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 $$\sqrt{3}\times \sqrt{3}\times 2$$ 3 × 3 × 2 CDW that is dominant in the kagome metal ScV6Sn6 above TCDW ≈ 91 K, competing with the $$\sqrt{3}\times \sqrt{3}\times 3$$ 3 × 3 × 3 CDW that orders below TCDW. 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 $$\sqrt{3}\times \sqrt{3}\times 2$$ 3 × 3 × 2 CDW is energetically favored, while a wavevector-dependent electron-phonon coupling (EPC) promotes the $$\sqrt{3}\times \sqrt{3}\times 3$$ 3 × 3 × 3 CDW as the ground state, and leads to enhanced electron scattering above TCDW. These findings underscore EPC-driven correlated many-body physics in ScV6Sn6 and motivate studies of emergent quantum phases in the strong EPC regime.