Axion electrodynamics in topological materials

One of the intriguing properties characteristic to three-dimensional topological materials is the topological magnetoelectric phenomena arising from a topological term called the theta term. Such magnetoelectric phenomena are often termed the axion electrodynamics since the theta term has exactly the same form as the action describing the coupling between a hypothetical elementary particle, axion, and a photon. The axion was proposed about 40 years ago to solve the so-called strong CP problem in quantum chromodynamics and is now considered a candidate for dark matter. In this Tutorial, we overview theoretical and experimental studies on the axion electrodynamics in three-dimensional topological materials. Starting from the topological magnetoelectric effect in three-dimensional time-reversal invariant topological insulators, we describe the basic properties of static and dynamical axion insulators whose realizations require magnetic orderings. We also discuss the electromagnetic responses of Weyl semimetals with a focus on the chiral anomaly. We extend the concept of the axion electrodynamics in condensed matter to topological superconductors, whose responses to external fields can be described by a gravitational topological term analogous to the theta term.

Automated summary

Axion electrodynamics in three-dimensional topological materials is characterized by a coupling between the axion and photon, resulting in fascinating magnetoelectric phenomena. This phenomenon is closely related to the topological magnetoelectric effect in such materials, involving static and dynamical axion insulators, as well as the electromagnetic responses of Weyl semimetals.