  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.
