Speaker
Description
In this talk we introduce magnetic topological insulators as a condensed-matter laboratory for exploring extensions of axion electrodynamics beyond its conventional relativistic formulation. The low-energy electronic structure of these systems is described by massive Dirac fermions coupled to electromagnetic and other symmetry-breaking background fields, providing a direct field-theoretic route to magnetoelectric (ME) response. When either time-reversal or inversion symmetry is preserved, the isotropic ME response is quantized and described by the familiar pseudoscalar axion field. When both symmetries are broken, however, the ME response is no longer restricted to this form.
We show that the most general ME coupling -- constrained only by the crystal point group -- contains, in addition to the pseudoscalar, axial-vector and symmetric-pseudotensor structures. These can therefore be viewed as generalized, or non-standard, axion fields emerging from Dirac fermions in a symmetry-breaking medium. This raises the possibility that analogous structures may occur in the ME response of other relativistic fermion systems with nontrivial backgrounds, such as vortical matter relevant to heavy-ion collisions, supernovae, and the early Universe.
We discuss magnetically polarized three-dimensional topological insulators as a platform for realizing and probing these generalized axion responses. We identify the symmetry-breaking mechanisms that generate them, determine their relation to magnetization and inversion breaking, and estimate their magnitude. Their characteristic signatures, such as anisotropic and non-reciprocal electromagnetic collective modes, should be experimentally accessible. We also discuss magnon-driven nonlinear optical effects, including second-harmonic generation, as dynamical signatures of non-standard axion electrodynamics.
