Strain-Induced Enhancement of the Charge Density Wave in the Kagome Metal ScV6Sn6

The kagome geometry is an example of a frustrated configuration in which rich physics takes place, including the emergence of superconductivity and charge density wave (CDW). Among the kagome metals, $\mathrm{S}\mathrm{c}{\mathrm{V}}_{\mathrm{6}}\mathrm{S}{\mathrm{n}}_{\mathrm{6}}$ hosts an unconventional CDW, with its electronic order showing a different periodicity from the leading lattice instability. In this material, a CDW-softened flat phonon band has a second-order collapse at the same time that the first-order transition occurs. This phonon band originates from the out-of-plane vibrations of the Sc and Sn atoms, and it is at the base of the electron-phonon-coupling driven CDW phase of $\mathrm{S}\mathrm{c}{\mathrm{V}}_{\mathrm{6}}\mathrm{S}{\mathrm{n}}_{\mathrm{6}}$. Here we use uniaxial strain to tune the frequency of the CDW amplitude mode, which originates from the collapse of the flat phonon band, tracking its evolution via time-resolved optical spectroscopy and first-principles calculations. Our findings emphasize the capability to induce an enhancement of the unconventional CDW properties in $\mathrm{S}\mathrm{c}{\mathrm{V}}_{\mathrm{6}}\mathrm{S}{\mathrm{n}}_{\mathrm{6}}$ kagome metal through control of strain.