Growth and characterization of CaFe1-xCoxAsF single crystals by CaAs flux method

• High-quality single crystals of CaFe1−xCoxAsF are grown successfully using a new flux CaAs.
• The crystals can be grown from three approaches with different initial raw materials.
• Tc can be enhanced for about 1 K under the very small hydrostatic pressure of 0.22 GPa.

Millimeter sized single crystals of CaFe1−xCoxAsFCaFe1−xCoxAsF 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. Compared with the undoped parent phase CaFeAsF, the crystal lattice along the c-axis is suppressed by the Co substitution while that along the a-axis expands slightly. Superconductivity with the critical transition Tc as high as 21 K was confirmed by both the resistivity and magnetic susceptibility measurements in the sample with x=0.118. Moreover, it is found that Tc can be enhanced for about 1 K under the very small hydrostatic pressure of 0.22 GPa, which is more quick than that reported in the polycrystalline samples. Our results are a promotion for the physical investigations of 1111 phase iron-pnictide superconductors.