We study the low-energy properties of a triangular triple quantum dot connected to two noninteracting 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 $N_{\text{tot}}$, or the level position ${ϵ}_d$ of the triple dot. The SU(4) Kondo behavior is seen in the half-filled case $N_{\text{tot}}\simeq 3.0$ 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 quasiparticle 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 $N_{\text{tot}}\simeq 4.0$, 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.

• Received 20 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.155330