  The low energy band structure and Fermi surface of the newly discovered
superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved
photoemission spectroscopy. Compared with iron pnictide superconductors,
AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only
electron pockets are observed with an almost isotropic superconducting gap of
~10.3 meV, while there is no hole Fermi surface near the zone center, which
indicates the inter-pocket hopping or Fermi surface nesting is not a necessary
ingredient for the unconventional superconductivity in iron-based
superconductors. Thus, the sign changed s$_\pm$ pairing symmetry, a leading
candidate proposed for iron-based superconductors, becomes conceptually
irrelevant in describing the superconducting state here. A more conventional
s-wave pairing is a better description.
