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 (6) over bar m2). A soft c-axis polarized mode is clearly observed along the Gamma-A-L symmetry directions. The soft mode is strongly anharmonically broadened at room temperature, but at 10 K, its linewidth narrows and comes in good agreement with calculations of linear electron-phonon coupling. This establishes a coherent description of the detailed phonon properties into this system and links them clearly and consistently with the superconductivity.