Gap structure in the electron-doped iron-arsenide superconductor Ba(Fe0.92Co0.08)2As2: low-temperature specific heat study

In this paper, we report the field and temperature dependence of low-temperature specific heat down to 400 mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe0.92Co0.08)2As2 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 field-induced 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.