PLASMON DISPERSION IN A(3)C(60) (A=K, RB)

We present experimental and theoretical results for the dispersion of the 0.5 eV charge-carrier plasmon in A(3)C(60) (A = K, Rb) compounds. The experimental dispersion in K3C60 is zero within the experimental a ccuracy, and the theoretical dispersion is slightly negative. We show that the small dispersion results from two large, opposing effects. Lo cal-field effects tend to lead to a large negative dispersion, while i nterband transitions largely cancel this effect. The interband effects can be thought of in terms of the dielectric function due to classica l, polarizable spheres describing the C-60 molecules. This dielectric function reduces the plasmon energy, and the reduction of the dielectr ic function with increasing q tends to lead to a large, positive dispe rsion.