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 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 , 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.
- 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)
Synopsis
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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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 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.