Quantum mechanics

Magnetic field

Antiferromagnetism

Particle physics

Magnetism

Axion

Physics

Topological order

Mathematics

Topological insulator

Condensed matter physics

Quantum

Combinatorics

Topology (electrical circuits)

Dark matter

Nature Materials

The intricate interplay between nontrivial topology and magnetism in two-dimensional (2D) materials has led to the emergence of many novel phenomena and functionalities. An outstanding example is the quantum anomalous Hall (QAH) effect, which was realized in magnetically doped topological insulators (TIs) in the absence of magnetic field. Recently, the layered van der Waals compound MnBi2Te4 has been theoretically predicted and experimentally verified to be a TI with interlayer antiferromagnetic (AFM) order. It is a rare stoichiometric material with coexisting topology and magnetism, thus represents a perfect building block for complex topological-magnetic structures. Here we investigate the quantum transport behaviors of both bulk crystal and exfoliated MnBi2Te4 flakes in a field effect transistor geometry. In the 6 septuple layers (SLs) device tuned into the insulating regime, we observe a large longitudinal resistance and zero Hall plateau, which are characteristic of the axion insulator state. The robust axion insulator state occurs in zero magnetic field, over a wide magnetic field range, and at relatively high temperatures. Moreover, a moderate magnetic field drives a quantum phase transition from the axion insulator phase to a Chern insulator phase with zero longitudinal resistance and quantized Hall resistance h/e2 (h is the Plank constant and e is the elemental charge). These results pave the road for using even-number-SL MnBi2Te4 to realize the quantized topological magnetoelectric effect and axion electrodynamics in condensed matter systems.

Robust axion insulator and Chern insulator phases in a two-dimensional antiferromagnetic topological insulator