Coexistence of phase transitions and hysteresis near BEC

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Multiple phases occurring in a Bose gas with finite-range interaction are investigated. In the vicinity of the onset of Bose-Einstein condensation (BEC) the chemical potential and the pressure show a van-der-Waals like behavior indicating a first-order phase transition although there is no long-range attraction. Furthermore the equation of state becomes multivalued near the BEC transition. For a Hartree-Fock or Popov (Hartree-Fock-Bogoliubov) approximation such a multivalued region can be avoided by the Maxwell construction. For sufficiently weak interaction the multivalued region can also be removed using a many-body T-matrix approximation. However, for strong interactions there remains a multivalued region even for the T-matrix approximation and after the Maxwell construction, what is interpreted as a density hysteresis. This unified treatment of normal and condensed phases becomes possible due to the recently found scheme to eliminate self-interaction in the T-matrix approximation, which allows to calculate properties below and above the critical temperature.