Variations of electron-phonon coupling in alkali-metal–C60solid solutions

We report on in situ, interference-enhanced Raman-scattering studies of ultrathin, several monolayer thick films that demonstrate solid-solution formation of ${\mathit{A}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ for A=NA, K, Rb, Cs and x≤3. In contrast to ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$, which indicates moderate, disorder-related vibrational broadening, the other alkali-metal–fulleride Raman spectra exhibit larger changes in intramolecular ${\mathrm{C}}_{60}$ derived Hg(2) and Hg(8) modes due to substantial electron-phonon coupling. These observations are consistent with the absence of superconductivity in ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ and comparable, substantial electron-phonon coupling in ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$. For ${\mathrm{Cs}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$, the largest electron-phonon coupling for Hg(2) is observed for x≃1, in contrast to values of x≃2 for K and Rb doped materials.