  The combination of non-trivial band topology and symmetry breaking phases
gives rise to novel quantum states and phenomena such as topological
superconductivity, quantum anomalous Hall effect and axion electrodynamics.
Evidence of intertwined charge density wave (CDW) and superconducting order
parameters has recently been observed in a novel kagome material AV3Sb5
(A=K,Rb,Cs) that features a Z2 topological invariant in the electronic
structure. However, the origin of the CDW and its intricate interplay with
topological state has yet to be determined. Here, using hard x-ray scattering,
we demonstrate a three-dimensional (3D) CDW with 2*2*2 superstructure in
(Rb,Cs)V3Sb5. Unexpectedly, we find that the CDW fails to induce acoustic
phonon anomalies at the CDW wavevector but yields a novel Raman mode which
quickly damps into a broad continuum below the CDW transition temperature. Our
observations exclude strong electron-phonon coupling driven CDW in AV3Sb5 and
point to an unconventional and electronic-driven mechanism that couples the CDW
and the topological band structure.
