Crossover from phonon- to photon-mediated charge transport observed in metal-cluster compounds

We have studied the frequency-dependent conductivity of randomly packed aggregates of ligand-stabilized, monodisperse metal clusters from dc to the infrared over 12 decades in frequency. At low frequencies the ac conductivity of these compounds is dominated by phonon-assisted hopping between localized states, and shows scaling behavior as a function of temperature and frequency. Due to the unique structure of these cluster aggregates, the transition from phonon- to photon-mediated charge transport is clearly observed. The data are found to be in excellent quantitative agreement with the two-site tunneling model.