Chapter 6 - Kondo effect in quantum dots with an even number of electrons

Abstract

This chapter investigates the Kondo effect in semiconductor quantum dots when the number of electrons in the dots, N, is even. The chapter begins with an overview of the Kondo effect with odd N which strongly influences various transport properties. Following this, a theory for the Kondo effect is constructed with even N when the spin-singlet and -triplet states are almost degenerate. The Kondo temperature is evaluated as a function of the energy difference between the states, A, by the scaling method, and it is illustrated that the Kondo effect is significantly enhanced by the competition between the spin states. This is in agreement with the experimental results using “vertical” quantum dots in which it can be tuned by applying a magnetic field. The chapter also examines the Kondo effect by the mean-field theory. The enhancement of the Kondo effect can be understood in terms of the overlap between the Kondo resonant states created around the Fermi level. These resonant states provide the unitary limit of the conductance; G∼2e2/h.