Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study

TL;DR

This study uses numerical renormalization group methods to analyze how ferromagnetic leads affect the Kondo resonance in quantum dots, revealing ways to restore the resonance via magnetic fields or gate voltages.

Contribution

It extends NRG techniques to handle spin asymmetric lead densities and explores mechanisms to recover the Kondo effect in ferromagnetic environments.

Findings

Ferromagnetic leads suppress and split the Kondo resonance.

Magnetic fields can compensate for lead-induced asymmetry.

Gate voltages can also restore the Kondo resonance.

Abstract

We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchange interaction with ferromagnetic leads leading to a suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry restoring the Kondo effect. We study both the gate-voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in presence of leads with an energy dependent density of states is not only possible by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.