Gap structure of the overdoped iron-pnictide superconductor Ba(Fe_0.942Ni_0.058)₂as₂: A low-temperature specific-heat study

Low-temperature specific heat (SH) is measured on the postannealed Ba(Fe_1-xNi_x)₂As₂ single crystal with x = 0.058 under different magnetic fields. The sample locates on the overdoped sides and the critical transition temperature T c is determined to be 14.8 K by both the magnetization and SH measurements. A simple and reliable analysis shows that, besides the phonon and normal electronic contributions, a clear T² term emerges in the low temperature SH data. Our observation is similar to that observed in the Co-doped system in our previous work and is consistent with the theoretical prediction for a superconductor with line nodes in the energy gap.