Screened spin-1 and-1/2 Kondo effect in a triangular quantum dot system with interdot Coulomb repulsion

By means of the numerical renormalization group (NRG) technique, we study the low temperature transport property and the phase transition for a triangular triple quantum dot system, including two centered dots (dot 1 and 2) and one side dot (dot 3). We focus on the effect of interdot repulsion V between two centered dots in a wide range of the interdot hopping t(ij) (ij = 1,2,3). When the hoppings between the centered dot and the side dot are symmetric, i.e., t(13) = t(23), and that between two centered dots t(12) is small, two centered dots form a spin triplet when V is absent, and a totally screened spin-1 Rondo effect is observed. In this case, one has a spin 1 that is partially screened by the leads as in the usual spin-1 Rondo model, and the remaining spin 1/2 degree of freedom forms a singlet with the side dot. As V is large enough, one of the centered dots is singly occupied, while the other one is empty. The spin-1/2 Rondo effect is found when t(13) is small. For large t(12), two centered dots form a spin singlet when V = 0, leading to zero conductance. As V is large enough, the spin-1/2 Rondo effect is recovered in the case of small t(13). For asymmetric t(13)# t(23) and small t(12), a crossover is found as V increases in comparison with a first order quantum phase transition for the symmetric case. In the regime of large V, the spin-1/2 Kondo effect could also be found when both t(13) and t(23) are small. We demonstrate the present model is similar to the side-coupled double dot system in some appropriate regimes, and it appears as a possible realization of side-controllable molecular electronics and spintronics devices. (C) 2017 Elsevier Ltd. All rights reserved.