Quantum Chemistry, Localization, Superconductivity, and Mott-Hubbard U

Abstract

A model for superconductivity and delocalisation of electrons based on fundamental principles is discussed and critically compared to other models. Mott insulators contain one electron per site and have a spin density wave (SDW) ground state. Local electron pair formation on alternant sites is possible by disproportionation, if Mott-Hubbard U≈ ΔG° < 0. A non-degenerate charge density wave (CDW) state is formed. Oxidation state is characterized by geometry and integrated charge. Transfer of electrons is therefore coupled to nuclear motion. An electron pair can move to adjacent site with or without activation energy. The activation barrier is absent if the geometry change is small and the inter-site electronic coupling via the SDW state sufficiently large. A degenerate, delocalised ground state is formed with an energy gap, zero resistivity and currents in a magnetic field. Whether the electrons are localised or delocalised depends on the relative values of coupling and reorganization energy, and can be calculated without involving U.