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

Abstract

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 ${\hat{I}}_e^3$. Remarkably, we found a continuing temperature dependent ${\hat{I}}_e^3(T)$ which follows the quadratic relation ${\hat{I}}_e^3={\hat{I}}_0^3+\hat{I}\pm T^2$ in the low temperature limit. Together with the very small residual term ${\hat{I}}_0^3$, linear magnetic field dependence of ${\hat{I}}_e^3$, it is concluded that there are either small segments of nodal lines, or point-like nodes in these samples.

5 pages, 5 figures