Angle-resolved photoemission evidence of s-wave superconducting gap in   KxFe2-ySe2 superconductor
  Although nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2-ySe2, 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 KxFe2-ySe2, which favors the s-wave pairing symmetry. 
M. Xu, Q. Q. Ge, R. Peng, Z. R. Ye, Juan Jiang, F. Chen, X. P. Shen,   B. P. Xie, Y. Zhang, and D. L. Feng
cond-mat.supr-con cond-mat.str-el