Single-crystal growth and extremely high H_c2 of 12442-type Fe-based superconductor KCa_2Fe_4As_4F_2

TL;DR

This study reports the growth of high-quality single crystals of KCa_2Fe_4As_4F_2, revealing an extremely high upper critical magnetic field and advancing understanding of 12442-type Fe-based superconductors.

Contribution

It presents a new method for growing large single crystals of KCa_2Fe_4As_4F_2 and characterizes its superconducting properties, especially the high upper critical field.

Findings

Superconducting transition temperature T_c = 33.5 K confirmed.

Observed steep increase in in-plane H_c2^ab near T_c.

Identified extremely high upper critical field H_c2.

Abstract

Millimeter sized single crystals of KCa_2Fe_4As_4F_2 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. A rather steep increase for the in-plane H_c2^ab with cooling, d\mu_0H_c2^a/dT|T_c = -50.9 T/K, was observed, indicating an extremely high upper critical field. Possible origins for this behavior were discussed. The findings in our work is a great promotion both for understanding the physical properties and applications of 12442-type Fe-based superconductors.