Kondo screening in high-spin side-coupled two-impurity clusters
Abstract
We study the system of two magnetic impurities described by a two-impurity Kondo model where only the first impurity couples directly to the conduction band, while the second impurity interacts with the first through Heisenberg exchange coupling in a "side-coupled" configuration. We consider various choices of the impurity spins (S1<S2, S1=S2, and S1>S2) and we contrast the regimes where the inter-impurity exchange coupling J is either lower or higher than the Kondo temperature TK(0) of the first impurity in the absence of the second. This model is a high-spin generalization of the two-impurity model for side-coupled double quantum dots which corresponds to the simplest S1=S2=1/2 case, where the moments are Kondo screened successively in two stages for J<TK(0) (the ``two-stage Kondo effect''). We show that the two-stage Kondo screening occurs generically for S2 >= S1. For S1 >= 1, the second Kondo temperature TK(2) is not exponentially reduced, as for S1 = 1/2, but is approximately a power-law function of the coupling J. Furthermore, for S1 >=1 1 all three scales (TK(0), J, TK(2)) explicitly appear in the temperature-dependence of the thermodynamic properties. For S1 > S2, there is no second stage of screening for J<TK(0), however in the opposite limit J>TK(0) the Kondo screening of the effective spin S1-S2 is found.