Direct visualization of edge state in even-layer MnBi₂Te₄ at zero magnetic field

Being the first intrinsic antiferromagnetic (AFM) topological insulator, MnBi₂Te₄ is argued to be a topological axion state in its even-layer form due to the antiparallel magnetization between the top and bottom layers. Here we combine both transport and scanning microwave impedance microscopy (sMIM) to investigate such axion state in atomically thin MnBi₂Te₄ with even-layer thickness at zero magnetic field. While transport measurements show a zero Hall plateau signaturing the axion state, sMIM uncovers an unexpected edge state raising questions regarding the nature of the “axion state”. Based on our model calculation, we propose that the even-layer MnBi₂Te₄ at zero field is in an AFM quantum spin Hall (QSH) state hosting a pair of helical edge states. Such novel AFM QSH is originated from the combination of half translation symmetry and time-reversal symmetry in MnBi₂Te₄. Our finding thus signifies the richness of topological phases in MnB₂Te₄ that has yet to be fully explored.