Nodal multigap superconductivity in KCa2Fe4As4F2

We find evidence that the newly discovered Fe-based superconductor KCa2Fe4As4F2 (Tc = 33.36(7) K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation (μSR) 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 μSR 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.