Opposite current-induced spin polarizations in bulk-metallic Bi2Se3 and bulk-insulating Bi2Te2Se topological insulator thin flakes

TL;DR

This study compares current-induced spin polarizations in bulk-insulating Bi2Te2Se and bulk-metallic Bi2Se3 thin flakes, revealing opposite signs linked to different conduction channels, aiding in distinguishing topological surface states from other effects.

Contribution

It demonstrates a method to differentiate topological surface state spin polarization from bulk and trivial conduction channels in 3D topological insulators.

Findings

CISP sign in BTS221 matches TSS helicity but opposes bulk SHC.

Opposite CISP sign in Bi2Se3 aligns with Rashba-Edelstein and bulk SHE effects.

Provides an electrical approach to identify conduction channels in spin transport.

Abstract

One of the most fundamental and exotic properties of 3D topological insulators (TIs) is spin-momentum-locking (SML) of their topological surface states (TSSs), promising for potential applications in future spintronics. However, other possible conduction channels, such as a trivial two-dimensional electron gas (2DEG) with strong Rashba type spin-orbit interaction (SOI) and bulk conducting states that may possess a spin Hall effect (SHE), can coexist in 3D TIs, making determining the origin of the current induced spin polarization (CISP) difficult. In this work, we directly compared the CISP between bulk-insulating Bi2Te2Se (BTS221) and bulk-metallic Bi2Se3 thin flakes using spin potentiometry. In the bulk insulating BTS221, the observed CISP has a sign consistent with the expected helicity of the SML of the TSS, but an opposite sign to its calculated bulk spin Hall conductivity (SHC). However, compared to BTS221, an opposite CISP is observed in the bulk metallic Bi2Se3, consistent instead with both the expectations of its Rashba-Edelstein effect of the band-bending induced 2DEG and bulk spin Hall Effect (SHE). Our results provide an electrical way to distinguish the TSS from other possible conducting channels in spin transport measurements on 3D TIs, and open ways for the potential applications in charge-spin conversion devices.