Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory

The optical conductivity of LaFeAsO, BaFe₂ As₂, SrFe₂ As₂, and EuFe₂ As₂ in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spin-polarized generalized gradient approximation (GGA) and GGA+U. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, GGA+ U_eff with a suitable choice of negative on-site interaction U_eff =U-J was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small U compared to the Hund's rule coupling J, as well as a strong Holstein-type electron-phonon interaction. Moreover, reminiscent of the fact that GGA+ U _eff with a positive U_eff is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative U_eff can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital-resolved analysis of the optical conductivity reveals significant contributions from all Fe3d orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.