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 spin-ful ground state, a strong coupling regime, which can be called mixed-valence (MV), 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. The Kondo effect arises from the coherent screening between a localized spin and that of surrounding mobile electrons, producing for example anomalous transport properties in metals with magnetic impurities [1]. Recently, however, there has been a great deal of experimental activity in systems where an individual localized spin is probed directly in quantum dots defined in semiconductor systems [2, 3]. Previous investigations on both single-and multi-level models have uncovered interesting features of the Kondo effect, including Kondo peaks in the conductance and the associated density of states, their temperature dependence, and other features [2, 4]. However, the typical approximation made in models of multilevel quantum dots with discrete energy levels is to neglect the strong mixing between the energy levels of the dot due to the interaction with the external fermionic system. This approach incorporates the external fermionic system only as a broadening of the levels in the quantum dot [4]. In this paper we analyze the Kondo effect in a quantum dot coupled to an external fermionic system in addition to the coupling to the measuring leads. We explicitly take into account the mixing of the states in the dot due to the coupling to an external reservoir. A typical experimentally accessible example of such a system is a dot coupled to three leads with one lead playing the role of the reservoir and drawing no current. It is assumed that the coupling to the fermionic reservoir can be varied independently of the coupling to the measuring leads. In the case of weak coupling, the reservoir just leads to broadening of the energy levels in the dot, whereas strong coupling results in a qualitative rearrangement of the energy levels which influences the Kondo effect in a nontrivial manner. In what follows we consider a two-level quantum dot in the …