Andreev interferometers in a strong radio-frequency field

We experimentally study the influence of 1-40 GHz radiation on the resistance of normal (N) mesoscopic conductors coupled to superconducting (S) loops (Andreev interferometers). At low radio-frequency (RF) amplitudes we observe the usual h/2e superconducting phase periodic resistance oscillations as a function of applied magnetic flux. We find that the oscillations acquire a π-shift with increasing RF amplitude, and consistently with this result the resistance at fixed phase is an oscillating function of the RF amplitude. The results are explained qualitatively as a consequence of two processes. The first is the modulation of the phase difference between the N/S interfaces by the RF field, with the resistance adiabatically following the phase. The second process is the change in the electron temperature caused by the RF field. From the data, the response time of the Andreev interferometer is estimated to be τ_f < 40 ps. However there are a number of experimental features which remain unexplained; these include the drastic difference in behaviour of the resistance at phiv = π and 0 as a function of the RF frequency and amplitude, and the existence of a 'window of transparency' where heating effects are weak enough to allow for the π-shift. A microscopic theory describing the influence of RF radiation on Andreev interferometers is required.