  Helicons are transverse electromagnetic waves propagating in
three-dimensional (3D) electron systems subject to a static magnetic field. We
present a theory of helicons propagating through a 3D Weyl semimetal. Our
approach relies on the evaluation of the optical conductivity tensor from
semiclassical Boltzmann transport theory, with the inclusion of certain Berry
curvature corrections that have been neglected in the earlier literature (such
as the one due to the orbital magnetic moment). We demonstrate that the axion
term characterizing the electromagnetic response of Weyl semimetals
dramatically alters the helicon dispersion with respect to that in
nontopological metals. We also discuss axion-related anomalies that appear in
the plasmon dispersion relation.
