Quasiparticle-phonon downconversion in nonequilibrium superconductors

We have developed a theory of quasiparticle and phonon energy downconversion in nonequilibrium superconductors following the absorption of an energetic photon. This stage of energy downconversion cascade is important for the production of quasiparticles and is shown to split into two phases. The first is controlled by the evolution of the phonon distribution while the second is dominated by quasiparticle downconversion. The relative durations of the two phases and hence the rates of quasiparticle generation depend on material parameters, and most common superconductors could be classified into three different groups. For typical superconductors used for x-ray detection the downconversion cascade was shown to be fast compared to various time scales in the tunneling regime.