Crossover from photon-assisted tunneling to classical behavior in single-electron transistors

We present a quantitative study of the frequency and amplitude dependence of the response of single-electron transistors to microwave radiation. At high frequencies, photon-assisted tunneling of electrons and, in all-superconducting samples, of Cooper pairs is observed. At lower frequencies, a nonadiabatic classical response is found, which depends on the microwave amplitude but not on frequency. We relate the crossover frequency to the widths of the tunneling spectrum features, which depend on the electron temperature and the quasiparticle tunneling rates. [S0163-1829(98)03216-0].