Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe_0.6Se_0.4 superconductor

We report on inelastic neutron-scattering measurements that find itinerantlike incommensurate magnetic excitations in the normal state of superconducting FeTe_0.6Se_0.4 (T_c=14 K) at wave vector Q_inc=(1/2±ε, 1/2-+ε) with ε=0.09 (1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal-state excitations. We use a four band model to describe the Fermi-surface topology of this iron-based superconductors with the extended s (±) symmetry and find that it qualitatively captures the salient features of these data.