Grigoreva, Anna - University of Washington

Axion electrodynamics and giant magnetic birefringence in Weyl excitonic insulators

We show that in the excitonic insulator phase of time-reversal (TR) invariant Weyl semimetals (WSMs), the phase of the exciton condensate couples to the electromagnetic (EM) field as a massive dynamical axion. This is a consequence of the chiral anomaly and the chiral magnetic effect in the parent WSM.

The mass of the axion is generated by the inter-band matrix elements of electron-electron interactions and is much smaller than the gap in the single-particle spectrum.

Because of this, in the presence of magnetic field the axion coupling to EM fields results in giant anisotropic polarizability and birefringence. The photon-axion hybridization produces a magneto-induced polariton resonance near the axion gap.