Competing charge-density wave instabilities in the kagome metal ScV6Sn6

Owing to its unique geometry, the kagome lattice hosts various many-body quantum states including frustrated magnetism, superconductivity, and charge-density waves (CDWs). In this work, using inelastic X-ray scattering, we discover a dynamic short-range root 3x root 3x root 2 CDW that is dominant in the kagome metal ScV6Sn6 above T-CDW approximate to 91 K, competing with the 3x3x3 CDW that orders below T-CDW. The competing CDW instabilities lead to an unusual CDW formation process, with the most pronounced phonon softening and the static CDW occurring at different wavevectors. First-principles calculations indicate that the root 3x root 3x root 2 CDW is energetically favored, while a wavevector-dependent electron-phonon coupling (EPC) promotes the root 3x root 3x root 3 CDW as the ground state, and leads to enhanced electron scattering above T-CDW. These findings underscore EPC-driven correlated many-body physics in ScV6Sn6 and motivate studies of emergent quantum phases in the strong EPC regime.