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Frozen Silicon "Trees" tell Story of Melting

Scientists have long predicted that some crystals don't melt all at once. Instead, the topmost layer of atoms melts before the other layers, creating a liquid that is only one atom thick. Seeing this liquid is fiendishly difficult, though. Researchers at Cornell have verified this prediction using a clever trick — they watch the melting of ultraflat silicon crystals. The silicon crystal is heated to a temperature just below its normal melting point, then cooled. If surface melting does not occur, the surface should remain ultraflat. Instead, tree-like protrusions develop. These