Measurement of a double neutron-spin resonance and an anisotropic energy gap for underdoped superconducting NaFe_0.985Co_0.015As using inelastic neutron scattering

We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe_0.985Co_0.015As induces a dispersive sharp resonance near E_r1=3.25 meV and a broad dispersionless mode at E_r2=6 meV. However, similar measurements on overdoped superconducting NaFe_0.935Co_0.045As find only a single sharp resonance at E_r=7 meV. We connect these results with the observations of angle-resolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.