Unconventional electromagnetic mode in neutral Weyl semimetals

We study light propagation in a neutral Weyl semimetal with the Fermi level lying at the Weyl nodes in the weak self-interacting regime. The nontrivial topology induces a screening effect in one of the two transverse gauge fields, for which we find two branches of attenuated collective excitations. In addition to the known topologically gapped photon mode, a different massless and slightly damped excitation appears. Strikingly, at low energies, this excitation has a linear dispersion and it propagates with the same velocity as the electrons, while at energies well above the electron-hole continuum threshold it behaves as a massive attenuated photon with velocity similar to the speed of light in the material. There is a crossover at a certain momentum in the direction perpendicular to the separation of the Weyl nodes above which this gapless mode enters into an overdamped regime. Regarding the unscreened gauge field, we show that it is also attenuated, which is a nontopological property shared by Dirac semimetals as well.

Figure 1

(a) Spectral function $\rho$ for the unscreened gauge field as a function of the frequency $\omega$ and momentum $\left|\mathit{p}\right|$ in units of ${\mu }_0$. (b) Spectral function $\rho$ for the screened gauge field as a function of $\omega$ and $|p_3|$ in units of ${\mu }_0$, for zero transverse momentum $|{\mathit{p}}_{\perp }|=0$. (c), (d) Spectral function $\rho$ for the screened gauge field as a function of the frequency $\omega$ and the transverse momentum $|{\mathit{p}}_{\perp }|$ in units of ${\mu }_0$, for (c) $|p_3|=0$ and (d) $|p_3|/{\mu }_0=3\times {10}^{-3}$. All plots for the screened gauge field are presented for $b^2/{\mu }_0^2=3$.

Figure 2

Both plots are presented for a value $b^2/{\mu }_0^2=3$ and in a logarithmic scale for the horizontal axis. (a) Spectral function $\rho$ for the screened gauge field as a function of the frequency $\omega$ in units of ${\mu }_0$ for ${\mathit{p}}_{\perp }=0$ and different values of $|p_3|$. (b) Spectral function $\rho$ for the screened gauge field as a function of the frequency $\omega$ in units of ${\mu }_0$ for $p_3=0$ and different values of $|{\mathit{p}}_{\perp }|$.