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

Two transition concentrations of the conducting component in RuO2-glass films are defined. The first is identified with the classical percolation threshold v(c) and its value has been found as lying usually in the range 0.02-0.04. At the second, denoted v(q), the temperature coefficient of resistance, measured at room temperature, changes its sign and it is found that v(q) is most often between 0.12 and 0.16. The relatively small value of v(c) is interpreted within the framework of a percolation model including segregation. A segregation coefficient is defined as particle size ratio chi = D/d of the glass and RuO2 particle mean diameters D and d, respectively, with chi ranging from 10 to 150. Our computer simulation results v(c)(chi = 1)congruent to 0.16 and v(c)(chi = infinity)congruent to 0.02 agree with the literature data for uncorrelated site-percolation and random-void models, respectively. The transition observed at v(q) is interpreted as quantum percolation threshold, separating a system with localized states for v < v(q) from a system with extended states for v > v(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-Biittiker 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 congruent to 0.33 for compositions below and above v(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.