XXZ model as an effective Hamiltonian for generalized Hubbard models with broken eta -symmetry

We consider the limit of strong Coulomb attraction for generalized Hubbard models with eta -symmetry. In this limit these models are equivalent to the ferromagnetic spin- 1/2 Heisenberg quantum spin chain. In order to study the behaviour of the superconducting phase in the electronic model under perturbations which break the eta -symmetry we investigate the ground state of the ferromagnetic non-critical XXZ-chain in the sector with fixed magnetization. It turns out to be a large bound state of N magnons. We find that the perturbations considered here lead to the disappearance of the off-diagonal long-range order.