Anharmonic strong-coupling effects at the origin of the charge density wave in CsV₃Sb₅

The formation of charge density waves is a long-standing open problem, particularly in dimensions higher than one. Various observations in the vanadium antimonides discovered recently further underpin this notion. Here, we study the Kagome metal CsV₃Sb₅ using polarized inelastic light scattering and density functional theory calculations. We observe a significant gap anisotropy with 2Δ_max/k_BT_CDW≈20, far beyond the prediction of mean-field theory. The analysis of the A_1g and E_2g phonons, including those emerging below T_CDW, indicates strong phonon-phonon coupling, presumably mediated by a strong electron-phonon interaction. Similarly, the asymmetric Fano-type lineshape of the A_1g amplitude mode suggests strong electron-phonon coupling below T_CDW. The large electronic gap, the enhanced anharmonic phonon-phonon coupling, and the Fano shape of the amplitude mode combined are more supportive of a strong-coupling phonon-driven charge density wave transition than of a Fermi surface instability or an exotic mechanism in CsV₃Sb₅.