Melting and Freezing of Microclusters from Analytics and Simulations

Abstract

Analytic theory and numerical simulations indicate that microclusters may exhibit solid-like and liquid-like phases which can coexist over a non zero range of temperature. The lower bound for existence of a stable liquid-like phase is a sharp freezing temperature T_f and the upper bound for existence of a stable solid-like phase is a sharp melting temperature T_m. The temperatures T_f and T_m are unequal for small systems; between them is the temperature T_eq at which the free energies of the solid-like and liquid-like clusters of a specified composition are equal. Observable phase-like behavior also requires that a dynamical criterion be satisfied, i.e. that the cluster persist in each phase long enough to develop well-defined equilibrium properties of that phase. This criterion is met by small clusters of argon of most but not all sizes. Taking averages inadvertently over both phases can hide the coexistence phenomenon; averaging properties separately for the two phases exhibits the double-valued character of the equation of state and other properties in the coexistence range.