Distinct Fermi Surface Topology and nearly Isotropic Superconducting Gap in A$_{x}$Fe$_{2-y}$ Se$_{2}$ (A=K, Tl, Rb) Superconductors

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the newly discovered A$_{x}$Fe$_{2-y}$Se$_{2}$ [A=K, (Tl,K) and (Tl,Rb)] superconductors[1,2,3] 1. Distinct Fermi surface topology, consisting of two electron-like Fermi surface sheets around the $\Gamma $(0,0) point and an electron-like Fermi surface sheet near the M($\pi $,$\pi )$ point, was revealed in all these samples. This is in strong contrast to the Fermi surface topology of other Fe-based superconductors where hole-like Fermi surface sheets are present near the $\Gamma $(0,0) point. 2. Both the electron-like Fermi surface sheet near M point and the large electron-like Fermi surface sheet near $\Gamma $ point show nearly isotropic superconducting gap without nodes 3. The doping evolution of the electronic structure from insulating samples to the superconducting samples is consistent with a phase separation picture. The information on the Fermi surface topology and superconducting gap of this new A$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor will provide key insights and constraints to understand the superconductivity mechanism in iron-based superconductors. \\[4pt] [1]. D. X. Mou, X. J. Zhou et. al, Phys. Rev. Lett. \textbf{106}, 107001 (2011). \\[0pt] [2]. L. Zhao, X. J. Zhou et. al, Phys. Rev. B \textbf{83}, 140508(R) (2011). \\[0pt] [3]. L. Yu, X. J. Zhou et al., unpublished.