Nonadiabatic dynamics in strongly driven diffusive Josephson junctions

By measuring the Josephson emission of a diffusive superconductor-normal metal-superconductor (SNS) junction we access the harmonic content of the current-phase relation (CPR). We experimentally identify a nonadiabatic regime in which the CPR is modified by high frequency microwave irradiation. This observation is explained by the excitation of quasiparticles in the normal wire induced by the electromagnetic field. The distortion of the CPR originates from the phase-dependent out-of-equilibrium distribution function which is strongly affected by the ac response of the spectral supercurrent. For a phase difference approaching pi, transitions across the minigap are dynamically favored, leading to a supercurrent reduction. This finding is supported by a comparison with the quasiclassical Green's function theory of superconductivity in diffusive SNS junctions under microwave irradiation.