Robust s+/- pairing in CaK[Fe(1-x)Ni(x)]4As4 (x = 0 and 0.05) from the response to electron irradiation

Controlled point-like disorder introduced by 2.5 MeV electron irradiation was used to probe the superconducting state of single crystals of \CaKx\ superconductor at $x = 0$ and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, $T_c$ at the rate of $dT_c/d\rho(T_c) \approx$ 0.19 K(\textmu$\Omega$cm)$^{-1}$ for $x=0$ and 0.38 K(\textmu$\Omega$cm)$^{-1}$ for $x=\:$0.05, respectively. In Ni - doped, $x = 0.05$, compound the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of $dT_N/d\rho(T_N)\approx$ 0.16 K(\textmu$\Omega$cm)$^{-1}$. Low - temperature variation of London penetration depth is well approximated by the power law, $\Delta \lambda (T) = AT^n$ with $n\approx\,$2.5 for $x=0$ and $n\approx\,$1.9 for $x=0.05$ in the pristine state. Electron irradiation leads to the exponent $n$ increase above 2 in $x=0.05$ suggesting superconducting gap with significant anisotropy that is smeared by the disorder scattering. Detailed analysis of $\lambda (T)$ and \(T_{c}\) evolution with disorder is consistent with two effective nodeless superconducting energy gaps due to robust s$_{\pm}$ pairing. Overall the behavior of \CaKx\ at $x = 0$ is similar to a slightly overdoped \BaK\ at $y \approx$ 0.5 and at $x= 0.05$ to an underdoped composition at $y \approx$ 0.2.