Niobium is a low-κ superconductor with GL parameters close to 1/√2, the critical value separating type-I and type-II. In high-purity niobium, the vortex-vortex interaction becomes attractive in the intermediate range where the inter-vortex distance is several times the magnetic field penetration length λ, so that a so-called intermediate mixed state appears where the vortex lattice phase with aggregated vortices and the vortex-free Meissner phase are separated . When high-purity niobium is cooled in a low magnetic field less than 100 Oe, vortex clusters are formed due to the attractive interaction. In view of its application to superconducting accelerating (SRF) cavities fabricated with high-purity niobium, contribution of the remanent vortices during cooling is one of the factors limiting the Q-value at sufficiently low temperatures. Therefore, understanding the morphology and dynamics of vortex clusters is helpful to effectively expel vortices from the SRF cavities.
In order to investigate how the clusters form depending on the magnetic field, in present study, we performed magneto-optical imaging observations using (Bi,Lu)3(Fe,Ga)5O12 garnet films fabricated by an Eclipse-PLD (pulsed laser deposition) method. As a result, we found that the cluster size decreased monotonically with decreasing magnetic field. This tendency was also confirmed by molecular dynamics simulations assuming a theoretically predicted vortex-vortex interaction.
 S. Ooi et al., “Observation of intermediate mixed state in high-purity cavity-grade Nb by magneto-optical imaging”, Phys. Rev. B 104, 064504 (2021).
Keywords: Vortex cluster, Niobium, Magneto-optical imaging, Pulsed laser deposition