Kondo effect in a two-level quantum dot coupled to an external fermionic reservoir

We investigate theoretically the linear conductance of a two-level quantum dot as a function of the gate voltage and different strength of coupling to the external electronic system (the reservoir). Apart from the weak coupling regime, characterized by the Kondo-enhancement of the conductance in the spinful ground state, a strong coupling regime, which can be called mixed-valence, is found. This regime is characterized by a qualitative change of the energy level structure in the dot, resulting in sub- and super-tunneling coupling of the levels, which in turn yield a novel temperature dependence of the Kondo effect in the quantum dot.