Tunable Fano-Kondo resonance in side-coupled double quantum dot systems

Abstract

We study the interference between the Fano and Kondo effects in a side-coupled double quantum dot system where one of the quantum dots couples to conduction-electron bath while the other dot only side couples to the first dot via antiferromagnetic (AF) spin-exchange coupling. We apply both the perturbative renormalization-group (RG) and numerical renormalization-group (NRG) approaches to study the effect of AF coupling on the Fano line shape in the conduction leads. With particle-hole symmetry, the AF spin-exchange coupling competes with the Kondo effect and leads to a local spin-singlet ground state for arbitrary small coupling, so-called "two-stage Kondo effect." As a result, via NRG we find the spectral properties of the Fano line shape in the tunneling density of states rho(c)(omega) of conduction-electron leads shows double dip-peak features at the energy scale around the Kondo temperature and the one much below it, corresponding to the two-stage Kondo effect; it also shows an universal scaling behavior at very low energies. We find the qualitative agreement between the NRG and the perturbative RG approaches. Relevance of our work to the experiments is discussed.