'Spin-disentangled' exact diagonalization of repulsive Hubbard systems: superconducting pair propagation

By means of a novel exact-diagonalization technique, we show that bound pairs propagate between repulsive Hubbard clusters in a superconducting fashion. The size of the matrices that must be handled depends on the number of fermion configurations per spin, which is of the order of the square root of the overall size of the Hilbert space. We use CuO₄ units connected by weak O–O links to model the interplanar coupling and c-axis superconductivity in cuprates. The numerical evidence on Cu₂O₈ and Cu₃O₁₂ prompts the proposal of a new analytic scheme describing the propagation of bound pairs and also the superconducting flux quantization in a 3D geometry.