We report on theoretical calculations of the optical conductivity of Ba(Fe1−xCox)2As, as obtained from density functional theory within the full potential LAPW method. A thorough comparison with experiment shows that we are able to reproduce most of the observed experimental features, in particular a magnetic peak located at about 0.2 eV which we ascribe to antiferromagnetic ordered magnetic stripes. We also predict a large in-plane anisotropy of this feature, which agrees very well with measurements on detwinned crystals. The effect of Co doping as well as the dependence of plasma frequency on the magnetic order is also investigated.
PACS numbers:The recent discovery of superconductivity in ironbased pnictide compounds 1 has triggered intense experimental and theoretical research activity. A great deal of attention has been devoted to the relationship between superconductivity and magnetism. Similar to cuprates, the parent compounds exhibit antiferromagnetic spindensity-wave (SDW) order that disappears upon doping or pressure, giving rise to superconductivity. A possible coexistence of magnetism and superconductivity has also been observed 2 , although this issue is still under debate 2-12 . Despite the many similarities to cuprates, important differences characterize the pnictides; most relevant being that their parent magnetic phase is not Mott insulating but rather metallic in nature. The preponderance of magnetic properties of FeAs compounds can be described by local/semilocal approximations to exchange correlation functionals within density functional theory (DFT), however the debate on the nature of magnetism (itinerant or localized) in this class of materials is still an open question 13 . The degree and type of correlation in these materials is also under debate, due mostly to the ARPES data which suggests a strong renormalization of the Kohn-Sham energy bands 14 . The pairing mechanism in pnictides is not yet universally agreed upon -but the possibility of standard phononic superconductivity has now been ruled out 15,16 . It has been suggested that the superconductivity in these materials could be mediated by magnetic excitations coupling electron and hole pockets of the Fermi surface and favouring s-wave order parameters, with opposite sign on different sheets of the Fermi surface (s ± coupling).What is clear is that the main physics in FeAs materials is controlled by a subtle interplay of magnetism and Fermi surface topology. One of the tools that can be used to probe these properties is optical measurement. In this work we report on our results of the optical properties of pure and Co-doped BaFe 2 As 2 using DFT. A detailed comparison of this work with infrared optical conductivity experiments is also performed, and provides interpretation of most of the main features of the experimentally observed spectra [17][18][19][20][21][22] .