Effect of magnetic field on the spin resonance in FeTe0.5Se 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 0.5 Se 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. © 2010 The American Physical Society.