Stability and structure of a supercooled liquid mixture in two dimensions

The structural and thermodynamic properties of a two-dimensional binary mixture of soft discs are reported over a range of temperatures down to large supercoolings using constant NPT molecular dynamics simulations. It is shown that the four orders of magnitude increase in the structural relaxation time is not accompanied by any significant increase in translational or orientational order. The phase diagram, calculated in the temperature/composition plane using thermodynamic integration, exhibits a deep eutectic point that is responsible for stabilizing the amorphous state. Voronoi analysis of the low-temperature ground state reveals a structure characterized by a network of linear arrays of fivefold and sevenfold sites. The heat capacity C-P exhibits an asymmetric peak with a maximum at T* = 0.55. It is argued that the initial rapid drop in C-P for T* < 0.55 is an equilibrium result and, hence, the peak in the heat capacity corresponds to the existence of an "enthalpy gap" with a characteristic temperature of T* approximate to 0.35. This gap results from a minimum volume change associated with an anharmonic fluctuation. [S1063-651X(99)07705-3].