Axion insulator and helical Chern insulator phases in MnBi₂Te₄

The recently discovered MnBi₂Te₄ combines intrinsic magnetism and nontrivial topology in one material, providing an ideal platform for exploring novel topological phases. In this talk, we report transport studies of exfoliated MnBi₂Te₄ flakes with varied layer structure and magnetic fields. In the 6-SL (septuple layer) sample tuned into the bulk-insulating regime, a robust zero Hall plateau and insulating longitudinal resistance exist in a wide range of magnetic fields and gate voltages. These are transport characteristics of an axion insulator when the top and bottom surfaces of MnBi₂Te₄ have opposite magnetization. An external magnetic field polarizes the magnetization of all the layers, and drives a transition from the axion insulator phase to a Chern insulator with zero longitudinal resistance and quantized Hall resistance. In a 7-SL sample subjected to pulsed magnetic field up to 60 T, we observe systematic and yet uniquely complex evolution of quantized Hall plateaus with Chern numbers from C = -3 to +1. More surprisingly, a novel phase characterized by an extremely broad zero Hall plateau emerges as the most robust ground state in the high field limit. Non-local transport measurements and theoretical calculations reveal that this C = 0 phase arises from the coexistence of a ferromagnetic-order-induced Chern band with C = -1 and a Zeeman-effect-induced Chern band with C = +1. This helical Chern insulator phase with broken time-reversal-symmetry represents a new type of quantum Hall effects originated from topologically nontrivial band structure.