  The layered MnBi2nTe3n+1 family represents the first intrinsic
antiferromagnetic topological insulator (AFM TI, protected by a combination
symmetry ) ever discovered, providing an ideal platform to explore novel
physics such as quantum anomalous Hall effect at elevated temperature and axion
electrodynamics. Recent angle-resolved photoemission spectroscopy (ARPES)
experiments on this family have revealed that all terminations exhibit (nearly)
gapless topological surface states (TSSs) within the AFM state, violating the
definition of the AFM TI, as the surfaces being studied should be -breaking and
opening a gap. Here we explain this curious paradox using a surface-bulk band
hybridization picture. Combining ARPES and first-principles calculations, we
prove that only an apparent gap is opened by hybridization between TSSs and
bulk bands. The observed (nearly) gapless features are consistently reproduced
by tight-binding simulations where TSSs are coupled to a Rashba-split bulk
band. The Dirac-cone-like spectral features are actually of bulk origin, thus
not sensitive to the-breaking at the AFM surfaces. This picture explains the
(nearly) gapless behaviour found in both Bi2Te3- and MnBi2Te4-terminated
surfaces and is applicable to all terminations of MnBi2nTe3n+1 family. Our
findings highlight the role of band hybridization, superior to magnetism in
this case, in shaping the general surface band structure in magnetic
topological materials for the first time.
