Order-disorder transitions take place in many physical systems, but observingthem in detail in real materials is difficult. In two- or quasi-two-dimensionalsystems, the transition has been studied by computer simulations andexperimentally in electron sheets, dusty plasmas, colloidal and other systems.Here I show the different stages of defect formation in the vortex lattice of asuperconductor while it undergoes an order-disorder transition by presentingreal-space images of the lattice from scanning tunneling spectroscopy. When thesystem evolves from the ordered to the disordered state, the predominant kindof defect changes from dislocation pairs to single dislocations, and finally todefect clusters forming grain boundaries. Correlation functions indicate ahexatic-like state preceding the disordered state. The transition in themicroscopic vortex distribution is mirrored by the well-known spectacularsecond peak effect observed in the macroscopic current density of thesuperconductor.
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