Vortex Pinning and the Mechanism in CaKFe4As4 Revealed by Dynamical Magnetization Relaxation

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

“…These experimental results can also be explained by the change of interlayer coupling between the CaKFe 4 As 4 and KFe 2 As 2 with reducing d. If there is strong interlayer coupling, the intercalation of KFe 2 As 2 can be also superconductive even at higher temperature due to the charge transfer between them [24]. Thus, The intercalations of KFe 2 As 2 cannot work as effective planar pinning centers under these circumstances, and this may be the reason why there is small ratio of Γ J when the sample with large d, as observed in present work and reported in [9,11]. However, the intergrowths of KFe 2 As 2 can become weakly (or not) superconductive as decreasing d due to the relaxation of the electronic structures.…”

“…There are always inherent intergrowths such as KFe 2 As 2 and/or CaFe 2 As 2 in CaKFe 4 As 4 matrix because of their similarities under low temperature and low field [6,9]. Another exception is that the fishtail effect is observed in some CaKFe 4 As 4 crystals but absent in others, and the reasons are still unknown [9][10][11]. All these phenomena suggest that the flux dynamics is much more complex in CaKFe 4 As 4 .…”

From δl- to δT c-pinning in CaKFe4As4 single crystals obtained by adjusting their defect structures

“…Thus we observe that both the δl-and δT c -pinning take function in the No.1 samples. This may be also the reason that both flux pinning mechanisms are in functioning as observed in Ba(Fe 1−x Co x ) 2 As 2 , some CaKFe 4 As 4 , Ba 1−x K x Fe 2 As 2 and others [18,34,48,49]. For the No.2 samples, the KFe 2 As 2 intergrowths become non-superconducting phase and result in the abrupt change of T c in their interface, which will lead to strong fluctuation of order parameter as well as the variation of condensation energy in the CaKFe 4 As 4 superconductor.…”

“…Recently, we observed a novel crystal-thicknessdependent vortex pinning behaviors in KFe 2 As 2 intercalated CaKFe 4 As 4 superconductor [17]. It has been proposed that the extraordinary phenomena of vortex dynamics should have close relationship with the defect structures [18]. Unfortunately, the relevant pinning mechanisms are still unclear, which motivated us to give in depth research on these phenomena.…”

1144-type iron-based superconducting materials: Status and prospect

“…1144 型铁基超导体单晶的研究进展 助熔剂法生长 CaK-1144 单晶的方式与 CaFe 2 As 2 (Ca-122)等 122 型铁基超导单晶生 长的方式相似 [28] 。该方法先预制 FeAs 作为助熔剂材料，原料使用 Ca 颗粒、K 块及 FeAs 粉末，混合后封装在钽管/氧化铝坩埚组件中，之后经烧结、离心，分离单晶和助熔剂， 最终获得 CaK-1144 单晶。Meier 等人 [11,29] 采用助熔剂法制备了大尺寸 CaK-1144 单晶(10 [29] . (c) The R-T relationship of CaK-1144 single crystal, in which the superconducting transition temperature T c ~ 35 K [11] 一步法生长 CaK-1144 单晶的方式和助熔剂法相似，但不需预制 FeAs 助熔剂。该方 法的初始材料为四种单质元素，即 Ca 颗粒、K 块、Fe 粉及 As 颗粒。经过混合、研磨 及封装后，直接置于管式炉内进行烧结。一步法的热处理过程同样与助熔剂法类似，即 将温度升至 1150~1200°C 后进行缓慢降温，最终冷却至 930°C。Wang 等人 [30,31] 采用助熔剂法制备了 CaK-1144 单晶，通过测试单晶样品(1.365 mm×1.09 mm×0.0025 [35] 、 BaK-122 [36] 、 BaCo-122 [37,38] 、 LiFeSe [39] 、 YBCO [40] 、 Bi-Si-Ca-Cu-O(BSCCO) [41] 等，但 1144 型超导体产生鱼尾效应的机制尚不清楚。当磁场 变为 H//ab 时，磁滞回线较平行于 c 轴方向时形状的变化较大，曲线在自场附近下降明 显，并且呈倾斜状，Pyon 等人 [34] 认为倾斜的原因是受到柱状缺陷方向上涡旋的错排列 [32] . (b)…”