Zeeman Shift and Injection Currents under Light Irradiation
The conversion of light into electrical currents enables energy harvesting and photodetection. Conventional second-order dc photocurrents are driven by the electric component of light through electric-dipole coupling and require broken inversion symmetry. Here, we show that the electric and magnetic components of light jointly generate Zeeman shift and injection currents. Using quantum perturbation theory, we derive explicit microscopic expressions for their conductivity tensors. We systematically classify the symmetry constraints on these responses and show how they differ from those governing conventional photocurrents. In particular, both responses can occur in centrosymmetric systems with spin-orbit coupling or noncollinear magnetic order, irrespective of time-reversal symmetry. A centrosymmetric kagome model exhibits broadband responses. First-principles calculations for Mn$_3$Sn further predict Zeeman injection and shift current densities of $10^{2}$-$10^{3}~\mathrm{A/m^{2}}$ and $1$-$10~\mathrm{A/m^{2}}$, respectively, under illumination with an electric-field amplitude $E=10^{4}~\mathrm{V/m}$. These magnitudes place both currents within experimental reach.