Softening of a flat phonon mode in the kagome ScV6Sn6

Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at the kz = π plane in ScV6Sn6. The low energy longitudinal phonon collapses at ~98 K and q = $$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ 1 3 1 3 1 2 due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vector qCDW = $$\frac{1}{3}\frac{1}{3}\frac{1}{3}$$ 1 3 1 3 1 3 . Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at $$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ 1 3 1 3 1 2 of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics.