Underscreened Kondo effect in quantum dots coupled to ferromagnetic leads

TL;DR

This paper investigates the underscreened Kondo effect in a two-level quantum dot with ferromagnetic leads, revealing how spin polarization and external magnetic fields influence the Kondo phenomena using NRG calculations.

Contribution

It provides a detailed analysis of the gate voltage dependence of spectral functions and demonstrates the extreme sensitivity of the underscreened Kondo effect to magnetic fields.

Findings

Dot polarization is highly sensitive to lead spin imbalance.

Sign change of polarization occurs in the S=1 Coulomb valley.

External magnetic fields can partially restore the Kondo effect.

Abstract

We analyze the equilibrium transport properties of underscreened Kondo effect in the case of a two-level quantum dot coupled to ferromagnetic leads. Using the numerical renormalization group (NRG) method, we have determined the gate voltage dependence of the dot's spin and level-resolved spectral functions. We have shown that the polarization of the dot is very susceptible to spin imbalance in the leads and changes sign in the middle of the S=1 Coulomb valley. Furthermore, we have also found that by fine-tuning an external magnetic field one can compensate for the presence of ferromagnetic leads and restore the Kondo effect in the case of $S=1/2$ Coulomb valley. However, the underscreened Kondo effect cannot be fully recovered due to its extreme sensitivity with respect to the magnetic field.