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We discuss the generic phase diagrams of pure systems that remain fluid nearzero temperature. We call this phase a quantum fluid. We argue that thesignature of the transition is the change of sign of the chemical potential,being negative in the normal phase and becoming positive in the quantum fluidphase. We show that this change is characterized by a phenomenon that we callcondensation, in which a macroscopic number of particles is in their ownmany-body ground state, a situation common to Fermi and Bose gases. We showthat the ideal Bose-Einstein Condensation fits in this scenario, but that italso permits the occurrence of a situation that we may call "Fermi-DiracCondensation". In addition, we argue that this phenomenon is also behind thedevelopment of superfluid phases in real interacting fluids. However, onlyinteracting systems may show the change from a thermal fluid to a quantum oneas a true phase transition. As a corollary, this argument shows the necessityof the appearance of a "supersolid" phase. We also indicate how these ideas maybe useful in the description of of experiments with ultracold alkali gases.

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