Kondo effects in a triangular triple quantum dot with lower symmetries

[摘要] We study the low-energy properties and characteristic Kondo energy scale of a triangular triple quantum dot, connected to two non-interacting leads, in a wide parameter range of a gate voltage and distortions which lower the symmetry of an equilateral structure, using the numerical renormalization group approach. For large Coulomb interactions, the ground states with different characters can be classified according to the plateaus of Theta equivalent to (delta(e) - delta(o))(2/pi), where delta(e) and delta(o) are the phase shifts for the even and odd partial waves. At these plateaus of Theta, both Theta and the occupation number N-tot = (delta(e) + delta(o))(2/pi) take values close to integers, and thus the ground states can be characterized by these two integers. The Kondo effect with a local moment with total spin S = 1 due to a Nagaoka mechanism appears on the plateau, which can be identified by Theta similar or equal to 2.0 and N-tot similar or equal to 4.0. For large distortions, however, the high-spin moment disappears through a singlet-triplet transition occurring within the four-electron region. It happens at a crossover to the adjacent plateaus for Theta similar or equal to 0.0 and Theta similar or equal to 4.0, and the two-terminal conductance has a peak in the transient regions. For weak distortions, the SU(4) Kondo effect also takes place for N-tot similar or equal to 3.0. It appears as a sharp conductance valley between the S = 1/2 Kondo ridges on both sides. We also find that the characteristic energy scale T* reflect these varieties of the Kondo effect. Particularly, T* is sensitive to the distribution of the charge and spin in the triangular triple dot.

[关键词] RENORMALIZATION-GROUP APPROACH;DILUTE MAGNETIC-ALLOYS;STATIC PROPERTIES;ANDERSON MODEL;RESERVOIRS;ELECTRONS;TRANSPORT;METALS