Hybridization-induced gapped and gapless states on the surface of magnetic topological insulators

The layered Mn Bi2n Te3n+1 family represents the first intrinsic antiferromagnetic (AFM) topological insulator (protected by a combination symmetry S ) ever discovered, providing an ideal platform to explore novel areas of physics such as the quantum anomalous Hall effect at elevated temperature and axion electrodynamics. Some of the recent angle-resolved photoemission spectroscopy (ARPES) experiments on this family have revealed that all terminations exhibit (nearly) gapless topological surface states (TSSs) in the AFM state. The gapless behavior is inconsistent with the theoretical expectation, as the surfaces being studied are S-breaking and shall therefore open a gap. Here we explain this curious paradox using a surface-bulk band hybridization picture. Combining circular dichroism ARPES and first-principles calculations on Mn Bi6 Te10 , we prove that gaplike features are induced through hybridization between TSSs and certain bulk bands with Rashba character. The observed (nearly) gapless features are consistently reproduced by tight-binding simulations where TSSs are coupled to a pair of Rashba-split bands (RSBs). The Dirac-cone-like spectral features actually originate from the RSBs. Our findings highlight the role of band hybridization, superior to magnetism in this case, in shaping the general surface band structure in this family of magnetic topological materials.