Phonon-induced enhancement of the energy gap and critical current of superconducting aluminum films

Enhancements of the energy gap $\Delta$ and the critical current $I_c$ have been induced in thin superconducting aluminum films near the transition temperature $T_c$ by pulses of phonons at approximately 9 GHz. In terms of the change in temperature $\left|\frac{\delta T}{T_c}\right|$ necessary to produce the same enhancement in equilibrium, the gap enhancement increased smoothly with phonon power at fixed temperature and decreasing temperature at fixed phonon power; however, very close to $T_c$ the enhancement rolled off. At relatively low phonon powers, the data were in good agreement with the theory of Eckern, Schmid, Schmutz, and Schön, but at higher power levels the data fell markedly below the predictions of the theory. The critical-current enhancements in terms of $\left|\frac{\delta T}{T_c}\right|$ were always larger than the gap enhancements at the same temperature and phonon power. At fixed phonon power the critical-current enhancements were nearly independent of temperature, except very close to $T_c$ where the enhancement became small. The inclusion of the nonequilibrium quasiparticle distribution and the kinetic energy of the supercurrent in the theory relating the critical-current enhancement to the gap enhancement did not resolve the discrepancies between the two enhancements. It appears likely that there is an additional mechanism for critical-current enhancement that has not yet been identified.