Vortex pinning in Nb thin films modulated by nanospheres

摘要

Superconducting Nb thin films deposited on top of self-assembled spherical nanoparticles have been investigated by transport measurements. Colloid polystyrene nanospheres have been organized by convective self-assembly into colloid crystals. These crystals serve as modulation templates for the Nb thin films which are deposited on top. These periodic arrays provide a natural topographic pattern to modulate laterally the superconducting order parameter in deposited Nb films. The nanoparticle diameter effectively controls the matching fields and flux pinning properties of these films. The superconducting phase boundary and matching effects have been studied for Nb films grown on the templates formed by polystyrene particles with diameter d = 202 and d = 357 nm. The triangular colloid lattice geometry induces strong modulation in thin Nb layers. In combination with the short range crystalline order of the templates, this leads to geometrical vortex ordering effects and vortex density dependent matching phenomena.