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 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 C-60 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.