DISLOCATION-MEDIATED MELTING OF A 2-DIMENSIONAL CRYSTAL

MELTING of three-dimensional solids usually starts at the free surface , which typically melts at a lower temperature than the bulk material( 1). In two dimensions the starting point of many studies is the Koster litz-Thouless theory(2,3), in which melting is initiated through dislo cation unbinding. Langmuir monolayers-single layers of amphiphilic mol ecules formed at the air-water interface-should provide an ideal model for studying melting in two dimensions. Here we show that for monolay er crystals of fatty acids coexisting with their liquid phase, the int erior melts before the edges. The melting of crystals under mechanical stress is initiated along the line at which the internal stress vanis hes. We suggest that this apparently counterintuitive result arises fr om defect migration to the region of zero stress, where they accumulat e and nucleate melting. These results support the idea that defects pl ay a crucial role in melting of two-dimensional systems.