We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe$_{ 0.92}$Co$_{ 0.08}$)$_{ 2}$As$_{ 2}$ 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 $\gamma_{ 0}$=3.6 mJ/mol K$^{ 2}$ and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 $^{ o}C$ for two weeks and shows $T_{ c}$ = 25 K and $\gamma_{ 0}$=1.4 mJ/mol~K$^{ 2}$. By subtracting the lattice specific heat, from pure BaFe$_{ 2}$As$_{ 2}$, 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
Categories: cond-mat.supr-con