Measurement of Characteristic Time Constants of Microwave-Enhanced Superconductivity in Aluminum Films

Time constants of the processes causing the microwave enhancement of the critical current in superconducting Al films have been determined by measuring the response to an amplitude-modulated microwave signal. The dominant time constant corresponds to the quasiparticle-recombination time, while for $\frac{T}{T_c}<0.9\mathrm{}$ a second time constant similar to the inelastic scattering time is also found. These observations confirm the results of Chang and Scalapino that increased recombination is more important than redistribution in enhancing the energy gap.