Axion electrodynamics in a topologically trivial antiferromagnet

Axion electrodynamics, proposed by Frank Wilczek, manifests in condensed matter as an isotropic linear magnetoelectric (ME) response[1]. Yet a continuous and purely monopolar ME response has remained elusive. Here, we achieve such a response by transforming the topologically trivial but ME anisotropic antiferromagnet chromia into an isotropic medium by field-cooling a chromia single crystal, followed by low-temperature powdering and solidification. Then we characterize it experimentally using low-frequency AC ME susceptometry and corroborate with Monte Carlo simulations. This procedure suppresses quadrupolar contributions and produces an isotropic ME tensor . Above the Néel temperature, the pure axion response becomes irretrievable, with no known route to recovery, revealing a remarkable asymmetry between the effects of ME annealing before and after powdering. We establish a condensed-matter platform for axion electrodynamics and open routes toward ME Hall phenomena, magnetophotovoltaic effects, and ME monopolar-based devices.

1.  Wilczek F., Phys. Rev. Lett. 58, 1799 (1987)