Axion physics in condensed-matter systems

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.