Classical and Tunneling Percolation Transitions in Nanoparticle Composites

Electrical conductivity in nanoparticle composites, which consist of insulating and conducting nanoparticles, is determined by two competing mechanisms, percolation in a continuous conducting network (governed by classical percolation with universal critical exponents) and tunneling between isolated conducting particles (described by tunneling percolation with non-universal critical exponents). In this work, we investigate a flexible model system based on half-metallic spherocylindrical CrO₂ nanoparticles, which can be gradually converted by annealing from metallic (CrO₂) to insulating (Cr₂O₃) state. Hence we can study a transition from classical to tunneling percolation with the variation of insulating Cr₂O₃ shell barrier (which can be controlled with sub-nanometer precision), through both experimental measurements and computer simulations - by utilizing a combination of hard-particle Monte Carlo and mechanical contraction methods. Our experimental and theoretical results for the percolation thresholds and critical exponents for classical percolation are in good agreement. We will also report the results of tunneling percolation, as well as magnetoresistance measurements for different tunnel barrier thicknesses.