Strong-coupling effects in alkali-metal-dopedC60

Various experiments indicate that ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ are in the strong-coupling regime. There is also both theoretical and experimental evidence for a large coupling of high-frequency intramolecular phonons with the electrons. If only these phonons are relevant, it is difficult to explain the relatively low ${\mathit{T}}_{\mathit{c}}$. This paradox can be resolved by introducing additional coupling with the low-lying intermolecular modes. Such a model also provides a consistent explanation of the tunneling experiments, and of the non-BCS behavior of the critical fields, penetration depth, and NMR relaxation time.