Low Temperature Specific Heat of Optimally Doped BaFe${}_{2-x}$$T{M}_x$As${}_2$ ($TM$ = Co and Ni) Single Crystals: Constraint on the Pairing Gap

Low temperature specific heat has been measured in optimally doped and highly overdoped non-superconducting BaFe${}_{2-x}$TM${}_x$As${}_2$ ($TM$ = Co and Ni) single crystals. By using the data of the overdoped samples, we successfully removed the phonon contribution of the optimally doped ones, and derived the electronic specific heat coefficient ${\gamma }_e$. Remarkably, we found a continuing temperature dependent ${\gamma }_e(T)$ which follows the quadratic relation ${\gamma }_e={\gamma }_0+\alpha T^2$ in the low temperature limit. Together with the very small residual term ${\gamma }_0$, linear magnetic field dependence of ${\gamma }_e$, it is concluded that there are either small segments of nodal lines, or point-like nodes in these samples.