LIMITING GIBBS-STATES AND PHASE-TRANSITIONS OF A BIPARTITE MEAN-FIELDHUBBARD-MODEL

In the frame of operator-algebraic quantum statistical mechanics we ca lculate the grand canonical equilibrium states of a bipartite, microsc opic mean-field model for bipolaronic superconductors (or anisotropic antiferromagenetic materials in the quasispin formulation). Depending on temperature and chemical potential, the sets of statistical equilib rium states exhibit four qualitatively different regions, describing t he normal, superconducting (spin-flopped). charge ordered (antiferroma gnetic), and coexistence phases. Besides phase transitions of the seco nd kind, the model also shows phase transitions of the first kind betw een the superconducting and the charge ordered phases. A unique limiti ng Gibbs state is found in its central decomposition For all temperatu res, even in the coexistence region, if the thermodynamic limit is per formed at fixed particle density (magnetization).