Local vortex--defect interaction in moving vortex lattices observed by STM

When applying a magnetic field to a type II superconductor, part of the magnetic flux penetrates the sample forming a current vortex. At high enough fields and low enough defect concentration the vortices form a 2D triangular lattice. We observed the vortex lattice on NbSe$_2$ single crystals using STM ($B=250-750$ mT, $T=4.2$ K). Due to a slow decay of the magnetic field of our superconducting magnet ($\sim -5$ nT/s) the vortices collectively move at an average speed of about 5 pm/s. The motion was observed by tracking the center of a vortex across consecutive images of the vortex lattice. The motion shows distinct acceleration/deceleration cycles we associate with collective pinning events on nearby defect sites. 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 defect sites. Since it takes an additional force to move a vortex out of position, we can identify subsurface defects and analyze the defect--vortex interaction. Results of the analysis will be presented.