We present experimental and theoretical results for the dispersion of the 0.5 eV charge-carrier plasmon in A3C60 (A=K,Rb) compounds. The experimental dispersion in K3C60 is zero within the experimental accuracy, and the theoretical dispersion is slightly negative. We show that the small dispersion results from two large, opposing effects. Local field effects tend to lead to a large negative dispersion, while interband transitions largely cancel this effect. The interband effects can be thought of in terms of the dielectric function due to classical, polarizable spheres describing the C60 molecules. This dielectric function reduces the plasmon energy, and the reduction of the dielectric function with increasing q tends to lead to a large, positive dispersion.
 

Journal of Physics: Condensed Matter, 8 2557-69, 1996.