In the high-temperature cuprate superconductors, only a single band is observed at the Fermi level; as a result the optical conductivity may be modeled using a single free-carrier (Drude) component. In a simple metal the Drude model is usually sufficient to describe the frequency-dependent conductivity; however, in the cuprates electronic correlations and electron-boson coupling require a more generalized form in which the scattering rate and the effective mass are both frequency dependent . The iron-based conductors and superconductors are multiband materials with several bands crossing the Fermi level, resulting in multiple hole and electron pockets at the center and corners of the Brillouin zone, respectively . The presence of multiple bands requires, at a minimum, a “two-Drude” model in which the electron and hole pockets are treated as separate contributions . In general, the two-Drude approach reveals: (i) a strong component associated with the hole pocket with a large scattering rate (nearly incoherent transport) that is essentially temperature independent; (ii) a weaker component associated with the electron pocket whose scattering rate has a strong temperature dependence. Some recent results using this approach are discussed for the iron-chalcogenide superconductor FeTe 0.55 Se 0.45 (T c ~14 K)  and the strongly-correlated parent material Fe 1+ d Te (T N ~68 K).
*This work done in collaboration with Yaomin Dai, Qiang Li, Jinsheng Wen, Zhijun Xu, Genda Gu, Ricardo Lobo, and Ana Akrap; work supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC02-98CH10886.
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