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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 [1] 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.

[1] A. Iyo et al., J. Am. Chem. Soc. 138, 3410 (2016).

[2] S. Ishida et al., npj Quantum Materials 4, 27 (2019).

[3] S. Pyon et al., Phys. Rev. B 99, 104506 (2019).

[4] L. Miu et al., Phys. Rev. B 78, 024520 (2008).

[5] R. Khasanov et al., Phys. Rev. B 97, 140503(R) (2018).


Lucica Miu
National Institute of Materials Physics