Suppression of the Kondo effect in quantum dots by even-odd asymmetry

We analyze here a model for single-electron charging in semiconductor quantum dots that includes the standard Anderson on-site repulsion (U) as well as the spin exchange (Jd) that is inherently present among the electrons occupying the various quantum levels of the dot. We show explicitly that for ferromagnetic coupling (Jd0), an s-d exchange for an S=1 Kondo problem is recovered. In contrast, for the antiferromagnetic case, Jd<0, we find that the Kondo effect is present only if there are an odd number of electrons on the dot. In addition, we find that spin exchange produces a second period as well as additional fine structure in the conductance that is consistent with experimental measurements.