Nodeless superconducting gap in K$_x$Fe$_{2-y}$Se$_2$ and its evolution with doping probed from angle-resolved photoemission

The nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in K$_x$Fe$_{2-y}$Se$_2$, whether its pairing symmetry is s wave or nodeless d wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in K$_x$Fe$_{2-y}$Se$_2$, which favors the s-wave pairing symmetry [1-3]. At the same time, we will present some of the recent data on the evolution of the band structure and superconducting gap of iron chalcogenides K$_x$Fe$_{2-y}$Se$_2$ as a function of electron and hole doping.\\[4pt] [1] M. Xu, et.al, Phys Rev B 85 (22), 220504(R) (2012).\\[0pt] [2] F. Chen, et.al, Phys Rev X 1, 021020 (2011).\\[0pt] [3] Y. Zhang, et.al, Nature Mater. 10, 273 (2011).