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| Main Authors: | , , |
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| Format: | Preprint |
| Published: |
2025
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2502.20456 |
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Table of Contents:
- 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.