Ferromagnetic and underscreened Kondo behavior in quantum dot arrays

TL;DR

This paper investigates the complex low-temperature behavior of a star-configured quantum dot array, revealing a transition from ferromagnetic to underscreened Kondo effects and characterizing the associated strongly correlated regimes.

Contribution

It extends previous models to N quantum dots, demonstrating a ferromagnetic Kondo model and a two-stage Kondo effect with detailed numerical analysis.

Findings

Identification of ferromagnetic Kondo behavior for N>1

Observation of a two-stage Kondo effect at low temperatures

Numerical characterization of thermodynamic and spectral properties

Abstract

We analyze the low energy properties of a device with $N+1$ quantum dots in a star configuration. A central quantum dot is tunnel coupled to source and drain electrodes and to $N$ quantum dots. Extending previous results for the $N=2$ case we show that, in the appropriate parameter regime, the low energy Hamiltonian of the system is a ferromagnetic Kondo model for a $S=(N-1)/2$ impurity spin. For small enough interdot tunnel coupling, however, a two-stage Kondo effect takes place as the temperature is decreased. The spin $1/2$ in the central quantum dot is Kondo screened first and at lower temperatures the antiferromagnetic coupling to the side coupled quantum dots leads to an underscreened $S=N/2$ Kondo effect. We present numerical results for the thermodynamic and spectral properties of the system which show a singular behavior at low temperatures and allow to characterize the different strongly correlated regimes of the device.