Group members publish results on magnetic topological insulators in Physical Review X



Recently, collaboration between our group and the group of Qihang Liu from the Department of Physics, and Chaoyu Chen from the Institute for Quantum Science and Engineering, SUSTech had led to important progress in the field of magnetic topological materials. Our results have been published in the top physics journal Physical Review X. This work is also featured in the review magazine Physics.

Topological insulators are a hot material in the research of condensed matter physics in recent years. It has an insulating internal and conductive surfaces, and has potential applications in the fields of topological electronics, nonlinear optics, and topological quantum computing. With the introduction of long-range magnetic orders in topological insulators, theoretically, specific surfaces of these magnetic topological insulators (MTIs) will no longer conduct electrons, but will instead exhibit insulating behavior (to be precise, their surface energy bands form an energy gap). Antiferromagnetic topological insulators with certain specific spin arrangements are particularly important in this field because they may exhibit two peculiar phenomena at the same time: the "quantum anomalous Hall effect" and the "axion insulator state", and the insulating behavior of specific surfaces is a prerequisite for achieving the latter. However, the research team found through systematic experiments that, in an MTI, the surface that should be insulating is actually conductive (the surface energy gap is zero). This behavior shows that the handling of magnetic topological materials in nature is much more complicated and interesting than previously thought.

Figure: Existence and stability of a Dirac cone topological surface state in MnBi2Te4 without an energy gap. Top: X-shaped topological surface state (left) and the kz dispersion (right) of adjacent energy bands. Second line: Stability of the topological surface state.

To reveal this surprising behavior, the team used a powerful surface physics experimental tool called angle resolved photoelectron spectroscopy (ARPES) to study the antiferromagnetic topological insulator MnBi2Te4. Theoretical predictions and previous experiments show that this recently synthesized material is the first ideal antiferromagnetic topological insulator, and its natural cleavage surface should be insulating. However, the team's high-quality data clearly shows that its surface energy band forms a complete X shape – which proves that the surface conducts electricity in an unusual, topologically protected manner. This data directly overturned previous experimental conclusions, and led the study of magnetic topological systems into a new stage. The research group believes that this strange behavior may be caused by surface magnetic or structural reconstruction. First-principles calculations in the paper reveal several possible surface magnetic structures that support this unusual state of conduction.

The results of this study pave the way for future research works on antiferromagnetic topological materials. For example, future work may find a way to overcome this surface reconstruction to obtain the axion insulator state, or make use of this reconstruction to build a device with novel quantum transport behavior.

Part of the authors. From left: Cai Liu (SIQSE), Yu-Jie Hao, Xiao-Ming Ma, Chaoyu Chen (SIQSE), Chang Liu and Rui'e Lu.

This result was published online in Physics Review X on November 21, 2019, and Physics published a synopsis article entitled "Skimming the Surface of Magnetic Topological Insulators" on the same day. Yu-Jie Hao, Ph.D. student in Chang Liu's group, Pengfei Liu, an undergraduate in Qihang Liu's group, and Yue Feng, an undergraduate in Chang Liu's group, are the co-first authors. Chaoyu Chen, Qihang Liu and Chang Liu are the corresponding authors. This article was completed in cooperation with the Department of Physics and the Institute for Quantum Science and Engineering of SUSTech, the Hiroshima Synchrotron Radiation Center, the Department of Physics of UCLA, and the Guangdong Key Laboratory of Computational Science and New Materials Design. The Department of Physics and the Institute for Quantum Science and Engineering of SUSTech is the leading affiliation of the article.

This work was supported by the National Natural Science Foundation of China (NSFC), NSFC Guangdong, the Guangdong Innovative and Entrepreneurial Research Team Program, the Shenzhen Peacock Plan Team, the Shenzhen Key Laboratory, and the Technology and Innovation Commission of Shenzhen Municipality. The research team would like to thank the Hiroshima Synchrotron Radiation Center for their strong support for the work.

This work is also highlighted in the SUSTech website.

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