| Transport through quantum dots is in general characterized by asym- metric couplings to the leads. In particular, in molecules and nanowires the coupling of each individual orbital level can be different. Motivated by recent experiments, we therefore study the effects of asymmetric hopping parameters on the non-equilibrium current and occupation probabilities of a two-level quantum dot. Starting from a two-level Anderson model, we perform a generalized Schrieffer-Wolff transformation to derive an effective Kondo model. A first perturbative analysis of the cotunneling current allows to determine a regime of negative differential conductance arising for couplings being both asymmetric with respect to the leads as well as to the quantum dot levels. Due to the non-equilibrium occupation of the quantum dot levels inelastic cotunneling transitions are allowed not only from the ground state but from the excited state. The dependence of this so-called cascade resonance on the magnetic field is discussed in detail. Since we expect the cotunneling lines to be measured experimentally for strong values of the couplings to the leads, we study the logarith- mic enhancement of the ascribed signatures by means of a poor-man's renormalization-group treatment out of equilibrium. |
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