Andreev interferometers in a strong radio-frequency field

Abstract

We experimentally study the influence of 1–40 GHz radiation on the resistance ofnormal (N) mesoscopic conductors coupled to superconducting (S) loops (Andreevinterferometers). At low radio-frequency (RF) amplitudes we observe the usualh/2e superconducting phase periodic resistance oscillations as a functionof applied magnetic flux. We find that the oscillations acquire aπ-shift with increasing RF amplitude, and consistently with this result the resistance at fixedphase is an oscillating function of the RF amplitude. The results are explained qualitativelyas a consequence of two processes. The first is the modulation of the phase differencebetween the N/S interfaces by the RF field, with the resistance adiabatically following thephase. The second process is the change in the electron temperature caused by the RFfield. From the data, the response time of the Andreev interferometer is estimated to beτf < 40 ps. However there are a number of experimental features which remainunexplained; these include the drastic difference in behaviour of the resistance atφ = 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 theπ-shift. A microscopic theory describing the influence of RF radiation on Andreevinterferometers is required.