Visualizing Dynamics of Vortex Clusters in Multiband Superconductor CaSb₂

In the intermediate-mixed state of type-II superconductor with a Ginzburg-Landau(GL) parameter near 1/√2=0.707, Bitter decoration experiments show that vortices tend to form clusters. In both multi-component and extended single-component GL theory, clusters theoretically originate from vortex-vortex interactions that are attractive at long ranges and repulsive at short ranges, but experimental evidence to confirm this mechanism is still lacking. CaSb₂, a multiband superconductor with a GL parameter of approximately 0.7, is a good model system to study this phenomenon.

Using a scanning superconducting quantum interference device (SQUID) susceptometer, we observed vortex clusters in bulk single crystals of CaSb₂, studied the dynamics of both isolated vortices and vortex clusters, and imaged the diamagnetic response. The diamagnetic response was relatively homogeneous, supporting the hypothesis that the vortex clusters arise from vortex-vortex interactions and not from inhomogeneity. The temperature dependence of the fitted superfluid density fits well with a weak-coupling s-wave model. In this presentation, we delve into the finer details of the observed vortex and vortex cluster dynamics, comparing the observations to simulations that include various models of vortex-vortex interactions.