Coulomb-blocked transport through a quantum dot with spin-split level: Increase of differential conductance peaks by spin relaxation

Nonequilibrium transport through a quantum dot with one spin-split single-particle level is studied in the cotunneling regime at low temperatures. The Coulomb diamond can be subdivided into parts differing in at least one of two respects: what kind of tunneling processes (i) determine the single-particle occupations and (ii) mainly contribute to the current. No finite systematic perturbation expansion of the occupations and the current can be found that is valid within the entire Coulomb diamond. We therefore construct a nonsystematic solution, which is physically correct and perturbative in the whole cotunneling regime, while smoothly crossing over between the different regions. With this solution, the impact of an intrinsic spin-flip relaxation on the transport is investigated. We focus on peaks in the differential conductance, which mark the onset of cotunneling-mediated sequential transport. It is shown that these peaks are maximally pronounced at a relaxation roughly as fast as sequential tunneling. The approach as well as the presented results can be generalized to quantum dots with few levels.