Unconventional superconducting gap in NaFe${}_{0.95}$Co${}_{0.05}$As observed by angle-resolved photoemission spectroscopy

Abstract

We have performed high resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe${}_{0.95}$Co${}_{0.05}$As ($T_c\sim$18 K). We observed a hole-like Fermi surface around the zone center and two electron-like Fermi surfaces around the M point which can be connected by the $Q=(\ddot{I}€,\ddot{I}€)$ wavevector, suggesting that scattering over the near-nested Fermi surfaces is important to the superconductivity of this "111" pnicitide. 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 $2\hat{I}”/k_B{T}_c\sim 8$ 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.

4 pages, 4 figures