Quantum Dot as a Spin--Current Diode

TL;DR

This paper investigates a quantum dot system that acts as a spin-current diode, showing bias-dependent spin polarization suppression and enhancement due to Coulomb interactions and spin accumulation.

Contribution

It introduces a quantum dot-based spin-current diode model demonstrating bias-dependent spin polarization control using Coulomb interactions.

Findings

Current polarization is suppressed to zero at positive bias.

Maximum spin polarization occurs at negative bias.

The effect is explained by Coulomb interaction and spin accumulation.

Abstract

We report a study of spin dependent transport in a system composed of a quantum dot coupled to a normal metal lead and a ferromagnetic lead (NM-QD-FM). We use the master equation approach to calculate the spin-resolved currents in the presence of an external bias and an intra-dot Coulomb interaction. We find that for a range of positive external biases (current flow from the normal metal to the ferromagnet) the current polarization $\wp=(I_\uparrow-I_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to zero, while for the corresponding negative biases (current flow from the ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The system thus operates as a rectifier for spin--current polarization. This effect follows from an interplay between Coulomb interaction and nonequilibrium spin accumulation in the dot. In the parameter range considered, we also show that the above results can be obtained via nonequilibrium Green functions within a Hartree-Fock type approximation.