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 $\ensuremath{\omega}<\frac{2\ensuremath{\Delta}(T)}{\ensuremath{\hbar}}$ increases the superconducting energy gap.

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