Resistance relaxation in carbon and RuO 2 based thermometers

The long-term time dependent relaxation of the electrical resistance in several carbon and RuO2 based thermometers at helium temperatures was investigated. The relative change of the resistance at a fixed low temperature measured after cooldown is always found to be linear in an appropriate logarithmic time scale. No tendency to saturation of this drift was found for any tested sensor. A theoretical model explaining this relaxation and based on the variable range hopping conductivity mechanism as well as the standard two-level theory of glasses at low temperatures is developed. The predictions of this model agree with experimentally obtained data for the isothermal relative resistance drift both immediately after cooldown and after low temperature changes.