Tunneling percolation staircase in nanoparticle composite systems

Electrical conduction in composite systems consisting of conducting fillers embedded in insulating matrix is defined by two mechanisms, an electron current flow via a direct contact between metallic particles/clusters (classical percolation) and the flow through tunnel barriers separating metallic particles/clusters (tunneling percolation). Classical percolation obeys scaling law with universal critical exponents while tunneling percolation obeys scaling law with a non-universal critical exponent for system conductivity. In this work, we investigated tunneling percolation in CrO₂ (half-metal) and Cr₂O₃ (insulator) experimentally. Composite samples with different volume fractions were prepared and the four-wire electrical transport measurements were performed. We observed tunneling staircase at low volume fractions of CrO₂ due to conduction through interphase regions. We also studied tunneling percolation staircase in CrO₂ and CrO₂ particles with a Cr₂O₃ shell, and investigated the conductivity property at various temperatures.