Effect of annealing on the specific heat of optimally doped Ba(Fe0.92Co0.08)2As2

We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe0.92Co0.08)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 Tc=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 Tc=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 Cel(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.