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+ Ueff with a suitable choice of negative on-site interaction Ueff =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 Ueff is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative Ueff 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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Oct 1 2010|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics