Nature 384, 145 - 147 (14 November 1996); doi:10.1038/384145a0

Dislocation-mediated melting of a two-dimensional crystal

L. Pauchard, D. Bonn & J. Meunier

Laboratoire de Physique Statistique de I'ENS, URA 1306 du CNRS, associé aux universités Paris VI et Paris VII, 24 rue Lhomond, 75231 Paris cedex 05, France

MELTING of three-dimensional solids usually starts at the free surface, which typically melts at a lower temperature than the bulk material¹. In two dimensions the starting point of many studies is the Kosterlitz–Thouless theory^2,3, in which melting is initiated through dislocation unbinding. Langmuir mono-layers—single layers of amphiphilic molecules formed at the air-water interface—should provide an ideal model for studying melting in two dimensions. Here we show that for monolayer crystals of fatty acids coexisting with their liquid phase, the interior melts before the edges. The melting of crystals under mechanical stress is initiated along the line at which the internal stress vanishes. We suggest that this apparently counterintuitive result arises from defect migration to the region of zero stress, where they accumulate and nucleate melting. These results support the idea that defects play a crucial role in melting of two-dimensional systems.

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