Real two-stage Kondo effect in parallel double quantum dot

We study the two-stage Kondo effect in asymmetric parallel double quantum dots. In the triplet, the magnetic moment screenings on two dots occur at the same Kondo temperature. In the critical regime of the triplet-singlet quantum phase transition, a two-stage Kondo screening accompanied with two kinds of Kondo resonance with two energy scales is observed. This is contrast to previous works, in which the Kondo peak in the second screening has not been observed. For large asymmetry of the Kondo coupling, the Kondo resonance in the second step is very weak, which indicates that the screening occurs mainly between two dots and is not a real Kondo screening. Therefore, the side-coupled double quantum dots, which have been extensively studied in the literature, are not an ideal candidate to show a real two-stage Kondo screening.