transition at T ~ 94 K is driven by the large electron-phonon coupling within the material, while the eventual melting of the CDW state is attributed to the robust anharmonic effects of the lattice. Additionally, the CDW is primarily triggered by the unstable phonons at the L point, with the M phonons not playing any essential role. Despite the second-order nature of the phase transition, our examination of the spectral function at the L point suggests that observing softening experimentally may prove to be exceedingly challenging due to the large electron-phonon linewidth. Finally, the second-order nature of the CDW transition, enables us to narrow down the potential space groups that may emerge.

Taken together, our theoretical framework successfully anticipates various facets of the CDW transition, encompassing the transition temperature, the absence of M mode condensation, and the observed lack of softening. This significant agreement with experimental data carries two noteworthy implications:

(1) The study of the CDW mechanism can be decoupled from phenomena arising in the CDW phase.

(2) Quantum effects of the lattice may also exert a crucial influence on the emergent physics in the CDW state, akin to their role in the CDW mechanism.