Analysis of low energy response and possible emergent SU(4) Kondo state in a double quantum dot

TL;DR

This paper investigates the low energy behavior of a double quantum dot system, revealing conditions under which an emergent SU(4) Kondo state appears and explaining experimental conductance observations.

Contribution

It demonstrates that SU(4) symmetry emerges only when interdot and on-site interactions are equal and sufficiently strong, providing a detailed analysis of the low energy fixed point and conductance behavior.

Findings

SU(4) symmetry appears only when U_{12}=U and both are > D

Phase shift of π/4 at degeneracy point indicates 4-fold degeneracy

Temperature-dependent conductance can be modeled with renormalized quasiparticle parameters

Abstract

We examine the low energy behavior of a double quantum dot in a regime where spin and pseudospin excitations are degenerate. The individual quantum dots are described by Anderson impurity models with an on-site interaction $U$ which are capacitively coupled by an interdot interaction $U_{12}<U$. The low energy response functions are expressed in terms of renormalized parameters, which can be deduced from an analysis of the fixed point in a numerical renormalization group calculation. At the point where the spin and pseudospin degrees of freedom become degenerate, the free quasiparticle excitations have a phase shift of $\pi/4$ and a 4-fold degeneracy. We find, however, when the quasiparticle interactions are included, that the low energy effective model has SU(4) symmetry only in the special case $U_{12}=U$ unless both $U$ and $U_{12}$ are greater than $D$, the half-bandwidth of the conduction electron bath. We show that the gate voltage dependence of the temperature dependent differential conductance observed in recent experiments can be described by a quasiparticle density of states with temperature dependent renormalized parameters.