J‐model for magnetism and superconductivity of triangular, kagome, and related spin lattice systems

Abstract

AbstractA previous theoretical model, the J‐model, for superconductivity of triangular, Kagome, and related spin lattice systems, has been reinvestigated in relation to the recent discovery of superconductivity in NaxCoO2 · yH2O. To elucidate a possible interrelationship between magnetism and superconductivity, effective exchange integrals (Jab) for magnetic clusters with triangular lattices are calculated by ab initio generalized Hartree–Fock (GHF) and generalized density functional theory (GDFT) methods through the use of general spin orbitals (GSO). From the computational results, several model Hamiltonians such as J‐ and t–J‐models are derived for elucidation of the electronic properties of molecule‐based materials with triangular, Kagome, and related spin lattice systems. The theoretical possibilities of spin‐mediated superconductivity are examined on the basis of the J‐model in the intermediate region for metal–insulator transitions. The interplay of spin exchange or spin fluctuation and other effects for superconductivity of strong correlation systems is also feasible both on theoretical and on experimental grounds, supporting the cooperative mechanism toward high‐Tc superconductivity. The implications of the calculated results are discussed in relation to theoretical possibilities of non‐Bardeen–Cooper–Schrieffer (BCS) superconductivity on the basis of cooperative mechanisms of electron correlation (or spin fluctuation) and multi‐band effects in magnetic clusters with icosidodecahedral and related symmetries. Noncoplanarity of spins in molecule‐based magnets is finally discussed to elucidate spin chirality, which is related to a persistent electron current and anomalous Hall effect in triangular and Kagome spin lattices. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004