Crystalline films of ${\mathrm{C}}_{60}$ with ∼1 μm grain size and a preferred [111] texture have been doped with K and Rb to form superconducting compositions that have sharp zero-resistance transitions. The temperature dependence of the resistance above ${\mathit{T}}_{\mathit{c}}$ for both ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ films reveals metallic behavior up to temperatures as high as 520 K without any evidence of saturation. For ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$, electronic mean free paths significantly shorter than nearest-neighbor ${\mathrm{C}}_{60}$ distances and anomalously high values of the electron-phonon coupling strength are inferred. These results suggest that there is a strong interaction between the conduction electrons and the intramolecular vibrational modes and also bring into question the assumption that all of the donated charge (three electrons per ${\mathrm{C}}_{60}$) is itinerant.

• Received 30 June 1993

DOI:https://doi.org/10.1103/PhysRevB.48.9945