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 stares for an even number of electrons. We show that the Kondo temperature as a function of the energy difference between the states a reaches its maximum around Delta = 0 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.