Direct observation of low-field vortex patterns in stoichiometric CaKFe 4 As 4 single crystal

作者：He Tian, Xinsheng Gao, Kang-Hong Yin, Xingjian Liu, Hanyu Gu, Y.‐L. He, Jiaying Zhang, Chun-Lei Wang, Jun‐Yi Ge · 发表于：Physical review. B./Physical review. B · 年份：2025 · DOI：10.1103/physrevb.111.104512 · 被引用次数：5 · 研究领域：Iron-based superconductors research、Rare-earth and actinide compounds、Magnetic and transport properties of perovskites and related materials

The vortex distribution in a high-quality stoichiometric superconductor ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ is studied using low-temperature magnetic force microscopy. Across the entire range of magnetic fields up to 80 Oe, the vortex structures lack long-range order due to vortex pinning. As the vortex density increases, the ordering of the vortex lattice gradually improves, characterized by an increased proportion of sixfold coordination and ${60}^{\ensuremath{\circ}}$ angle. Continuous-field cooling experiments reveal a high density of equivalent pinning centers in ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$. In localized regions, vortex chains induced by surface steps are observed, from which the freezing temperatures of the vortex configurations are inferred and found to be consistent with direct experimental observations. Analysis of vortex interaction energies and pinning force distributions, based on the relative positions of vortices, suggests that the strong pinning in ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ originates from nanoscale spatial fluctuations in the superfluid density, which is closely linked to the unique structural characteristics of ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ single crystals. In this paper, we illustrate the relationship between disordered vortex patterns, critical current density, and nanoscale spatial fluctuations of superfluid density, providing insights into strong pinning in iron-based superconductors.