The electronic part eta (the Hopfield factor) of the electron-phonon coupling constant lambda for alkali-metal-doped fullerenes A3C60 is calculated within the rigid muffin-tin-potential approximation. It is found that eta is large for tangential atomic motions, while for the radial vibrations eta is 20 times smaller. The authors have calculated eta for three lattice constants (a=14.1, 14.4, and 14.6 AA), corresponding approximately to those of C60, Rb3C60, and (hypothetical) Cs3C60, and found eta =21, 32, and 36 eV/AA2. Using semiempirical nearest-neighbor force constants estimated lambda =0.49, 0.77, and 0.83 and ( omega log)=870 cm-1 for the average phonon frequency. The McMillan formula yields Tc=5, 36, and 44 K for these lattice constants, in reasonable agreement with the available experimental data. The relatively high-temperature superconductivity in A3C60, as well as the strong dependence of Tc on the dopant, is fully explained within the framework of the conventional superconductivity theory.
 

Physical Review B, 45 5114-17, 1992.