Optical signatures of Dirac nodal lines in NbAs ₂

Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of NbAs ₂ where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra σ ₁ (ω). This behavior is followed by the linear power-law scaling of σ ₁ (ω) at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines.