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 ($P/bar{6}m2$). 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 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. 
S. Kuroiwa, A. Q. R. Baron, T. Muranaka, R. Heid, K. -P. Bohnen, J.   Akimitsu