Superconducting heterostructures: from antipinning to pinning potentials

We study vortex lattice dynamics in a heterostructure that combines two type-II superconductors: a niobium film and a dense triangular array of submicrometric vanadium (V) pillars. Magnetic ac susceptibility measurements reveal a sudden increase in ac penetration, related to an increase in vortex mobility above a magnetic field, H^∗(T), that decreases linearly with temperature. Additionally, temperature independent matching effects that occur when the number of vortices in the sample is an integer of the number of V pillars, strongly reduce vortex mobility, and were observed for the first and second matching fields, H₁ and H₂. The angular dependence of H₁, H₂ and H^∗(T) shows that matching is determined by the normal applied field component, while H^∗(T) is independent of the applied field orientation. This important result identifies H^∗(T) with the critical field boundary for the normal to superconducting transition of V pillars. Below H^∗(T), superconducting V pillars repel vortices, and the array becomes an 'antipinning' landscape that is more effective in reducing vortex mobility than the 'pinning' landscape of the normal V sites above H^∗(T). Matching effects are observed both below and above H^∗(T), implying the presence of ordered vortex configurations for 'antipinning' or 'pinning' arrays. (paper)