Anisotropic neutron spin resonance in underdoped superconducting   NaFe1-xCoxAs
  We use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a-axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er2=6 meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the a- and c-axis, while the dispersionless mode is isotropic similar to that of C45. Since the a-axis polarized spin excitations of the anisotropic resonance appear below Tc, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity. 
Chenglin Zhang, Yu Song, L.P. Regnault, Yixi Su, M.Enderle, J. Kulda,   Guotai Tan, Zachary C. Sims, Takeshi Egami, Qimiao S...
cond-mat.supr-con