Hybridization-Induced Gapped and Gapless States on the Surfaces of Magnetic Topological Insulators

TL;DR

This paper reveals that the nearly gapless surface states observed in magnetic topological insulators are due to surface-bulk band hybridization rather than magnetic effects, challenging previous assumptions about their topological nature.

Contribution

It demonstrates that hybridization, not magnetism, explains the gapless surface states in MnBi2nTe3n+1, combining ARPES, first-principles calculations, and tight-binding simulations.

Findings

Hybridization causes apparent gap closure in surface states.

Bulk origin of Dirac-like features explains gapless behavior.

Applicable to all terminations of MnBi2nTe3n+1 family.

Abstract

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.