Axion electrodynamics in a topologically trivial antiferromagnet

Axion electrodynamics, originally proposed by Frank Wilczek in the context of high energy physics, can manifest in condensed matter systems as an isotropic linear magnetoelectric (ME) response. Yet, experimental realization of a continuous and purely monopolar ME response has remained elusive. Here, we demonstrate such a response by transforming the topologically trivial antiferromagnet chromia into an isotropic ME medium. Using low-frequency AC ME susceptometry, supported by Monte Carlo simulations, we reveal a monopolar ME response α _ij=θ δ _ij resulting from suppressed quadrupolar contributions. The zero crossing of the temperature-dependent θ at 168 K provides direct evidence that the measured isotropic ME response arises from the axion contribution in chromia. These results establish a condensed-matter platform for axion electrodynamics and open pathways toward ME Hall effects, magnetophotovoltaic responses, and monopolar-based devices. It is possible to realize axion electrodynamics in a material with an isotropic and linear magnetoelectric response, however, such a material has thus far evaded experimental realization. Here, Mishra et al succeed in creating such a linear isotropic magnetoelectric response in powdered Chromia.