Spin fluctuations and pairing symmetry in A$_{x}$Fe$_{2-y}$Se$_{2}$: dual effect of the itinerant and the localized nature of electrons

TL;DR

This paper explores how both itinerant and localized electrons influence spin fluctuations and pairing symmetry in A$_{x}$Fe$_{2-y}$Se$_{2}$, revealing that small changes in magnetic coupling can switch pairing from d-wave to s-wave.

Contribution

The study demonstrates the dual role of itinerant and localized electrons in determining pairing symmetry, highlighting the impact of next-nearest-neighbor coupling on superconducting gap structures.

Findings

Small $J_{2}$ coupling can switch pairing symmetry from d-wave to s-wave.

Both itinerant and localized electrons are crucial for understanding experimental results.

Real-space gap structures provide insights into the pairing mechanism.

Abstract

We investigate the spin fluctuations and the pairing symmetry in A$_{x}$Fe$_{2-y}$Se$_{2}$ by the fluctuation exchange approximation. Besides the on-site interactions, the next-nearest-neighbor antiferromagnetic coupling $J_{2}$ 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 $J_{2}$ 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.