Trapping and squeezing of vortices in voids directly observed by scanning tunneling microscopy and spectroscopy

Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied quantized magnetic fluxes (vortices) that penetrated nanosized superconductors with void structures inside. The void structures act as a pinning center for the vortices, and the trapped vortices are confined and squeezed inside. When multiple vortices are trapped in voids, a cluster of vortices and a giant vortex were formed as a result of the squeezing, depending on the depth and the size of the trapping potential.