Enhancement of Kondo Effect in Quantum Dots with an Even Number of Electrons

We investigate the Kondo effect in a quantum dot with almost degenerate spin-singlet and triplet states for an even number of electrons. We show that the Kondo temperature as a function of the energy difference between the states $\Delta$ reaches its maximum around $\Delta =0\mathrm{}$ and decreases with increasing $\Delta$. The Kondo effect is thus enhanced by competition between singlet and triplet states. Our results explain recent experimental findings. We evaluate the linear conductance in the perturbative regime.