Effect of annealing on the specific heat of optimally doped Ba(Fe _0.92Co_0.08)₂As₂

We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe_0.92Co_0.08) ₂As₂ superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows T_c=20 K, residual specific heat γ₀=3.6 mJ/mol K² and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 °C for two weeks and shows T_c=25 K and γ₀=1.4 mJ/mol K². By subtracting the lattice specific heat, from pure BaFe₂As₂, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of C_el(T) clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes.