Unconventional superconducting gap in NaFe0.95Co0.05As observed by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe0.95Co0.05As (T-c similar to 18 K). We observed a holelike Fermi surface around the zone center and two electronlike Fermi surfaces around the M point, which can be connected by the Q = (pi, pi) wave vector, suggesting that scattering over the near-nested Fermi surfaces is important in this 111 pnictide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below T-c on all three Fermi surfaces. Upon increasing temperature through T-c, the gap size shows little change while the coherence vanishes. Large ratios of 2 Lambda/k(B)T(c) similar to 8 are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.