SU(3) Kondo effect in spinless triple quantum dots

TL;DR

This paper proposes a triple quantum dot device to observe the SU(3) Kondo effect, characterized by a conductance plateau at 3/4 of the unitary limit, and analyzes its thermodynamic properties and robustness.

Contribution

It introduces a purely capacitively coupled spinless triple quantum dot system as a new platform to observe and analyze the SU(3) Kondo effect, including its thermodynamics and stability.

Findings

Conductance pinned at 3/4 of the unitary limit indicates SU(3) Kondo effect.

Effective impurity orbital moment increases from 1 to 4/3 as temperature decreases.

SU(3) Kondo effect is robust against certain perturbations and can be restored by tuning dot potentials.

Abstract

We discuss a device --- a purely capacitively coupled interacting spinless triple quantum dot system --- for the observation of the SU(3) Kondo effect. Unlike more familiar SU(2) and SU(4) Kondo effects in quantum dot devices which lead to unitary linear conductance at low temperatures, the SU(3) Kondo scenario can be easily identified by the conductance pinned to a characteristic value of 3/4 of the unitary limit. This is associated with the interesting fact that the SU(3) Kondo effect does not occur at the particle-hole symmetric point, where the system is found instead in the valence-fluctuating regime with the total dot occupancy flipping between 1 and 2, but for gate voltages in the two Kondo plateaux where the dot occupancy is pinned to an integer value, either 1 or 2. From the thermodynamic analysis in the Kondo regime we find that the effective impurity orbital moment, defined through the impurity orbital susceptibility (chi_imp) multiplied by the temperature, is T\chi_imp=1 at high temperatures and then it increases to the characteristic value of T\chi_imp=4/3 corresponding to the three-fold degenerate local-moment fixed point where the impurity entropy is S_imp=ln 3. Then, at much lower temperatures, the system flows to the non-degenerate strong-coupling fixed point in which the SU(3) Kondo effect takes place. We also report results about the robustness of the SU(3) Kondo effect against various perturbations present in real experimental setups. Finally, we describe possible mechanisms to restore the SU(3) Kondo physics by properly tuning the on-site dot potentials. We briefly comment on the spinfull case which has very different behavior and shows Kondo plateaus in conductance for all integer values of the occupancy, including at the particle-hole symmetric point.