Direct visualization of edge state in even-layer MnBi 2 Te 4 at zero magnetic field.

Being the first intrinsic antiferromagnetic (AFM) topological insulator (TI), MnBi 2 Te 4 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 MnBi 2 Te 4 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 edge state of even-layer MnBi 2 Te 4 at zero field is derived from gapped helical edge states of the quantum spin Hall effect with time-reversal-symmetry breaking, when a crossover from a three-dimensional TI MnBi 2 Te 4 to a two-dimensional TI occurs. Our finding thus signifies the richness of topological phases in MnB 2 Te 4 that has yet to be fully explored.