Calorimetric evidence for nodes in the overdoped Ba(Fe_0.9Co _0.1)₂As₂

We present low-temperature specific heat of the electron-doped Ba(Fe _0.9Co_0.1)₂As₂, which does not show any indication of an upturn down to 400 mK, the lowest measuring temperature. The lack of a Schottky-like feature at low temperatures or in magnetic fields up to 9 T enables us to identify enhanced low-temperature quasiparticle excitations and to study anisotropy in the linear term of the specific heat. Our results cannot be explained by a single or multiple isotropic superconducting gap, but are consistent with multi-gap superconductivity with nodes on at least one Fermi surface sheet.