Charge dynamics of Co-doped BaFe2As2

We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(CoxFe1- x)2As2 compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for x=0.025, we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051⩽x⩽0.11, we detect the superconducting gap, while at x=0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the combination of a coherent and an incoherent component, and extract the respective scattering rates. We establish that the dc transport properties in the normal phase are dominated by the coherent Drude term for 0⩽x⩽0.051 and by the incoherent one for 0.061⩽x⩽0.18, respectively. Finally, through spectral weight arguments, we give clear-cut evidence of moderate electronic correlations for 0⩽x⩽0.061, which then cross over to values appropriate for a regime of weak interacting and nearly-free electron metals for x⩾0.11.