Zhong Wang, Shou-Cheng Zhang
We study dynamical instability and chiral symmetry breaking in three dimensional Weyl semimetals, which turns Weyl semimetals into "axion insulators". Charge density waves (CDW) is found to be the natural consequence of the chiral symmetry breaking. The Goldstone mode of this charge density wave state is identified as the axion, which couples to electromagnetic field in the topological $\theta{\bf E}\cdot{\bf B}$ form. Our main finding is that the "axion strings" can be realized as the (screw or edge) dislocations in the charge density wave, which provides a simple physical picture for the elusive axion strings. These axion strings carry gapless chiral modes, therefore they have important implications for dissipationless transport properties of Weyl semimetals with broken symmetry.
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http://arxiv.org/abs/1207.5234
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