  We study the binding of a holon to a doublon in a half-filled Hubbard model
as the mechanism of the zero-temperature metal-insulator transition. In a spin
polarized system and a non-bipartite lattice a single holon-doublon (HD) pair
exhibits a binding transition (e.g., on a face-centred cubic lattice), or a
sharp crossover (e.g., on a triangular lattice) corresponding well to the
standard Mott transition in unpolarized systems. We extend the HD-pair study
towards non-polarized systems by considering more general spin background and
by treating the finite HD density within a BCS-type approximation. Both
approaches lead to a discontinuous transition away from the fully polarized
system and give density correlations consistent with numerical results on a
triangular lattice.
