DIPLOMARBEIT Vibrations and Diffusion in Colloidal Cluster Crystals

In soft matter physics novel kinds of “cluster phases” have attracted considerable attention recently. These phases are observed, for instance, in systems in which particles interact via bounded, purely repulsive potentials that originate from the effective interactions between soft colloids, such as dendrimers. At sufficiently high densities these particles overlap, thereby forming clusters which occupy the regular sites of a cubic lattice. These cluster crystals exhibit astonishing new features that have not been encountered in hard matter so far. Such systems react upon compression by increasing the average cluster size while the lattice spacings remain constant. In the present work, we study the dynamic properties of cluster crystals by means of molecular dynamics simulations. We employ advanced data analysis techniques to characterise the spectrum of vibrations in such systems. We identify two different kinds of oscillatory motion of particles in the system, corresponding to single-particle and collective motion. In the cluster crystals under consideration, diffusion takes place through so-called particle hopping processes between distinct cluster sites. We analyse the behaviour of hopping particles in detail and show that a significant fraction of hopping events is highly correlated and involves particles travelling over many nearest-neighbour distances. Our results shed some new light on the microscopic mechanisms of diffusion at work in cluster crystals.