Nanoteilchen als Ausgangspunkt für künstliche Pinningzentren in Supraleitern

Very small pinning centers in superconducting thin films were produced applying a patterning technique based on self-organization of salt-loaded spherical micelles made of diblock copolymers. In a subsequent plasma treatment, the polymer matrix is removed completely, while the metal salts are reduced to metallic nanoparticles used as etching mask to transfer the resulting pattern into silicon substrates. The pinning sites produced by growth of a niobium thin film on top of such substrates are much smaller than those produced by conventional methods like optical, electron-beam or focused ion beam lithography. To the best of our knowledge, this method delivers the smallest periodic artificial pinning centers studied yet. Due to their nanoscaled size, the artificial pinning sites guarantee single-vortex pinning even at temperatures well below the critical temperature. This is shown by the fact that Little-Parks oscillations can be observed only up to the first matching field, where each of the pinning sites contains one vortex, and not up to higher orders as observed for wire networks or perforated films with larger holes. Several samples containing such artificial pinning centers were characterized using a SQUID-magnetometer to obtain their magnetic moment, by high resolution electron microscopy and by transport measurements at low temperature. Although the order of the resulting artificial pinning centers is not perfect, pronounced integer and fractional matching features induced by commensurability effects when the vortex lattice matches the lattice of artificial pinning sites are observed within a wide temperature range. Experimental data of a niobium layer on top of silicon nanopillars shows an unexpected temperature dependence of matching features at the first matching field. This effect can be understood by applying a modified concept of collective pinning in a triangular array with non-perfect order.