Evolution of symmetry and structure of the gap in Fe-based superconductors with doping and interactions

TL;DR

This paper investigates how the symmetry and structure of the superconducting gap in Fe-based superconductors evolve with doping and interactions, analyzing pairing mechanisms and gap features using simplified harmonic models.

Contribution

It provides a detailed analysis of the pairing interactions, gap symmetry, and the conditions for nodal gaps in FeSC, comparing RPA and RG approaches.

Findings

Interaction approximated by lowest angular harmonics

Conditions for s+/- gap nodes identified

Relation between RPA and RG approaches clarified

Abstract

We present a detailed study of the symmetry and structure of the pairing gap in Fe-based superconductors (FeSC). We treat FeSC as quasi-2D, decompose the pairing interaction in the XY plane in s-wave and d-wave channels into contributions from scattering between different Fermi surfaces and analyze how each scattering evolves with doping and input parameters. We verify that each interaction is well approximated by the lowest angular harmonics. We use this simplification to analyze the interplay between the interaction with and without spin-fluctuation components, the origin of the attraction in the s+/-and d_{x2-y2} channels, the competition between them, the angular dependence of the s+/- gaps along the electron Fermi surface, the conditions under which s+/- gap develops nodes, and the origin of superconductivity in heavily electron- or hole-doped systems, when only Fermi surfaces of one type are present. We also discuss the relation between RPA and RG approaches for FeSC.