Electron-phonon coupling in the charge density wave state of CsV$_3$Sb$_5$

TL;DR

This study uses neutron scattering to show that the charge density wave in CsV$_3$Sb$_5$ involves lattice distortion and phonon mode hardening, indicating electron-phonon coupling's role alongside electronic effects.

Contribution

It provides experimental evidence linking lattice distortion and phonon behavior to the CDW in CsV$_3$Sb$_5$, highlighting the importance of electron-phonon interactions.

Findings

CDW order involves static lattice distortion

Sudden hardening of B3u phonon mode observed

Electron-phonon coupling is significant in CDW formation

Abstract

Metallic materials with kagome lattice structure are interesting because their electronic structures can host flat bands, Dirac cones, and van Hove singularities, resulting in strong electron correlations, nontrivial band topology, charge density wave (CDW), and unconventional superconductivity. Recently, kagome lattice compounds AV$_3$Sb$_5$ (A = K, Rb, Cs) are found to have intertwined CDW order and superconductivity. The origin of the CDW has been suggested to be purely electronic, arising from Fermi-surface instabilities of van Hove singularity (saddle point) near the M points. Here we use neutron scattering experiments to demonstrate that the CDW order in CsV$_3$Sb$_5$ is associated with static lattice distortion and a sudden hardening of the B3u longitudinal optical phonon mode, thus establishing that electron-phonon coupling must also play an important role in the CDW order of AV$_3$Sb$_5$.