Spin Gap and Resonance at the Nesting Wave Vector in Superconducting FeSe0:4Te0:6

Neutron scattering is used to probe magnetic excitations in FeSe 0 . 4 Te 0 . 6 (T c =14 K). Low energy spin fluctuations are found with a characteristic wave vector (½ ½L) that corresponds to Fermi surface nesting and differs from Q m = (δ0 ½) for magnetic ordering in Fe 1+y Te. A spin resonance with ħΩ 0 = 6.51(4) meV≈ 5.3k B T c and ħΓ = 1.25(5) meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s± superconductivity and imperfect quasi-2D Fermi surface nesting.