Anharmonic Strong-Coupling Effects at the Origin of the Charge Density Wave in CsV3Sb5

Abstract The formation of charge density waves (CDW) is a long-standing open problem particularly in dimensions higher than one. Various observations in the vanadium antimonides discovered recently, such as the missing Kohn anomaly in the acoustic phonons or the latent heat at the transition TCDW = 95 K, further underpin this notion. Here, we study the Kagome metal CsV3Sb5 using polarized inelastic light scattering. The electronic energy gap 2Δ as derived from the redistribution of the continuum is much larger than expected from mean-field theory and reaches values above 20 for 2Δ/kBTCDW. The A1g phonon has a discontinuity at TCDW and a precursor starting 20 K above TCDW. Density functional theory qualitatively reproduces the redistribution of the electronic continuum at the CDW transition and the phonon energies of the pristine and distorted structures. The linewidths of all A1g and E2g phonon lines including those emerging below Tcdw were analyzed in terms of anharmonic symmetric decay revealing strong phonon-phonon coupling. In addition, we observe two CDW amplitude modes (AMs): one in A1g symmetry and one in E2g symmetry. The temperature dependence of both modes deviates from the prediction of mean-field theory. The A1g AM displays an asymmetric Fano-type lineshape, suggestive of strong electron-phonon coupling. The asymmetric A1g AM, along with the discontinuity of the A1g phonon, the large phonon-phonon coupling parameters and the large gap ratio, indicate the importance of anharmonic strong phonon-phonon and electron-phonon coupling for the CDW formation in CsV3Sb5.