Theoretical studies of molecule-based magnetic conductors

Our theoretical efforts towards molecule-based magnetic conductors and superconductors on the basis of ab initio Hamiltonians and effective model Hamiltonians are summarized in relation to recently developed pi-d systems, for which transition metal complexes have been used as spin sources. Organic pi-electron donors or acceptors coupling with organic radicals (R) are also investigated as possible pi-R interaction systems. In order to elucidate electronic and magnetic properties of these species, effective exchange integrals (J(ab)) for magnetic clusters are calculated by ab initio hybrid density functional (HDFT) methods. The ab initio J(ab) values are numerically reproduced by using model Hamitonians such as t-J, Kondo, Anderson and RKKY models. Theoretical possibilities of magnetic conductors and spin-mediated superconductors are elucidated on the basis of these models in the intermediate region for metal-insulator transitions. Interrelationships among magnetism, superconductivity and fluctuations in strongly correlated electron systems are discussed on the basis of the SO(5) and three-dimensional quantum electrodynamics (QED(3)) theories. External magnetic field effects such as Jaccarino-Peter effect are also examined for elucidation of magnetic field induced superconductivity. Implications of the calculated results are finally discussed in order to obtain a unified picture of many p-d, pi-d and pi-R molecule-based systems with strong electron correlations. (C) 2003 Elsevier Science Ltd. All rights reserved.