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

Neutron scattering is used to probe magnetic excitations in FeSe0.4Te0.6 (T-c = 14 K). Low energy spin fluctuations are found with a characteristic wave vector (1/2 1/2 L) that corresponds to Fermi surface nesting and differs from Q(m) = (delta 01/2) for magnetic ordering in Fe1+yTe. A spin resonance with h Omega(0) = 6.51(4) meV approximate to 5.3k(B)T(c) and h Gamma = 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.