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Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4

Yu-Jie Hao, Pengfei Liu, Yue Feng, Xiao-Ming Ma, Eike F. Schwier, Masashi Arita, Shiv Kumar, Chaowei Hu, Rui’e Lu, Meng Zeng, Yuan Wang, Zhanyang Hao, Hong-Yi Sun, Ke Zhang, Jiawei Mei, Ni Ni, Liusuo Wu, Kenya Shimada, Chaoyu Chen, Qihang Liu, and Chang Liu
Phys. Rev. X 9, 041038 – Published 21 November 2019
Physics logo See Synopsis: Skimming the Surface of Magnetic Topological Insulators
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Abstract

The recently discovered antiferromagnetic topological insulators in the Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with a half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi2Te4 exists inside the bulk band gap. Such an unexpected surface state remains unchanged across the bulk Néel temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab initio calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results unveil the experimental topological properties of MnBi2Te4, revealing that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases by tuning the magnetic or structural configurations, and thus push forward the comprehensive understanding of magnetic topological materials.

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  • Received 10 July 2019
  • Revised 7 August 2019

DOI:https://doi.org/10.1103/PhysRevX.9.041038

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

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Skimming the Surface of Magnetic Topological Insulators

Published 21 November 2019

Experiments by three separate groups show that the surface states of a magnetic topological insulator are not “gapped” as expected.  

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Authors & Affiliations

Yu-Jie Hao1, Pengfei Liu1, Yue Feng1, Xiao-Ming Ma1, Eike F. Schwier2, Masashi Arita2, Shiv Kumar2, Chaowei Hu3, Rui’e Lu1, Meng Zeng1, Yuan Wang1, Zhanyang Hao1, Hong-Yi Sun1, Ke Zhang2, Jiawei Mei1, Ni Ni3, Liusuo Wu1, Kenya Shimada2, Chaoyu Chen1,*, Qihang Liu1,4,†, and Chang Liu1,‡

  • 1Shenzhen Institute for Quantum Science and Engineering (SIQSE) and Department of Physics, Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong 518055, China
  • 2Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-0046, Japan
  • 3Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, California 90095, USA
  • 4Guangdong Provincial Key Laboratory for Computational Science and Material Design, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China

  • *chency@sustech.edu.cn
  • liuqh@sustech.edu.cn
  • liuc@sustech.edu.cn

Popular Summary

Topological insulators (TIs) are materials with insulating interiors and conducting surfaces. When aligned magnetic elements (or spins) are added, selected surfaces of the TI will no longer conduct electrons, instead showing insulating behavior. With some specific spin arrangements, such materials might present two exotic phenomena simultaneously: the “quantum anomalous Hall effect” and the “axion insulating state.” However, we find experimentally that an antiferromagnetic TI whose surface is supposed to insulate in fact conducts, showing that nature deals with magnetic TIs in a more intricate way than previously thought.

To reveal this surprising behavior, we study the antiferromagnetic MnBi2Te4 with a powerful surface-sensitive experimental tool called angle resolved photoemission. Theoretical predictions indicate that this recently synthesized material is the first ideal antiferromagnetic TI. However, our data clearly reveal that the “energy bands” at the surface form an integrated X shape: smoking-gun evidence that the surface is conductive in a nontrivial way. We propose that this unexpected behavior likely results from surface magnetic or structural reconstruction. Our calculations indicate several possible surface magnetic structures that support such a nontrivial conductive state.

Motivated by our results, future works on antiferromagnetic TIs could either find a way to overcome such surface reconstruction in favor of the long-sought axion insulators, or make use of such reconstruction to build devices with novel transport phenomena.

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See Also

Dirac Surface States in Intrinsic Magnetic Topological Insulators EuSn2As2 and MnBi2nTe3n+1

Hang Li et al.
Phys. Rev. X 9, 041039 (2019)

Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4

Y. J. Chen, L. X. Xu, J. H. Li, Y. W. Li, H. Y. Wang, C. F. Zhang, H. Li, Y. Wu, A. J. Liang, C. Chen, S. W. Jung, C. Cacho, Y. H. Mao, S. Liu, M. X. Wang, Y. F. Guo, Y. Xu, Z. K. Liu, L. X. Yang, and Y. L. Chen
Phys. Rev. X 9, 041040 (2019)

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Vol. 9, Iss. 4 — October - December 2019

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