Distinct Fermi Surface Topology in A$_x$Fe$_{2-y}$Se$_2$ Revealed by ARPES

The discovery of superconductivity with a transition temperature above 30 K in A$_x$Fe$_{2-y}$Se$_2$ (A=alkali metal or Thallium) triggered a new wave of research on the iron-based superconductors. The new A$_x$Fe$_{2-y}$Se$_2$ superconductor exhibits many unique characteristics which make it a new platform to uncover the pairing mechanism of the iron-based superconductors. In this talk, we will show the electronic structures of A$_x$Fe$_{2-y}$Se$_2$ by means of ARPES, including Fermi Surface topology, superconducting gap structure and electron dynamics. The observed Fermi surface topology with only electron-like pockets is distinct from other iron-based superconductors and challenges the pairing mechanism based on scattering between electron-like and hole-like Fermi surface sheets. Nearly isotropic superconducting gap without nodes is revealed for all Fermi surface sheets which favors s-wave pairing symmetry. Other related topics, such as Fe vacancy order and phase separation, will also be discussed from viewpoint of our ARPES results. \\[4pt] [1] D. Mou, S. Liu, J. He, et. al, Phys. Rev. Lett. 106, 107001 (2011).\\[0pt] [2] L. Zhao, D. Mou, S. Liu, et al., Phys. Rev. B 83, 140508(R) (2011).