Polar Kerr effect in antiferromagnets by optical axion electrodynamics*

While the polar Kerr effect is considered a measure of the net magnetic moment in materials, it may also occur in fully compensated antiferromagnets because of magneto-electric coupling. Since electromagnetic response properties unique to antiferromagnets are rarely known, the magneto-electric Kerr effect is potentially an important tool to detect and manipulate the antiferromagnetic order. However, theoretical analysis of this effect is challenging because of the difficulty in calculating the isotropic part (i.e., the axion part) of the optical magneto-electric coupling in periodic systems. The peculiar property of the axion magneto-electric coupling leads to the quantization in the static limit, allowing calculations based on topological analysis, which is hard to generalize to optical frequencies beyond the band gap. In this talk, I will introduce a theory of optical axion electrodynamics that allows for a simple quantitative analysis. Then I will apply our theory to the Kerr effect in a topological antiferromagnet MnBi2Te4.