Physical origins of current and temperature controlled negative differential resistances in NbO2

Negative differential resistance behavior in oxide memristors, especially those using NbO2, is gaining renewed interest because of its potential utility in neuromorphic computing. However, there has been a decade-long controversy over whether the negative differential resistance is caused by a relatively low-temperature non-linear transport mechanism or a high-temperature Mott transition. Resolving this issue will enable consistent and robust predictive modeling of this phenomenon for different applications. Here we examine NbO2 memristors that exhibit both a current-controlled and a temperature-controlled negative differential resistance. Through thermal and chemical spectromicroscopy and numerical simulations, we confirm that the former is caused by a ~400 K non-linear-transport-driven instability and the latter is caused by the ~1000 K Mott metal-insulator transition, for which the thermal conductance counter-intuitively decreases in the metallic state relative to the insulating state. The development of future computation devices will be aided by a better understanding of the physics underlying material behaviors. Using thermoreflectance and spatially resolved X-ray microscopy, Kumar et al. elucidate the origin of two types of negative differential resistance in NbO2 memristors.