Temperature (T) -dependent Raman scattering from ${\mathrm{C}}_{60}^{3\mathrm{-}}$ intramolecular modes and simultaneous electrical resistivity measurements were made on ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ films which exhibited a superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$=28 K. ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ films were studied for comparison, but at T=300 K only. Weak, or no T dependence in the intramolecular modes is observed through ${\mathit{T}}_{\mathit{c}}$, consistent with the fact that the mode frequencies ω>2Δ (superconducting gap). By comparing the linewidths of the modes in ${\mathit{M}}_3$${\mathrm{C}}_{60}$ to values in pristine ${\mathit{C}}_{60}$ we have estimated their contributions to the electron-phonon coupling constant λ. Values of λ have been found to be 0.6 and 0.5 for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$, respectively, in good agreement with the theoretical value λ=0.6 obtained previously by Schlüter and co-workers. The ${\mathit{H}}_{\mathit{g}}$(2)-derived mode is found to make the dominant contribution to λ. A line-shape analysis of the ${\mathit{H}}_{\mathit{g}}$(1)-derived mode suggests that this mode is a Breit-Wigner-Fano resonance. The radial ${\mathit{A}}_{\mathit{g}}$(1)-derived mode linewidth in ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ is anomalously narrow (∼0.1 ${\mathrm{cm}}^{\mathrm{-}1}$), and found to be very sensitive to sample quality, i.e., strongly correlated with the width of the superconducting transition.

• Received 2 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.8412