Spin fractionalization of an even number of electrons in a quantum dot

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 to along the axis is applied. B-perpendicular to lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B-perpendicular to 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.