Spectroscopic studies of the superconducting gap in the 12442 family of iron-based compounds

TL;DR

This study investigates the superconducting gap structure in the 12442 iron-based compounds, revealing at least two gaps with one being nodal, using spectroscopic techniques to enhance understanding of their pairing symmetry.

Contribution

The paper provides new experimental evidence of multiple gaps, including a nodal gap, in Rb-12442, contributing to the understanding of pairing symmetry in this unique family of superconductors.

Findings

Rb-12442 has at least two superconducting gaps.

One of the gaps in Rb-12442 is definitely nodal.

Results are consistent with certain theoretical gap structures.

Abstract

The iron-based compounds of the so-called 12442 family are very peculiar in various respects. They originate from the intergrowth of 122 and 1111 building blocks, display a large in-plane vs. out-of-plane anisotropy, possess double layers of FeAs separated by insulating layers, and are generally very similar to double-layer cuprates. Moreover, they are stoichiometric superconductors because of an intrinsic hole doping. Establishing their superconducting properties, and in particular the symmetry of the order parameter, is thus particularly relevant in order to understand to what extent these compounds can be considered as the iron-based counterpart of cuprates. In this work we review the results of various techniques from the current literature and compare them with ours, obtained in Rb-12442 by combining point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements of the superfluid density. It turns out that the compound possesses at least two gaps, one of which is certainly nodal. The compatibility of this result with the theoretically allowed gap structures, as well as with the other results in literature, is discussed in detail.