Title: On the Structure of Superconducting Order Parameter in High-Temperature Fe-Based Superconductors,
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.