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Microscopic mechanisms of equilibrium melting of a solid

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Science  07 Nov 2014:
Vol. 346, Issue 6210, pp. 729-732
DOI: 10.1126/science.1253810

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Melting can follow many pathways

Melting involves the loss of order as additional kinetic energy is added to a system. Although simple models of this sort of phase transition exist, it can be very difficult to observe the initial stages either experimentally or using simulations. Samanta et al. developed a robust rareevent sampling technique that makes it possible to examine melting events without needing excessive computing time (see the Perspective by van de Walle). For both copper and aluminum, they observed the formation of defects that act as starting points for the melting process rather than the homogeneous loss of order assumed in classic nucleation theory.

Science, this issue p. 729

Abstract

The melting of a solid, like other first-order phase transitions, exhibits an intrinsic time-scale disparity: The time spent by the system in metastable states is orders of magnitude longer than the transition times between the states. Using rare-event sampling techniques, we find that melting of representative solids—here, copper and aluminum—occurs via multiple, competing pathways involving the formation and migration of point defects or dislocations. Each path is characterized by multiple barrier-crossing events arising from multiple metastable states within the solid basin. At temperatures approaching superheating, melting becomes a single barrier-crossing process, and at the limit of superheating, the melting mechanism is driven by a vibrational instability. Our findings reveal the importance of nonlocal behavior, suggesting a revision of the perspective of classical nucleation theory.

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