Nonequilibrium Kondo effect in a multilevel quantum dot near the singlet-triplet transition

The linear and nonlinear transport through a multilevel lateral quantum dot connected to two leads is investigated using a generalized finite-$U$ slave-boson mean-field approach. For a two-level quantum dot, our calculation demonstrates a substantial conductance enhancement near the degeneracy point of the spin singlet and triplet states, a nonmonotonic temperature dependence of conductance, and a sharp dip and nonzero bias maximum of the differential conductance. These agree well with recent experiment observations. This two-stage Kondo effect in an out-of-equilibrium situation is attributed to the interference between the two energy levels.