Linkage between scattering rates and superconductivity in doped ferropnictides

We report an angle-resolved photoemission study of a series of hole- and electron-doped iron-based superconductors, their parent compound BaFe$_{2}$As$_{2}$, and their cousins BaCr$_{2}$As$_{2}$ and BaCo$_{2}$As$_{2}$. We focus on the inner hole pocket, which is the hot spot in these compounds. More specifically, we determine the energy (E)-dependent scattering rate (E) as a function of the 3d count. Moreover, for the compounds K$_{0.4}$Ba$_{0.6}$Fe$_{2}$As$_{2}$ and BaCr$_{2}$As$_{2}$, we derive the energy dependence of the renormalization function Z(E) and the imaginary part of the self-energy function Im(E). We obtain a non-Fermi liquidlike linear in energy scattering rate (E>>k$_{B}$T ), independent of the dopant concentration. The main result is that the slope β = (E>>k$_{B}$T )/E reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit T(E)/E ≈ 1. ... mehrThis result, together with the energy dependence of the renormalization function Z(E), signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.