Effect of magnetic field on the spin resonance in FeTe_0.5Se _0.5 as seen via inelastic neutron scattering

Inelastic neutron scattering and susceptibility measurements have been performed on the optimally doped Fe-based superconductor FeTe _0.5Se_0.5, which has a critical temperature, Tc of 14 K. The magnetic scattering at the stripe antiferromagnetic wave vector Q= (0.5,0.5) exhibits a "resonance" at ∼6 meV, where the scattering intensity increases abruptly when cooled below Tc. In a 7-T magnetic field parallel to the a-b plane, Tc is slightly reduced to ∼12K, based on susceptibility measurements. The resonance in the neutron-scattering measurements is also affected by the field. The resonance intensity under field cooling starts to rise at a lower temperature ∼12K, and the low-temperature intensity is also reduced from the zero-field value. Our results provide clear evidence for the intimate relationship between superconductivity and the resonance measured in magnetic excitations of Fe-based superconductors.