Superconductivity of Co-Doped CaKFe4As4 Investigated via Point-Contact Spectroscopy and London Penetration Depth Measurements

TL;DR

This study investigates how cobalt doping affects the superconducting properties of CaKFe4As4, revealing a robust multigap structure that diminishes in amplitude with decreasing critical temperature, using spectroscopy and penetration depth measurements.

Contribution

It provides detailed experimental insights into the effects of Co substitution on the superconducting gaps and superfluid density in CaKFe4As4, highlighting the robustness of the multigap structure.

Findings

Superconducting critical temperature decreases with Co doping.

Multigap structure remains nodeless and robust upon doping.

Gap amplitudes decrease linearly with T_c, maintaining constant ratios.

Abstract

The iron-based superconductors (IBSs) of the recently discovered 1144 class, unlike many other IBSs, display superconductivity in their stoichiometric form and are intrinsically hole doped. The effects of chemical substitutions with electron donors are thus particularly interesting to investigate. Here, we study the effect of Co substitution in the Fe site of CaKFe$_4$As$_4$ single crystals on the critical temperature, on the energy gaps, and on the superfluid density by using transport, point-contact Andreev-reflection spectroscopy (PCARS), and London penetration depth measurements. The pristine compound (T$_\mathrm{c}\simeq$ 36 K) shows two isotropic gaps whose amplitudes ($\Delta_1$ = 1.4$-$3.9 meV and $\Delta_2$ = 5.2$-$8.5 meV) are perfectly compatible with those reported in the literature. Upon Co doping (up to $\approx$7% Co), T$_\mathrm{c}$ decreases down to $\simeq$ 20 K, the spin-vortex-crystal order appears, and the low-temperature superfluid density is gradually suppressed. PCARS and London penetration depth measurements perfectly agree in demonstrating that the nodeless multigap structure is robust upon Co doping, while the gap amplitudes decrease as a function of T$_\mathrm{c}$ in a linear way with almost constant values of the gap ratios 2$\Delta_i$/k$_\mathrm{B}$T$_\mathrm{c}$.