Visualizing Vortex Cluster Dynamics in the Weak Type-II Superconductor CaSb$_2$

TL;DR

This study uses scanning SQUID imaging to visualize vortex cluster dynamics in CaSb$_2$, revealing behaviors that challenge existing theories of vortex interactions in superconductors.

Contribution

First local visualization of vortex cluster magnetic dynamics in a weakly pinned superconductor, highlighting physics beyond current single-band models.

Findings

Dense vortex clusters with boundary susceptibility

Suppressed internal vortex motion within clusters

Observations inconsistent with isolated vortex models

Abstract

Scanning SQUID imaging of CaSb$_2$ reveals dense vortex clusters with enhanced boundary susceptibility and suppressed internal vortex motion, which features inconsistent with both isolated vortex and flux tube behaviors. These measurements provide the first local visualization of magnetic dynamics within vortex clusters in a weakly pinned superconductor, offering a new route to probe non-monotonic vortex-vortex interactions that are typically expected in single-band type-II/1 or multiband type-1.5 superconductors. Although the superfluid density follows a single-gap BCS model and the Ginzburg-Landau parameter of CaSb$_2$ lies slightly outside the type-II/1 regime, vortex clustering and spatially inhomogeneous dynamics are clearly observed, indicating physics beyond existing microscopic theories for single-band superconductors.