CROSSOVER FROM PHONON-MEDIATED TO PHOTON-MEDIATED CHARGE-TRANSPORT OBSERVED IN METAL-CLUSTER COMPOUNDS

We have studied the frequency-dependent conductivity of randomly packe d aggregates of ligand-stabilized, monodisperse metal clusters from de to the infrared over 12 decades in frequency. At low frequencies the ac conductivity of these compounds is dominated by phonon-assisted hop ping between localized states, and shows scaling behavior as a functio n of temperature and frequency. Due to the unique structure of these c luster aggregates, the transition from phonon- to photon-mediated char ge transport is clearly observed. The data are found to be in excellen t quantitative agreement with the two-site tunneling model.