Spin resonance in AFe₂Se₂ with s-wave pairing symmetry

We study spin resonance in the superconducting state of recently discovered alkali-intercalated iron selenide materials A_xFe _2-ySe₂ (A=K, Rb, Cs) in which the Fermi surface has only electron pockets. Recent angle-resolved photoemission spectroscopy (ARPES) studies were interpreted as strong evidence for s-wave gap in these materials, while the observation of the resonance peak in neutron scattering measurements suggests that the gap must have different signs at Fermi surface points connected by the momentum at which the resonance has been observed. We consider recently proposed unconventional s^+- superconducting state of A_xFe_2-ySe₂ with superconducting gap changing sign between the hybridized electron pockets. We argue that such a state supports a spin resonance. We compute the dynamical structure factor and show that it is consistent with the results of inelastic neutron scattering.