Spin-mediated superconductivity in cuprates, organic conductors and π-d conjugated systems

Previous theoretical models of organometallic magnetic conductors and superconductors are examined on the basis of ab initio Hamiltonians and Hubbard models for clusters of p-d and π-d systems in relation to the recently developed π-d systems such as (BEDT-TTF)₂Y and (BETS)₂Y (Y = Cu(NCS)₂, Cu[N(CN)₂]X, Fe(III)X₄ (X = halogens, etc.)). The Fe(III) complexes have been used as spin sources in these systems. The phase diagrams observed for the species are similar to those of cuprate and heavy fermion superconductors because of the existence of a magnetic phase near superconducting phase. In order to elucidate the characteristic electronic structures of these species, effective exchange integrals (J_ab) for magnetic clusters are calculated by ab initio density functional (DFT) methods. From the computational results, several model Hamiltonians such as t-J, Kondo and RKKY models are examined for a theoretical understanding of the experimental phase diagrams. Theoretical possibilities of magnetic conductors and spin-mediated superconductors are discussed on the basis of these models in the intermediate region for metal-insulator transitions. The importance of electron correlation and lattice dimensionality is emphasized in relation to high-T_c superconductivity.