Visualization of Vortex Configurations in Surface-planarized Nb Antidot Arrays

Superconducting thin films with artificial structures provide a versatile platform for manipulating the macroscopic quantum states of superconductivity. To study the macroscopic quantum states, it is very helpful to manipulate the field distribution and directly visualize the vortex configuration in order to understand the underlying physics. However, the pronounced surface topography introduced by the nanoscale structures can produce substantial topographic contributions in magnetic force microscopy (MFM) due to the height-related atomic force, making it diffcult to unambiguously identify the magnetic signal associated with individual vortices. Here, we develop a surface-planarization approach by filling the antidot lattice with insulating non-magnetic CaF 2 after nano-fabrication, substantially reducing the topographic corrugation while preserving the underlying antidot geometry and its superconducting properties. This approach enables high-fidelity MFM imaging of the field distribution in an 80-nm-thick Nb superconducting film with an antidot array by removing the inhomogeneous atomic force background from the magnetic force. It is shown that well-defined vortex configurations at integer and fractional matching fields can be clearly visualized. These results demonstrate that surface planarization can effectively overcome a major experimental limitation in MFM studies of nano-structured superconductors, providing a reliable route for real-space imaging of vortex configurations in artificial structures in superconductors.