EXACT MEAN-FIELD THEORY OF THE EXTENDED SIMPLIFIED HUBBARD-MODEL

The recently introduced concept of exact mean-field theories is applied to the extended simplified Hubbard model, which is a one-band extended Hubbard model (including nearest-neighbor interaction) where only the down electrons can hop. An exact mean-field Hamiltonian is derived for the model on a Bethe lattice in the limit of a large coordination number (Z --> infinity). The mean-field Hamiltonian is used to calculate the Green functions and the free energy at half-filling. This exact solution is analyzed analytically at small and large values of the on-site (U) and nearest-neighbor (V) interaction strengths. One finds that a phase transition occurs at sufficiently low temperatures for all U, V > 0. The low-temperature phase is a charge-density wave for V > 1/2 U and a spin-density wave for V < 1/2 U. Special attention is paid to (i) the critical temperature as a function of U and V, (ii) the order parameter as a function of temperature, and (iii) the density of states. The density of states shows that the low-temperature phase is an insulator for all U, V provided that V/U not-equal 1/2, while the high-temperature phase is a paramagnetic metal for all V if U < 2, and a paramagnetic insulator if U > 2. Many detailed results are given.