Real Space Images of the Vortex Lattice Structure in a Type II Superconductor during Creep over a Barrier

We report on Bitter decoration studies of the magnetic flux line lattice in the type II superconductor ${\mathrm{NbSe}}_2$ during flux creep experiments over barriers, a surface step. We find that these steps can act as “vortex diodes.” By imaging the real space structures of the vortex lattices as they flow over these steps and measuring the change in vortex density, we are able to measure the energetic height of the barrier as well as the elastic correlation length of the vortices along their length. At low fields, we find that the vortices are correlated over distances of roughly $5\mu \mathrm{m}$.