Yong P. Chen

Affiliations

Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA

Title

Electrically generated spin polarization in topological insulators: Where are the spins from?

Abstract

One of the most important properties of topological insulators (TI) is the helical spin-momentum locking of the topological surface states (TSS), giving rise to a current induced helical spin polarization (CISP). Various recent experiments including ours have measured a CISP in TIs and demonstrated the promise of TIs as electrically-controlled spin injectors and source of spin polarization for spintronics applications. On the other hand, CISP is not unique to TIs, and has been measured previously in other spin-orbit coupled (SOC) systems such as heavy metals with spin Hall effects (SHE) and 2D electron gases (2DEG) with Rashba SOC in InSb or InAs based semiconductor structures. Since many commonly studied TI materials often possess conducting bulk and/or band-bending-induced (topologically trivial) surface 2DEGwith Rashba SOC, it is important to ask whether such channels and mechanisms (bulk SHE, surface Rashba 2DEG) may also give rise to a CISP measurable in experiments. I will discuss our spin-sensitive transport measurements (such as spin potentiometry) using ferromagnetic electrodes to detect spin polarization, where we have found qualitatively different types and behaviors of CISP in various different TI materials. For example, the CISP found in some of the materials has opposite spin helicity than that of TSS, and is more consistent with the Rashba surface 2DEG. If time permits, I may also discuss possible effects of hyperfine coupling to nuclear spins that has been revealed in NMR but largely neglected in transport measurements. Our studies reveal a rich variety of entities in real TI materials that may play important roles in potential applications of TIs in spintronics.

References

• [1] J. Tian, I. Miotkowski, S. Hong, and Y. P. Chen, Electrical injection and detection of spin-polarized currents in topological insulator Bi₂Te₂Se, arXiv:1501.05050.