  We use inelastic neutron scattering to show that superconductivity in
electron-underdoped NaFe0.985Co0.015As induces a dispersive sharp resonance
near Er1 = 3:25 meV and a broad dis- persionless mode at Er2 = 6 meV. However,
similar measurements on overdoped superconducting NaFe0:955Co0:045As find only
a single sharp resonance at Er = 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 pnic- tides. Our discovery also shows the importance of the inelastic
neutron scattering in detecting the multiorbital superconducting gap structures
of iron pnictides.
