Spin-Charge Locking and Tunneling into a Helical Metal

TL;DR

This paper develops a kinetic model for electrons on a topological insulator surface, revealing spin-charge locking and how tunneling properties depend on magnetic orientation and layer configurations.

Contribution

It introduces a kinetic equation capturing spin-charge locking in helical metals and analyzes tunneling effects with ferromagnets and nonmagnetic layers.

Findings

Backward scattering is suppressed due to helical nature.

Tunnel resistance varies with magnetization angle.

Nonmagnetic layers enhance spin polarization.

Abstract

We derive a kinetic equation for the electrons moving on the surface of a three-dimensional topological insulator. Due to the helical nature of the excitations backward scattering is suppressed in the collision integral, and the spin dynamics is entirely constrained by that of the charge. We further analyze the tunneling between the helical and a conventional metal or ferromagnet. We find that the tunnel resistance strongly depends on the angle between the magnetization in the ferromagnet and the current in the helical metal. A nonmagnetic layer on top of the helical metal amplifies the current-induced spin polarization.