Abstract
Spin–orbit coupling (SOC) in materials plays a crucial role in interconversion between spin and charge currents. In reduced dimensions, SOC effects are enhanced and have been the focus of intensive experimental and theoretical research, both for their novel spin-dependent phenomena and for their potential exploitation in new spintronics devices. Thanks to the discovery of a family of two-dimensional materials, extensive research has been conducted to explore potential material systems to achieve high spin–charge interconversion rates as well as to allow detection and accurate measurement. This article reviews the prospect of topological insulators as a reliable material system for efficient spin–charge interconversion and recent experimental advances in detecting the charge-to-spin and spin-to-charge conversions on topological insulator surfaces via spin-torque ferromagnetic resonance and spin-pumping techniques, respectively.
Highlights
Spin–charge interconversion in materials with strong spin–orbit interactions has received great attention in the past few years, stimulated by their rich spin-transport phenomena and inspiring a plethora of promising applications in spintronics
The spin current in these materials can be detected by the inverse spin-Hall effect (ISHE)[23] and the inverse Rashba–Edelstein effect (IREE).[24]. Another important phenomenon that was observed in these materials is the spin-Seebeck effect,[25–27] which can be utilized for efficient spin-to-charge power generation and has led to the emergence of the field of spin caloritronics.[28]
The readers can refer to detailed reviews on topological insulators (TIs) related to band theory,[35–37] material growth,[38,39] device applications,[40] magneto-transport measurements and associated quantum phenomenon,[41–46] and exotic physics related to TI-superconductor systems.[43,47]
Summary
Spin–charge interconversion in materials with strong spin–orbit interactions has received great attention in the past few years, stimulated by their rich spin-transport phenomena and inspiring a plethora of promising applications in spintronics. The energy-momentum dispersion relation of these topological surface states in the narrow bulk bandgap energy range can be modeled by an effective Dirac Hamiltonian of the form HD = hvF (kyσx − kxσy) for common Bi/Sb–Se/Te-based TIs neglecting the effects of hexagonal warping.[29–31] His the reduced Planck’s constant, vF is the Fermi velocity, kx and ky are the in-plane momentum components, and the σx and σy are the in-plane Pauli spin matrices. Hexagonal warping in TIs can give rise to out-of-plane (z-directional) spin-polarization components as well.[29–31] The Fermi-level position can be varied from the CB branch to the VB branch of the Dirac cone through the Dirac point either by the electrostatic field effect (gating) or by chemical doping (composition tuning).[29–31] The existence of metallic Dirac surface states in a TI, the spin-momentum locking, and the momentum-space spin orientations of the TI surface states have been directly observed by angleresolved photoemission spectroscopy, spin- and angle-resolved photoemission spectroscopy, and spin-polarized photo-current measurements.[29–34] The readers can refer to detailed reviews on TIs related to band theory,[35–37] material growth,[38,39] device applications,[40] magneto-transport measurements and associated quantum phenomenon,[41–46] and exotic physics related to TI-superconductor systems.[43,47]
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