Novel Pinning Phenomena in Nb Thin Films with Square Pinning Arrays

[[abstract]]Electron-beam lithography and reaction dry etching techniques have been used to fabricate superconductor thin films with regular square hole arrays to explore the vortex pinning behavior. Magnetotransport measurements were carried out by a four-probe technique. Periodic critical current matching peaks are observed. We found that the height of the third matching peak is close to that of the fourth matching peak for film with holes of diameter of 200 nm at certain temperatures, and it appears that the third matching peak is missing. Similar peculiar phenomena are observed for films with holes of different diameters, but the missing peaks are at the fourth or fifth matching field for larger holes. We made molecular dynamic simulations to study the ground state vortex distributions and the dynamics of the vortex motion. The simulation results offer good explanations to the features we found in the experiments