Kondo effects in a triangular triple quantum dot: NRG study in the whole region of the electron filling

TL;DR

This paper investigates the diverse Kondo effects in a triangular triple quantum dot system across various electron fillings using NRG, revealing SU(4) behavior, charge redistribution, and a two-stage Kondo screening of a local moment.

Contribution

It provides a comprehensive NRG analysis of Kondo phenomena in a triangular triple quantum dot, highlighting the conditions for SU(4) symmetry and the emergence of a two-stage Kondo effect.

Findings

SU(4) Kondo behavior at half-filling (N_{tot} = 3.0)

Charge redistribution between orbitals at low energies

Two-stage Kondo screening of a local S=1 moment at N_{tot} = 4.0

Abstract

We study the low-energy properties of a triangular triple quantum dot connected to two non-interacting leads in a wide parameter range, using the numerical renormalization group (NRG). Various kinds of Kondo effects take place in this system depending on the electron filling N_{tot}, or the level position $\epsilon_d$ of the triple dot. The SU(4) Kondo behavior is seen in the half-filled case N_{tot} = 3.0 at the dip of the series conductance, and it causes a charge redistribution between the even and odd orbitals in the triangle. We show generally that the quasi-particle excitations from a local Fermi-liquid ground state acquire a channel symmetry at zero points of the two-terminal conductance, in the case the system has time-reversal and inversion symmetries. It causes the SU(4) behavior at low energies, while the orbital degeneracy in the triangle determines the high-energy behavior. At four-electron filling N_{tot} = 4.0, a local S=1 moment emerges at high temperatures due to a Nagaoka ferromagnetic mechanism. It is fully screened by the electrons from the two conducting channels via a two-stage Kondo effect, which is caused by a difference in the charge distribution in the even and odd orbitals.