Enhancement of Kondo effect in multilevel quantum dots

We theoretically study enhancement mechanisms of the Kondo effect in multilevel quantum dots. In quantum dots fabricated on semiconductors, the energy difference between discrete levels Δ is tunable by applying a magnetic field. With two orbitals and spin 1/2 in the quantum dots, we evaluate the Kondo temperature T _K as a function of Δ, using the scaling method. T _K is maximal around Δ = 0 and decreases with increasing |Δ|, following a power law, T _K(Δ) = T _K(0)·(T _K(0)/|Δ|) ^γ, which is understood as a crossover from SU(4) to SU(2) Kondo effect. The exponents on both sides of a level crossing, γ _L and γ _R, satisfy a relation of γ _L·γ _R = 1. We compare this enhanced Kondo effect with that by spin-singlet-triplet degeneracy for an even number of electrons, to explain recent experimental results using vertical quantum dots.