Direct observation of stress accumulation and relaxation in small bundles of superconducting vortices in tungsten thin films.

We study the behavior of bundles of superconducting vortices when increasing the magnetic field using scanning tunneling microscopy and spectroscopy at 100 mK. Pinning centers are given by features on the surface corrugation. We find strong net vortex motion in a bundle towards a well-defined direction. We observe continuous changes of the vortex arrangements, and identify small displacements, which stress and deform the vortex bundle, separated by larger rearrangements or avalanches, which release accumulated stress.