77Se NMR study of pairing symmetry and spin dynamics in KyFe2-xSe2
  We present a 77Se NMR study of the newly discovered iron selenide superconductor KyFe2-xSe2, in which Tc = 32 K. Below Tc, the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above Tc, Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-Tc superconductivity in this iron selenide system. 
Weiqiang Yu, L. Ma, J. B. He, D. M. Wang, T.-L. Xia, G. F. Chen, Wei   Bao
cond-mat.supr-con cond-mat.str-el