Mapping the chemical potential dependence of current-induced spin polarization in a topological insulator

TL;DR

This study measures how current-induced spin polarization in topological insulators varies with chemical potential, revealing significant enhancement near the Dirac point and disentangling surface and bulk contributions.

Contribution

It provides the first electrical gating measurements of spin polarization in topological insulators, demonstrating tunability and quantifying spin signals near the Dirac point.

Findings

Spin polarization increases up to 300% near the Dirac point.

Hysteretic voltage signals confirm spin-momentum locking.

Linear relationship between voltage change and bias current.

Abstract

We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disen- tangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/Al2O3) as a spin detector on a back-gated (Bi,Sb)2Te3 channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or anti-parallel to the spin polariza- tion of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied DC bias current in the (Bi,Sb)2Te3 channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e. the Dirac point).