Superconducting anisotropy and vortex pinning in CaKFe₄As₄ and KCa₂Fe₄As₄F₂ *

Abstract

The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials. For layered superconductors, the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy, which remains to be investigated in iron based superconductors (FeSCs) with the layered structure. Here, based on the transport and magnetic torque measurements, we experimentally investigate the vortex pinning in two bilayer FeSCs, CaKFe4As4(Fe1144) and KCa2Fe4As4F2(Fe12442), and compare their superconducting anisotropy γ. While the anisotropy γ ≈ 3 for Fe1144 is much smaller than γ ≈ 15 in Fe12442 around T c, a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144, as compared with the case of Fe12442. In combination with the literature data of Ba0.72K0.28Fe2As2 and NdFeAsO0.82F0.18, we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these FeSCs. Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in FeSCs.

Vortex glass phase transition in two-dimensional Bi2Sr2Ca2Cu3O 10+δ sub-microbridge

“…It is found that the U of 3.5 UC single crystal is slightly smaller than that of 2.5 UC single crystal, which suggests that the flux pinning in 2.5 UC single crystal is stronger than that in 3.5 UC single crystal. U decreases with the increase of fields, which is similar to iron-based superconductors [39,40] and other cuprates [41,42]. In addition, the thermal activation energy U is almost proportional to lnH at all measurement ranges, indicating a 2D structure of the vortices [39,42].…”

Influences of quenching treatment and cobalt-doping on the thermally activated flux-flow behavior in KCa2(Fe 1−x Co x )4As4F2

“…Typically the defects induced by impurities or ion irradiations should have a size of several to tens of nanometers to supply the effective pinning centers [13,44]. Another factor worthy of attention is that the activation energy shows an anti-correlation with the anisotropy in the FeSCs [27]. Thus, the enhancement of Γ from 8.0 to 9.2 induced by Co-doping may be the dominant and intrinsic reason for the reduction of U 0 in Co-doped samples.…”

“…In addition, we have observed a significant suppression of the activation energy for the flux flow in the ultra-thin sample of KCa 2 Fe 4 As 4 F 2 [12]. The correlation between the activation energy and the SC anisotropy in FeSCs was also uncovered [27].…”

Role of grain boundary networks in vortex motion in superconducting films

“…Pinning plays a crucial role in determining the vortex dynamics in the mixed state of type-II superconductors, which is important for lossless transport in the applications of superconductors. [1][2][3][4][5][6][7][8][9] It has been suggested to introduce many defects as effective pinning centers against the driving force of magnetic flux flow. Of them, twin boundaries may be the most significant anisotropic pinning defects in the cuprate compound YBa 2 Cu 3 O 7−x (YBCO).…”