  Kagome lattice materials offer a fertile ground to discover novel quantum
phases of matter, ranging from unconventional superconductivity and quantum
spin liquids to charge orders of various profiles. However, understanding the
genuine origin of the quantum phases in kagome materials is often challenging,
owing to the intertwined atomic, electronic, and structural degrees of freedom.
Here, we combine angle-resolved photoemission spectroscopy, phonon mode
calculation, and chemical doping to elucidate the driving mechanism of the
root3*root3 charge order in a newly discovered kagome metal ScV6Sn6. In
contrast to the case of the archetype kagome system AV3Sb5 (A= K, Rb, Cs), the
van Hove singularities in ScV6Sn6 remain intact across the charge order
transition, indicating a marginal role of the electronic instability from the V
kagome lattice. Instead, we identified a three-dimensional band with dominant
planar Sn character opening a large charge order gap of 260 meV and strongly
reconstructing the Fermi surface. Our complementary phonon dispersion
calculations further emphasize the role of the structural components other than
the V kagome lattice by revealing the unstable planar Sn and Sc phonon modes
associated to the root3*root3 phase. Finally, in the constructed phase diagram
of Sc(V1-xCrx)6Sn6, the charge order remains robust in a wide doping range x =
0 ~ 0.10 against the Fermi level shift up to ~ 120 meV, further making the
electronic scenarios such as Fermi surface or saddle point nesting unlikely.
Our multimodal investigations demonstrate that the physics of ScV6Sn6 is
fundamentally different from the canonical kagome metal AV3Sb5, uncovering a
new mechanism to induce symmetry-breaking phase transition in kagome lattice
materials.
