Point contact Andreev reflection spectroscopy of superconducting energy gaps in 122-type family of iron pnictides

TL;DR

This paper reviews point contact Andreev reflection spectroscopy studies on 122-type iron pnictides, highlighting the presence of multiple nodeless gaps in hole-doped samples and a single strong-coupling gap in electron-doped samples.

Contribution

It provides a comprehensive overview of superconducting gap measurements in 122-type iron pnictides, emphasizing the differences between hole- and electron-doped systems and clarifying the gap structures.

Findings

Hole-doped Ba1-xKxFe2As2 has multiple nodeless gaps.

Electron-doped Ba(Fe1-xCox)2As2 shows a single strong-coupling gap.

ARPES confirms two close gaps with strong coupling in electron-doped samples.

Abstract

A brief overview of the superconducting energy gap studies on 122-type family of iron pnictides is given. It seems that the situation in the hole-doped Ba1-xKxFe2As2 is well resolved. Most of the measurements including the presented here point-contact Andreev reflection spectra agree on existence of multiple nodeless gaps in the excitation spectrum of this multiband system. The gaps have basically two sizes - the small one with a strength up to the BCS weak coupling limit and the large one with a very strong coupling with 2Delta/kTc ~ 6 - 8. In the electron doped Ba(Fe1-xCox)2As2 the most of the experiments including our point contact measurements reveal in quite broadened spectra only a single gap with a strong coupling strength. The high precision ARPES measurements on this system identified two gaps but very close to each other, both showing a strong coupling with 2Delta/kTc ~ 5 and 6, respectively.