Activating Nonstandard Axion Electrodynamics in Topological Insulator Materials

The conventional axion response of topological insulators (TIs) is characterized by an isotropic pseudoscalar magnetoelectric (ME) coupling rooted in band topology: the condensed matter analog of the axion field. However, as we reveal here, this and other more general ME responses can arise in existing TI materials from symmetry breaking without requiring nontrivial topology. Indeed, we show that breaking parity and time-reversal symmetries using combined electric and magnetic fields unlocks nonstandard axion electrodynamics via the controllable activation of various tensorial ME couplings, thus, going beyond the standard axion picture. Using a well-established first-principles-informed effective model for Bi-and Sb-based TI materials, we estimate the resulting ME coefficients and make concrete predictions for experiments. In particular, we identify nonreciprocal directional dichroism as a distinctive magneto-optical signature of these ME effects, therefore opening a route to engineer and probe symmetry-induced ME phenomena beyond the conventional topological response.