Real space visualisation of defect annealing in disordered vortex lattice

Scanning tunnelling spectroscopy (STS) is a powerful technique to probe local density of states at atomic length scales. Using this technique one can image vortices in superconductors. NbSe2 is a well known type II superconductor. In clean NbSe2 single crystals vortex lattice is found to have perfectly ordered hexagonal Abrikosov pattern. However if we cool the sample from above its critical temperature (Tc)  down to its superconducting state in presence of external magnetic field, the vortex lattice is found to be disordered. This disorder comes due to pinning of certain vortices by local impurities. However this field cooled (FC) disordered vortex lattice state is not stable as shown in our experiment. A small magnetic field pulse anneals these vortex lattice defects and the subsequent lattice becomes ordered. However these defects are found to be resilient to temperature annealing as with increasing temperature (~Tc) the defects become mobile but do not anneal.