Phase transitions in clusters

The solid to liquid transition of clusters is discussed, mainly from an experimental point of view. An experiment is sketched which allows to measure the caloric curve of size selected sodium clusters. Melting temperatures, latent heats, and entropies of melting are determined in the size range between 55 and 357 atoms per cluster. The melting temperatures are about 30% less than the bulk value and fluctuate strongly, one additional atom can change it by ±10 K. Latent heats and entropies show a similar behaviour. From the change in entropy one can deduce the increase of phase space upon melting, which is about twelve orders of magnitude already for Na+55 and increases exponentially for larger clusters. The theoretical prediction that a finite particle can have a negative heat capacity is verified experimentally, showing that their are differences between a canonical and a microcanonical description. Finally the analogue of the boiling phase transition is shown for a finite system. To cite this article: M. Schmidt, H. Haberland, C. R. Physique 3 (2002) 327–340.