Imaging the vortex-lattice melting process in the presence of disorder

General arguments(1) suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can trans form them into second-order transitions; but the atomic level details of th is process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition(2-6) on an inter -particle scale, as well as over a wide range of particle densities. Here w e use a differential magneto-optical technique to obtain direct experimenta l visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid -liquid interface coarsening and pinning. Although the local melting is fou nd to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temper atures, at scales down to the mesoscopic level. We also resolve local hyste retic supercooling of microscopic liquid domains, a nonequilibrium process that occurs only at selected sites where the disorder-modified melting temp erature has a local maximum. By revealing the nucleation process, we are ab le to experimentally evaluate the solid-liquid surface tension, which we rn d to be extremely small.