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Axion electrodynamics and giant magnetic birefringence in Weyl excitonic insulators

Anna Grigoreva, Anton Andreev, Leonid Glazman

Abstract

We study the electromagnetic (EM) response of the excitonic insulator phase of a time-reversal (TR) invariant Weyl semimetal (WSM). At low temperatures, the system develops two exciton condensates. The condensates are related to each other by TR symmetry and weakly coupled by a Josephson tunneling term. The latter leads to the formation of the Leggett mode [Number-phase fluctuations in two-band superconductors, Prog. Theor. Phys. 36, 901 (1966).] with a small gap. Our main finding is that the Leggett mode couples to the EM fields as a massive dynamical axion. This is a consequence of the chiral anomaly and the chiral magnetic effect in the parent WSM. Because of the small axion mass, its coupling to EM fields results in a giant anisotropic polarizability and birefringence in the presence of a static magnetic field. The photon-axion hybridization produces a polariton resonance near the axion gap.

Axion electrodynamics and giant magnetic birefringence in Weyl excitonic insulators

Abstract

We study the electromagnetic (EM) response of the excitonic insulator phase of a time-reversal (TR) invariant Weyl semimetal (WSM). At low temperatures, the system develops two exciton condensates. The condensates are related to each other by TR symmetry and weakly coupled by a Josephson tunneling term. The latter leads to the formation of the Leggett mode [Number-phase fluctuations in two-band superconductors, Prog. Theor. Phys. 36, 901 (1966).] with a small gap. Our main finding is that the Leggett mode couples to the EM fields as a massive dynamical axion. This is a consequence of the chiral anomaly and the chiral magnetic effect in the parent WSM. Because of the small axion mass, its coupling to EM fields results in a giant anisotropic polarizability and birefringence in the presence of a static magnetic field. The photon-axion hybridization produces a polariton resonance near the axion gap.

Paper Structure

This paper contains 25 equations, 1 figure.

Figures (1)

  • Figure 1: a) Fermi surface (FS) evolution caused by the spectral flow associated with the chiral anomaly, $\propto \bm{E}\cdot \bm{B}$. The equilibrium FSs (small ellipses) evolve to new positions shown by the large ellipses. For an isotropic spectrum, the FSs remain congruent, and the system remains unstable to exciton pairing for $\mu_h \neq \mu_e$. b) Reconstructed spectrum and Berry curvature flux (shown by blue arrows).