Nonequilibrium transport via spin-induced subgap states in superconductor/quantum dot/normal metal cotunnel junctions

We study low-temperature transport through a Coulomb blockaded quantum dot (QD) contacted by a normal (N) and a superconducting (S) electrode. Within an effective cotunneling model the conduction electron self-energy is calculated to leading order in the cotunneling amplitudes and subsequently resummed to obtain the nonequilibrium T matrix, from which we obtain the nonlinear cotunneling conductance. For even-occupied dots the system can be conceived as an effective S/N-cotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd-occupied dot, however, leads to the formation of subgap resonances inside the superconducting gap which give rise to a characteristic peak-dip structure in the differential conductance, as observed in recent experiments.