Gap Structure of the Overdoped Iron-Pnictide Superconductor Ba(Fe_{0.942}Ni_{0.058})₂As₂: A Low-Temperature Specific-Heat Study

Low-temperature specific heat (SH) is measured on the postannealed Ba(Fe_{1-x}Ni_x)_2As_2 single crystal with x = 0.058 under different magnetic fields. The sample locates on the overdoped sides and the critical transition temperature 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 termemerges 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.