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When applying a magnetic field to a type II superconductor, part of the magnetic flux penetrates the sample in form of current vortices. At high enough fields and low enough defect concentration the vortices form a 2D triangular lattice. We observed the vortex lattice on NbSe2 single crystals using a low temperature scanning tunneling microscope (B=250-750 mT, T=4.2 K). Due to a slow decay of the magnetic field of our superconducting magnet the vortices move coherently at an average velocity of 1-5 pm/s. The motion was observed by tracking the center of a vortex through consecutive images of the vortex lattice. The movement is not constant but shows distinct acceleration/deceleration cycles we associate with collective pinning events. A more subtle observation was the deviation of the vortex positions from their 'expected' location within the lattice of up to 3 nm. A similar effect was found in 2D simulations of a moving vortex lattice near point defect sites. We can identify subsurface defects and analyze the defect--vortex interaction. Results of the analysis will be presented.
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