Kondo effect in a quantum dot coupled to ferromagnetic leads and side-coupled to a nonmagnetic reservoir

TL;DR

This paper investigates the Kondo effect in a quantum dot system with ferromagnetic leads and a nonmagnetic reservoir, revealing quantum phase transitions and the possibility of restoring the Kondo effect with magnetic fields.

Contribution

It provides a theoretical analysis of the Kondo regime in a complex quantum dot setup, highlighting the effects of magnetic configurations and exchange coupling on conductance.

Findings

Quantum phase transition in antiparallel configuration with increasing exchange coupling.

Suppressed conductance in parallel configuration due to spin splitting.

Restoration of the Kondo effect using compensating magnetic fields.

Abstract

Equilibrium transport properties of a single-level quantum dot tunnel-coupled to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are analyzed theoretically in the Kondo regime. The equilibrium spectral functions and conductance through the dot are calculated using the numerical renormalization group (NRG) method. It is shown that in the antiparallel magnetic configuration, the system undergoes a quantum phase transition with increasing exchange coupling $J$, where the conductance drops from its maximum value to zero. In the parallel configuration, on the other hand, the conductance is generally suppressed due to an effective spin splitting of the dot level caused by the presence of ferromagnetic leads, irrespective of the strength of exchange constant. However, for $J$ ranging from J=0 up to the corresponding critical value, the Kondo effect and quantum critical behavior can be restored by applying properly tuned compensating magnetic field.