Novel sample-thickness-dependent flux pinning behaviors of KFe2As2 intercalations in CaKFe4As4 single crystals

We report the strong dependence of critical current density (J(c)) on the sample thickness (d) along the crystalline c-axis in CaKFe4As4 single crystals. The J(c) can be respectively enlarged more than 3 and 50 times in H parallel to c-axis (J(c)(c)) and ab-plane (J(c)(ab)) at temperature of 5 K and high magnetic fields of 8 T when the d is reduced from 88 to 2.0 mu m. It is surprising to find that, accompanied by the enhanced J(c), the fishtail effect fades away with the decreasing d. We believe that both the origins of enhanced J(c) and the fishtail effect in CaKFe4As4 may have close relationship with the interlayer coupling between CaKFe4As4 and the intercalations of KFe2As2. Our results not only demonstrate an encouraging prospect for application of CaKFe4As4, but also illuminate new light on the origin of fishtail effect in iron-based superconductors.