Competitive critical current densities in iron-based superconducting CaKFe_4As_4 single crystals
The DC magnetic hysteresis curves m(H) of CaKFe_4As_4 single crystals  for different magnetic field H orientations relative to the crystallographic axes and the magnetization relaxation have been thoroughly investigated to find the origin of the unexpectedly high critical current density J_c at large H around the liquid hydrogen temperature. While the J_c values of the order of 10^6-10^7 A/cm^2 with H parallel to the (a, b) plane clearly result from the presence of planar defects (CaFe_2As_2 intergrowhts [2, 3]), the peculiar intersection of the isothermal m(H) curves leading the a nonmonotonic temperature T variation of J_c when H is oriented along the c axis is not yet understood. It is shown that the occurrence of the J_c(T) maximum (~10^5 A/cm^2) around the liquid hydrogen temperature in magnetic fields as high as 50-100 kOe can be explained by considering the development of a second magnetization peak in the conditions of the multi-band structure of CaKFe_4As_4 [4, 5].
The kind assistance of the Alexander von Humboldt Foundation if gratefully acknowledged.
 A. Iyo et al., J. Am. Chem. Soc. 138, 3410 (2016).
 S. Ishida et al., npj Quantum Materials 4, 27 (2019).
 S. Pyon et al., Phys. Rev. B 99, 104506 (2019).
 L. Miu et al., Phys. Rev. B 78, 024520 (2008).
 R. Khasanov et al., Phys. Rev. B 97, 140503(R) (2018).