77Se NMR Study of the Pairing Symmetry and the Spin Dynamics in KyFe2-xSe2

We present a Se-77 NMR study of the newly discovered iron selenide superconductor KyFe2-xSe2, in which T-c = 32 K. Below Tc, the Knight shift K-77 drops sharply with temperature, providing strong evidence for singlet pairing. Above T-c, Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-T-c superconductivity in this iron selenide system.