Soft-core binary fluid exhibiting a λ-line and freezing to a highly delocalized crystal

We show that a simple soft-core binary fluid mixture with purely repulsive interactions exhibits a λ-line, i.e., a line of continuous transitions to a state characterized by undamped periodic concentration fluctuations (microphase separation). For states in the disordered fluid phase the bulk pair correlation functions exhibit strong 1-2 ordering, similar to that found in ionic fluids, which is also reflected in the density profiles of confined fluids. The latter display fluid layers alternating from rich in species 1 to rich in species 2. We argue that the λ-instability drives a freezing transition to a highly delocalized crystal, with Lindemann ratios that can exceed 90% near melting.