Magnetic excitation in Co-doped NaFeAs studied by neutron scattering

Even though NaFeAs ``111'' shares many similarities with BaFe2As2 ``122'' such as magnetic structure and phase diagram with doping , actually they are quite different from many aspects. For one example, the spin resonance is very sharp like delta function and well below 2detal, in sharp contrast with the broaden resonance observed in doped-122 systems. Our result provide a strong piece of evidence to support S$+$\_ and exclude S$++$ pairing symmetry in Fe-based superconductors.