Ground state of the parallel double quantum dot system

TL;DR

This paper resolves a controversy about the ground state of a double quantum dot system near half filling, showing that the system exhibits an underscreened Kondo state with residual magnetic moment and conductance.

Contribution

The study combines NRG and quantum Monte Carlo methods to demonstrate the presence of an underscreened Kondo state, clarifying the nature of the ground state.

Findings

NRG predicts an underscreened Kondo state with residual spin-1/2.

Quantum Monte Carlo results agree with NRG predictions.

The dark state on the dots causes the unconventional behavior.

Abstract

We resolve the controversy regarding the ground state of the parallel double quantum dot system near half filling. The numerical renormalization group (NRG) predicts an underscreened Kondo state with residual spin-1/2 magnetic moment, ln 2 residual impurity entropy, and unitary conductance, while the Bethe Ansatz (BA) solution predicts a fully screened impurity, regular Fermi-liquid ground state, and zero conductance. We calculate the impurity entropy of the system as a function of the temperature using the hybridization-expansion continuous-time quantum Monte Carlo technique, which is a numerically exact stochastic method, and find excellent agreement with the NRG results. We show that the origin of the unconventional behavior in this model is the odd-symmetry "dark state" on the dots.