title: Measurement of a double neutron-spin resonance and an anisotropic energy gap for underdoped superconducting NaFe0.985Co0.015As using inelastic neutron scattering.
abstract: We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe0.985Co0.015As 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 NaFe0.935Co0.045As 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.
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA and Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA.
authors.forename: Chenglin
authors.initials: C
authors.lastname: Zhang
authors.forename: Rong
authors.initials: R
authors.lastname: Yu
authors.forename: Yixi
authors.initials: Y
authors.lastname: Su
authors.forename: Yu
authors.initials: Y
authors.lastname: Song
authors.forename: Miaoyin
authors.initials: M
authors.lastname: Wang
authors.forename: Guotai
authors.initials: G
authors.lastname: Tan
authors.forename: Takeshi
authors.initials: T
authors.lastname: Egami
authors.forename: J A
authors.initials: JA
authors.lastname: Fernandez-Baca
authors.forename: Enrico
authors.initials: E
authors.lastname: Faulhaber
authors.forename: Qimiao
authors.initials: Q
authors.lastname: Si
authors.forename: Pengcheng
authors.initials: P
authors.lastname: Dai