Combined single-electron and coherent-Cooper-pair tunneling in voltage-biased Josephson junctions

Small-capacitance tunnel junctions show single-electron effects and, in the superconducting state, the coherent tunneling of Cooper pairs. We study these effects in a system of two voltage-biased Josephson junctions in series. Novel features show up in the I-V characteristics, in particular, pronounced structures at subgap voltages. These are due to combined quasiparticle and pair tunneling and to higher-order Cooper-pair tunneling processes. Several of them have been observed in experiments.