Angle-resolved photoemission studies of the superconducting gap symmetry in Fe-based superconductors

The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here we discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. We show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of momentum. We show that while this observation is inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. We also suggest that the strong anisotropies measured by other probes sensitive to the residual density of states are not related to the pairing interaction itself, but rather emerge naturally from the smaller lifetime of the superconducting Cooper pairs that is a direct consequence of the momentum dependent interband scattering inherent to these materials.