Axions are hypothetical particles that were proposed to solve the strong charge–parity problem in high-energy physics. Although they have long been known in quantum field theory, axions have so far not been observed as elementary particles in nature. Yet, in condensed-matter systems, axions can also emerge as quasiparticles in certain materials such as strong topological insulators. The corresponding axion field is expected to lead to exciting physical phenomena in condensed-matter systems, such as a fractional quantum anomalous Hall effect, the chiral anomaly, exotic Casimir–Lifshitz repulsion and a linear magnetoelectric response quantized in units of the fine-structure constant. First signatures of electronic states that permit axion dynamics have been reported in condensed-matter systems. In this Review, we explore the concepts that introduce axion fields in condensed-matter systems and present experimental findings. We discuss predicted and realized material systems, the prospects of using axion electrodynamics for next-generation devices and the search for axions as a possible constituent of dark matter.

轴子是为解决高能物理中强电荷奇偶性问题而提出的假设粒子。尽管轴在量子场论中早已为人所知，但迄今为止，轴尚未被视为自然界中的基本粒子。但是，在凝聚态系统中，轴突也可能以准粒子的形式出现在某些材料中，例如强拓扑绝缘体。预计相应的轴场将导致凝聚态系统中令人兴奋的物理现象，例如分数量子异常霍尔效应，手性异常，奇异的卡西米尔-利夫希兹斥力和以精细结构常数为单位量化的线性磁电响应。在凝聚态系统中已经报道了允许轴运动的电子状态的第一个特征。在这篇评论中，我们探索在凝聚态系统中引入轴突场的概念，并介绍实验结果。我们讨论了预测的和已实现的材料系统，将轴力电动力学用于下一代设备的前景以及寻找作为暗物质可能成分的轴力的前景。