Impurity Scattering Effect and Pairing Symmetry Revealed by Low Temperature Specific Heat in Iron Pnictide Superconductors*

Low-temperature specific heat C$_{p}$ was measured on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals in wide doping region. A sizeable residual specific heat coefficient $\gamma _{0}$ was observed in the low temperature limit of all samples. The specific heat jump near T$_{c}$, i.e. $\Delta $C$_{p}$/T$\vert _{Tc}$ was also determined. It is found that -$\gamma _{0}$,$_{ }\Delta $C$_{p}$/T$\vert _{Tc}$ and T$_{c}$ all share a similar evolution with doping. These can be well understood within the model of S$^{\pm }$ pairing manner when accounting the cobalt-dopants as unitary scattering centers in the FeAs planes. Our results revealed a non-trivial impact of impurity scattering in FeAs-based superconductors. Angle resolved low temperature specific heat was measured in FeSe$_{1-x}$Te$_{x}$ single crystals. Oscillations with periodicity of $\pi $/2 was observed at 9 T. This constrains the pairing symmetry with the two possibilities: either d-wave or S$^{\pm }$.