We report the results of a muon-spin rotation ($\mu \mathrm{SR}$) experiment to determine the superconducting ground state of the iron-based superconductor ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ with $T_{\mathrm{c}}\approx 28.3\mathrm{K}$. This compound is related to the fully gapped superconductor ${\mathrm{CaCsFe}}_4{\mathrm{As}}_4$, but here the Ca-containing spacer layer is replaced with one containing ${\mathrm{Ca}}_2{\mathrm{F}}_2$. The temperature evolution of the penetration depth strongly suggests the presence of line nodes and is best modeled by a system consisting of both an $s$- and a $d$-wave gap. We also find a potentially magnetic phase which appears below $\approx 10\mathrm{K}$ but does not appear to compete with the superconductivity. This compound contains the largest alkali atom in this family of superconductors, and our results yield a value for the in-plane penetration depth of ${\lambda }_{ab}(T=0)=244\left(3\right)\mathrm{nm}$.

• Received 13 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.060506