Enhancement of the energy gap in superconducting aluminum by tunneling extraction of quasiparticles

Al-Al2O3-Al-Al2O3-Al tunnel junctions have been used to induce and detect enhancements of up to 40% in the energy gap of superconducting aluminum. Quasiparticles are extracted form the middle aluminum film through the first tunnel junction into an aluminum film with a larger energy gap, and the gap enhancement in the middle film is measured from the characteristics of the second tunnel junction. A theory developed for the enhancement is good agreement with the experimental results for bias voltages below the difference of the energy gaps, but overestimates the enhancement at higher voltages. This discrepancy is believed to arise from the effects of nonequilibrium phonons, which are neglected in the calculation. The low-voltage data are consistent with a value of about 80 ns for the characteristic electron-phonon scattering time, π0, in aluminum films with an average transition temperature of 1.34 K