Most superconducting electronics based on films exhibit granular structures. It has been suggested that grain boundaries form a network with relatively weak superconductivity, potentially acting as pinning centers. Yet, so far, detailed microscopic studies of the pinning landscape and its relation to vortex behavior remain scarce. Here, we imaged the vortex lattices (VL) in granular Nb films using magnetic force microscopy over large scanning areas at various magnetic fields. A non-monotonic evolution in the degree of vortex lattice ordering was observed with increasing vortex density, driven by a combination of vortex-vortex interactions and pinning effects. The spatial distribution of pinning potential within the film was directly mapped using a recently developed scanning quantum vortex microscope (SQVM). Instead of the network formed by grain boundaries, the pinning landscape presents a network-like structure, yet with domains significantly larger than the individual grains. The results of numerical simulations based on pinning landscape revealed by SQVM well reproduce our experiments. The pinning force per unit length at low magnetic fields was calculated. The critical current density, estimated from the relative positions of vortices, aligns well with the critical state model. Our work illustrates the relationship between the evolution of the vortex lattice with magnetic field and the structural features of granular Nb film, providing new insights into the design of high-performance superconducting electronic devices.

中文翻译：

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揭示颗粒超导薄膜中的钉扎景观和相关涡流模式演变


大多数基于薄膜的超导电子器件都表现出颗粒结构。有人认为晶界形成了一个超导性相对较弱的网络，可能充当固定中心。然而，到目前为止，关于钉扎景观及其与涡旋行为关系的详细微观研究仍然很少。在这里，我们使用磁力显微镜在各种磁场下的大扫描区域上对颗粒 Nb 薄膜中的涡旋晶格 （VL） 进行成像。在涡旋-涡旋相互作用和固定效应的共同驱动下，随着涡旋密度的增加，观察到涡旋晶格有序程度的非单调演变。使用最近开发的扫描量子涡旋显微镜 （SQVM） 直接绘制了薄膜内固定电位的空间分布。钉扎景观不是由晶界形成的网络，而是呈现出类似网络的结构，但其域明显大于单个晶粒。SQVM 揭示的基于钉扎景观的数值模拟结果很好地再现了我们的实验。计算了低磁场下每单位长度的钉接力。根据涡流的相对位置估计的临界电流密度与临界状态模型非常吻合。我们的工作阐明了涡旋晶格随磁场的演变与颗粒 Nb 薄膜的结构特征之间的关系，为高性能超导电子器件的设计提供了新的见解。