Scaling analysis of the Kondo effect in quantum dots with an even number of electrons

The Kondo effect is investigated theoretically when spin singlet and triplet states are involved. This Kondo effect is realized in quantum dots with an even number of electrons, where the energy difference between the spin states, Δ, can be controlled experimentally using a magnetic field. Based on the scaling analysis for general situations, we show that the Kondo effect is significantly enhanced when the spin states are nearly degenerate (Δ ≈ 0). An analytical expression is given for a lower limit of the Kondo temperature as a function of Δ.