Optimized unconventional superconductivity in a molecular Jahn-Teller metal

Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above T(c) is a major challenge for all unconventional superconductors. The molecular A(3)C(60) fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C(60)(3–) electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of T(c) with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.