Exact results related to extended Hubbard model containing couplings up to next-nearest-neighbour terms were deduced for restricted regions of parameter space in arbitrary D > 1 dimensions at half-filling and T = 0 temperature. Different types of commensurate charge-density waves and spin-density waves, a ferromagnetic domain and a phase separation region were found, the last being for the first time signalled at the level of approximation-free results in dimensions higher than one. In connection with other presented phases we extended their stability domains in comparison with previously published data. The deduced phase diagram is complete in the localized limit. The dependences on the interaction range and dimension were studied. The obtained results emphasize the importance of next-nearest neighbour terms in the development of ordered phases within the studied system.
Exact results related to extended Hubbard model containing couplings up to next-nearest-neighbour terms were deduced for restricted regions of parameter space in arbitrary D > 1 dimensions at half-filling and T = 0 temperature. Different types of commensurate charge-density waves and spin-density waves, a ferromagnetic domain and a phase separation region were found, the last being for the first time signalled at the level of approximation-free results in dimensions higher than one. In connection with other presented phases we extended their stability domains in comparison with previously published data. The deduced phase diagram is complete in the localized limit. The dependences on the interaction range and dimension were studied. The obtained results emphasize the importance of next-nearest neighbour terms in the development of ordered phases within the studied system.
We have modified an existing real space renormalization group scheme to remove some physically inconsistent features and used the rectified method to study the ground state of the one-dimensional Hubbard model with nearest-neighbour repulsion and the bond-charge interaction for half filling. For the nearest-neighbour interaction we find a phase diagram which agrees nicely with the Monte Carlo results. For the case of bond-charge interaction we have reproduced some available exact results and found some interesting information in the parameter regime where no exact solution is available.
Phase diagrams of the one-dimensional extended Hubbard model with a nearest neighbor interaction
V and site-off-diagonal (bond-charge
X and bond-bond
W) interactions at half-filling are obtained numerically, using the level-crossing approach for finite-size clusters. This analysis is straightforward extension of the preceding work [cond-mat/9909277], where only the
X term is considered as a site-off-diagonal term. Similarly to the
X term, the
W term enlarges bond-charge-density-wave (BCDW) and bond-spin-density-wave (BSDW) phases for
W<0 and
W>0 cases, respectively. In the strong-coupling region, however, the
W term causes ferromagnetism for
W>0, and shifts the boundary of the phase separation in the
U/t≪−1 region, which are different from the roles of the
X term. In addition, we discuss a parameter region where the BSDW state is the exact ground state, and show that the exactly solvable point appears just on the phase boundary between the BSDW and the ferromagnetic phases.