Spin fluctuations and pairing symmetry in AₓFe₂₋ySe₂: dual effect of the itinerant and the localized nature of electrons

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We investigate the spin fluctuations and the pairing symmetry in AₓFe₂₋ySe₂ by the fluctuation exchange approximation. Besides the on-site interactions, the next-nearest-neighbor antiferromagnetic coupling J₂ is also included. We find that both the itinerant and the localized natures of electrons are important to describe the recent experimental results of the spin fluctuations and the pairing symmetry. In particular, a small J₂ coupling can change the pairing gap from the d-wave symmetry to the s-wave symmetry. We have also studied the real-space structures of the gap functions for different orbits in order to gain more insight on the nature of the pairing mechanism.