Analysis of the London penetration depth in Ni-doped CaKFe4As4

TL;DR

This study combines experimental measurements and theoretical analysis to investigate the superconducting properties of Ni-doped CaKFe4As4, revealing a nodeless s± order parameter and increased interband interaction with doping.

Contribution

It provides a comprehensive analysis of the London penetration depth in Ni-doped CaKFe4As4 using multiple experimental techniques and Eliashberg theory, highlighting the nature of the superconducting gap and doping effects.

Findings

Superfluid density was accurately measured using TDR and microwave resonators.

The London penetration depth at 5 K was approximately 170-196 nm.

Superconductivity is described by a nodeless s± order parameter with increased interband interaction upon doping.

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, $\lambda_L (\mathrm{5~K}) \approx 170 \pm 20$ nm, which agreed well with the NV-centers in diamond optical magnetometry that gave $\lambda_L (\mathrm{5~K}) \approx 196 \pm 12$ 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.