Transport experiments on crystals grown by Yingkai Huang showed that the resistivity of carefully annealed single crystals decreased, while the critical temperature increased significantly. This motivated us to investigate the optical properties of as-grown and annealed single crystals, which could provide important clues to the changes induced by annealing. Such information is usually hidden in the optical scattering rate and mass enhancement that can be extracted from the optical conductivity. Such an analysis is however complicated for materials where low energy optical transitions between different bands are present (such as is the case for the iron-pnictides). In the paper we point out this difficulty and provide a new method that circumvents this problem. By comparing the experimental results with detailed calculations we conclude that the normal state has all the hallmarks of a weakly correlated Fermi liquid. More information can be found under Highlights.
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