On the structure of the superconducting order parameter in high-temperature Fe-based superconductors
a Lomonosov Moscow State University, Department of Physics, Vorobevy gory, Moscow, 119992, Russian Federation
b Lebedev Physical Institute, Russian Academy of Sciences, Leninsky prosp. 53, Moscow, 119991, Russian Federation
c Lomonosov Moscow State University, Vorobevy Gory, Moscow, 119991, Russian Federation
d International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka str. 95, Wroclaw, 53-421, Poland
e National Research University Higher School of Economics, ul. Myasnitskaya 20, Moscow, 101000, Russian Federation
This paper discusses the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various Tc's. 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 kxky-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.