Magnetic Order in 3D Topological Insulators -- Wishful Thinking or Gateway to Emergent Quantum Effects?

TL;DR

This paper reviews the potential of magnetic order in 3D topological insulators to induce quantum effects like the quantum anomalous Hall effect, discussing experimental challenges and future prospects.

Contribution

It provides a comprehensive analysis of methods to break time-reversal symmetry in 3D TIs and evaluates their effectiveness in realizing emergent quantum states.

Findings

Magnetic doping can induce exchange gaps in TIs.

Proximity coupling offers an alternative TRS breaking method.

Challenges remain in achieving robust quantum anomalous Hall states.

Abstract

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counter-propagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI, and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS), and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)$_2$Te$_3$, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or by proximity coupling to a magnetically ordered layer or substrate, in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)$_2$(Se,Te)$_3$ family, and try to answer the question whether these materials can live up to the hype surrounding them.