Spin Fractionalization of an Even Number of Electrons in a Quantum Dot

Abstract

We find that Kondo resonant conductance can occur in a quantum dot in the Coulomb blockade regime with an even number of electrons N. The contacts are attached to the dot in a pillar configuration, and a magnetic field B( perpendicular) along the axis is applied. B( perpendicular) lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B( perpendicular) to the value B(*) where levels with different total spin cross restores both the degeneracy and the Kondo effect. We analyze a dot charged with N = 2 electrons. Coupling to the contacts is antiferromagnetic due to a spin selection rule and, in the Kondo state, the charge is unchanged while the total spin on the dot is S = 1/2.

Cotunneling Spectroscopy in Few-Electron Quantum Dots

“…Effects of lifting spin degeneracy of the triplet have also been theoretically investigated [14]. For the degenerate triplet case, a characteristic asymmetric peak in conductance at the ST crossing has been predicted [15,16].…”

“…The interplay of electron-electron interactions, electron spin, and coupling to a Fermi sea makes transport in the few-electron regime a subtle problem in many-body physics [12,13,14,15,16,17]. Of particular importance is the two-electron case ("quantum dot helium") [17] since this is a paradigm for the preparation of entangled electronic states [18], and in double quantum dots is the basis of a quantum gate proposal [19].…”

Tunneling and magnetic properties of triple quantum dots

“…In this case the quantum with dot a singlet ground state may become magnetically active due to external forces, and Kondo effect arises either at finite energy [19] or in external magnetic field [20][21][22]. Later on it was recognized that in many cases the direct mapping of the original Kondo model into such system is impossible, because the effective symmetry of the pertinent nanoobject is neither SU(2) nor SU(2N).…”

“…Another facet of Kondo physics in nanoobjects was unveiled, when the possibility of Kondo effect in quantum dots with even electron occupation number was considered in several theoretical publications [19][20][21][22]. In this case the quantum with dot a singlet ground state may become magnetically active due to external forces, and Kondo effect arises either at finite energy [19] or in external magnetic field [20][21][22].…”

Possibility of two-channel spin-½ Kondo conductance in a quantum dot

“…If N is even, the ground state (GS) of the QD is usually a spin singlet and the ordinary Kondo effect cannot occur. Nevertheless, it has been shown that level crossing between states at different S induced by a magnetic field B orthogonal to the dot can restore the degeneracy required for the Kondo effect to take place [4][5][6]. The occurrence of Kondo physics in quantum dots was predicted long ago in analogy to magnetic impurities in diluted metal alloys at very low temperatures [7].…”