Vortex Doping Into Superconducting Square Network

We have fabricated the finite-sized amorphous Mo₈₀Ge₂₀ superconducting networks with 10×10 square holes and directly observed the vortex structures by means of using the scanning SQUID microscope. On the increase of the field in a stepwise manner, the vortex penetration into the network is likely to follow the Bean critical model, wherein vortices first enter the network from the edges and penetrate deeper toward the center of the network as the field increases. We observed that the evolution of vorticity mainly occurred along the diagonal direction of the network, which has a narrow wire compared with the hole size. The vortex pattern was strongly modified as the system was cooled down below T_c. A diagonal vortex array appeared at intermediate temperatures, and is transformed into the isolated vortices at lower temperatures due to the strong repulsive interaction between vortices.