Growth and characterization of CaFe$_{1-x}$Co$_x$AsF single crystals by CaAs flux method

TL;DR

This study reports the growth of high-quality CaFe$_{1-x}$Co$_x$AsF single crystals using a self-flux method, characterizes their structure, and explores how Co doping and pressure influence their superconducting transition temperature.

Contribution

It introduces a reliable method for growing CaFe$_{1-x}$Co$_x$AsF single crystals and investigates the effects of Co doping and hydrostatic pressure on their superconductivity.

Findings

High-quality single crystals can be grown from different initial materials.

Co substitution suppresses the c-axis lattice constant.

Superconductivity with $T_c$ up to 21 K was observed, and $T_c$ can be enhanced by small pressure.

Abstract

Millimeter sized single crystals of CaFe$_{1-x}$Co$_x$AsF were grown using a self-flux method. It is found that high-quality single crystals can be grown from three approaches with different initial raw materials. The chemical compositions and crystal structure were characterized carefully. The c-axis lattice constant is suppressed by the Co substitution. Superconductivity with the critical transition $T_c$ as high as 21 K was confirmed by both the resistivity and magnetic susceptibility measurements in the sample with $x$ = 0.12. Moreover, it is found that $T_c$ can be enhanced for about 1 K under the very small hydrostatic pressure of 0.22 GPa, which is more quickly than that reported in the polycrystalline samples. Our results is a promotion for the physical investigations of 1111 phase iron-pnictide superconductors.