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We study spin resonance in the superconducting state of recently discoveredalkali-intercalated iron selenide materials A_xFe_(2-y)Se_2 (A=K,Rb,Cs) inwhich the Fermi surface has only electron pockets. Recent angle-resolvedphotoemission spectroscopy (ARPES) studies [M. Xu et al., Phys. Rev. B 85,220504(R) (2012)] were interpreted as strong evidence for s-wave gap in thesematerials, while the observation of the resonance peak in neutron scatteringmeasurements [G. Friemel et al., Phys. Rev. B 85, 140511 (2012)] suggests thatthe gap must have different signs at Fermi surface points connected by themomentum at which the resonance has been observed. We consider recentlyproposed unconventional s+- superconducting state of A_xFe_(2-y)Se_2 withsuperconducting gap changing sign between the hybridized electron pockets. Weargue that such a state supports a spin resonance. We compute the dynamicalstructure factor and show that it is consistent with the results of inelasticneutron scattering.

Spin resonance in AFe2Se2 with s-wave pairing symmetry