A model for a spin-1/2 ladder system with two legs is introduced. It is demonstrated that this model is solvable via the Bethe ansatz method for arbitrary values of the rung coupling J. This is achieved by a suitable mapping from the Hubbard model with appropriate twisted boundary conditions. We determine that a phase transition between gapped and gapless spin excitations occurs at the critical value Jc=1/2 of the rung coupling.
Physical review. B, Condensed matter and materials physics. Melville. Vol. 62, no. 1 (July 2000), p. 65-68