From <i>δl</i> - to <i>δT</i> _c -pinning in CaKFe ₄ As ₄ single crystals obtained by adjusting their defect structures

Abstract

Abstract We have investigated the vortices pinning behaviors of CaKFe 4 As 4 with two types of defect structures by alternating current and direct current susceptibility measurements under the Ginzburg-Landau theory and collective pinning model. We observed a new dissipative peak at temperature T J in each out-of-phase ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>χ</mml:mi> <mml:mrow> <mml:msup> <mml:mi/> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:msup> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>T</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> ) curve even at low magnetic field in the samples with thick CaFe 2 As 2 intercalations, which has been ascribed to the weak Josephson-like link. The unrelaxed current J c and the normalized pinning energy g ( t ) have been investigated for both types of the CaKFe 4 As 4 crystals based on the dynamic relaxation rate Q and transient current J s by means of generalized inversion scheme. For the samples with monolayer-KFe 2 As 2 intercalations, the temperature dependence of J c and g ( t ) is found to be in agreement well with the model of single vortex δT c -pinning. But for the samples with thick CaFe 2 As 2 intergrowths, both the δT c - and δl -pinning are in functioning at low magnetic field, but the δl pinning will play a leading role at magnetic field above 3.0 T. Differential charge analysis shows that the intergrowth of KFe 2 As 2 should be normal phase and the CaFe 2 As 2 will become superconducting one due to the charge transfer. The non-superconducting KFe 2 As 2 will induce an abruptly spatial change of T c resulting to strong δT c -pinning, and the superconducting CaFe 2 As 2 phase can mainly cause to fluctuate of mean free path leading to δl -pinning. The present work illustrates how the defect structure affects the flux pinning behaviors and can provide a new clue to design high-performance superconducting wire/tapes and films for strong magnetic field applications.