Local-moment magnetism in superconducting FeTe_0.35Se _0.65 as seen via inelastic neutron scattering

The nature of the magnetic correlations in Fe-based superconductors remains a matter of controversy. To address this issue, we use inelastic neutron scattering to characterize the strength and temperature dependence of low-energy spin fluctuations in FeTe_0.35Se_0.65 (T_c∼14 K). Integrating magnetic spectral weight for energies up to 12 meV, we find a substantial moment (_{M2 LE}∼0.07μB2/Fe)that shows little change with temperature, from below T_c to 300 K. Such behavior cannot be explained by the response of conduction electrons alone; states much farther from the Fermi energy must have an instantaneous local spin polarization. It raises interesting questions regarding the formation of the spin gap and resonance peak in the superconducting state.