Chiral electromagnetic waves in Weyl semimetals

We show that Weyl semimetals with broken time-reversal symmetry can host chiral electromagnetic waves. The magnetization that results in a momentum-space separation of a pair of opposite chirality Weyl nodes is also responsible for the nonzero 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.

Figure 1

(a) Schematics of the domain wall separating two opposite magnetizations $\pm \vec{m}$ (shown in red) at point $x=0$. The magnetization is parallel to the $z$ axis. The profile $\propto e^{-\left|x\right|/\ell }$ of a localized electromagnetic wave at the domain wall and direction of propagation is shown in blue, where $\ell$ is the localization length of the wave. (b) Schematics of the magnetization-defined chirality of an electromagnetic wave. Black curves are the domain walls separating regions with different magnetization. Arrows are the directions of the propagation of the electromagnetic wave.