内禀磁性的拓扑反铁磁绝缘体Mn2Bi2Te5是另一种理想的磁性拓扑绝缘体系统，而且实验样品已经成功合成，有望成为第一个实现动力学轴子场的量子材料。与Mn2Bi2Te4和Bi2Te3的异质结可产生更多具有可调磁相互作用和拓扑性质的新材料，必将成为研究磁性拓扑物态、二维材料调控、凝聚态/光学交叉等研究的理想平台。

以上工作已发表在CPL Express Letters栏目

Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn2Bi2Te5

Jinlong Zhang (张金龙), Dinghui Wang (王丁辉), Minji Shi (施敏吉), Tongshuai Zhu (朱同帅), Haijun Zhang (张海军), Jing Wang (王靖)

Chin. Phys. Lett. 2020, 37 (7): 077304

应编辑部邀请，美国宾夕法尼亚州立大学刘朝星教授为本文作了点评！

Chaoxing Liu (刘朝星)

Chin. Phys. Lett. 2021, 38 (1): 010101

“Axion” was predicted as a hypothetical elementary particle to resolve the strong conjugation-parity problem in particle physics, and it is also an attractive candidate for the as-yet-unobserved dark matter in cosmology. While the detection of axions still remains elusive in particle physics and cosmology, it was recently proposed that the elegant physics of axions, known “axion electrodynamics”, can emerge in certain condensed matter systems, particularly topological insulator materials, in which a variety of exotic physical phenomena (e.g. topological magnetoelectric effect) have been theoretically predicted. The recent discovery of magnetic topological insulators in MnBi2Te4 family of materials provides an excellent platform to explore these physical phenomena induced by axion electrodynamics. Current research mainly focuses on the phenomena related to a static quantized axion field (also called θ field), of which the parameter θ is independent of time. This is because the axion field is normally fixed to a quantized value (θ=π) by certain symmetry in MnBi2Te4 family of materials. On the other hand, several intriguing phenomena, such as axionic polariton and axion instability induced by nonlinear electromagnetic effect, relies on the dynamics of axion field. Therefore, the ability of controlling the θ value of the axion field and inducing a large fluctuation of θ is of great importance for the experimental test of axion electrodynamics, as well as the potential applications. More recently, it was suggested that the phason mode of the charge density wave in Weyl semimetal (TaSe4)2I can also play the role of dynamical axion field. However, since (TaSe4)2I is non-magnetic and respects time reversal, it remains challenging to induce a large fluctuation of the axion field for the experimental probe of axionic polariton and axion instability.

The paper by Zhang et al. provides a guiding principle to search for materials with a large dynamical axion field, based on which they identify a series of van der Waals layered Mn2Bi2Te5-related topological materials as the candidates. The key insight is that the parameter θ is fixed to a quantized value by either time reversal T or inversion P, and thus breaking both T and P is required to drive θ away from the quantized value for the fluctuation. It is further noticed that θ can rapidly vary between the values of 0 and π when the system is close to a topological phase transition. Therefore, a non-centrosymmetric magnetic topological insulator with a small gap (close to topological phase transition) is preferable for a large dynamical axion field. Based on this guiding principle, Zhang et al. propose that a large dynamic axion field can exist in a variation of the MnBi2Te4 compounds, namely X2A2B5, X = Mn/Eu, A = Sb/Bi, B = Se/Te, through the first-principles calculations. A systematic study on the variation of the gap as a function of element substitution (Bi/Sb and Te/Se) is also carried out in order to guide the optimization of the material compounds experimentally to maximize the effect of dynamical axion field.

Zhang et al. also explore the possible experimental detection of dynamical axion field. It is interesting to notice that they predict a double frequency signature induced by dynamical axion field in nonlinear optical spectroscopy. Current efforts in topological nonlinear optics mainly focus on the phenomena related to Berry phase and Berry curvature, while nonlinear optical response induced by axion electrodynamics remains largely unexplored. If these proposed phenomena can be observed in Mn2Bi2Te5 family of materials, this may also pave the way to a new generation of axion-based devices for the applications of electronics, optronics and spintronics.

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