Superconductivity from doping a spin-liquid insulator: A simple one-dimensional example

We study the phase diagram of a one-dimensional Hubbard model where, in addition to the standard nearest-neighbor hopping t, we also include a next-to-nearest-neighbor hopping t'. For strong enough on-site repulsion, this model has a transition at half filling from a magnetic insulator with gapless spin excitations at small t'/t to a dimerized insulator with a spin gap at larger t'/t. We show that upon doping this model exhibits quite interesting features, which include the presence of a metallic phase with a spin gap and dominant superconducting fluctuations, in spite of the repulsive interaction. More interestingly, we find that this superconducting phase can be reached upon hole doping the magnetic insulator. The connections between this model and the two chain models, recently object of intensive investigations, are also discussed.