On the Structure of Superconducting Order Parameter in High-Temperature Fe-Based Superconductors

TL;DR

This study investigates the superconducting properties of Ba-122 single crystals, revealing the coexistence of two nodeless s-wave gaps on different Fermi surface sheets and supporting an s^{++} pairing model.

Contribution

It provides a comprehensive experimental analysis confirming the two-gap, s-wave symmetry and the coexistence of two superconducting condensates in Ba-122 high-temperature superconductors.

Findings

Two nodeless s-wave gaps on different Fermi surface sheets.

Coexistence of two superconducting condensates with different electron-boson couplings.

Support for the s^{++} pairing model due to moderate interband coupling.

Abstract

This paper discusses the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various $T_c$. The paper uses five complementary techniques to obtain a self-consistent set of data on the superconducting properties of Ba-122. A major conclusion of the authors work is the coexistence of two superconducting condensates differing in the electron-boson coupling strength. The two gaps that develop in distinct Fermi surface sheets are nodeless in the $k_xk_y$-plane and exhibit s-wave symmetry, the two-band model represents a sufficient data description tool. A moderate interband coupling and a considerable Coulomb repulsion in the description of the two-gap superconducting state of barium pnictides favor the $s^{++}$-model.