Angle-resolved photoemission spectroscopy studies on new iron-based superconductors

The recent discovery of new iron chalcogenide superconductors with the 122 crystal structure has generated fresh excitement in the investigation of iron-based high-Tc superconductors. Here we present the results of our measurements of their band structure, Fermi surface (FS) and superconducting gap studied by angle-resolved photoemission spectroscopy. We find that the band structure and FS topology at the center of the Brillouin zone are significantly different from those of other iron-based superconductors, resulting in the disappearance of FS nesting in the particle-hole channel. In addition, we have observed strong and isotropic superconducting gaps on the electron-like FS sheets around the zone corner. All of these results impose severe constraints on possible superconducting pairing mechanisms.