Ballistic Transport in Superconducting Weak Links in a Microwave Field

Nonequilibrium effects and their impact on a charge transport in superconducting ballistic weak links biased by an ac voltage are investigated within the framework of the Keldysh technique. We demonstrate that the microwave field destroys the phase coherence during the multiple Andreev reflection cycle and leads to the effective cooling of subgap quasiparticles accelerated due to multiple Andreev reflection. For small bias voltages this effect results in a strong supression of both the excess current and the conductance of the weak link. In the opposite limit of large bias voltages the excess current remains unaffected. We also demonstrate that a simple Boltzmann kinetic approach becomes inadequate if an ac voltage bias is applied to the weak link.