  Being the first intrinsic antiferromagnetic(AFM) topological insulator(TI),
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 edge state of even-layer MnBi2Te4 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 MnBi2Te4 to a two-dimensional TI occurs. Our finding thus
signifies the richness of topological phases in MnB2Te4 that has yet to be
fully explored.
