Sequential tunneling through a two-level semiconductor quantum dot system coupled to magnetic leads

Abstract

Sequential transport through a semiconductor quantum dot (QD) weakly coupled to two collinear magnetic leads is analyzed theoretically based on the master equation method. The transport model considers two discrete energy levels in the dot, i.e., the lowest unoccupied ϵe (electron) and the highest occupied ϵh (hole) energy levels, with associated Coulomb correlation energies Ue and Uh, respectively, and the spin-flip effect within the dot. The charge and spin currents and the tunneling magnetoresistance are evaluated in the sequential tunneling regime. It was found that when both energy levels, ϵe and ϵh, contribute to the tunneling transport, there is enhancement of the current as well as tunneling magnetoresistance, compared to the case of a single-level QD transport considered previously. The tunneling transport through the two levels of the QD also results in additional bias-dependence: it was observed that spin accumulation and magnetoresistance of the system are particularly suppressed by spin flip within the bias range corresponding to single QD occupancy.