A theory of the normal and superconducting states of doped solid C60

In this paper the authors identify the Stability of Molecular Singlets (SMS) as an important feature of MxC60, and argue that it arises from the intramolecular carbon pπ-electron system of the C60 molecule. Alternatively SMS is shown to arise from the exchange of virtual triplet intra-molecular excitons. SMS is modelled as an effective intra-molecular pairing resulting in a narrow band model with an effective negative Hubbard U, which explains the observed superconductivity as well as predicts anomalous boson metal like normal state properties. Disorder plays an important role and restricts superconductivity to a narrow range of x around 3. The narrowness of the conduction band puts limitations on the maximum attainable Tc