CaSb₂ superconductor’s quantum vortex clusters defy theory

Researchers at Stanford and the University of British Columbia have captured the first local visualization of magnetic dynamics within vortex clusters in the weakly pinned superconductor CaSb₂. Scanning SQUID imaging revealed “dense vortex clusters with enhanced boundary susceptibility and suppressed internal vortex motion,” a behavior inconsistent with expected patterns for isolated vortices or simple flux tubes. Despite a Ginzburg-Landau parameter that places CaSb₂ just outside the typical type-II/1 regime, the material exhibits clear vortex clustering and spatially inhomogeneous dynamics, suggesting underlying physics beyond current single-band superconductor theories. These measurements offer a new way to probe vortex interactions and potentially reveal non-monotonic interactions usually found only in more complex materials. The Ginzburg-Landau parameter for CaSb₂ falls within a range of 0.707 to 3, depending on crystal direction, yet the material…