Single-Crystal Growth and Extremely High Hc2 of 12442-Type Fe-Based Superconductor KCa2Fe4As4F2

The 12442-type Fe-based superconductor is the only system that possesses two FeAs layers between neighboring insulating layers, which is worth the in-depth investigations. In this work, millimeter-sized single crystals of KCa2Fe4As4F2 were grown using a self-flux method. The chemical compositions and crystal structure were characterized carefully. Superconductivity with the critical transition T-c = 33.5 K was confirmed by both the resistivity and magnetic susceptibility measurements. Moreover, the upper critical field H-c2 was studied by the resistivity measurements under different magnetic fields, where an anisotropy of 8 was revealed near the superconducting transition. Importantly, a rather steep increase for the in-plane H-c2(ab) with cooling, d mu H-0(c2)ab/dT vertical bar(Tc) = -50.9 T/K, was observed. This value is several times higher than that of other systems of Fe-based superconductor and indicates an extremely high upper critical field. Possible origins for this behavior were discussed. The finding in our work is a great promotion both for understanding the physical properties and for the high-field applications of 12442-type Fe-based superconductors.