Plasmon dispersion and broadening in A(3)C(60) (A=K,Rb)

Inelastic electron energy loss measurements of the 0.5-eV charge carrier plasmon in K3C60 are presented, showing that the plasmon has a very large broadening but a negligible dispersion. The response function is calculated in a self-consistent, conserving approximation, showing that the plasmon broadening can be explained in terms of a decay in an electron-hole pair dressed by phonons. The plasmon dispersion can be understood within the random-phase approximation. Local-field effects tend to give a large negative dispersion, which is compensated by interband effects.