Topological insulators: Potential devices and applications

Following the tutorial sessions on materials and physics of topological insulators, we will discuss important unique properties of TI and potential applications. In particular, we will outline the properties that are related to the spin-momentum lock for spin injection applications. I will then discuss the use of the magnetic order in topological insulators (TIs) to break the time-reversal-symmetry (TRS) to drive the non-trivial topological surface into a new massive Dirac-fermions state. An overview of the utilization of both the strong spin-orbit coupling (SOC) and the ferromagnetic (FM) orders in such TRS-breaking systems will be introduced. I will then summarize the progress on magnetic TIs and related TRS-breaking physics. In Bi₂Te₃/Sb₂Te₃-based TI materials, FM moments can be developed through two major mechanisms: the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling and the van Vleck mechanism. In the former case, we find the neighboring magnetic ions can also be coupled through the mediation of conduction carriers, indicating that the magnetic strength can be modulated by electric-field; on the other hand, unlike conventional dilute magnetic semiconductors (DMS), due to the unique band inversion in magnetic TIs, valence electrons themselves can generate large spin susceptibility, and the magnetic ions can thus be directly coupled through these local valence electrons without the assistance of the itinerant electrons. With gate-controlled magneto-transport measurements, the hole-mediated RKKY coupling and the carrier-independent van Vleck magnetism may be separated by controlling, for example, the Cr doping level.