A minimum single-band model for low-energy excitations in superconducting A

We propose a phenomenological single-band model for the newly discovered iron-selenide superconductors AxFe2−ySe2 (A = Tl, K, Rb, Cs). Based on this minimum model and the random phase approximation, the dx² − y² pairing symmetry is revealed theoretically, which may be understood in the framework of Fermi surface topology. A common origin of superconductivity is elucidated for this compound and other high-Tc materials. The spin excitations at (π,π/2) in superconducting states are observed, in good agreement with the neutron scattering experiments. The spin resonance is indicated to show up only for the d-wave pairing, which provides an additional indication for the d-wave pairing symmetry in this family of superconductors