Gap structure in the electron-doped iron-arsenide superconductor Ba(Fe _0.92Co_0.08)₂As₂: Low-temperature specific heat study

In this paper, we report the field and temperature dependence of low-temperature specific heat down to 400mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe_0.92Co_0.08)₂As ₂ superconductor. Using the phonon specific heat obtained from pure BaFe₂As₂, we found a normal state Sommerfeld coefficient of 18 mJ mol^-1 K^-2 and a condensation energy of 1.27 J mol^-1. The temperature dependence of electronic specific heat clearly indicates the presence of low-energy excitations in the system. The magnetic field variation of fieldinduced specific heat cannot be described by single clean s- or d-wave models. Rather, the data require an anisotropic gap scenario that may or may not have nodes. We discuss the implications of these results.