Tuning the Intrinsic Anisotropy with Disorder in the CaKFe4As4 Superconductor

We report on the anisotropy of the London penetration depth of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by $\mathrm{Ni}$ substitution) and irradiation induced (by 3.5-MeV protons). Indeed, ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ is particularly suitable for the study of fundamental superconducting properties due to its stoichiometric composition, exhibiting clean-limit behavior in the pristine samples and having a fairly high critical temperature, ${T}_{c}\ensuremath{\approx}35$ K. The London penetration depth ${\ensuremath{\lambda}}_{L}$ is measured with a microwave-coplanar-resonator technique that allows us to deconvolve the anisotropic contributions ${\ensuremath{\lambda}}_{L,ab}$ and ${\ensuremath{\lambda}}_{L,c}$ and obtain the anisotropy parameter ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}={\ensuremath{\lambda}}_{L,c}/{\ensuremath{\lambda}}_{L,ab}$. The ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ found for the undoped pristine sample is in good agreement with previous literature and is here compared to ab initio density-functional-theory and Eliashberg calculations. The dependence of ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ on both chemical and irradiation-induced disorder is discussed to highlight which method is more suitable to decrease the direction dependence of the electromagnetic properties while maintaining a high critical temperature. Lastly, the relevance of an intrinsic anisotropy such as ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$ on application-related anisotropic parameters (critical current, pinning) is discussed in light of the recent employment of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ in the production of wires.