Phonon-induced increase in the energy gap of superconducting films

We have observed large increases in the critical current of Josephson junctions under 10-GHz phonon excitation. Eliashberg has predicted that absorption of microwaves in a superconducting film will increase the energy gap by creating a nonthermal quasiparticle distribution. We modify this theory for phonon absorption and calculate the increase in the critical current of a Josephson junction corresponding to the increase in the energy gap. We obtain good agreement between our data and the modified Eliashberg theory. We conclude that absorption of phonons of energy ω<2Δ(T) increases the superconducting energy gap.