UC San Diego

Vortex pinning in superconductors with artificially introduced pinning arrays provides a controllable method for studying periodic systems moving in the presence of a periodic potential. Vortex pinning has applications that include enhancing critical current densities for superconducting transmission lines and reducing noise in a variety of devices based on superconductivity. Modern nanolithography enables the positioning of pinning structures at length scales similar to the vortex interaction length in superconductors. This provides the means to manipulate the potential-energy landscape and to study vortex dynamics under tunable conditions. Although the role of disorder and defects in a lattice is of great importance in condensed-matter physics, it has received little attention in the context of vortices. While the extreme case of intrinsic defects lacking any order has previously been studied, there have been few attempts to study partially ordered systems. This dissertation studies the effects on the vortices of different types of controlled partial order in pinning site lattices. Experiments clarifying the origin of the ratchet effect in vortex systems have been performed. It has previously been shown that in a non-symmetric vortex potential, an AC current can be applied to a superconducting system and a DC voltage will emerge. The DC voltage can undergo a ratchet reversal, where the sign of the DC voltage changes. An experiment testing the origin of the DC ratchet reversal has been performed and compared to several theories. The effects of different current density configurations are explored in circular geometries. In the Corbino geometry, current is injected in the center of a superconducting disc and travels radially outwards towards the edges causing a tangential shearing force on the vortex lattice. This dissertation studies the Corbino geometry in the presence of different periodic potentials, and shows that the vortex lattice undergoes a shearing transition due to the pinning sites. Finally, several ongoing projects and international collaborations are presented. Measurements of the vortex lattice using microwave magnetic susceptibility measurements, neutron scattering, and magnetic force microscopy are discussed. A computer simulation of the vortex lattice is presented, and an experiment involving varying pinning site densities is described