Role of Bond-Bond Interaction in the Extended Hubbard Chain
  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   $U/t\ll-1$ 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. 
Masaaki Nakamura, Kazuhito Itoh and Norihiro Muramoto (ISSP Univ. of   Tokyo)