  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 longe-range order.
