Doping-dependent specific heat study of the superconducting gap in Ba(Fe_1-x Co_x)₂As₂

We report a doping, magnetic field, and low-temperature-dependent study of the specific heat of the iron-arsenide Ba(Fe_1-x Co_x) ₂As₂ at underdoped (x=0.045), optimal-doped (x=0.08) and overdoped (x=0.103 and 0.105) regimes. By subtracting the lattice specific heat the temperature and magnetic field dependence of the electronic specific heat has been studied. The temperature and field dependencies of the superconducting part of C_p exhibit similar behavior for all doping concentrations. The temperature variation in the electronic specific heat as well as its field dependence cannot be described by a single isotropic s -wave gap, pointing to a complex gap structure in the system. The lack of doping dependence indicates that the gap structure does not change significantly as a function of doping. We also observe a significant residual linear term of unknown origin in the specific heat of Ba(Fe_1-x Co_x)₂As₂ which suggests that inhomogeneity may be an important factor in Co-doped BaFe₂ As₂.