Unconventional superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As observed by ARPES

The size, the symmetry and the temperature evolution of the superconducting (SC) gap in a given material are directly related to the SC pairing mechanism. The momentum-resolution capability of angle-resolved photoemission spectroscopy (ARPES) allows precise determination of these key parameters, even for complex multi-band systems such as the iron-based superconductors. We performed an ARPES study of NaFe$_{0.95}$Co$_{0.05}$As. The fermiology of this electron-doped 111-pnictide is similar to that of other pnictides. Similarly, the measured SC gaps are nearly isotropic and their size indicates that the system is in the strong coupling regime. Surprisingly, the SC gaps show little change upon increasing temperature towards $T_{c}$, while coherence vanishes.