Spin injection from topological insulator tunnel-coupled to metallic leads

TL;DR

This paper theoretically investigates how contacts influence transport in topological insulators, proposing a structure that generates pure spin currents and analyzing how various length scales affect current magnitudes.

Contribution

It introduces a simple device structure for pure spin current generation and analyzes the effects of contact-induced damping on transport in topological insulators.

Findings

Transport properties are strongly affected by contacts acting as heat baths.

Pure spin current can be generated with a simple structure.

Currents depend on characteristic length scales like damping length and contact size.

Abstract

We study theoretically helical edge states of 2D and 3D topological insulators (TI) tunnel-coupled to metal leads and show that their transport properties are strongly affected by contacts as the latter play a role of a heat bath and induce damping and relaxation of electrons in the helical states of TI. A simple structure that produces a pure spin current in the external circuit is proposed. The current and spin current delivered to the external circuit depend on relation between characteristic lengths: damping length due to tunneling, contact length and, in case of 3D TI, mean free path and spin relaxation length caused by momentum scattering. If the damping length due to tunneling is the smallest one, then the electric and spin currents are proportional to the conductance quantum in 2D TI, and to the conductance quantum multiplied by the ratio of the contact width to the Fermi wavelength in 3D TI.