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 spi n-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 st ates a reaches its maximum around Delta = 0 and decreases with increasing D elta. The Kondo effect is thus enhanced by competition between singlet and triplet states. Our results explain recent experimental findings. We evalua te the linear conductance in the perturbative regime.