Soft-phonon-driven superconductivity in CaAlSi as seen by inelastic x-ray scattering

Inelastic x-ray scattering and $ab$-$initio$ calculation are applied to investigate the lattice dynamics and electron-phonon coupling of the ternary silicide superconductor CaAlSi ($P\mathrm{/}bar6m2$). A soft c-axis polarized mode is clearly observed along the $\mathrm{/}Gamma$-$A$-$L$ symmetry directions. The soft mode is strongly anharmonically broadened at room temperature, but, at 10 K, its linewidth narrows and becomes in good agreement with calculations of linear electron-phonon coupling. This establishes a coherent description of the detailed phonon properties in this system and links them clearly and consistently with the superconductivity.