Low temperature specific heat of the codoped iron-arsenic superconductor Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$

Despite large experimental and theoretical efforts the structure of the superconducting gap and the origin of the pairing mechanism in iron-based superconductors in still unresolved. Measurements of the low temperature specific heat and its magnetic response inside the superconducting state give important information about the symmetry of the gap. Here, we present results of our studies of codoped Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$ with a $T_{c}$ of 32.5 K. The high quality of the material is marked by a pronounced peak at $T_{c}$ as well as by a low residual specific heat $\gamma_{0}$ = 2.4 mJ/mol K$^{2}$. We will discuss the implications of the new specific heat results on the symmetry of the order parameter in this system.