Nodal multigap superconductivity in KCa$_2$Fe$_4$As$_4$F$_2$

TL;DR

This study provides evidence of multigap superconductivity with line nodes in KCa$_2$Fe$_4$As$_4$F$_2$, revealing a new pathway for nodal superconductivity in iron-based materials through structural modifications.

Contribution

It demonstrates the presence of nodal multigap superconductivity in KCa$_2$Fe$_4$As$_4$F$_2$ and explores how structural changes influence the pairing state.

Findings

Superfluid density shows non-saturation at low temperatures.

Data fits well with two-gap models with line nodes.

No time reversal symmetry breaking detected.

Abstract

We find evidence that the newly discovered Fe-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ ($T_c~=~33.36(7)$~K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation ($\mu$SR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully-gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gaps. Meanwhile the zero-field $\mu$SR results indicate a lack of time reversal symmetry breaking in the superconducting state, but suggest the presence of magnetic fluctuations. These results demonstrate a different route for realizing nodal superconductivity in iron-based superconductors. Here the gap structure is drastically altered upon replacing one of the spacer layers, indicating the need to understand how the pairing state is tuned by changes of the asymmetry between the pnictogens located either side of the Fe planes.