Mean-field theory of the Kondo effect in quantum dots with an even number of electrons - art. no. 085322

We investigate the enhancement of the Kondo effect in quantum dots with an even number of electrons, using a scaling method and a mean field theory. W e evaluate the Kondo temperature T-K as a function of the energy difference between spin-singlet and -triplet states in the dot, Delta, and the Zeeman splitting, E-z. If the Zeeman splitting is small, E-z<<T-K, the competitio n between the singlet and triplet states enhances the Kondo effect. T-K rea ches its maximum around Delta =0 and decreases with Delta obeying a power l aw. If the Zeeman splitting is strong, E-z>>T-K, the Kondo effect originate s from the degeneracy between the singlet state and one of the components o f the triplet state at - Delta similar to E-z. We show that T-K exhibits an other power-law dependence on E-z. The mean field theory provides a unified picture to illustrate the crossover between these regimes. The enhancement of the Kondo effect can be understood in terms of the overlap between the Kondo resonant states created around the Fermi level. These resonant states provide the unitary limit of the conductance G similar to 2e(2)/h.