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

Abstract

The layered MnBi2nTe3n+1 family represents the first intrinsic antiferromagnetic topological insulator (AFM TI, protected by a combination symmetry S ) 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 S -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 S -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.

Facet dependent surface energy gap on magnetic topological insulators

“…45 and 50, more states appear inside the surface hybridization gap, which can be interpreted as bulk states according to our theory. We also notice that most previous calculations based on slab models [18,43,47,51,53,54,56] found gapped surface states for the QL1 surface. These calculations adopted slab models with thicknesses from 5 to 13 nm.…”

“…However, many ARPES experiments and calculations concluded that the QL1 surface is gapped [42,43,45,47,48,51,54] while some other works claimed the gapless feature for the same surface [35,45,46,49,52,53]. We note that the gapped nature of surface states is crucial to realizing QAHE, Chern insulator, or AI, which requires insulating thin films including their surfaces.…”

“…1(c)]. So far, both experiments and calculations agree that the QL2 surface is gapless [35,42,43,[51][52][53][54].…”

An Integrated Quantum Material Testbed with Multi-Resolution Photoemission Spectroscopy

“…5(c) and 5(d)). While the focus of this work is on the technical capabilities of MRPES, we note that the interpretations of various spectral features from MnBi 4 Te 7 are currently under debates 44,45,48 . Based on our previous studies 49,50 , we attribute the large electronlike pocket on the MBT termination to a pair of Rashbasplit states.…”

Light-Tunable Surface State and Hybridization Gap in Magnetic Topological Insulator MnBi$_8$Te$_{13}$

“…While MnBi 4 Te 7 (m=1) [12][13][14][15][16][17][18][19] and MnBi 6 Te 10 (m=2) [17][18][19][20][21] have been found to be AFM similar to MnBi 2 Te 4 (m=0), MnBi 8 Te 13 (m=3) clearly distinguishes from other compounds with a unique FM interaction between neighboring MnBi 2 Te 4 layers and QAHE at zero field has been predicted at low temperature, 22,23 highlighting the importance of interlayer interaction.…”