Since the Ginzburg-Landau (GL) parameter kGL of high-purity Nb is close to the critical value that separates type-I from type-II, the magnetic flux quantum (vortex) in this material has a rather unique property, i.e. non-monotonic intervortex interaction. This regime of superconductivity is often called type-II/1. Recently, the properties of vortices in this material have been reinvestigated using state-of-the-art techniques. Besides the academic interest, the understanding of the vortex dynamics in high-purity Nb is important for superconducting applications. For example, the performance of niobium superconducting radiofrequency (SRF) accelerator cavities is closely related to the behavior of remanent vortices that are not expelled during cooling. In fact, the contribution of remanent vortices is one of the factors limiting the Q-value at operating temperatures. Therefore, in order to further improve the cavity performance, it is necessary to clarify the unique vortex state of cavity-grade high-purity Nb and the behavior of the vortices during cooling.
To study the dynamic behavior of the vortices, we performed in-situ observations by magneto-optical imaging (MOI) technique during field cooling. We observed the intermediate mixed state (IMS), which is a manifestation of the attractive vortex interaction, in the cavity-grade high-purity Nb in a magnetic field of 100-400 Oe [1]. More recently, we have dynamically visualized the clustering process of a small number of vortices in lower fields by MOI with single vortex resolution [2]. In this talk, I would like to present some MOI results on the vortex behavior in high-purity Nb. The observed features can mostly be explained by the combined influence of the vortex-vortex interaction, the Lorentz force due to the screening current, and the pinning potential landscape.
[1] 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).
[2] S. Ooi et al., submitted.
Keywords: Vortex cluster, Nb, Intermediate mixed state, Magneto-optical imaging