Tunable Kondo effect in double quantum dots coupled to ferromagnetic contacts

TL;DR

This paper explores how spin polarization in ferromagnetic contacts affects the Kondo effect in double quantum dots, demonstrating control over spin conductance and revealing phase transitions in related models.

Contribution

It introduces a method to restore the Kondo resonance via super-exchange tuning and analyzes the impact of ferromagnetic exchange on quantum phase transitions.

Findings

Effective magnetic fields suppress the Kondo effect.

Super-exchange interaction can restore the Kondo resonance.

Ferromagnetic exchange leads to a quantum phase transition.

Abstract

We investigate the effects induced by spin polarization in the contacts attached to a serial double quantum dot. The polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The super-exchange interaction ($J_{\mathrm{AFM}}$), tuned by the inter-dot tunnelling rate $t$, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Furthermore, we study the associated two-impurity Kondo model and show that a ferromagnetic exchange coupling ($J_{\mathrm{FM}}$) leads to an effective spin-1 exchange-anisotropic Kondo model which exhibits a quantum phase transition in the presence of partially polarized contacts.