Phonon softening in lanthanum under pressure

Superconducting tunneling experiments on double-hexagonal close-packed La show that the ratio $\frac{2{\Delta }_0}{k{T}_c}$ increases with pressure. The low-frequency part of the phonon density of states shifts to lower energies. Phonon softening is thus, at least partially, responsible for the strong increase of the superconducting T_c with pressure.