Classical percolation threshold and resistance versus temperature behaviour of RuO2-glass films

Abstract Two transition concentrations of the conducting component in RuO 2 -glass films are defined. The first is identified with the classical percolation threshold ν c and its value has been found as lying usually in the range 0.02–;0.04. At the second, denoted ν q , the temperature coefficient of resistance, measured at room temperature, changes its sign and its is found that percolation ν q is most often between 0.12 and 0.16. The relatively small value of ν c is interpreted within the framework of a model including segregation. A segregation coefficient is defined as particle size ratio χ = D / d of the glass and RuO 2 particle mean diameters D and d , respectively, with χ ranging from 10 to 150. Our computer simulation results ν c ( χ = 1)≅0.16 and ν c ( χ = ∞)≅0.02 agree with the literature data for uncorrelated site-percolation and random-void models, respectively. The transition observed at ν q is interpreted as quantum percolation threshold, separating a system with localized states for ν ν q from a system with extended states for ν > q . This is illustrated by results of numerical studies of the dimensionless conductance g in a quantum percolation model; the calculations are made using Landauer-Buttiker formula and a Green's function method. Studies of conductance versus temperature characteristics from 4.2 to 300 K show G = a + bT y low-temperature behaviour with y ≅0.33 for compositions below and above ν q but close to it. An interpretation of this behaviour is given within a localization/delocalization picture assuming that inelastic scattering processes play a dominant role in the transport properties. Other approaches like those based on electron-electron interaction effects and hopping conduction are also discussed.