Superconductivity, Mott Hubbard states, and molecular orbital order in intercalated fullerides

This article reviews the current status of chemically doped fullerene superconductors and related compounds, with particular focus on Mott–Hubbard states and the role of molecular orbital degeneracy. Alkaline-earth metal fullerides produce superconductors of several kinds, all of which have states with higher valence than (C60)6−, where the second lowest unoccupied molecular orbital (the LUMO + 1 state) is filled. Alkali-metal-doped fullerides, on the other hand, afford superconductors only at the stoichiometry A3C60 (A denotes alkali metal) and in basically fcc structures. The metallicity and superconductivity of A3C60 compounds are destroyed either by reduction of the crystal symmetry or by change in the valence of C60. This difference is attributed to the narrower bandwidth in the A3C60 system, causing electronic instability in Jahn–Teller insulators and Mott–Hubbard insulators. The latter metal–insulator transition is driven by intercalation of ammonia molecules into A3C60-type superconductors. Furthermore, the triple degeneracy of the LUMO state of C60 plays a crucial role in the metal–insulator transition and in controlling the magnetic structures of insulating states, possibly providing novel properties of degenerate orbitals. The goal of this article is to establish a unifying picture of fullerene intercalation compounds, and to clarify the underlying physics: competing energy scales and orbital properties of molecule-based systems.