Direct comparison of current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states

TL;DR

This study compares current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states, demonstrating the dominant role of Dirac surface states in Bi2Se3 and revealing opposite spin polarization signs.

Contribution

It provides the first direct electrical comparison of spin polarization from Dirac surface states and Rashba 2DEG states, clarifying their individual contributions and signs.

Findings

Spin polarization has opposite signs in Bi2Se3 and InAs.

Dirac surface states dominate spin polarization in Bi2Se3.

Model confirms the sign and origin of spin signals.

Abstract

Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry protected, linearly dispersing Dirac surface states. Band bending at the TI surface may also lead to coexisting trivial two-dimensional electron gas (2DEG) states with parabolic energy dispersion that exist as spin-split pairs due to Rashba spin-orbit coupling (SOC). A bias current is expected to generate spin polarization in both systems arising from their helical spin-momentum locking. However, their induced spin polarization is expected to be different in both magnitude and sign. Here, we compare spin potentiometric measurements of bias current-generated spin polarization in Bi2Se3(111) films where Dirac surface states coexist with trivial 2DEG states, with identical measurements on InAs(001) samples where only trivial 2DEG states are present. We observe spin polarization arising from spin-momentum locking in both cases, with opposite signs of the spin voltage. We present a model based on spin dependent electrochemical potentials to directly derive the signs expected for the TI surface states, and unambiguously show that the dominant contribution to the current-generated spin polarization measured in the TI is from the Dirac surface states. This direct electrical access of the helical spin texture of Dirac and Rashba 2DEG states is an enabling step towards the electrical manipulation of spins in next generation TI and SOC based quantum devices.