The Two-stage Kondo Effect And Magnetotransport Properties In Asymmetric Coupled Double Quantum Dots Systems

Using numerical renormalization group method, we study the critical phenomena of quantum phase transition(QPT), the effect of the two-stage Kondo screening, and the transport properties with and without magnetic field in an strongly correlated asymmetric double quantum dot(DQD) system in this thesis.We first study the critical phenomena and the two-stage Kondo screening. It is find that the competition between the ferromagnetic induced by RKKY interaction and the interdot antiferromagnetic interaction induced by t leads to an effective interdot interaction, which induces a local triplet-singlet KT-type QPT at the critical point t=tc. In the triplet, the electrons of the leads screenings the local magnetic moment on two dots and the dots are screened at the same Kondo temperature. At the critical point t=tc, the two dots decouple with each other. In the critical regime, we could observe a two-stage Kondo screening dependence with two kinds of characteristic energy scales. It is find that the two kinds of Kondo peaks of two dots with the width corresponds to two different Kondo temperatures. The two kinds of peaks with two energy scales manifest that a two-stage Kondo screening really happens. As t increases continuously, the antiferromagnetic of effective interdot interaction is enhanced, the Kondo peak on the dot which weaken coupled leads becomes weaker and wider, and finally disappeared, a molecular-type singlet between two dots forms, which indicates that the two-stage Kondo screening have vanished.Then we study the transport properties in the DQD attached to leads with and without magnetic field. We focus on the situation in which the second dot (QD2) couples with the leads with a weak hybridization function. The results shows that by tuning the energy level ε2 of QD2 can control the conductance and its spin polarization of the system. In absence of magnetic field B, with increasing s2, the conductance shows a dip structure. In the triplet, the Kondo effect is a collective effect of two dots while in the doublet, the Kondo effect occurs just on QD1. In presence of B, we obtain spin-polarized conductance in different regime of ε2. In the regime far away the critical point of the QPT, the spin polarization of the conductance is positive and negative by the two sides of the critical point, respectively. In the critical regime, the behavior of the conductance is a mixing of above two cases. Especially, for a definite ε2, the spin polarization of the conductance may change from-1 to 1 by increasing the magnetic field. One can obtain a perfect spin filtering with a fully polarized conductance of up-spin or down-spin.