Electron-doping evolution of the low-energy spin excitations in the iron arsenide superconductor BaFe_2-x Ni_x As₂

We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe_2-x Ni_x As₂as a function of nickel doping x. In the undoped state, BaFe₂ As ₂ exhibits a tetragonal-to-orthorhombic structural phasetransition and simultaneously develops a collinear antiferromagnetic (AF) order below TN =143K. Upon electron doping of x=0.075 to induce bulk superconductivity with Tc =12.2K, the AF ordering temperature reduces to TN ≈58K. We show that the appearance of bulk superconductivity in BaFe1.925 Ni0.075 As2 coincides with a dispersive neutron spin resonance in the spin excitation spectra and a reduction in the static ordered moment. For optimally doped BaFe_1.9 Ni _0.1 As₂ (Tc =20K) and overdoped BaFe_1.85 Ni_0.15 As₂ (Tc =14K) superconductors, the static AF long-range order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the three-dimensional characters of the electronic superconducting gaps are prevalent throughout the phase diagram and may be critical for superconductivity in these materials.