  Order-disorder transitions take place in many physical systems, but observing
them in detail in real materials is difficult. In two- or quasi-two-dimensional
systems, the transition has been studied by computer simulations and
experimentally in electron sheets, dusty plasmas, colloidal and other systems.
Here I show the different stages of defect formation in the vortex lattice of a
superconductor while it undergoes an order-disorder transition by presenting
real-space images of the lattice from scanning tunneling spectroscopy. When the
system evolves from the ordered to the disordered state, the predominant kind
of defect changes from dislocation pairs to single dislocations, and finally to
defect clusters forming grain boundaries. Correlation functions indicate a
hexatic-like state preceding the disordered state. The transition in the
microscopic vortex distribution is mirrored by the well-known spectacular
second peak effect observed in the macroscopic current density of the
superconductor.
