Superconductivity in ladders and coupled planes

We discuss an electronic model consisting of two chains or planes, each described by a t-J model, coupled by t’-J’ interactions between them. For J’∼J or larger we show the presence of a spin-gap and hole-pair formation upon doping. The model exhibits superconducting pairing correlations away from half filling. We support our claims by numerical studies of the spin gap, the binding energy of holes, and the pairing correlations on finite clusters. The hole-pairing operator is a spin singlet with one member of the pair in each chain or plane. Our model belongs to the universality class of the U<0 Hubbard model with ‖U‖≫t. We argue that this model may be physically realized by doping the orthorhombic compound (VO)₂P₂O₇.