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The formation of charge density waves (CDW) is a long-standing open problemparticularly in dimensions higher than one. Various observations in thevanadium antimonides discovered recently, such as the missing Kohn anomaly inthe acoustic phonons or the latent heat at the transition $T_{\rm CDW}$ = 95 K, further underpin this notion. Here, we study the Kagome metal CsV$_3$Sb$_5$using polarized inelastic light scattering. The electronic energy gap $2\Delta$as derived from the redistribution of the continuum is much larger thanexpected from mean-field theory and reaches values above 20 for $2\Delta/k_{\rmB}T_{\rm CDW}$. The A$_{1g}$ phonon has a discontinuity at $T_{\rm CDW}$ and aprecursor starting 20 K above $T_{\rm CDW}$. Density functional theoryqualitatively reproduces the redistribution of the electronic continuum at theCDW transition and the phonon energies of the pristine and distortedstructures. The linewidths of all A$_{1g}$ and E$_{2g}$ phonon lines includingthose emerging below $T_{\rm CDW}$ were analyzed in terms of anharmonicsymmetric decay revealing strong phonon-phonon coupling. In addition, weobserve two CDW amplitude modes (AMs): one in A$_{1g}$ symmetry and one inE$_{2g}$ symmetry. The temperature dependence of both modes deviates from theprediction of mean-field theory. The A$_{1g}$ AM displays an asymmetricFano-type lineshape, suggestive of strong electron-phonon coupling. Theasymmetric A$_{1g}$ AM, along with the discontinuity of the A$_{1g}$ phonon,the large phonon-phonon coupling parameters and the large gap ratio, indicatethe importance of anharmonic strong phonon-phonon and electron-phonon couplingfor the CDW formation in CsV$_3$Sb$_5$.

Anharmonic Strong-Coupling Effects at the Origin of the Charge Density Wave in CsV$_3$Sb$_5$