Illuminating liquid polymorphism in silicon.

P hase transitions are phenomena of enduring fascination for scientists studying condensed matter (e.g., the ubiquitous solids and liquids). Much of our ability to rationalize phenomena in the natural world and to design or make new materials depends on our ability to characterize the properties of different phases that substances can exist in and predict conditions under which transitions occur between them. A unique phase transition that may occur between two liquid states of a pure substance (1–4) has been the focus of considerable attention recently. Beye et al. (5), in work reported in PNAS, study such a liquid–liquid transition in silicon, using femtosecond (fs) pump-probe spectroscopy that allows them to follow the structural evolution of a silicon crystal over tens of picoseconds (ps) after it has been optically excited by a laser. Apart from the impressive developments in technique that make these ultrafast measurements possible, measurements on such short time scales turn out to be quite essential to observe the transition of interest, and therein lies the significance of the reported results. The possibility of a liquid–liquid transition has been studied for silicon, germanium, water, silica, carbon, hydrogen, etc. —substances that form a significant component of our natural world and of importance for technology. Understanding a phenomenon common to these substances has broad implications, including for biomolecular systems (6). Most of the substances mentioned above possess energetically stabilized open structures (e.g., fourfold coordinated tetrahedral bonding geometry in water and silicon), which characterize the liquid (and solid) at low temperature and pressure, whereas at high pressures and temperatures the liquid is characterized by a denser packing of atoms or molecules. The proposed liquid phases are thus termed low-density liquid (LDL) and high-density liquid (HDL). From calorimetric data, Spaepen and Turnbull (7) and Bagley and Chen (8) long ago proposed an unusual transition between amorphous solid and liquid silicon, 200–300 °K below the melting temperature of 1,685 °K. After more recent investigations, this transition has been reinterpreted as a liquid–liquid transition and demonstrated in computer simulations (3, 9–11) but at a lower temperature around 1,100 °K. Clear experimental verification is hard (12) because the transition is expected to occur under conditions in which the crystal is the stable phase. The liquid is therefore metastable (Fig. 1A) and transforms to the stable crystal phase on nanosecond time scales. Such short life times make reliable measurements near the expected transition hard, although indirect evidence has been obtained using, for example, shortduration laser or heavy ion irradiation (13, 14). [Similarly, the estimated transition for water, at 220 °K, lies in the “no man’s land” that is inaccessible due to crystallization (15, 16), and attempts to circumvent this limit, for example using confined