Probing the superconducting gap structure of (Li${}_{1-x}$Fe${}_x$)OHFeSe

We report measurements of the London penetration depth [$\mathrm{\Delta }\lambda (T)$] of the recently discovered iron-based superconductor (Li${}_{1-x}$Fe${}_x$)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, $\mathrm{\Delta }\lambda (T)$ displays nearly temperature independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density ${\rho }_s(T)$ which cannot be well accounted for by a single-gap isotropic $s$-wave model but are consistent with either two-gaps, a model for the orbital selective $s\times {\tau }_3$ state or anisotropic $s$-wave superconductivity.