Charge dynamics of Co-doped BaFe₂As₂

Abstract

We report on a thorough optical investigation over a broad spectral range and\nas a function of temperature of the charge dynamics in\nBa(Co$_x$Fe$_{1-x}$)$_2$As$_2$ compounds for Co-doping ranging between 0 and\n18%. For the parent compound as well as for $x$=0.025 we observe the opening of\na pseudogap, due to the spin-density-wave phase transition and inducing a\nreshuffling of spectral weight from low to high frequencies. For compounds with\n0.051$\\le x \\le$ 0.11 we detect the superconducting gap, while at $x$=0.18 the\nmaterial stays metallic at all temperatures. We describe the effective metallic\ncontribution to the optical conductivity with two Drude terms, representing the\ncombination of a coherent and incoherent component, and extract the respective\nscattering rates. We establish that the $dc$ transport properties in the normal\nphase are dominated by the coherent Drude term for 0$\\le x \\le$0.051 and by the\nincoherent one for 0.061$\\le x \\le$0.18, respectively. Finally through spectral\nweight arguments, we give clear-cut evidence for moderate electronic\ncorrelations for 0$\\le x \\le$0.061, which then crossover to values appropriate\nfor a regime of weak interacting and nearly-free electron metals for\n$x\\ge$0.11.\n