Tuning London penetreation depth anisotropy with Disorder in the CaKFe₄As₄ Superconductor

We report on the anisotropic behavior of the London penetration depth of CaK(Fe_1−xNi_x)₄As₄ (CaK1144), discussing how it relates to its electronic structure and how it can be tuned by disorder to match specific requirements for applications.
The London penetration depth was measured with a microwave technique [1] that allowed us to deconvolve its anisotropic contributions and obtain the anisotropy parameter γ_λ [2]. Single crystals with Ni doping level x=0, 0.017, and 0.034 were analyzed both in the pristine state and after 3.5 MeV proton irradiation to introduce scattering centers.
The data found are in good agreement with previous literature [3] and are here compared to ab-initio density functional theory and Eliashberg calculations, showing that gap anisotropy in CaK1144 is present.
The dependence of γ_λ on both chemical and irradiation-induced disorder is discussed to highlight which methods are more suitable to optimize the material, in light of the recent employment of CaK1144 in the production of wires [4].

References
[1] G. Ghigo et al., Phys. Rev. B 96 (2017) 014501
[2] D. Torsello et al., Phys. Rev. B 100 (2019) 094513
[3] R. Khasanov et al., Phys. Rev. B 99 (2019) 140507
[4] S. Pyon et al., Appl. Phys. Express 11 (2018) 123101