Local distortions in slow moving vortex lattices

We observed slow moving vortex lattices in NbSe_2 using a home built STM at a temperature of 4.2 K and magnetic fields in the range of 250-500 mT. The vortex lattices move coherently at speeds of 1-10 pm/s due to a slow decay of our magnetic field of 0.5 mT/day. We observe collective speed variations which indicate pinning/depinning events. Furthermore, we found -- to our knowledge for the first time -- small and local deviations from the coherent behavior. Most noticeable were local lattice distortions with displacements of the vortices of ~2 nm from their ideal lattice position. We compared the observations to a 2-D simulation of a vortex lattice moving under similar conditions and found similar distortion effects near pinning sites. Thus, studying the distortion patterns could lead to a better understanding of the vortex-defect interaction.