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Topological surface states with intrinsic magnetic ordering in the MnBi2Te4(Bi2Te3)n compounds have been predicted to host rich topological phenomena including quantized anomalous Hall effect and axion insulator state. Here we use scanning tunneling microscopy to image the surface Dirac fermions in MnBi2Te4 and MnBi4Te7. We have determined the energy dispersion and helical spin texture of the surface states through quasiparticle interference patterns far above Dirac energy, which confirms its topological nature. Approaching the Dirac point, the native defects in the MnBi2Te4 septuple layer give rise to resonance states which extend spatially and potentially hinder the detection of a mass gap in the spectra. Our results demonstrate that regulating defects is essential to realize exotic topological states at higher temperatures in these compounds.

By means of the first-principles calculations and magnetic topological quantum chemistry, we demonstrate that the low-energy physics in the checkerboard antiferromagnetic (AFM) monolayer FeSe, very close to an AFM topological insulator that hosts robust edge states, can be well captured by a double-degenerate nearly flat band with fragile topology just below the Fermi level. The Wilson loop calculations identify that such fragile topology is protected by the S4z symmetry, which gives rise to a 2D second-order topological insulator that supports the bound state with fractional charge e/2 at the sample corner. This work provides a platform to study the intriguing properties of magnetic fragile topological electronic states. Previous observations of the edge states and bound states in checkerboard AFM monolayer FeSe can also be well understood in our work.

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