Analysis of the London penetration depth in Ni-doped CaKFe4As4

Abstract

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ (CaK1144) for $x=$0, 0.017 and 0.034. To obtain the superfluid density, $\rho ={\left(1+\Delta {\lambda }_L(T)/{\lambda }_L(0)\right)}^{-2}$, the temperature dependence of the London penetration depth, $\Delta {\lambda }_L(T)$, was measured by using tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of ${\lambda }_L(T\ll T_c)\approx {\lambda }_L(0)$ was measured by using MWR, ${\lambda }_L(5\mathrm{K})\approx 170\pm 20$ nm, which agreed well with the NV-centers in diamond optical magnetometry that gave ${\lambda }_L(5\mathrm{K})\approx 196\pm 12$ nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless s${}_{\pm }$ order parameter and that upon Ni doping the interband interaction increases.

8 pages, 5 figures