Phase diagram of quantum fluids. The role of the chemical potential and the phenomenon of condensation

Abstract: We discuss the generic phase diagrams of pure systems that remain fluid near zero temperature. We call this phase a quantum fluid. We argue that the signature of the transition is the change of sign of the chemical potential, being negative in the normal phase and becoming positive in the quantum fluid phase. We show that this change is characterized by a phenomenon that we call condensation, in which a macroscopic number of particles is in their own many-body ground state, a situation common to Fermi and Bose gases. We show that the ideal Bose-Einstein Condensation fits in this scenario, but that it also permits the occurrence of a situation that we may call "Fermi-Dirac Condensation". In addition, we argue that this phenomenon is also behind the development of superfluid phases in real interacting fluids. However, only interacting systems may show the change from a thermal fluid to a quantum one as a true phase transition. As a corollary, this argument shows the necessity of the appearance of a "supersolid" phase. We also indicate how these ideas may be useful in the description of of experiments with ultracold alkali gases.