Using developed eclipse pulsed laser deposition (PLD) technique, we fabricated magnetic insulating garnet thin films exhibiting a large Faraday effect by increasing the Bi substitution level while suppressing the deposition of droplet-like particulates. Using these films, we further succeeded in developing a high-performance magneto-optical (MO) sensor with more than four times the magnetic-field sensitivity of commercially available sensors at low temperatures (Fig. 1).
　Using a high-performance real-time magneto-optical microscope equipped with this MO sensor, we have also succeeded in real-time imaging of vortices in Nb, a practical low-temperature superconducting material, with single-vortex resolution. Nb is an important material for a wide range of advanced technologies, including superconducting radio-frequency cavities for particle accelerators, superconducting quantum devices, and ultrafast Josephson devices.
　In particular, we achieved the world’s first dynamic observation of the process by which vortices form an Abrikosov triangular lattice just below the superconducting transition temperature. In conventional type-II superconductors, the interaction between vortices is predominantly described by magnetic repulsion. In contrast, ultra-high-purity Nb, which is used in superconducting accelerator cavities and related applications, has a Ginzburg–Landau parameter κ close to the boundary value of 1/√2 between type-I and type-II superconductivity and is classified as a type-II/1 superconductor. In this regime, an attractive component can emerge in the interaction between vortices under certain conditions, resulting in unusual vortex interactions that differ from those in conventional type-II superconductors. Consequently, characteristic vortex configurations such as vortex molecules and vortex clusters can form. We have also succeeded in dynamically observing the formation of these vortex molecules and clusters with single-vortex resolution. This represents another world-first achievement (Fig. 2).
　On the other hand, the microscopic origin of the attractive interaction between vortices in type-II/1 Nb superconductors cannot be fully described by phenomenological theories such as Ginzburg–Landau theory alone and therefore requires an understanding based on a more microscopic theoretical framework. One such framework is the boson theory of superconductivity developed by Dr. Hiroomi Umezawa and his collaborators. In this study, we perform molecular-dynamics simulations of vortices using the vortex–vortex interaction potential energy derived from this theory and theoretically analyze the mechanisms responsible for the formation of the vortex molecules and clusters observed experimentally (Fig. 3).