Low temperature electronic transport in RuO2-based cermet resistors

We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (<-10% wt.), high lead-silicate glass particles and Mn (≤1.4% wt.). We found that the resistance fits the exponential lawR=R0 exp (T0/T)x with x=1/4 and the most resistive samples show a cross-over to the x=1/2 regime as the temperature decreases. Both the fitting parametersR0 andT0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar to that observed inn-type GaAs [Phys. Rev. B39, 8059 (1989)] andn-type CdSe [Phys. Rev. Lett.64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys.48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.