Generic mechanism for generating a liquid-liquid phase transition

Recent experimental results(1) indicate that phosphorus-a single component system-can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order transition between two liquids of different densities( 2) is consistent with experimental data for a variety of materials(3,4), in cluding single-component systems such as water(5-8), silica(9) and carbon(1 0). Molecular dynamics simulations of very specific models for supercooled water(2,11), liquid carbon(12) and supercooled silica(13) predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL t ransition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part an d two characteristic short-range repulsive distances. This kind of interact ion is common to other single-component materials in the liquid state (in p articular, liquid metals(2,14-21)), and such potentials are often used to d escribe systems that exhibit a density anomaly(2). However, our results sho w that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liqui d-liquid transition in systems like liquid metals, regardless of the presen ce of a density anomaly.