Collective and plastic vortex motion in superconductors at high flux densities

The 'mixed state' of type II superconductors occurs when magnetic flux penetrates the material (in the form of vortices) without destroying the superconducting ground state. Zero resistivity is retained if the vortices are pinned by crystalline defects, but is destroyed by vortex motion. This provides the practical motivation for studying vortices in random pinning potentials(1-7). But the insights so obtained also bear on the more general class of problems involving the dynamics of elastic media in the presence of-quenched disorders (for example, mechanical friction). Moreover, the magnetic vortex system is highly tunable and permits questions concerning frictional, plastic and elastic flow(9) to be investigated on the,scale of single vortices. Remarkable results have been obtained on-the dynamics of this system(10-17), but have. been largely restricted to well separated vortices at very low flux densities. Scanning tunnelling microscopy has the potential to resolve individual vortices at much higher flux densities(18-21), and here we show that the imaging rates can be sufficiently high to resolve the dynamics in this flux regime. We find that, in the presence of strongly pinning line defects, the vortex lattice remains pinned until the,number of vortices is about twice that of the defects, at which point plastic creep commences. But in the presence of weak intrinsic point disorder, the vortices creep coherently along one of the principle axes of the vortex lattice, where they exhibit striking and unanticipated velocity modulations that appear to be related to the lattice periodicity.