Cluster-forming systems of ultrasoft repulsive particles: statics and dynamics

In this paper, we present a short review as well as novel results on a recently established counterintuitive phenomenon of cluster aggregation of particles that interact via purely repulsive interactions. We demonstrate how repulsion can lead to clustering provided that the interaction allows full particle overlaps and also displays negative Fourier components. The formation of crystals with average site occupancy that scales linearly with density, and which is in general a noninteger number, is demonstrated by means of density functional theory and Monte Carlo simulations. Noninteger average occupancy of lattice sites is sustained by incessant hopping processes in the crystal, which we observe and quantitatively analyze by means of Molecular Dynamics simulations. The simultaneous presence of relaxative dynamics on the single-particle and arrested dynamics on the collective level leads to an unusual interplay between the two and to novel scenarios of dynamical arrest in the cluster crystal. (C) 2008 Elsevier B.V. All rights reserved.