Phase Transition in Small System
  Everybody knows that when a liquid is heated, its temperature increases until the moment when it starts to boil. The increase in temperature then stops, all heat being used to transform the liquid into vapor. What is the microscopic origin of such a strange behavior? Does a liquid drop containing only few molecules behave the same? Recent experimental and theoretical developments seem to indicate that at the elementary level of very small systems, this anomaly appears in an even more astonishing way: during the change of state - for example from liquid to gas - the system cools whereas it is heated, i.e. its temperature decreases while its energy increases. This paper presents a review of our understanding of the negative specific heat phenomenon. 
Ph. Chomaz, F. Gulminelli