Hartree theory for the negative-Uextended Hubbard model. II. Finite temperature

The zero-temperature Hartree theory is extended to finite temperature to investigate the phase diagram, the order parameters, and the chemical potential of the negative-$U$ extended Hubbard model in the weak-coupling regime. All the transitions between the charge-ordered, the singlet-superconducting, the mixed, and the nonordered phases are second order. In a special region of the parameter space, we discover the heat-charge-order process, which is the main source for the qualitative difference between the results of the present weak-coupling regime and the strong-coupling regime investigated earlier. Phenomena associated with the heat-charge order are compared to other work in which an effect similar to the heat-charge order exists.