Spin transport across carbon nanotube quantum dots

TL;DR

This paper studies spin-dependent electron transport in carbon nanotube quantum dots with ferromagnetic contacts, deriving equations for the density matrix and analyzing conductance behaviors, including nonlinear effects and negative differential conductance.

Contribution

It develops a comprehensive theoretical framework for spin transport in SWCNT quantum dots with arbitrary contact polarization, including analytical and numerical results for various configurations.

Findings

Identification of four resonant tunneling regimes with symmetry considerations.

Analytical expressions for conductance in collinear configurations.

Observation of negative differential conductance in nonlinear regimes.

Abstract

We investigate linear and nonlinear transport in interacting single wall carbon nanotubes (SWCNTs) that are weakly attached to ferromagnetic leads. For the reduced density matrix of a SWCNT quantum dot, equations of motion which account for an arbitrarily vectored polarisation of the contacts are derived. We focus on the case of large diameter nanotubes where exchange effects emerging from short-ranged processes can be excluded and the four-electron periodicity at low bias can be observed. This yields in principle four distinct resonant tunnelling regimes, but due to symmetries in the involved groundstates, each two possess a mirror-symmetry. With a non-collinear configuration, we recover at the 4N <-> 4N+1 / 4N+3 <-> 4N resonances the analytical results known for the angular dependence of the conductance of a single level quantum dot or a metallic island. The two other cases are treated numerically and show on the first glance similar, yet not analytically describable dependences. In the nonlinear regime, negative differential conductance features occur for non-collinear lead magnetisations.