We have measured the normal-state susceptibility χ for both ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{Cu}}_{60}$ and the specific heat jump ΔC at the superconducting ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. Within a phonon-mediated pairing model, the combined effects of χ and ΔC are shown to give strong support to pairing via high-energy modes (>1000 K) associated with intramolecular vibrations. In addition, the small ΔC along with the known upper critical field slope provide evidence for an extreme dirty-limit superconductor, with mean free path l≊10 Å.

• Received 8 April 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1687