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

Abstract

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 $(P\overline{6}m2)$. A soft $c$-axis polarized mode is clearly observed along the $\ensuremath{\Gamma}\text{\ensuremath{-}}A\text{\ensuremath{-}}L$ symmetry directions. The soft mode is strongly anharmonically broadened at room temperature, but at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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.