Phonon-Induced Enhancement of the Superconducting Energy Gap

We have observed large increases of the critical current in 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. By modifying this theory to account for phonon absorption we obtain a good agreement with our results on the phonon-induced enhancement of the critical current. We conclude that absorption of phonons of frequency $\omega <\frac{2\Delta \mathrm{}\left(T\right)\mathrm{}}{\hslash }$ increases the superconducting energy gap.