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

Abstract

The optical conductivity of LaFeAsO, ${\text{BaFe}}_{2}{\text{As}}_{2}$, ${\text{SrFe}}_{2}{\text{As}}_{2}$, and ${\text{EuFe}}_{2}{\text{As}}_{2}$ 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 $\text{GGA}+U$. 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, $\text{GGA}+{U}_{\text{eff}}$ with a suitable choice of negative on-site interaction ${U}_{\text{eff}}=U\ensuremath{-}J$ 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 $\text{GGA}+{U}_{\text{eff}}$ with a positive ${U}_{\text{eff}}$ is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative ${U}_{\text{eff}}$ 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 $\text{Fe}\text{ }3d$ 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.