Mapping two-dimensional melting

At absolute zero, a two-dimensional array of identical particles will crystallize into a hexagonal close-packed lattice. At high temperature, the lattice melts. What happens in between has interested physicists for decades. In the 1970s, theorists predicted that 2D melting would proceed via a so-called hexatic state in which the crystal breaks up into patches of local orientational order. The hexatic state’s existence has been inferred from changes in resistance and other sample-averaged quantities. Now, the whole melting process—from a crystal through a hexatic state to a liquid--has been directly imaged. Isabel Guillamón of the Autonomous University of Madrid, Spain, and her collaborators melted a lattice of superconducting vortices that forms in a thin film of tungsten under a magnetic field. For their imager, they used a scanning tunneling microscope, which can distinguish the non-superconducting vortex cores from the vortices themselves. As the movie shows, the vortices start off with hexagonal order. As the temperature increases, pentagonal (gold) and heptagonal (green) defects appear that cause dislocations (solid magenta lines). On further heating, the vortices become mobile. Just above 2 K, they move too quickly for the STM to track; they appear as white stripes, whose ordering resembles a liquid crystal’s smectic phase. By 3 K, which is 1 K below the film’s T _c, the lattice melts completely and the vortices move freely in a gray, undifferentiated blur. (I. Guillamón et al., Nat. Phys. , in press.)--Charles Day