Sizable Residual Quasiparticle Density of States Induced by Impurity Scattering Effect in Ba(Fe_1-xCo_x)₂As₂ Single Crystals

Low-temperature specific heat C_p is measured on Ba(Fe_1-xCo_x)₂As₂ single crystals in a wide doping region. A sizeable residual specific heat coefficient γo is observed in the low temperature limit of all samples. The specific heat jump near T_c, i.e. ΔC_p/T\_{T_c}, is also determined. It is found that -γo, ΔC_p/T\_{T_c} and T_c all share a similar evolution with doping. These can be well understood within the model of S^± pairing symmetry when accounting for the cobalt-dopants as unitary scattering centers in the FeAs planes. Our results reveal a non-trivial impact of impurity scattering in FeAs-based.