Soft-mode behavior in ternary silicides MAlSi (M=Ca, Sr, Ba)

We present a combined experimental and theoretical investigation of the lattice dynamics and electron-phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron-phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T (c) , among the investigated compounds can be explained by the presence of the soft mode.