施加电场产生磁化以及施加磁场产生电极化，这就是典型的磁电效应。具有该效应的磁电材料在传感、存储等领域有着广泛的应用价值。近年来，人们在三维拓扑绝缘体中发现了一种全新的拓扑电子态起源的磁电效应。这一效应来源于受时间反演对称性或空间反演对称性保护的量子化的静态轴子场。更为有趣的是，如果时间反演与空间反演对称性都被破坏时，静态轴子场不再是量子化的，可以变为动态轴子场，从而产生如手征磁效应和轴子型极化激元等有趣的物理现象。尽管人们已经提出多个动态轴子绝缘体体系的理论设计和预言，然而，至今还没有任何一个真实的材料体系可以实现具有拓扑起源的动态轴子场及动态磁电效应。
近日，来自南京大学物理学院的张海军教授团队，基于他们之前在磁性拓扑绝缘体MnBi2Te4的系列研究工作，提出在偶数层MnBi2Te4薄膜中可以实现可调控的拓扑起源的动态磁电效应。该工作中，基于MnBi2Te4薄膜的低能有效模型，结合第一性原理计算，揭示了偶数层MnBi2Te4薄膜的有限尺寸效应导致时间和空间反演对称性自发破缺，进而反铁磁自旋波可以诱导出动态轴子场及动态磁电效应。通过对磁电响应系数的详细计算，该研究中分别讨论了薄膜厚度、温度以及元素替换对于动态磁电效应的调控行为，并进而预言了最佳的薄膜厚度、温度和元素配比。进一步地，基于手征磁效应，该研究工作还提出了实验探测MnBi2Te4薄膜中动态磁电效应的一种可行方案。该文近期发布于npj Computational Materials 7: 121 (2021)。
Fig. 2 Finite-size effect of MnBi2Te4 thin fifilms. a The evolution of the band structure of the 12-SL MnBi2Te4 thin fifilm as a function of thestrength of the SOC λ/λ0.
The application of an electric field can induce a magnetization, and reversely, the application of a magnetic field can lead to a polarization, which is the typical magnetoelectric effect. Magnetoelectric materials have important potential applications, such as sensing and storage. Recently, topological magnetoelectric effect has been discovered in three-dimensional topological insulators, which originates from the quantized axion field protected by the time-reversal symmetry (T) or inversion symmetry (P). Interestingly, when both T and P are broken, the axion field will become unquantized and dynamical. This dynamical axion field can lead to rich phenomena, such as the chiral magnetic effect and the axion polariton. However, realistic materials possessing dynamical axion field are still lacking.
A team led by Prof. Haijun Zhang from school of physics, Nanjing University, China, predicted the tunable dynamical magnetoelectric effect in antiferromagnetic topological insulator MnBi2Te4 thin films. Based on the low-energy effective model combined with first-principles calculations, this work revealed how time-reversal and inversion symmetries are spontaneously broken in even-layer MnBi2Te4 thin films with the finite-size effect, which leads to the dynamical axion field and the dynamical magnetoelectric effect. Through the detailed calculations of the magnetoelectric response coefficient, this work studied the tunability of the dynamical magnetoelectric effect from film thickness, temperature, and element replacement, and predicted the most suitable choice of each parameter. Furthermore, motivated by the chiral magnetic effect, this work also proposed a feasible experimental setup to detect the dynamical magnetoelectric effect in MnBi2Te4 thin films. This work was recently published in npj Computational Materials 7: 121 (2021).
原文Abstract及其翻译
Tunable dynamical magnetoelectric effect in antiferromagnetic topological insulator MnBi2Te4 films (反铁磁拓扑绝缘体MnBi2Te4薄膜中可调控的动态磁电效应)
Tongshuai Zhu, Huaiqiang Wang, Haijun Zhang & Dingyu Xing
Abstract Axion was postulated as an elementary particle to solve the strong charge conjugation and parity puzzle, and later axion was also considered to be a possible component of dark matter in the universe. However, the existence of axions in nature has not been confirmed. Interestingly, axions arise out of pseudoscalar fields derived from the Chern–Simons theory in condensed matter physics. In antiferromagnetic insulators, the axion field can become dynamical due to spin-wave excitations and exhibits rich exotic phenomena, such as axion polariton. However, antiferromagnetic dynamical axion insulator has yet been experimentally identified in realistic materials. Very recently, MnBi2Te4 was discovered to be an antiferromagnetic topological insulator with a quantized static axion field protected by inversion symmetry P and magnetic-crystalline symmetry S. Here, we studied MnBi2Te4 films in which both the P and S symmetries are spontaneously broken and found that substantially enhanced dynamical magnetoelectric effects could be realized through tuning the thickness of MnBi2Te4 films, temperature, or element substitutions. Our results show that thin films of MnBi2Te4 and related compounds could provide a promising material platform to experimentally study axion electrodynamics.
摘要 轴子是为了解决强电荷共轭-宇称问题而假设的一种基本粒子，并且被认为是宇宙中暗物质的可能组成部分。然而，自然界中轴子的存在尚未得到证实。有趣的是，轴子作为赝标量粒子，凝聚态物理中通过陈-西蒙斯理论得到的赝标量场可以等效为一种轴子场。在反铁磁绝缘体中，自旋波激发可以使得轴子场变为动态的，并表现出丰富的奇异现象，例如轴子型极化激元。然而，具有动态轴子场的反铁磁绝缘体尚未在真实材料体系中实验实现。最近，人们发现 MnBi2Te4 是一种反铁磁拓扑绝缘体，并且它具有由空间反演对称性(P)和磁晶对称性(S)保护的量子化的静态轴子场。本文中，我们研究了P和S对称性都自发破缺的MnBi2Te4 薄膜。我们发现通过调节薄膜的厚度、温度或元素替代，可以在MnBi2Te4薄膜中实现显著增强的动态磁电效应。我们的结果表明MnBi2Te4和相关化合物的薄膜有望为实验研究轴子电动力学提供一个有前途的材料平台。
Fig. 4 Experimental setup for detecting the tunable dynamicalME effect.
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