Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi2Te4

In a magnetic topological insulator, the interplay of nontrivial band topology and magnetic order gives rise to intriguing states of matter, most notably exemplified by quantum anomalous Hall (QAH) insulators and axion insulators. These magnetic topological insulators are typically obtained by doping magnetic atoms into topological insulators. However, the presence of random magnetic dopants inevitably introduces disorders that impede further exploration of topological quantum effects within the material.

In this context, we focus on MnBi2Te4, a stoichiometric topological insulator that possesses an innate magnetic order. This layered van der Waals crystal is an antiferromagnet in the bulk form, with each layer exhibiting ferromagnetic order (a layer refers to a single structural unit in the out-of-plane direction). Atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has odd number of layers. We have observed the zero-field QAH effect in specimens composed of five layers. As we further gate the surface states of the QAH insulator under a strong magnetic field, conventional quantum Hall (QH) states emerge. I will discuss the intriguing array of topological states that arise from the intricate interplay between the QAH and QH effects.