Evidence for ans-wave superconducting gap in KxFe2−ySe2from angle-resolved photoemission

Although a 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.