The Disorder-Free Non-BCS Superconductor Cs3C60 Emerges from an Antiferromagnetic Insulator Parent State

The body-centered cubic A15-structured cesium fulleride Cs3C60 is not superconducting at ambient pressure and is free from disorder, unlike the well-studied face-centered cubic A(3)C(60) alkali metal fulleride superconductors. We found that in Cs3C60, where the molecular valences are precisely assigned, the superconducting state at 38 kelvin emerges directly from a localized electron antiferromagnetic insulating state with the application of pressure. This transition maintains the threefold degeneracy of the active orbitals in both competing electronic states; it is thus a purely electronic transition to a superconducting state, with a dependence of the transition temperature on pressure-induced changes of anion packing density that is not explicable by Bardeen-Cooper-Schrieffer (BCS) theory.