Microscopic origin of preferred vortex-trapping sites

Determine the relative importance of local topography, film-thickness variation, stress, spatial variations in superconducting properties, layer-dependent superconducting transition behavior, and cooldown-induced thermal gradients in producing the preferred Pearl-vortex trapping sites near via arrays and layer boundaries in multilayer superconducting circuits, and identify their microscopic origin.

Background

Scanning SQUID microscopy measurements showed that the first off-moat Pearl vortices repeatedly appeared near via arrays, layer transitions, and edges of the multilayer on-chip field coil, even when the moat geometries differed. This correlation indicates that the complete multilayer circuit structure, rather than moat geometry alone, shapes the vortex-pinning landscape.

The paper identifies several possible mechanisms for these preferred sites: local topography, film-thickness variation, mechanical stress, spatial variations in superconducting properties, and differences in the superconducting transition temperatures of different layers. Thermal gradients during cooldown may also cause layers to become superconducting at different times, altering the paths available for flux expulsion and trapping. The relative contributions of these mechanisms and the microscopic origin of the preferred trapping sites are left unresolved.

References

While the Figs.~\ref{fig5} and Figs.~\ref{fig6} establish a clear correlation between preferred trapping sites and the multilayer device geometry, rather than with the moat geometry alone, the microscopic origin of these preferred sites remains unclear, and several mechanisms may contribute. The via arrays and layer transition can introduce local topography, film-thickness variation, or stress, leading to spatial variation in the superconducting properties that can modify the local vortex-pinning energy. In addition, thermal gradient during cooldown, together with variations in the superconducting properties and critical temperatures among layers may cause some layers to become superconducting before others. The geometry of the layers that have already transitioned may dominate the available paths for flux expulsion and trapping, thereby modifying the eventual vortex locations. Determining the relative importance of these effects and the microscopic origin of the preferred trapping sites will require further investigation.

— Correlating DC SQUID Performance with the Location of Trapped Magnetic Flux Using Scanning SQUID Microscopy  (2610.10442 - Xu et al., 7 Oct 2026) in Results section, final paragraph before Section Conclusion