GANIL, Caen

Phase transitions in finite systems

Phase transitions are universal phenomena of the every day life. Water boils in kettles. Snow melts in the sun light and the fog makes London so special! An amazing observation is that phase coexistences are present at all scales. Elementary particles deconfine in particle accelerators while self- gravitating systems collapse in the cosmos.

However, do we really know what is a phase transition? How does the water go from liquid to gas on a molecular level? How is it possible that the temperature of a ^Óbain-marie^Ô remains perfectly fixed at 100°C?

Recent conceptual progresses suggest that at the elementary level of mesoscopic systems phase transitions are even more astonishing: when boiling or melting small systems cool down when heated!

This phenomenon has been now observed in 4 mesoscopic systems: the melting of a metallic sodium cluster containing 147 atoms, the boiling of different atomic nuclei, the fragmentation of Hydrogen clusters and the superfluid transition in atomic nuclei. But this phenomenon is in fact analogous to what happen in stars which loose energy when they collapse and yet get hotter. Therefore, collapse of self-gravitating systems might be seen as a 5th example of phase transition with negative heat capacity.

In this paper, we will explain these recent discoveries and revisit the idea of phase transition showing that these strange behaviors are in fact the genuine characteristic of phase transitions.