Microscopic aspects of conduction in discontinuous metal systems

Electron Transport in Discontinuous Metal Thin Films

The structure and basic experimental electrical properties of vacuum evaporated discontinuous (island) metal thin films of discrete metal nanoparticles on insulating substrates are briefly reviewed. Then the widely accepted Neugebauer and Webb (N&W) electrostatically activated electron tunneling conduction model is covered (with enhancements) before the numerous discrepancies between this model and experimental observations are identified, e.g. minimal substrate bias effect, non-linear field distribution, anomalous AC effects, asymmetrical contact effects, and switching. A modified model, based on contact electron injection and extraction, and computer simulations are introduced which explain these discrepancies at a qualitative level. However, quantitative experimental verification of the model is not possible without stable, reproducible films of known structures. The paper concludes with a review of possible preparation techniques which could yield satisfactory samples, especially self-assembly of organically protected metal nanoparticles. One of these has already demonstrated electrostatically activated conduction.

Conductivity of the Granular Metal Films Obtained by High Dose Ion Implantation into Pmma

Thin granular metal films in polymethylmethacrylate(PMMA) have been synthesized by 40 keV Fe+, ag+ or Pb+ ions implantation with fluencies up to 6*1017 ion/cm2. the resistivity of synthesized films was measured in the temperature range from 300K. to 5K. the temperature dependence of the resistivity of PMMA implanted with ag+, Pb+ and small fluence Fe+ obeys the well known law lnR~(l/T)1/2. the samples implanted by high fluence Fe+ reveal rather a different behaviour. at low temperature (T<100K) the curves R(T) fit the formulae inR~lnT. the two mechanisms of conductivity of a granular film are considered: direct tunneling and thermally activated hopping. Combined with the morphology features of films, obtained by high fluence Fe+ implantation, the above mentioned consideration offers a satisfactory explanation of the observed temperature dependence R(T).

Percolation transition in a two-dimensional system of Ni granular ferromagnets

We model magnetotransport features of the quenched condensed granular Ni thin films by a random two-dimensional resistor network in order to test the condition where a single bond dominates the system. The hopping conductivity is assumed to depend on the distance between neighboring ferromagnetic grains and the mutual orientation of the magnetic moments of these grains. We find that the quantity characterizing the transition from weak disorder (not sensitive to a change of a single bond resistivity) to strong disorder (very sensitive to such changes) scales as kappa/L(1/1.3), where L is the size of the system and kappa is a measure of disorder.