Unique defect structure and advantageous vortex pinning properties in superconducting CaKFe4As4

Abstract

The lossless current-carrying capacity of a superconductor is limited by its critical current density (Jc). A key to enhance Jc towards real-life applications is engineering defect structures to optimize the pinning landscape. For iron-based superconductors (IBSs) considered as candidate materials for high-field applications, high Jc values have been achieved by various techniques to introduce artificial pinning centres. Here we report extraordinary vortex pinning properties in CaKFe4As4 (CaK1144) arising from the inherent defect structure. Scanning transmission electron microscopy revealed the existence of nanoscale intergrowths of the CaFe2As2 phase, which is unique to CaK1144 formed as a line compound. The Jc properties in CaK1144 are found to be distinct from other IBSs characterized by a significant anisotropy with respect to the magnetic field orientation as well as a remarkable pinning mechanism significantly enhanced with increasing temperature. We propose a comprehensive explanation of the Jc properties based on the unique intergrowths acting as pinning centres.

Effect of Ni doping on vortex pinning in CaK(Fe1−xNix)Haberkorn1, Xu2, Meier3 et al. 2019Phys. Rev. B184290View full textAdd to dashboardCite

“…(2)]. However, as we mentioned earlier, it has also been associated with pinning provided by CaFe 2 As 2 inclusions [16]. The effect is not evidenced in Ni-doped samples because the chemical substitution improves the vortex pinning and masks little changes in the pinning landscape.…”

“…On the other hand, because of thermally activated vortex motion, J c data usually decrease with temperature. An exception to that has been recently observed in CaKFe 4 As 4 single crystals [16,17,22,23]. For instance, as a consequence of smoother J c (H ) dependences, the J c value for μ 0 H = 5 T at 20 K is higher than that observed at 10 K. This unusual behavior has been related to the presence of planar CaFe 2 As 2 intergrowths [16].…”

“…The undoped crystals display an unusual maximum at J c (T ) at intermediate temperatures [see Fig. 2(a)] [16,17,22,23]. The effect 064524-3 disappears for Ni-doped samples.…”

“…Vortex dynamics in Fe-SCs is usually well described by the collective creep theory [12][13][14]. The sources of flux pinning include random point defects [15], precipitates [16], planar defects [17], and correlated disorder such as twin boundaries [18]. Notwithstanding the coexistence between superconductivity and magnetism usually present in many Fe-SCs [19][20][21], its influence on the resulting vortex dynamics has been little explored.…”

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

“…This system lacks long-ranged magnetic order or a nematic electronic state at low temperatures [5][6][7][8][9][10][11] but upon doping with Ni a hedgehog magnetic structure is stabilized [9,12,13]. CaKFe 4 As 4 has an exceptionally large critical current density due to the strong point-like defects caused by local structural site effects as well as surface pinning [14][15][16]. Due to reduced symmetry compared with the 122 family of iron-based superconductors, CaKFe 4 As 4 is predicted to have up to ten different bands ( Fig.…”