Dynamical visualization of attractively interacting single vortices in type-II/1 superconducting Nb by magneto-optical imaging

In type-II/1 superconductors, the intervortex interaction has a potential minimum, resulting in an attractive force at certain distances. To better understand this unique vortex system, in situ observations in high-purity Nb have been performed using magneto-optical imaging with single-vortex resolution. We have dynamically visualized the behavior of the vortices during cooling, including clustering of attractively interacting single vortices, the hopping motion of vortices, the formation of an Abrikosov triangular lattice, and various degrees of disordered states with or without clusters. From the direct observations of clustering vortices, a conversion temperature for the boundary between type-II/2 and type-II/1 states is evaluated. The diverse vortex configurations arise from the interplay of the vortex-vortex interaction, the screening current, and the pinning potential landscape.