1. Observation of Flux Quanta Using Low-Temperature Magneto-Optical Imaging Microscopy

The behavior of magnetic flux quanta in ultra-high-purity niobium crystals has attracted considerable attention because of its importance for improving the performance of superconducting accelerating cavities. The purity of niobium used in accelerators is extremely high, with a residual resistivity ratio (RRR) of several hundred. Such high-purity niobium is expected to behave as a type-II/1 superconductor, in which an attractive interaction exists between magnetic flux quanta. However, direct observation of the attraction between flux quanta had not previously been achieved. Using a magneto-optical imaging microscope (Fig. a), we successfully performed dynamical observations of the intermediate mixed state expected for a type-II/1 superconductor. As the temperature decreases, the flux quanta aggregate and phase-separate into a vortex-lattice phase and a Meissner phase, as shown in Fig. b and c.[1] [1] S. Ooi, et al., PRB 104, 064504 (2021); PRB 111, 094519 (2025).

1. Flux Quanta in Mesoscopic Regular-Polygon High-Temperature Superconductors

In superconductors whose dimensions are reduced to the scale of the magnetic penetration depth, flux quanta experience strong confinement effects. Therefore, understanding such effects is essential for the precise control of flux quanta. Meanwhile, in high-temperature superconductors, vortex matter is known to undergo a melting transition from a solid to a liquid phase, making the thermodynamic properties of flux-quanta matter a subject of considerable interest. We prepared a variety of mesoscopic Bi2_22​Sr2_22​CaCu2_22​O8+δ_{8+\delta}8+δ​ (Bi2212) samples with different geometries (Fig. a) and investigated the melting transition of systems containing only a small number of flux quanta (vortex clusters). Reflecting the geometric commensurability between the vortex configuration and the sample shape, the melting temperature was found to oscillate as a function of the number of flux quanta (Fig. b). We further identified that edge dislocations, which are forced to form within the vortex lattice due to geometrical constraints, are responsible for the suppression of the melting temperature.[2] [2] S. Ooi, et al., PRB 100, 144509 (2019).