Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman and interference effects

TL;DR

This paper investigates how magnetic fields influence the spin-polarized conductance in a double quantum dot system, revealing complex interactions between Kondo effects, interference, and Zeeman splitting, with potential for spin filter applications.

Contribution

It provides a detailed analysis of magnetic field effects on spectral and transport properties in a double quantum dot, including a generalized Friedel sum rule and spin polarization control.

Findings

Magnetic field causes nonuniversal spectral function dependence.

Interdot exchange coupling leads to antiparallel magnetic moments.

Complete spin polarization achievable via gate voltages and magnetic field.

Abstract

We study the effect of an external magnetic field in the Kondo regime of a double-quantum-dot system in which a strongly correlated dot (the "hanging dot") is coupled to a second, noninteracting dot that also bridges the gap between two external leads. In zero field, the spectral function of the hanging dot has previously been shown to exhibit a split-peak structure near the Fermi level due to "Kondo resonance filtering" by the bridging dot. We show using the numerical renormalization group that application of a magnetic field leads to a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for the spectral and transport properties. The value of the hanging-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be explained using a generalized Friedel sum rule for a Kondo system with energy-dependent hybridization. The magnetic field also accentuates the exchange-mediated interdot coupling, which dominates the ground state at intermediate fields leading to the formation of antiparallel magnetic moments on the dots. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter. The system's low-energy properties are qualitatively unchanged by the presence of weak on-site Coulomb repulsion within the bridging dot.