A new route to negative refractive index from topological metals

We discuss the possibility of realizing a negative refractive index inWeyl/Dirac semimetals.We consider the Maxwell equations with the plasma gap and chiral magnetic effect. By computing the dispersion relations of electromagnetic waves, we show that the refractive index becomes negative at frequencies (just) below the plasma frequency. We find that the axial anomaly, or, more specifically, the negative magnetoresistance (an electric current parallel to magnetic fields) opens a new route to realizing a negative refractive index. As an experimental implication, we compute the reflection and transmission coefficients of electromagnetic waves in a slab of Weyl/Dirac semimetals.