  Long-range electronic order descending from a metallic parent state
constitutes a rich playground to study the intricate interplay of structural
and electronic degrees of freedom. With dispersive and correlation features as
multifold as topological Dirac-like itinerant states, van-Hove singularities,
correlated flat bands, and magnetic transitions at low temperature, kagome
metals are located in the most interesting regime where both phonon and
electronically mediated couplings are significant. Several of these systems
undergo a charge density wave (CDW) transition, and the van-Hove singularities,
which are intrinsic to the kagome tiling, have been conjectured to play a key
role in mediating such an instability. However, to date, the origin and the
main driving force behind this charge order is elusive. Here, we use the
topological bilayer kagome metal ScV6Sn6 as a platform to investigate this
puzzling problem, since it features both kagome-derived nested Fermi surface
and van-Hove singularities near the Fermi level, and a CDW phase that affects
the susceptibility, the neutron scattering, and the specific heat, similarly to
the siblings AV3Sb5 (A = K, Rb, Cs) and FeGe. We report on our findings from
high-resolution angle-resolved photoemission, density functional theory, and
time-resolved optical spectroscopy to unveil the dynamics of its CDW phase. We
identify the structural degrees of freedom to play a fundamental role in the
stabilization of charge order. Along with a comprehensive analysis of the
subdominant impact from electronic correlations, we find ScV6Sn6 to feature an
instance of charge density wave order that predominantly originates from
phonons. As we shed light on the emergent phonon profile in the low-temperature
ordered regime, our findings pave the way for a deeper understanding of
ordering phenomena in all CDW kagome metals.
