Energy spectrum of extended Hubbard model with spin-dependent hopping and related spin ladder model

We studied the energy spectrum of the 1-D extended Hubbard model with spin-dependent hopping and related spin ladder system formed by two coupled XXZ spin 1/2 chains with the interchain interaction of Ising type. It was proved that the model excitation spectrum has no gap excepting some special values of z-projection of the ground-state total spin. The thorough analytic consideration of two-magnon states was given. The existence tip to five bound states at specified value of quasimomentum of the pair of inverted spins was shown. We also present the results of density matrix renormalization group calculations that showed nonadequacy of the pair approximation for n-magnon bound states of the extended model with the strong electron-electron interactions. (C) 2002 Wiley Periodicals, Inc.