Kondo-peak splitting and resonance enhancement caused by interdot tunneling in coupled double quantum dots

Abstract

In this paper, we report our recent work on the interdot tunneling-dependent Kondo effect for two serially coupled quantum dots (QDs) by adopting the hierarchical equations of motion approach. As is known, for such a system, when interdot tunneling $t$ increases, a continuous crossover from a Kondo singlet to a spin singlet is observed rather than a quantum phase transition, even though an induced antiferromagnetic interaction exists due to the interdot tunneling $t$. Our study focuses on the influence of the tunneling $t$ between two QDs at a finite temperature and especially on the smearing of the quantum phase transition. In this study, we find that, at small $t$, there is an enhanced Kondo effect, which results in the most noticeable Kondo effect appearing at finite $t$ rather than at $t=0$. When $t$ is larger, the Kondo peak splits into two subpeaks. Because of the weak coupling to the leads, the splitting space exactly corresponds to the energy difference between the original degenerated local spin singlet and the local spin triplet of the two isolated QDs. The Kondo peak can be preserved when the energy difference between the local spin singlet and the local spin triplet is smaller than the enhanced Kondo temperature. Finally, interdot tunneling also introduces a doubly occupied charge singlet, which causes charge fluctuations in the ground state at finite temperature and thus, therefore, smears the phase transition. This can also explain why no phase transition is observed in experiments in other QDs systems, even though the phase transition is always predicted in theory.