  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$_5$.
