Low-energy doublet states, quantum phase transition, and partial Kondo screening in the triple dot system

Abstract We consider a triple dot system with one of the dots coupled to a lead whose density of states varies in power-law fashion. Using the poor-man's scaling and the numerical renormalization group method, we investigate a quantum phase transition between local moment and partially Kondo-screened strong coupling phases with the variation of non-thermal control parameters. These quantum phases are closely related to low-energy doublet states formed by the triangular symmetry breaking. Since the lead has power-law density of states, the transition fixed point separating odd-parity and even-parity doublets depends on pseudo-gap exponent. The quantum phase in the even-parity doublet is determined by the magnitude of the hybridization width; the triple dot system lies in partially Kondo-screened strong coupling phase only for large enough hybridization width. Scaling result for the critical hybridization width is compared with that of numerical renormalization group method.