Temperature dependence of the resonance and low-energy spin excitations in superconducting FeTe _0.6Se _0.4

We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe _0.6Se _0.4 (T _c=14 K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi-surfaces nesting wave vector Q=(0.5,0.5), χ ^′′(Q,ω), has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing ω for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above T _c, the energy of the mode is weakly temperature dependent and vanishes concurrently above T _c. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe _2-x(Co,Ni) _xAs ₂, the mode in FeTe _0.6Se _0.4 is not directly associated with the superconducting electronic gap.