Chiral Electromagnetic Waves in Weyl Semimetal

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We show that Weyl semimetals with broken time-reversal symmetry can host chiral electromag- netic waves. The magnetization that results in a momentum space separation of a pair of opposite chirality Weyl nodes is also responsible for the non-zero gyrotropy parameter in the system. It is then shown that a chiral electromagnetic wave can propagate in a region of space where the gyrotropy parameter changes sign. Such waves are analogs of quantum Hall edge states for photons.