Interacting spin systems exhibiting spin plateaus and gaps

We study a spin chain and a three-dimensional spin lattice with ground stat es given by a product of spin-singlet nearest-neighbor pairs. In a magnetic field, the ground state and low-lying states are a mixture of spin singlet s and spin-polarized triplets; these states are highly degenerate. When the lattice is distorted, hopping terms are introduced which lift the degenera cy. For particular parameter ranges, the magnetization versus magnetic-fiel d curve exhibits spin plateaus, when the excited states are separated from the ground state by a spin gap. At a critical point, the plateau and the co rresponding gap vanish. This spin system is embedded in a more general inte racting electron system, and when doped, the charged carriers are found to bind in pairs.