Enhancement of Kondo effect in multilevel quantum dots

TL;DR

This paper investigates how the Kondo effect is enhanced in multilevel quantum dots by tuning energy levels with magnetic fields, revealing a crossover from SU(4) to SU(2) symmetry and comparing with experimental data.

Contribution

It provides a theoretical analysis of the enhancement mechanisms of the Kondo effect in multilevel quantum dots, including the dependence of Kondo temperature on level splitting and symmetry crossover.

Findings

Kondo temperature peaks at zero level splitting

T_K follows a power law decay with level difference ||

Exponents satisfy _L _R=1

Abstract

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 \Delta 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 \Delta, using the scaling method. T_K is maximal around \Delta=0 and decreases with increasing |\Delta|, following a power law, T_K(\Delta)=T_K(0) (T_K(0)/|\Delta|)^\gamma, which is understood as a crossover from SU(4) to SU(2) Kondo effect. The exponents on both sides of a level crossing, \gamma_L and \gamma_R, satisfy a relation of \gamma_L \gamma_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.