⇦Chinese Physics Letters, 2021, Vol. 38, No. 1, Article code 010101Viewpoint Making Axion Dynamical in Non-Centrosymmetric Magnetic Topological Insulators Chaoxing Liu (刘朝星)* Affiliations Department of Physics, The Pennsylvania State University, University Park, PA, USA Received 4 November 2020; accepted 5 November 2020; published online 16 November 2020 *Corresponding author. Email: cxl56@psu.edu Citation Text: Liu C X 2021 Chin. Phys. Lett. 38 010101    Abstract DOI:10.1088/0256-307X/38/1/010101 © 2021 Chinese Physics Society Article Text “Axion” was predicted as a hypothetical elementary particle to resolve the strong conjugation-parity problem in particle physics,[1] and it is also an attractive candidate for the as-yet-unobserved dark matter in cosmology.[2] 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”,[3] can emerge in certain condensed matter systems, particularly topological insulator materials,[4] in which a variety of exotic physical phenomena (e.g. topological magnetoelectric effect) have been theoretically predicted.[5,6] The recent discovery of magnetic topological insulators in MnBi$_{2}$Te$_{4}$ family of materials[7,8] provides an excellent platform to explore these physical phenomena induced by axion electrodynamics.[9–13] Current research mainly focuses on the phenomena related to a static quantized axion field (also called $\theta$ field), of which the parameter $\theta$ is independent of time. This is because the axion field is normally fixed to a quantized value ($\theta =\pi$) by certain symmetry in MnBi$_{2}$Te$_{4}$ family of materials. On the other hand, several intriguing phenomena, such as axionic polariton[14] and axion instability induced by nonlinear electromagnetic effect,[15] relies on the dynamics of axion field. Therefore, the ability of controlling the $\theta$ value of the axion field and inducing a large fluctuation of $\theta$ 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 (TaSe$_{4})_{2}$I can also play the role of dynamical axion field.[16] However, since (TaSe$_{4})_{2}$I 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.[17] 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 Mn$_{2}$Bi$_{2}$Te$_{5}$-related topological materials as the candidates. The key insight is that the parameter $\theta$ 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 $\theta$ away from the quantized value for the fluctuation. It is further noticed that $\theta$ can rapidly vary between the values of 0 and $\pi$ 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 MnBi$_{2}$Te$_{4}$ compounds, namely X$_{2}$A$_{2}$B$_{5}$, 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[18–21] 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 Mn$_{2}$Bi$_{2}$Te$_{5}$ 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. References CP Conservation in the Presence of PseudoparticlesTwo applications of axion electrodynamicsTopological insulators and superconductorsTopological field theory of time-reversal invariant insulatorsMagnetoelectric Polarizability and Axion Electrodynamics in Crystalline InsulatorsExperimental Realization of an Intrinsic Magnetic Topological Insulator *Prediction and observation of an antiferromagnetic topological insulatorQuantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi 2 Te 4Intrinsic magnetic topological insulators in van der Waals layered MnBi 2 Te 4 -family materialsRobust axion insulator and Chern insulator phases in a two-dimensional antiferromagnetic topological insulatorUnique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi 2 Te 4 FilmsTopological Axion States in the Magnetic Insulator MnBi 2 Te 4 with the Quantized Magnetoelectric EffectDynamical axion field in topological magnetic insulatorsInstability in Magnetic Materials with a Dynamical Axion FieldAxionic charge-density wave in the Weyl semimetal (TaSe4)2ILarge Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn 2 Bi 2 Te 5Topological nature of nonlinear optical effects in solidsIn situ click chemistry generation of cyclooxygenase-2 inhibitorsQuantum Nonlinear Hall Effect Induced by Berry Curvature Dipole in Time-Reversal Invariant MaterialsEnhancement of the Bulk Photovoltaic Effect in Topological Insulators
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