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, ScV6Sn6 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-ofplane vibrations of the Sc and Sn atoms, and it is at the base of the electron-phonon-coupling driven CDW phase of ScV6Sn6. 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 ScV6Sn6 kagome metal through control of strain.