Conduction-band plasmons in cluster-type compounds: Application to fullerides and quantum-dot arrays

We analyze collective excitations of carriers in three- and two-dimensional crystals made up of cluster-type units (fullerenes, quantum dots, etc.). This class of compounds is unique in that the electrons are essentially confined to the individual clusters, while the latter are both large in size and close packed. We derive a general expression for the dispersion law of conduction-band plasmons, and apply it to alkali-metal-doped fullerides and quantum-dot arrays. We find that the dispersion relation is modified from the case of a homogeneous electron gas: it becomes measurably steeper and acquires a downward slope. This change is due to the appearance of a term which is determined by the density distribution of electrons within a unit cluster. Related electron-energy-loss spectroscopic data on the doped fullerides are discussed.