Real-space d-wave superconductivity from weak attraction

It is shown that even a weak out-of-plane attraction, V -> 0, can form real-space pairs in the body-centered tetragonal lattice despite the presence of a Hubbard repulsion and a fully developed three-dimensional kinetic energy. In the hole channel, the pairs have d orbital symmetry which, following Bolgoliubov's argument, translates into a d-symmetric macroscopic superconducting order parameter. The findings help us to understand key features of La_{2-x}Sr_xCuO_4 and further support the real-space mechanism of superconductivity in cuprates. The rules of real-space superconductivity are formulated at the end.